PKR ELECTRICAL ENGINEERING
101. What is battery?
The combination of two or more cells is called the battery.
102. What are the classifications of cell?
a. Primary cells.
b. Secondary cells.
103. What are the differences between primary cell and secondary cell?
Primary cells are those cells, which cannot be re-charged after the substances
(electrolyte, electrode and container) used in it becomes useless.
The common primary cells in use are,
a. Simple voltaic cell (one fluid cell).
b. Daniel cell (two fluid cell).
c. Leclanche cell (two fluid cell).
d. Dry cell.
e. Standard cell or Weston cadmium cell.
Secondary cells are those cells, which can be re-charged and use again once they
discharged or used for the work for number of times with out re-newing it’s
materials.
Most commonly used secondary cells are,
a. Lead acid cell.
b. Nickel iron alkaline cell.
c. Nickel cadmium alkaline cell.
104. What is polarization? What is local action?
Polarization
The hydrogen bubbles which are clinging over the surface of copper electrode
(anode) becomes a thin film of hydrogen over the copper electrode. This hydrogen
film increases the internal resistance and reduces the emf of the cell and hence the
cell soon becomes inactive. This effect is known as polarization
.
Local action
In voltaic cell it is observed that, when the cell is not connected to the load and not
supplying any current zinc will continuously dissolving in the electrolyte. This is
due to the impurities (copper, iron, tin, and lead) in the commercial zinc. So that
whenever commercial zinc is used as a electrode, separate small cells are
developed between the impurities and zinc with the presence of electrolyte. These
local cells consume always zinc and the emf developed by those local cells are
always opposite to the main emf. The action of these cells is known as local action.
105. What are the advantages of secondary cells over primary cell?
a. It gives high current capacity.
b. Its internal resistance is very low.
c. It gives a constant current.
d. It posses very high efficiency.
e. It posses fairly constant emf.
f. It posses good mechanical strength.
g. It posses large storage capacity.
h. It can be renewed by charging after it is discharge.
i. It is durable.
106. What is Plante plate and Faure plate?
There are two types of positive plate preparation. They are Plante plate and Faure
plate.
Plante plate
As per plante process positive plate PbO2 are prepared by a process of repeated
charging and discharging of pure lead. Positive plates, which are made by this
process, are also called ‘formal plates’. This process of positive plate preparation
required very long time for it’s manufacturing and so it is very costly.
Faure plate
Faure plates are generally made up of rectangular lead grid into which the active
material lead peroxide PbO2 is filled in the form of paste.
107. How negative plate is made up of?
The negative plate of a lead acid cell is made up of spongy lead ‘Pb’. The negative
plates are also of rectangular lead grid and the active material Pb in the form of
paste is held firmly in this lead grid.
108. Why negative plates are one more than positive plates?
Negative plates are one more than positive plates so as to get negative plates on
both the sides of positive plates. This is to prevent the buckling action of the lead
on positive plate in the multi plate lead acid cell. The other reason is that both the
sides of positive plates will become active and the efficiency of the positive plate
and the cell will increase.
109. What is electrolyte?
Electrolyte is the medium through which the current produces chemical changes.
Electrolyte is a mixture of sulphuric acid o 1.85 specific gravity (concentrated
sulphuric acid) diluted with distilled water in the ratio of 1:3 approximately, so the
specific gravity of the dilute sulphuric acid is 1.280.
110. What are the types of grouping of cells?
There are three main ways of grouping.
a. Series grouping.
b. Parallel grouping.
c. Series parallel grouping.
111. What are the advantages of series grouping and parallel grouping?
Advantages of series grouping.
a. The total emf increases and is equal to ‘nE’. Where n – total number of cells in
series and E – emf of one cell.
b. The internal resistance ‘r’ also increases and equal to ‘nr’. So total resistance of
the circuit also increases and is equal to R + nr ohms. Where R – external load
resistance.
c. Total current is equal to one cell current. That is there is no current increase. If
the internal resistance is negligible or less then current will be maximum.
Advantages of parallel grouping.
a. In parallel grouping emf of one cell will be the total emf of the grouping.
b. Total internal resistance of the parallel group is equal to r/n.
c. Total resistance of the group is equal to R + r/n.
d. Total current = E / (R +r/n) amps.
So we can understand that parallel useful when the external resistance is small as
compared to internal resistance of the parallel group. But at the same time series
grouping is useful when the internal resistance is small compared to the external
resistance of the group.
112. What are the methods of charging of battery.
Mainly there are three types of charging of battery.
a. Constant current charging system.
In this system the charging current is kept to constant by varying the supplied DC
voltage by the help of rheostat or filament lamps in series with the battery, so as to
over come the increased back emf of the battery or of the cell.
Charging current = V – Eb / R + r amps.
b. Constant voltage or potential charging.
In this system the voltage is kept to constant, so the charging current in the
beginning will be high when the back emf or counter emf of the battery is low and
current will be small when the back or counter emf increases as the battery gets
charge.
c. Trickle charging system.
The continuous charging of a battery at a very low rate for keeping the battery
ready in good working condition is called the trickle charging. This is to maintain
the losses occurring at the idle period. The value of the trickle charging current is
approximately 2% of the full charging current of the battery.
113. What are the factors on which the capacity of the battery depends?
The capacity of the battery is measured in ampere-hour. The capacity of the battery
depends upon the following factors.
a. Number and area of the positive plate.
b. Discharge voltage. A cell should not be discharged below 1.8 V. If it is
discharged below 1.8 V it may cause to reduce the capacity.
c. Discharge rate. Capacity decreases with increase rate of discharge.
d. Specific gravity of electrolyte. With rapid rate of discharge causes to weaken
the electrolyte so the chemical action also weakens and there by the capacity
decreases. When the specific gravity increases the capacity of the battery
increases.
e. Quantity of electrolyte. Electrolyte level should be at the top plate level.
f. The design of separator. The design of the separator should be thin.
g. Temperature. When the temperature increases the resistance of the battery
decreases and the capacity increases.
114. Explain Kirchhoff’s laws.
Kirchhoff’s laws are used in complex network circuits to determine the equivalent
total resistance and the current flowing in various conductors of that circuit.
Mainly there are two laws.
a. Point law or current law.
b. Mesh law or voltage law.
Point law or current law.
The point law states that, the algebraic sum of the currents meeting at any point or
junction or node of a network is zero. In other words the sum of the currents
flowing towards the junction or node or any point of network is equal to the total
current flowing away from that junction.
Mesh law or voltage law.
The mesh law states that, in any closed electrical circuit the algebraic sum of the
potential drops is equal to the sum of the impressed emf’s acting in that close
circuit. In this the important factor is to determine the emf sign to calculate the total
emf.
115. What are the types of wiring?
Mainly there are two types of wiring systems.
a. Tree system.
b. Distribution system.
116. What are the systems of wiring?
Following are the general systems of domestic wiring and industrial wiring.
a. Cleat system wiring.
b. Casing and capping system wiring.
c. Lead sheathed system wiring.
d. C.T.S, T.R.S, P.V.C sheathed system wiring.
e. Conduit system wiring.
117. What are the testing of wiring installation?
Following are the tests to be done after installation of wiring.
a. Polarity test.
b. Short circuit test.
c. Continuity test.
d. Insulation test between conductors and conductors to the earth.
e. Earth continuity test.
118. What are the advantages of AC over DC?
a. For the same capacity alternators are cheaper than DC generators, because
alternator is not having commutator arrangement and there by small in size.
b. Alternating current produces pulsating magnetic field and there by it posses the
property of inductance and capacitance.
c. Alternating current can be step-up or step-down by static transformer.
d. AC can be transmitted with very less cost in comparing to DC transmission.
e. Alternating line losses are very less comparing to DC line losses.
f. An alternators and AC motor requires very less maintenance.
g. Charge per unit for AC is less than DC.
119. Define AC.
Alternating current is that type of electric current, which changes it’s magnetude
and direction periodically.
120. What is cycle?
One complete set of changes in value and direction of alternating quantity and emf
or current is called a cycle.
121. What is periodic time?
Periodic time is the time taken to complete on cycle. Its symbol is ‘T’. For example
Indian standard frequency is 50 cycles per second. So the periodic time T = 1/50
seconds. That is equal to 20 m seconds.
122. What is frequency?
Number of cycles per second is called frequency.
123. What is amplitude value or peak value?
It is the maximum value of an alternating quantity that can be obtained in any one
direction.
124. What is instantaneous value?
The value of an alternating quantity at a particular instant is called instantaneous
value.
125. What is average value or mean value?
Average of all instantaneous values of emf or current over a half cycle is known as
average value or mean value.
Average value = 0.637 * Emax or Imax
126. What is root mean square value (R.M.S)?
The R.M.S value is also known as effective value or virtual value. The
instantaneous value of both the directions will all be squared up and will be added
together. Then divide to get the average with the number of instantaneous values
and find the square root of this average to calculate the R.M.S value of the emf or
current.
Or
The R.M.S value of an alternating current or emf is equal to the same value of
direct current (DC), which produces the same amount of heat with the same time
when applied the DC through the same circuit as AC is produced.
R.M.S value = maximum value / √2 = 1/√2 = 0.707.
∴ R.M.S value or effective value = 0.707 * Emax or Imax
127. What is form factor?
The ratio of the R.M.S value to the average value is called the form factor.
∴ Form factor = 0.707 * Emax or Imax : 0.637 * Emax or Imax
= 0.707 * Emax or Imax / 0.637 * Emax or Imax
= 1.11
So that R.M.S value = average value * 1.11
Or average value = R.M.S value / 1.11
128. What is crest factor or peak factor?
The ratio of maximum value to the R.M.S value is known as crest factor. So the
crest factor = maximum value / R.M.S value.
= Emax or Imax / (Emax or Imax / √2)
= Emax or Imax * √2 / Emax or Imax = √2 = 1.414
129. What is vector quantity and what is scalar quantity?
Vector quantity
A quantity, which has both the direction and magnitude is said to be a vector
quantity. Examples are force, emf, current etc.
Scalar quantity
A scalar quantity is that, which has only magnitude but no direction. Examples are
temperature, mass, volume etc.
130. What is phase?
The development of an AC quantity through different stages is known as Phase.
The term phase refers to the number of separate individual voltage setup in an AC
circuit.
131. What is in-phase?
When those two vectors (voltage and current) attain (reaches) their maximum and
minimum values simultaneously (at the same time), then those two quantities are
said in-phase. Here between those quantities there is no angle.
132. What is out of phase?
When two alternating quantities voltage and current do not reaches their maximum
and minimum values simultaneously, then they are called out of phase.
133. What is phase angle?
Phase angle is an angular displacement between two alternating quantities. Phase
angle is measured in electrical degrees or radians.
134. What is quadrature quantity?
When the phase angle between two vectors is 90° electrical, then they are said to be
quadrature quantity.
135. What anti-phase quantity?
When two quantities are out of phase by 180° electrical, then they are said to be
anti-phase quantities.
136. What is leading quantity?
The alternating quantity that reaches its maximum value earlier than the other
quantity is known as the leading quantity.
137. What is lagging quantity?
The alternating quantity that attains its maximum value later than the other quantity
is called the lagging quantity.
138. What is the relation between voltage and current in AC circuit containing only
resistance?
Current (I) is in-phase with the voltage.
I = V/R amps.
P = I * V * cosϕ or I2 R watts. (Where cosϕ is zero because the voltage and current
are in-phase. So cosϕ 0° (zero) = 1)
139. What is the relation between voltage and current in AC circuit containing only
inductance?
Current (I) is lags behind the voltage by 90°.
I = V/XL amps.
XL = 2πfL ohms.
P = I * V * cosϕ watts. (Where cosϕ is 90 because current lags behind voltage by
90°. So cosϕ 90° = 0)
∴ P = I * V * 0 = 0 watts.
140. What is the relation between voltage and current in AC circuit containing only
capacitance?
Current (I) is leading the voltage by 90°.
I = V/XC amps.
XC = 1/2πfC ohms.
P = I * V * cosϕ watts. (Where cosϕ is 90 because current is leading the voltage by
90°. So cosϕ 90° = 0)
∴ P = I * V * 0 = 0 watts.
141. What is inductance and inductive reactance?
Inductance
A coil carrying alternating current produces an alternating flux, which causes to
link with same coil and produces an emf in the coil, which opposes the applied
emf. This property is known as inductance. The unit for measurement is henry.
Inductive reactance
The opposition or the reactance offered by the property of inductance in the circuit
is known as inductive reactance and denoted by the letter XL. The unit for
measurement is ohm.
142. What is capacitance and capacitive reactance?
Capacitance
The property of a capacitor to store electrical energy in it, when it is connected to
an electric supply is called capacitance. Unit for measurement is farad. Capacitor
store an electric energy in the unit of charge and the unit of charge is coulomb.
Capacitive reactance
The opposition due to capacitance of capacitor in an electric circuit is called
capacitive reactance and it denoted by the letter XC. The unit for measurement is
ohm.
143. What is impedance?
The total opposition offered by an AC circuit for the flow of current through it is
called Impedance. The letter ‘Z’ denotes it and the unit is ohm.
∴ Z = √ R2 + (XL ∼ XC) 2
Z = √ R2 + (X) 2
Where ∼ indicates the difference of XL and XC and denoted in the letter X (net
reactance of the AC circuit).
144. What is ohm’s law for AC circuit?
I = V/Z amps.
Z = V/I ohms.
V = I * Z volts.
145. What is the current and power in an AC circuit?
Current
AC circuit contains resistance ‘R’ and reactance ‘X’.
In resistive circuit IR = I cosϕ. Because resistance current (IR) is in-phase with
voltage (ER).
In reactance circuit IX = I sinϕ. Because reactance current will lead or lag the
voltage (ER) by 90°.
So the resultant current (I) is the vector sum of I cosϕ and I sinϕ. So that circuit
current I = √ (I cosϕ)2 + (I sinϕ)2 amps.
I cosϕ is some times known as power component of current or the power current or
energy current and the I sinϕ is known as reactive component of current or wattless
current. Because I sinϕ is not taking any energy from the circuit.
Power
Power in watts = terminal voltage * power component of current.
a. True power = E * I * cosϕ watts.
This true power is some times known as energy component or active
component or watt-full component. Because this is the power used to produce
torque in motor and supplies heat, light etc. or this true power is the power
consumption of all source of electric circuit.
b. Reactive power = E * I * sinϕ watts.
This reactive power is some times known as reactive or in-active component or
watt less component or VARS.
c. Apparent power = E * I watts.
The terminal voltage multiplied by the actual resultant current (I) is called the
apparent power or volt-ampere or VA.
146. What is power factor?
So from the above power explanation,
Cosϕ = true power / apparent power = E * I * cosϕ / E * I.
So that power factor is equal to
a. Cosine of angle of lead and lag of the resultant current with the applied voltage.
b. The ratio of R/Z.
c. The ratio of true power to the apparent power.
147. What is resonance in series circuit?
If in an AC circuit inductive reactance XL and capacitive reactance XC is equal the
voltage across both will be equal and are 180° out of phase. So that each will
cancel each other and the current limiting component will be the resistance of the
circuit.
If we are in a position to alter the frequency of supply voltage at a particular
frequency named as ‘resonant frequency’, AC series circuit’s XL = XC and the net
reactance will be zero. So the current in the circuit is in-phase with the voltage.
Because the controlling component of the circuit is resistance only and the current
is maximum and equal to V/R amps.
This above said condition is called ‘series resonance’ and the frequency at which it
occurs is called resonant frequency and the resonant frequency (FR) is equal to
(FR) = 1/2π√LC cycles per second.
148. What is Q-factor?
The ratio of VL/V or VC/V at the resonant frequency is called the voltage
magnification denoted as Q-factor.
Q-factor = 1 √L/C
R
149. What is Admittance?
Admittance: Admittance is the reciprocal of impedance. It is denoted by the letter
‘Y’ and the unit of measurement is mho.
Y = I/E = RMS current / RMS voltage.
Equation used in admittance
a. Conductance ‘G’ = Y * cosϕ = 1/Z *R/Z = R/Z2 mho.
b. Susceptance ‘B’ = Y * sinϕ = 1/Z * X/Z = X/Z2 mho.
c. Admittance ‘Y’ = √G2 + B2 mho.
d. In special cases when X = zero, then G = 1/R and R = zero, then B = 1/X.
PKR ELECTRICAL ENGINEERING
150. What is the resonance frequency equation for parallel circuit?
In parallel circuit when XC = XL, the circuit is called the parallel resonance circuit.
That is 2πfL = 1/2πfC.
In term (FR) = 1/2π√1/LC – R2/L2 cycles per second.
If ‘R’ is negligible, then (FR) = 1/2π√LC cycles per second.
151. What is poly phase?
A system with two or more the two phases is known as poly phase system.
152. What is phase sequence?
The sequence of attaining the maximum value of the induced emf in each set of
winding among those three sets is known as phase sequence. This phase sequence
is usually indicated by the letters R, Y, B.
153. What is phase voltage?
The voltage between one of the phase and neutral is known as phase voltage and it
is denoted by VPh.
154. What is line voltage?
The voltage across any two phases of the supply system is called line voltage and it
is denoted by the letter VL.
155. What is phase current?
The current flowing through any of the phase winding is known as phase current
and it is denoted by IPh.
156. What is line current?
The current flowing between any two phases of the winding is called line current
and it is denoted by the letter IL.
157. What is balanced load and unbalance load?
Balanced load
In a three-phase system the power factors and the phase current or line currents of
the 3-phase are equal, then that load is called balanced load.
Unbalance load
If the three-phases have different power factors and the phase current, then the load
is called the unbalance load.
158. What is phase power and total power?
Phase power
The power measured between a phase and neutral is known as phase power.
Total power
The total power measured between the three phases is called total power.
159. What are the methods of connecting 3-phase windings?
There are two methods.
a. Star or wye (Y) connection.
b. Delta or mesh (<) connection.
160. What are the value of voltage and current in star connection and in delta
connection?
Star connection
a. IL = IPh.
b. VL = √3 VPh. ∴ VPh = VL/√3.
Note: in star connection we are getting neutral point and we can able to measure
the phase as well as line voltage.
Delta connection
a. VL = VPh.
b. IL = √3 IPh. ∴ IPh = IL/√3.
161. What is the power in 3-phase supply system?
In a single-phase system power ‘P’ = VPh * IPh * cosϕ watts.
In 3-ϕ system power ‘P’ = 3 * VPh * IPh * cosϕ watts.
In Star connection, IL = IPh and VPh = VL/√3. Substituting the value of IPh and VPh
in the above 3-ϕ power equation,
P = 3 * VPh * IPh * cosϕ watts.
P = 3 * VL/√3 * IL * cosϕ watts.
P = √3 * VL * IL * cosϕ watts.
In Delta connection, VL = VPh and IPh = IL/√3. Substituting the value of IPh and VPh
in the above 3-ϕ power equation,
P = 3 * VPh * IPh * cosϕ watts.
P = 3 * VL* IL/√3 * cosϕ watts.
P = √3 * VL * IL * cosϕ watts.
So that the power in three phase supply system whether star connected or delta
connected is same and power P = √3 * VL * IL * cosϕ watts.
So cosϕ = P/ √3 * VL * IL .
162. What are the advantages of rotating field system?
a. For rotating field alternators only two slip rings and brush gear assembly are
required irrespective of number of phases.
b. The DC excitation voltage is low and it is very easy to insulate. This intern
reduces the size of the machine.
c. Out put current can be taken directly from the fixed terminals on the stator. It is
easy to insulate high voltage stationary stator (armature).
d. The armature winding can be easily braced to prevent any deformation
produced by the mechanical stress set as a result of short circuit current and the
high centrifugal brought into play.
163. What are the types of alternator?
Depending upon the speed there are three types.
a. Low speed. b. Medium speed. c. High speed.
Depending on rotation there are two types.
a. Armature rotating b. Field rotating.
Depending on number of phases there are two types.
a. Single phase b. Poly phases.
With respect to excitation there are two types.
a. Self excited b. Separately excited.
164. What is the voltage equation for alternator?
166. How alternators are rated?
Alternators are rated in kVA.
167. What are the losses in an alternator?
Losses in alternators are same as DC generator and they are as follows.
a. Copper losses includes following losses
Armature copper losses (Ia2 ra).
Field copper losses (Ife2 rf).
Losses in brush.
b. Stray losses are as follows
Magnetic losses (Iron loss or core loss and pole shoes loss).
Mechanical losses includes bearing friction, slip ring friction and friction due to
windage.
168. When the efficiency of the alternator is maximum or on what factor the efficiency
of the alternator depends?
Efficiency of an alternator depends on its load power factor for a given load. As
the power factor decreases Ia increases and the copper losses increases and thus
efficiency decreases. The efficiency for given load is maximum only when the
power factor is unity and it decreases as the power factor fall.
169. What are the methods of synchronizing?
a. Lamp method.
Dark lamp method and bright lamp method.
b. Synchroscope method.
170. What is synchroscope?
Synchroscope is an instrument, which shows the phase relationship of emf of the
incoming alternator and at the same time it also indicates whether it is running slow
or fast. This instrument works on the principle of rotating magnetic fields. It
consists of a small motor with rotor and stator. Both wound for two phase. A
potential transformer connected to two of the main bus-bar give supply to the stator
‘A’ winding and another potential transformer of same type connected to the
corresponding terminals on the incoming machine supply to the stator ‘B’ winding.
The rotor rotates if the stator resultant flux in the ‘A’ and ‘B’ is different and the
exact time of synchronizing is the stand still position of the rotor. That means the
both the voltages in winding ‘A’ and ‘B’ are same and there is no resultant flux to
rotate the rotor. The speed of the rotor depends on the frequency of the alternator
and is too fast when alternator (incoming machine) speed is more and less when
alternator is too slow.
171. What is transformer?
Transformer is a static device by which AC power at one voltage in one circuit will
be transformed into AC power of same frequency at another (decreased or
increased voltage) or same voltage to an another circuit, which is in mutual
inductive influence with the previous circuit and it is based on mutual electro
magnetic induction.
172. What are the purposes or advantages of transformer?
Purposes
a. Electrical energy may be transmitted economically over long distance by
stepping up of voltages to reduce the line losses.
b. To distribute the low voltages at consumer side by stepping down the voltages.
Advantages
a. Transformer is a static machine and losses are very less. There by efficiency is
high and about 95 to 98%.
b. Practically maintenance is very less.
173. What is the working principle of transformer?
A transformer works under the principle of mutual electro magnetic induction
(Faraday’s laws of Electro-magnetic induction). It says that, when ever a changing
flux links with a coil an emf is induced in it and this induced emf is proportional to
the rate of change of flux and the number of turns in the coils linking the flux.
174. What are the types of transformer core?
a. Core type transformer core.
b. Shell type transformer core.
c. Berry type transformer core.
d. Spiral type transformer core.
175. What is the transformation ratio in transformer?
Equation for transformation ratio is,
E2/E1 = N2/N1 = k
k = >1 in step up transformer, where secondary turns are more and thus voltage is
more to reduce the transmission current.
k = <1 in step down transformer, where secondary turns are less than primary and
low voltage for consumer use.
If we include the current in transformation ration the equation is,
E2/E1 = N2/N1 = I1/I2 = k
176. What is the use of conservator in the transformer?
It is a drum type cylinder mounted on the top of the transformer through a small
pipe. … of the conservator is kept empty. To indicate the level of oil in the
transformer an indicator is fixed. Conservator will help the oil inside the tank by
providing sufficient space to expand and to contract as its temperature varies
without exposing much surface area. That is it limits the air with oil due to its less
surface area.
177. What is the use of breather in the transformer?
Breather is a bottle shaped steel tube, which is attached to one side of conservator
to allow the air to pass in and out of the tank or conservator through the calcium
chloride and silica gel, which is filled in it to absorb the moisture contained in the
air. When the silica gel absorb the moisture its colour changes from blue to pink.
178. What is the use of buchholz relay in the transformer?
It’s a protection relay used in oil immersed transformer to protect the transformer
from insulation failure, core heating or any other type of internal faults, which may
cause the heating of winding beyond the specified temperature. This relay is placed
in between the pipe connecting the conservator and the tank. Generally used in
power transformer of above 500 kVA.
It consists of two operating floats and is operated by two mercury switches
separately provided for the alarm and trip. Due to internal fault (collection of gases)
or leakage of oil if the oil level comes down the alarm relay first operates and then
the trip relay operates to isolate the transformer from the circuit.
179. What is the use of explosion vent in the transformer?
It is also a safety device of a transformer, which protects the transformer tank from
the high consequences of the high-pressure gases induced or developed by any type
of short circuit in the transformer by allowing the gas to escape by puncturing the
diaphragm.
180. What is the emf equation for transformer?
Always maximum flux reaches from zero to maximum in one quarter of the cycle.
That is in … of second. That is equal to 1/200 second.
Average rate of change of flux = Qm / … f. = Qm * 4 * f.
= 4 f Qm Weber / second.
As the coil has N turns the average emf induced in the coil = 4 f Qm N volts.
But the rms. Value = average value * form factor.
∴ rms. Value of emf = 1.11 * 4 f Qm N volts.
= 4.44 f Qm N volts.
181. What are the losses in transformer?
In transformer there are losses due to,
1. Resistance of the winding (copper losses).
2. Eddy current and Hysterisis in the iron parts and core (core and iron losses)
3. Losses due to leakage reactance (leakage flux).
At No load the copper losses and leakage flux losses are negligible due to the very
less primary current.
At loaded condition copper losses and leakage flux losses will exist in cosiderable
manner. Copper losses are variable and can be calculated by Ip
2*rp and Is
2*rs.
182. What are the types of cooling in transformer?
1. Natural cooling.
a. Air natural cooling (Dry type).
b. Oil immersed natural cooling.
c. Oil immersed, forced oil circulation with natural cooling.
2. Artificial cooling.
a. Oil immersed forced air circulation with air blast cooling.
b. Oil immersed blast cooling.
c. Air blast cooling.
3. Artificial cooling (water).
a. Oil immersed water cooling.
b. Oil immersed forced oil circulation with water cooling.
4. Mixed cooling (water).
This is the method of cooling combining oil natural, water, air natural, air blast
and forced oil.
183. State the type transformers?
Transformers can be classified into different groups and types based on the
following factors.
1. Type of core.
a. Core type transformer core.
b. Shell type transformer core.
c. Berry type transformer core.
d. Spiral type transformer core.
2. Method of cooling.
a. Natural cooling transformer.
b. Artificial cooling transformer.
c. Artificial cooling (water) transformer.
d. Mixed cooling transformer.
3. As per transformer ratio.
a. One to one transformer.
b. Step down transformer.
c. Step up transformer.
4. Based on number of phases.
a. Single-phase transformer.
b. Two-phase transformer.
c. Three phase transformer.
5. As per winding connection.
a. Star-star connected.
b. Star-delta connected.
c. Delta-delta connected.
d. Delta-star connected.
e. Open delta connected.
f. Scott connected.
6. As per the size of the transformer.
a. Distribution transformer (upto 500 kVA).
b. Power transformer (above 500 kVA).
7. Based on function and utilization.
a. Auto transformer.
b. Potential transformer (instrument transformer).
c. Current transformer (instrument transformer).
184. What is the humming of transformer?
Humming is a sound, which is produced due to the vibration of the cores in the
transformer. The vibrations are produced due to the change in polarity of an
alternating current or voltage and by the loose of lamination of the core. Both can
be minimised by tightening the core of the transformer.
185. What are the types of AC three phase motors?
Mainly there are two types.
1. Synchronous motors.
a. Plain synchronous motors.
b. Auto synchronous motors.
2. A-synchronous motors.
a. Induction motors.
1. Single phase motors
• Shaded pole motor.
• Capacitor start capacitors run motor.
• Capacitor start induction’s run motor.
• Split face motor.
2. Three phase motors.
• 3φ single squirrel cage motor.
• 3φ double squirrel cage motor.
• Squirrel deep bar induction motor.
• Slipring induction motor.
b. Commutator motors.
1. Single phase commutator motors.
• Plain repulsion motor.
• Repulsion start induction’s run motor.
• Repulsion induction motor.
• Series motor or universal motor.
2. 3φ commutator motors.
• 3φ series motor
• Charge motor.
• Compensated motor.
186. What is the working principle of 3φ induction motor?
When 3φ supply is given to stator, a rotating magnetic field of constant magnitude
is produced. This rotating magnetic field produces induced emf in the rotor
winding as per faraday’s laws and this induced emf causes to circulate a heavy
induced current in the rotor winding due to very small resistance of rotor. At the
initial moment the frequency of induced emf is equal to the frequency of the stator
supply voltage, when the rotor is stationary as in the case of secondary of a
transformer. The rotor induced current according to lenz’s law flows in such a
direction that it opposes the cause, which is inducing it. In this case the cause
producing the rotor current is the relative speed between the rotating magnetic field
if stator and the rotor and is maximum when the rotor is stationary. Hence to reduce
this relative speed rotor conductor (rotor) starts to rotate in the same direction in
which the stator field is rotating and tries to catch it up. The rotation of this rotor is
developed due tog the torque developed in the rotor by interaction between the
rotating magnetic field of stator and the field produced by the rotor current.
187. What is torque?
As said above torque is a turning or twisting moment of a force about an axis and it
is measured by the product of force * radius at which the force acts.
There are two types of torques.
a. Starting torque: This is the torque, which is required to start the motor at
load or no-load.
b. Running torque: This is the torque, which is required to run the motor at
normal speed and at normal load.
The letter ‘T’ denotes torque in induction motor and torque is proportional to
Ir φ cosϕr.
That is T ∝ Ir φ cosϕr. Where Ir = rotor current.
φ = Flux = stator flux per pole in Weber.
Cosϕr = rotor power factor.
188. What is slip?
The difference in speed of stator magnetic speed ‘Ns’
(synchronous speed) and rotor speed ‘Nr’ is called slip
or absolute slip and it is denoted by the letter ‘S’.
∴ S = Ns – Nr / Ns.
Slip has no unit. Percentage of slip of induction
motor varies from 4 to 5% in small motors and 1.5 to
2.5% in big motors.
In other words slip ‘S’ = fr / f. Where fr is rotor
frequency and f is stator frequency.
189. What is the working principle of double squirrel cage
induction motor?
In double squirrel cage motor outer cage rotor winding
is of high resistance and low reactance. Inner cage
winding is of high reactance and low resistance.
At the time of starting rotor frequency is equal to
the stator frequency and there by the reactance of the
inner cage winding is comparatively high (XL = 2πfL) because
it is linking more inner winding than the outer winding. So the impedance of inner
cage winding is very high. Hence the current flow through inner cage winding is
very less comparing to the outer cage winding. That is a very high ratio of current
is passing through the outer cage winding at the time of starting and there by
produces very high starting torque.
When the rotor starts running the speed of the motor can be increased and the slip
will be decreased and there by the rotor frequency (‘S’ = fr / f). So that in
the running condition the reactance of the inner cage decreases to the lowest value
and hence the Impedance (XL = 2πfL). So the current in inner cage winding will be
comparatively more than the outer cage winding at the time of running. So now
inner cage winding produces more torque than outer cage at the time of running
and the motor running torque is good enough.
190. Why starter is necessary to start the AC motor?
a. At the time of starting motor starting current is
high (4 to 5 times). Therefore if motor is directly
started the supply voltage may be disturb.
b. By the help of starters starting and stopping of
motors can be made easily as we required. Because
starters provides overload tripping difficulties.
c. The help of starters can protect motor against the
single phasing by the action of overload
arrangements.
d. Protect the motor from no-voltage and its
difficulties.
e. Permits automatic control when required.
191. What are the types starters used for starting of
induction motor?
a. Direct on line starter (air break) mechanically.
b. Direct on line starter (air break or oil immersed)
electrically.
c. Star delta starter.
d. Slipring motor starter.
e. Auto transformer starter.
192. What are the speed control methods of induction motor?
a. By controlling the supply voltage.
b. By controlling the supply frequency (Ns = 120f / P).
c. By varying the number of poles (Ns = 120f / P).
d. By rotor rheostatic control (for small speed
variation).
193. What is magnetic locking or cogging effect of
induction motor?
In squirrel cage induction motor some times the rotor
and stator care teeth or slots are comes face to face
or parallel at stationary condition. If we are starting
the motor at this condition the motor get hesitated to
start or run due to the attraction developed between
those rotor and stator teeth or slots. This is known as
the magnetic locking or cogging effect of a squirrel
cage induction motor. This type of magnetic locking in
squirrel cage induction motor can be avoided either by
skewing the rotor slot or by selecting the rotor slot,
such that there is no common factor between the rotor
slot and stator slots.
194. What is skewing?
Skewing can be done by turning the rotor slots about 15°
from the parallel position of slots with the shaft.
That is rotor slots are not in parallel with the shaft
but there is an angle of about 15° with the shaft.
195. What are the losses in induction motor?
a. Stator losses (stator copper losses, stator iron
losses).
b. Rotor losses (rotor copper losses, rotor iron
losses).
c. Windage and friction losses.
196. What is synchronous motor?
An alternator, which is running as a motor can be
called as synchronous motor and it runs at synchronous
speed while it converts electrical energy into
mechanical energy. It requires both AC for armature and
DC supply for field.
197. What are the advantages and dis-advantages of
synchronous motor?
Advantages
a. It’s a constant speed motor and is equal to
synchronous speed from no load to full load.
b. It has good efficiency higher than induction motor.
c. It can be run as a motor and also as an alternator as
per the requirement. More over it can be used as
synchronous condenser.
Dis-advantages
a. It can not be used as a varying speed motor. Because
its speed can not be varied.
b. As a motor it is not self-starting type and it can
not be started on load.
c. It requires both AC and DC supply.
d. Hunting is also produced in this motor.
198. What are the applications of synchronous motor?
a. These motors are used in powerhouses, in sub stations
for the improvement of power factor by connecting it
in parallel to the supply and it is run without load
under over excitation of field.
b. Used in big industries where many induction motors
are installed to improve the power factor.
c. Used for constant mechanical loads.
199. What is hunting effect?
When the load is varied to the motor the oscillation
being setup in the rotor about the position of
equilibrium corresponding to change of load condition.
So the damper winding acts the magnetic lines of force
and causes to create the opposite torque, which keeps
the rotor in the same position of the particular load.
This oscillation of the rotor is known as Hunting or
Phase swinging. To reduce this hunting damper winding
is helpful.
200. What is synchronous condenser or phase advancer?
An over excited synchronous motor takes leading current
just like a condenser and gives leading power factor. A
synchronous motor, which I used only for the purpose of
improving power factor, can be called as synchronous
condenser or phase advancer.
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101. What is battery?
The combination of two or more cells is called the battery.
102. What are the classifications of cell?
a. Primary cells.
b. Secondary cells.
103. What are the differences between primary cell and secondary cell?
Primary cells are those cells, which cannot be re-charged after the substances
(electrolyte, electrode and container) used in it becomes useless.
The common primary cells in use are,
a. Simple voltaic cell (one fluid cell).
b. Daniel cell (two fluid cell).
c. Leclanche cell (two fluid cell).
d. Dry cell.
e. Standard cell or Weston cadmium cell.
Secondary cells are those cells, which can be re-charged and use again once they
discharged or used for the work for number of times with out re-newing it’s
materials.
Most commonly used secondary cells are,
a. Lead acid cell.
b. Nickel iron alkaline cell.
c. Nickel cadmium alkaline cell.
104. What is polarization? What is local action?
Polarization
The hydrogen bubbles which are clinging over the surface of copper electrode
(anode) becomes a thin film of hydrogen over the copper electrode. This hydrogen
film increases the internal resistance and reduces the emf of the cell and hence the
cell soon becomes inactive. This effect is known as polarization
.
Local action
In voltaic cell it is observed that, when the cell is not connected to the load and not
supplying any current zinc will continuously dissolving in the electrolyte. This is
due to the impurities (copper, iron, tin, and lead) in the commercial zinc. So that
whenever commercial zinc is used as a electrode, separate small cells are
developed between the impurities and zinc with the presence of electrolyte. These
local cells consume always zinc and the emf developed by those local cells are
always opposite to the main emf. The action of these cells is known as local action.
105. What are the advantages of secondary cells over primary cell?
a. It gives high current capacity.
b. Its internal resistance is very low.
c. It gives a constant current.
d. It posses very high efficiency.
e. It posses fairly constant emf.
f. It posses good mechanical strength.
g. It posses large storage capacity.
h. It can be renewed by charging after it is discharge.
i. It is durable.
106. What is Plante plate and Faure plate?
There are two types of positive plate preparation. They are Plante plate and Faure
plate.
Plante plate
As per plante process positive plate PbO2 are prepared by a process of repeated
charging and discharging of pure lead. Positive plates, which are made by this
process, are also called ‘formal plates’. This process of positive plate preparation
required very long time for it’s manufacturing and so it is very costly.
Faure plate
Faure plates are generally made up of rectangular lead grid into which the active
material lead peroxide PbO2 is filled in the form of paste.
107. How negative plate is made up of?
The negative plate of a lead acid cell is made up of spongy lead ‘Pb’. The negative
plates are also of rectangular lead grid and the active material Pb in the form of
paste is held firmly in this lead grid.
108. Why negative plates are one more than positive plates?
Negative plates are one more than positive plates so as to get negative plates on
both the sides of positive plates. This is to prevent the buckling action of the lead
on positive plate in the multi plate lead acid cell. The other reason is that both the
sides of positive plates will become active and the efficiency of the positive plate
and the cell will increase.
109. What is electrolyte?
Electrolyte is the medium through which the current produces chemical changes.
Electrolyte is a mixture of sulphuric acid o 1.85 specific gravity (concentrated
sulphuric acid) diluted with distilled water in the ratio of 1:3 approximately, so the
specific gravity of the dilute sulphuric acid is 1.280.
110. What are the types of grouping of cells?
There are three main ways of grouping.
a. Series grouping.
b. Parallel grouping.
c. Series parallel grouping.
111. What are the advantages of series grouping and parallel grouping?
Advantages of series grouping.
a. The total emf increases and is equal to ‘nE’. Where n – total number of cells in
series and E – emf of one cell.
b. The internal resistance ‘r’ also increases and equal to ‘nr’. So total resistance of
the circuit also increases and is equal to R + nr ohms. Where R – external load
resistance.
c. Total current is equal to one cell current. That is there is no current increase. If
the internal resistance is negligible or less then current will be maximum.
Advantages of parallel grouping.
a. In parallel grouping emf of one cell will be the total emf of the grouping.
b. Total internal resistance of the parallel group is equal to r/n.
c. Total resistance of the group is equal to R + r/n.
d. Total current = E / (R +r/n) amps.
So we can understand that parallel useful when the external resistance is small as
compared to internal resistance of the parallel group. But at the same time series
grouping is useful when the internal resistance is small compared to the external
resistance of the group.
112. What are the methods of charging of battery.
Mainly there are three types of charging of battery.
a. Constant current charging system.
In this system the charging current is kept to constant by varying the supplied DC
voltage by the help of rheostat or filament lamps in series with the battery, so as to
over come the increased back emf of the battery or of the cell.
Charging current = V – Eb / R + r amps.
b. Constant voltage or potential charging.
In this system the voltage is kept to constant, so the charging current in the
beginning will be high when the back emf or counter emf of the battery is low and
current will be small when the back or counter emf increases as the battery gets
charge.
c. Trickle charging system.
The continuous charging of a battery at a very low rate for keeping the battery
ready in good working condition is called the trickle charging. This is to maintain
the losses occurring at the idle period. The value of the trickle charging current is
approximately 2% of the full charging current of the battery.
113. What are the factors on which the capacity of the battery depends?
The capacity of the battery is measured in ampere-hour. The capacity of the battery
depends upon the following factors.
a. Number and area of the positive plate.
b. Discharge voltage. A cell should not be discharged below 1.8 V. If it is
discharged below 1.8 V it may cause to reduce the capacity.
c. Discharge rate. Capacity decreases with increase rate of discharge.
d. Specific gravity of electrolyte. With rapid rate of discharge causes to weaken
the electrolyte so the chemical action also weakens and there by the capacity
decreases. When the specific gravity increases the capacity of the battery
increases.
e. Quantity of electrolyte. Electrolyte level should be at the top plate level.
f. The design of separator. The design of the separator should be thin.
g. Temperature. When the temperature increases the resistance of the battery
decreases and the capacity increases.
114. Explain Kirchhoff’s laws.
Kirchhoff’s laws are used in complex network circuits to determine the equivalent
total resistance and the current flowing in various conductors of that circuit.
Mainly there are two laws.
a. Point law or current law.
b. Mesh law or voltage law.
Point law or current law.
The point law states that, the algebraic sum of the currents meeting at any point or
junction or node of a network is zero. In other words the sum of the currents
flowing towards the junction or node or any point of network is equal to the total
current flowing away from that junction.
Mesh law or voltage law.
The mesh law states that, in any closed electrical circuit the algebraic sum of the
potential drops is equal to the sum of the impressed emf’s acting in that close
circuit. In this the important factor is to determine the emf sign to calculate the total
emf.
115. What are the types of wiring?
Mainly there are two types of wiring systems.
a. Tree system.
b. Distribution system.
116. What are the systems of wiring?
Following are the general systems of domestic wiring and industrial wiring.
a. Cleat system wiring.
b. Casing and capping system wiring.
c. Lead sheathed system wiring.
d. C.T.S, T.R.S, P.V.C sheathed system wiring.
e. Conduit system wiring.
117. What are the testing of wiring installation?
Following are the tests to be done after installation of wiring.
a. Polarity test.
b. Short circuit test.
c. Continuity test.
d. Insulation test between conductors and conductors to the earth.
e. Earth continuity test.
118. What are the advantages of AC over DC?
a. For the same capacity alternators are cheaper than DC generators, because
alternator is not having commutator arrangement and there by small in size.
b. Alternating current produces pulsating magnetic field and there by it posses the
property of inductance and capacitance.
c. Alternating current can be step-up or step-down by static transformer.
d. AC can be transmitted with very less cost in comparing to DC transmission.
e. Alternating line losses are very less comparing to DC line losses.
f. An alternators and AC motor requires very less maintenance.
g. Charge per unit for AC is less than DC.
119. Define AC.
Alternating current is that type of electric current, which changes it’s magnetude
and direction periodically.
120. What is cycle?
One complete set of changes in value and direction of alternating quantity and emf
or current is called a cycle.
121. What is periodic time?
Periodic time is the time taken to complete on cycle. Its symbol is ‘T’. For example
Indian standard frequency is 50 cycles per second. So the periodic time T = 1/50
seconds. That is equal to 20 m seconds.
122. What is frequency?
Number of cycles per second is called frequency.
123. What is amplitude value or peak value?
It is the maximum value of an alternating quantity that can be obtained in any one
direction.
124. What is instantaneous value?
The value of an alternating quantity at a particular instant is called instantaneous
value.
125. What is average value or mean value?
Average of all instantaneous values of emf or current over a half cycle is known as
average value or mean value.
Average value = 0.637 * Emax or Imax
126. What is root mean square value (R.M.S)?
The R.M.S value is also known as effective value or virtual value. The
instantaneous value of both the directions will all be squared up and will be added
together. Then divide to get the average with the number of instantaneous values
and find the square root of this average to calculate the R.M.S value of the emf or
current.
Or
The R.M.S value of an alternating current or emf is equal to the same value of
direct current (DC), which produces the same amount of heat with the same time
when applied the DC through the same circuit as AC is produced.
R.M.S value = maximum value / √2 = 1/√2 = 0.707.
∴ R.M.S value or effective value = 0.707 * Emax or Imax
127. What is form factor?
The ratio of the R.M.S value to the average value is called the form factor.
∴ Form factor = 0.707 * Emax or Imax : 0.637 * Emax or Imax
= 0.707 * Emax or Imax / 0.637 * Emax or Imax
= 1.11
So that R.M.S value = average value * 1.11
Or average value = R.M.S value / 1.11
128. What is crest factor or peak factor?
The ratio of maximum value to the R.M.S value is known as crest factor. So the
crest factor = maximum value / R.M.S value.
= Emax or Imax / (Emax or Imax / √2)
= Emax or Imax * √2 / Emax or Imax = √2 = 1.414
129. What is vector quantity and what is scalar quantity?
Vector quantity
A quantity, which has both the direction and magnitude is said to be a vector
quantity. Examples are force, emf, current etc.
Scalar quantity
A scalar quantity is that, which has only magnitude but no direction. Examples are
temperature, mass, volume etc.
130. What is phase?
The development of an AC quantity through different stages is known as Phase.
The term phase refers to the number of separate individual voltage setup in an AC
circuit.
131. What is in-phase?
When those two vectors (voltage and current) attain (reaches) their maximum and
minimum values simultaneously (at the same time), then those two quantities are
said in-phase. Here between those quantities there is no angle.
132. What is out of phase?
When two alternating quantities voltage and current do not reaches their maximum
and minimum values simultaneously, then they are called out of phase.
133. What is phase angle?
Phase angle is an angular displacement between two alternating quantities. Phase
angle is measured in electrical degrees or radians.
134. What is quadrature quantity?
When the phase angle between two vectors is 90° electrical, then they are said to be
quadrature quantity.
135. What anti-phase quantity?
When two quantities are out of phase by 180° electrical, then they are said to be
anti-phase quantities.
136. What is leading quantity?
The alternating quantity that reaches its maximum value earlier than the other
quantity is known as the leading quantity.
137. What is lagging quantity?
The alternating quantity that attains its maximum value later than the other quantity
is called the lagging quantity.
138. What is the relation between voltage and current in AC circuit containing only
resistance?
Current (I) is in-phase with the voltage.
I = V/R amps.
P = I * V * cosϕ or I2 R watts. (Where cosϕ is zero because the voltage and current
are in-phase. So cosϕ 0° (zero) = 1)
139. What is the relation between voltage and current in AC circuit containing only
inductance?
Current (I) is lags behind the voltage by 90°.
I = V/XL amps.
XL = 2πfL ohms.
P = I * V * cosϕ watts. (Where cosϕ is 90 because current lags behind voltage by
90°. So cosϕ 90° = 0)
∴ P = I * V * 0 = 0 watts.
140. What is the relation between voltage and current in AC circuit containing only
capacitance?
Current (I) is leading the voltage by 90°.
I = V/XC amps.
XC = 1/2πfC ohms.
P = I * V * cosϕ watts. (Where cosϕ is 90 because current is leading the voltage by
90°. So cosϕ 90° = 0)
∴ P = I * V * 0 = 0 watts.
141. What is inductance and inductive reactance?
Inductance
A coil carrying alternating current produces an alternating flux, which causes to
link with same coil and produces an emf in the coil, which opposes the applied
emf. This property is known as inductance. The unit for measurement is henry.
Inductive reactance
The opposition or the reactance offered by the property of inductance in the circuit
is known as inductive reactance and denoted by the letter XL. The unit for
measurement is ohm.
142. What is capacitance and capacitive reactance?
Capacitance
The property of a capacitor to store electrical energy in it, when it is connected to
an electric supply is called capacitance. Unit for measurement is farad. Capacitor
store an electric energy in the unit of charge and the unit of charge is coulomb.
Capacitive reactance
The opposition due to capacitance of capacitor in an electric circuit is called
capacitive reactance and it denoted by the letter XC. The unit for measurement is
ohm.
143. What is impedance?
The total opposition offered by an AC circuit for the flow of current through it is
called Impedance. The letter ‘Z’ denotes it and the unit is ohm.
∴ Z = √ R2 + (XL ∼ XC) 2
Z = √ R2 + (X) 2
Where ∼ indicates the difference of XL and XC and denoted in the letter X (net
reactance of the AC circuit).
144. What is ohm’s law for AC circuit?
I = V/Z amps.
Z = V/I ohms.
V = I * Z volts.
145. What is the current and power in an AC circuit?
Current
AC circuit contains resistance ‘R’ and reactance ‘X’.
In resistive circuit IR = I cosϕ. Because resistance current (IR) is in-phase with
voltage (ER).
In reactance circuit IX = I sinϕ. Because reactance current will lead or lag the
voltage (ER) by 90°.
So the resultant current (I) is the vector sum of I cosϕ and I sinϕ. So that circuit
current I = √ (I cosϕ)2 + (I sinϕ)2 amps.
I cosϕ is some times known as power component of current or the power current or
energy current and the I sinϕ is known as reactive component of current or wattless
current. Because I sinϕ is not taking any energy from the circuit.
Power
Power in watts = terminal voltage * power component of current.
a. True power = E * I * cosϕ watts.
This true power is some times known as energy component or active
component or watt-full component. Because this is the power used to produce
torque in motor and supplies heat, light etc. or this true power is the power
consumption of all source of electric circuit.
b. Reactive power = E * I * sinϕ watts.
This reactive power is some times known as reactive or in-active component or
watt less component or VARS.
c. Apparent power = E * I watts.
The terminal voltage multiplied by the actual resultant current (I) is called the
apparent power or volt-ampere or VA.
146. What is power factor?
So from the above power explanation,
Cosϕ = true power / apparent power = E * I * cosϕ / E * I.
So that power factor is equal to
a. Cosine of angle of lead and lag of the resultant current with the applied voltage.
b. The ratio of R/Z.
c. The ratio of true power to the apparent power.
147. What is resonance in series circuit?
If in an AC circuit inductive reactance XL and capacitive reactance XC is equal the
voltage across both will be equal and are 180° out of phase. So that each will
cancel each other and the current limiting component will be the resistance of the
circuit.
If we are in a position to alter the frequency of supply voltage at a particular
frequency named as ‘resonant frequency’, AC series circuit’s XL = XC and the net
reactance will be zero. So the current in the circuit is in-phase with the voltage.
Because the controlling component of the circuit is resistance only and the current
is maximum and equal to V/R amps.
This above said condition is called ‘series resonance’ and the frequency at which it
occurs is called resonant frequency and the resonant frequency (FR) is equal to
(FR) = 1/2π√LC cycles per second.
148. What is Q-factor?
The ratio of VL/V or VC/V at the resonant frequency is called the voltage
magnification denoted as Q-factor.
Q-factor = 1 √L/C
R
149. What is Admittance?
Admittance: Admittance is the reciprocal of impedance. It is denoted by the letter
‘Y’ and the unit of measurement is mho.
Y = I/E = RMS current / RMS voltage.
Equation used in admittance
a. Conductance ‘G’ = Y * cosϕ = 1/Z *R/Z = R/Z2 mho.
b. Susceptance ‘B’ = Y * sinϕ = 1/Z * X/Z = X/Z2 mho.
c. Admittance ‘Y’ = √G2 + B2 mho.
d. In special cases when X = zero, then G = 1/R and R = zero, then B = 1/X.
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150. What is the resonance frequency equation for parallel circuit?
In parallel circuit when XC = XL, the circuit is called the parallel resonance circuit.
That is 2πfL = 1/2πfC.
In term (FR) = 1/2π√1/LC – R2/L2 cycles per second.
If ‘R’ is negligible, then (FR) = 1/2π√LC cycles per second.
151. What is poly phase?
A system with two or more the two phases is known as poly phase system.
152. What is phase sequence?
The sequence of attaining the maximum value of the induced emf in each set of
winding among those three sets is known as phase sequence. This phase sequence
is usually indicated by the letters R, Y, B.
153. What is phase voltage?
The voltage between one of the phase and neutral is known as phase voltage and it
is denoted by VPh.
154. What is line voltage?
The voltage across any two phases of the supply system is called line voltage and it
is denoted by the letter VL.
155. What is phase current?
The current flowing through any of the phase winding is known as phase current
and it is denoted by IPh.
156. What is line current?
The current flowing between any two phases of the winding is called line current
and it is denoted by the letter IL.
157. What is balanced load and unbalance load?
Balanced load
In a three-phase system the power factors and the phase current or line currents of
the 3-phase are equal, then that load is called balanced load.
Unbalance load
If the three-phases have different power factors and the phase current, then the load
is called the unbalance load.
158. What is phase power and total power?
Phase power
The power measured between a phase and neutral is known as phase power.
Total power
The total power measured between the three phases is called total power.
159. What are the methods of connecting 3-phase windings?
There are two methods.
a. Star or wye (Y) connection.
b. Delta or mesh (<) connection.
160. What are the value of voltage and current in star connection and in delta
connection?
Star connection
a. IL = IPh.
b. VL = √3 VPh. ∴ VPh = VL/√3.
Note: in star connection we are getting neutral point and we can able to measure
the phase as well as line voltage.
Delta connection
a. VL = VPh.
b. IL = √3 IPh. ∴ IPh = IL/√3.
161. What is the power in 3-phase supply system?
In a single-phase system power ‘P’ = VPh * IPh * cosϕ watts.
In 3-ϕ system power ‘P’ = 3 * VPh * IPh * cosϕ watts.
In Star connection, IL = IPh and VPh = VL/√3. Substituting the value of IPh and VPh
in the above 3-ϕ power equation,
P = 3 * VPh * IPh * cosϕ watts.
P = 3 * VL/√3 * IL * cosϕ watts.
P = √3 * VL * IL * cosϕ watts.
In Delta connection, VL = VPh and IPh = IL/√3. Substituting the value of IPh and VPh
in the above 3-ϕ power equation,
P = 3 * VPh * IPh * cosϕ watts.
P = 3 * VL* IL/√3 * cosϕ watts.
P = √3 * VL * IL * cosϕ watts.
So that the power in three phase supply system whether star connected or delta
connected is same and power P = √3 * VL * IL * cosϕ watts.
So cosϕ = P/ √3 * VL * IL .
162. What are the advantages of rotating field system?
a. For rotating field alternators only two slip rings and brush gear assembly are
required irrespective of number of phases.
b. The DC excitation voltage is low and it is very easy to insulate. This intern
reduces the size of the machine.
c. Out put current can be taken directly from the fixed terminals on the stator. It is
easy to insulate high voltage stationary stator (armature).
d. The armature winding can be easily braced to prevent any deformation
produced by the mechanical stress set as a result of short circuit current and the
high centrifugal brought into play.
163. What are the types of alternator?
Depending upon the speed there are three types.
a. Low speed. b. Medium speed. c. High speed.
Depending on rotation there are two types.
a. Armature rotating b. Field rotating.
Depending on number of phases there are two types.
a. Single phase b. Poly phases.
With respect to excitation there are two types.
a. Self excited b. Separately excited.
164. What is the voltage equation for alternator?
166. How alternators are rated?
Alternators are rated in kVA.
167. What are the losses in an alternator?
Losses in alternators are same as DC generator and they are as follows.
a. Copper losses includes following losses
Armature copper losses (Ia2 ra).
Field copper losses (Ife2 rf).
Losses in brush.
b. Stray losses are as follows
Magnetic losses (Iron loss or core loss and pole shoes loss).
Mechanical losses includes bearing friction, slip ring friction and friction due to
windage.
168. When the efficiency of the alternator is maximum or on what factor the efficiency
of the alternator depends?
Efficiency of an alternator depends on its load power factor for a given load. As
the power factor decreases Ia increases and the copper losses increases and thus
efficiency decreases. The efficiency for given load is maximum only when the
power factor is unity and it decreases as the power factor fall.
169. What are the methods of synchronizing?
a. Lamp method.
Dark lamp method and bright lamp method.
b. Synchroscope method.
170. What is synchroscope?
Synchroscope is an instrument, which shows the phase relationship of emf of the
incoming alternator and at the same time it also indicates whether it is running slow
or fast. This instrument works on the principle of rotating magnetic fields. It
consists of a small motor with rotor and stator. Both wound for two phase. A
potential transformer connected to two of the main bus-bar give supply to the stator
‘A’ winding and another potential transformer of same type connected to the
corresponding terminals on the incoming machine supply to the stator ‘B’ winding.
The rotor rotates if the stator resultant flux in the ‘A’ and ‘B’ is different and the
exact time of synchronizing is the stand still position of the rotor. That means the
both the voltages in winding ‘A’ and ‘B’ are same and there is no resultant flux to
rotate the rotor. The speed of the rotor depends on the frequency of the alternator
and is too fast when alternator (incoming machine) speed is more and less when
alternator is too slow.
171. What is transformer?
Transformer is a static device by which AC power at one voltage in one circuit will
be transformed into AC power of same frequency at another (decreased or
increased voltage) or same voltage to an another circuit, which is in mutual
inductive influence with the previous circuit and it is based on mutual electro
magnetic induction.
172. What are the purposes or advantages of transformer?
Purposes
a. Electrical energy may be transmitted economically over long distance by
stepping up of voltages to reduce the line losses.
b. To distribute the low voltages at consumer side by stepping down the voltages.
Advantages
a. Transformer is a static machine and losses are very less. There by efficiency is
high and about 95 to 98%.
b. Practically maintenance is very less.
173. What is the working principle of transformer?
A transformer works under the principle of mutual electro magnetic induction
(Faraday’s laws of Electro-magnetic induction). It says that, when ever a changing
flux links with a coil an emf is induced in it and this induced emf is proportional to
the rate of change of flux and the number of turns in the coils linking the flux.
174. What are the types of transformer core?
a. Core type transformer core.
b. Shell type transformer core.
c. Berry type transformer core.
d. Spiral type transformer core.
175. What is the transformation ratio in transformer?
Equation for transformation ratio is,
E2/E1 = N2/N1 = k
k = >1 in step up transformer, where secondary turns are more and thus voltage is
more to reduce the transmission current.
k = <1 in step down transformer, where secondary turns are less than primary and
low voltage for consumer use.
If we include the current in transformation ration the equation is,
E2/E1 = N2/N1 = I1/I2 = k
176. What is the use of conservator in the transformer?
It is a drum type cylinder mounted on the top of the transformer through a small
pipe. … of the conservator is kept empty. To indicate the level of oil in the
transformer an indicator is fixed. Conservator will help the oil inside the tank by
providing sufficient space to expand and to contract as its temperature varies
without exposing much surface area. That is it limits the air with oil due to its less
surface area.
177. What is the use of breather in the transformer?
Breather is a bottle shaped steel tube, which is attached to one side of conservator
to allow the air to pass in and out of the tank or conservator through the calcium
chloride and silica gel, which is filled in it to absorb the moisture contained in the
air. When the silica gel absorb the moisture its colour changes from blue to pink.
178. What is the use of buchholz relay in the transformer?
It’s a protection relay used in oil immersed transformer to protect the transformer
from insulation failure, core heating or any other type of internal faults, which may
cause the heating of winding beyond the specified temperature. This relay is placed
in between the pipe connecting the conservator and the tank. Generally used in
power transformer of above 500 kVA.
It consists of two operating floats and is operated by two mercury switches
separately provided for the alarm and trip. Due to internal fault (collection of gases)
or leakage of oil if the oil level comes down the alarm relay first operates and then
the trip relay operates to isolate the transformer from the circuit.
179. What is the use of explosion vent in the transformer?
It is also a safety device of a transformer, which protects the transformer tank from
the high consequences of the high-pressure gases induced or developed by any type
of short circuit in the transformer by allowing the gas to escape by puncturing the
diaphragm.
180. What is the emf equation for transformer?
Always maximum flux reaches from zero to maximum in one quarter of the cycle.
That is in … of second. That is equal to 1/200 second.
Average rate of change of flux = Qm / … f. = Qm * 4 * f.
= 4 f Qm Weber / second.
As the coil has N turns the average emf induced in the coil = 4 f Qm N volts.
But the rms. Value = average value * form factor.
∴ rms. Value of emf = 1.11 * 4 f Qm N volts.
= 4.44 f Qm N volts.
181. What are the losses in transformer?
In transformer there are losses due to,
1. Resistance of the winding (copper losses).
2. Eddy current and Hysterisis in the iron parts and core (core and iron losses)
3. Losses due to leakage reactance (leakage flux).
At No load the copper losses and leakage flux losses are negligible due to the very
less primary current.
At loaded condition copper losses and leakage flux losses will exist in cosiderable
manner. Copper losses are variable and can be calculated by Ip
2*rp and Is
2*rs.
182. What are the types of cooling in transformer?
1. Natural cooling.
a. Air natural cooling (Dry type).
b. Oil immersed natural cooling.
c. Oil immersed, forced oil circulation with natural cooling.
2. Artificial cooling.
a. Oil immersed forced air circulation with air blast cooling.
b. Oil immersed blast cooling.
c. Air blast cooling.
3. Artificial cooling (water).
a. Oil immersed water cooling.
b. Oil immersed forced oil circulation with water cooling.
4. Mixed cooling (water).
This is the method of cooling combining oil natural, water, air natural, air blast
and forced oil.
183. State the type transformers?
Transformers can be classified into different groups and types based on the
following factors.
1. Type of core.
a. Core type transformer core.
b. Shell type transformer core.
c. Berry type transformer core.
d. Spiral type transformer core.
2. Method of cooling.
a. Natural cooling transformer.
b. Artificial cooling transformer.
c. Artificial cooling (water) transformer.
d. Mixed cooling transformer.
3. As per transformer ratio.
a. One to one transformer.
b. Step down transformer.
c. Step up transformer.
4. Based on number of phases.
a. Single-phase transformer.
b. Two-phase transformer.
c. Three phase transformer.
5. As per winding connection.
a. Star-star connected.
b. Star-delta connected.
c. Delta-delta connected.
d. Delta-star connected.
e. Open delta connected.
f. Scott connected.
6. As per the size of the transformer.
a. Distribution transformer (upto 500 kVA).
b. Power transformer (above 500 kVA).
7. Based on function and utilization.
a. Auto transformer.
b. Potential transformer (instrument transformer).
c. Current transformer (instrument transformer).
184. What is the humming of transformer?
Humming is a sound, which is produced due to the vibration of the cores in the
transformer. The vibrations are produced due to the change in polarity of an
alternating current or voltage and by the loose of lamination of the core. Both can
be minimised by tightening the core of the transformer.
185. What are the types of AC three phase motors?
Mainly there are two types.
1. Synchronous motors.
a. Plain synchronous motors.
b. Auto synchronous motors.
2. A-synchronous motors.
a. Induction motors.
1. Single phase motors
• Shaded pole motor.
• Capacitor start capacitors run motor.
• Capacitor start induction’s run motor.
• Split face motor.
2. Three phase motors.
• 3φ single squirrel cage motor.
• 3φ double squirrel cage motor.
• Squirrel deep bar induction motor.
• Slipring induction motor.
b. Commutator motors.
1. Single phase commutator motors.
• Plain repulsion motor.
• Repulsion start induction’s run motor.
• Repulsion induction motor.
• Series motor or universal motor.
2. 3φ commutator motors.
• 3φ series motor
• Charge motor.
• Compensated motor.
186. What is the working principle of 3φ induction motor?
When 3φ supply is given to stator, a rotating magnetic field of constant magnitude
is produced. This rotating magnetic field produces induced emf in the rotor
winding as per faraday’s laws and this induced emf causes to circulate a heavy
induced current in the rotor winding due to very small resistance of rotor. At the
initial moment the frequency of induced emf is equal to the frequency of the stator
supply voltage, when the rotor is stationary as in the case of secondary of a
transformer. The rotor induced current according to lenz’s law flows in such a
direction that it opposes the cause, which is inducing it. In this case the cause
producing the rotor current is the relative speed between the rotating magnetic field
if stator and the rotor and is maximum when the rotor is stationary. Hence to reduce
this relative speed rotor conductor (rotor) starts to rotate in the same direction in
which the stator field is rotating and tries to catch it up. The rotation of this rotor is
developed due tog the torque developed in the rotor by interaction between the
rotating magnetic field of stator and the field produced by the rotor current.
187. What is torque?
As said above torque is a turning or twisting moment of a force about an axis and it
is measured by the product of force * radius at which the force acts.
There are two types of torques.
a. Starting torque: This is the torque, which is required to start the motor at
load or no-load.
b. Running torque: This is the torque, which is required to run the motor at
normal speed and at normal load.
The letter ‘T’ denotes torque in induction motor and torque is proportional to
Ir φ cosϕr.
That is T ∝ Ir φ cosϕr. Where Ir = rotor current.
φ = Flux = stator flux per pole in Weber.
Cosϕr = rotor power factor.
188. What is slip?
The difference in speed of stator magnetic speed ‘Ns’
(synchronous speed) and rotor speed ‘Nr’ is called slip
or absolute slip and it is denoted by the letter ‘S’.
∴ S = Ns – Nr / Ns.
Slip has no unit. Percentage of slip of induction
motor varies from 4 to 5% in small motors and 1.5 to
2.5% in big motors.
In other words slip ‘S’ = fr / f. Where fr is rotor
frequency and f is stator frequency.
189. What is the working principle of double squirrel cage
induction motor?
In double squirrel cage motor outer cage rotor winding
is of high resistance and low reactance. Inner cage
winding is of high reactance and low resistance.
At the time of starting rotor frequency is equal to
the stator frequency and there by the reactance of the
inner cage winding is comparatively high (XL = 2πfL) because
it is linking more inner winding than the outer winding. So the impedance of inner
cage winding is very high. Hence the current flow through inner cage winding is
very less comparing to the outer cage winding. That is a very high ratio of current
is passing through the outer cage winding at the time of starting and there by
produces very high starting torque.
When the rotor starts running the speed of the motor can be increased and the slip
will be decreased and there by the rotor frequency (‘S’ = fr / f). So that in
the running condition the reactance of the inner cage decreases to the lowest value
and hence the Impedance (XL = 2πfL). So the current in inner cage winding will be
comparatively more than the outer cage winding at the time of running. So now
inner cage winding produces more torque than outer cage at the time of running
and the motor running torque is good enough.
190. Why starter is necessary to start the AC motor?
a. At the time of starting motor starting current is
high (4 to 5 times). Therefore if motor is directly
started the supply voltage may be disturb.
b. By the help of starters starting and stopping of
motors can be made easily as we required. Because
starters provides overload tripping difficulties.
c. The help of starters can protect motor against the
single phasing by the action of overload
arrangements.
d. Protect the motor from no-voltage and its
difficulties.
e. Permits automatic control when required.
191. What are the types starters used for starting of
induction motor?
a. Direct on line starter (air break) mechanically.
b. Direct on line starter (air break or oil immersed)
electrically.
c. Star delta starter.
d. Slipring motor starter.
e. Auto transformer starter.
192. What are the speed control methods of induction motor?
a. By controlling the supply voltage.
b. By controlling the supply frequency (Ns = 120f / P).
c. By varying the number of poles (Ns = 120f / P).
d. By rotor rheostatic control (for small speed
variation).
193. What is magnetic locking or cogging effect of
induction motor?
In squirrel cage induction motor some times the rotor
and stator care teeth or slots are comes face to face
or parallel at stationary condition. If we are starting
the motor at this condition the motor get hesitated to
start or run due to the attraction developed between
those rotor and stator teeth or slots. This is known as
the magnetic locking or cogging effect of a squirrel
cage induction motor. This type of magnetic locking in
squirrel cage induction motor can be avoided either by
skewing the rotor slot or by selecting the rotor slot,
such that there is no common factor between the rotor
slot and stator slots.
194. What is skewing?
Skewing can be done by turning the rotor slots about 15°
from the parallel position of slots with the shaft.
That is rotor slots are not in parallel with the shaft
but there is an angle of about 15° with the shaft.
195. What are the losses in induction motor?
a. Stator losses (stator copper losses, stator iron
losses).
b. Rotor losses (rotor copper losses, rotor iron
losses).
c. Windage and friction losses.
196. What is synchronous motor?
An alternator, which is running as a motor can be
called as synchronous motor and it runs at synchronous
speed while it converts electrical energy into
mechanical energy. It requires both AC for armature and
DC supply for field.
197. What are the advantages and dis-advantages of
synchronous motor?
Advantages
a. It’s a constant speed motor and is equal to
synchronous speed from no load to full load.
b. It has good efficiency higher than induction motor.
c. It can be run as a motor and also as an alternator as
per the requirement. More over it can be used as
synchronous condenser.
Dis-advantages
a. It can not be used as a varying speed motor. Because
its speed can not be varied.
b. As a motor it is not self-starting type and it can
not be started on load.
c. It requires both AC and DC supply.
d. Hunting is also produced in this motor.
198. What are the applications of synchronous motor?
a. These motors are used in powerhouses, in sub stations
for the improvement of power factor by connecting it
in parallel to the supply and it is run without load
under over excitation of field.
b. Used in big industries where many induction motors
are installed to improve the power factor.
c. Used for constant mechanical loads.
199. What is hunting effect?
When the load is varied to the motor the oscillation
being setup in the rotor about the position of
equilibrium corresponding to change of load condition.
So the damper winding acts the magnetic lines of force
and causes to create the opposite torque, which keeps
the rotor in the same position of the particular load.
This oscillation of the rotor is known as Hunting or
Phase swinging. To reduce this hunting damper winding
is helpful.
200. What is synchronous condenser or phase advancer?
An over excited synchronous motor takes leading current
just like a condenser and gives leading power factor. A
synchronous motor, which I used only for the purpose of
improving power factor, can be called as synchronous
condenser or phase advancer.
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