100 MCQ OF Synchronous Generator or Alternator with Explanation
61.Alternators are usually designed to generate which type of ac voltage?
In
an alternating current electric power system, synchronization is the
process of matching the speed and frequency of a generator or other
source to a running network. An AC generator cannot deliver power to an
electrical grid unless it is running at the same frequency as the
network. If two segments of a grid are disconnected, they cannot
exchange AC power again until they are brought back into exact
synchronization. Hence alternators are usually designed with the fixed
frequency of AC voltage.
62.Two alternators rated 40 MVA and 60 MVA respectively are working in parallel and supplying a total load of 80 MW. Speed regulation of both the alternator is 5%. The load sharing between them will be
Change in frequency from No load to full load f = 50 x 0.05 = 2.5Hz Full load frequency ffl1 = 50 – 2.5 = 47.5Hz Since both alternators have same speed regulation then ffl1 = ffl2From the above diagram For machine 1 (50 – f)/(80 – x) = (50 – 47.5)/40 x – 16f = 80 – 16 x 50 x- 16f = -720—–1 For machine 2 (50 – f)/(80 – x) = (50 – 47.5)/60 x – 24f = -1200 ——-2 From equation (1) and (2) x= 48 MW f = 60 Hz So, machine A operates at a load of 48 MW While machine B will operate at a load of 80 – 48 = 32 MW
63.An alternator has a per unit impedance of 0.9 p.u. to a base of 20 MVA, 33 kV, Then the p.u. impedance to the base of 50 MVA and 11 kV.
The per unit impedance is given as
64.In an alternator, the effect of armature reaction is minimum at the power factor of
At unity p.f., the effect of armature reaction is merely to distort the main field; there is no weakening of the main field and the average flux practically remains the same. At zero p.f. lagging, armature reaction is directly demagnetizing and the armature reaction weakens the main flux. This causes a reduction in the generated e.m.f. At zero p.f. leading, armature flux is now in the same direction as the field flux and, therefore the armature reaction strengthens the main flux. This causes an increase in the generated voltage.
65.A 10 pole 25 Hz alternator is directly coupled to and is driven by 60 Hz synchronous motor then the number of poles in a synchronous motor is?
Number of poles of alternator Pa = 10 F = 25 Hz (alternator) F = 60 Hz (motor) Then the number of poles of motor Pm =? Since the synchronous motor is directly coupled hence Synchronous speed of an alternator = Synchronous speed of the motor (120 x 25)/10 = (120 x 60)/ Pm Pm = 24
66.Turbo alternators have rotors of
Smooth Cylindrical Type or Non-Salient Pole alternator rotor is used for steam-driven alternator i.e turbo alternator which runs at very high speed.
- This type of Rotor is used for steam-driven alternator i.e turbo alternator which runs at very high speed.
- The Rotor is made up of smooth solid forgings of alloy steel cylinder having the number of slots along the outer periphery.
- The field windings of cylindrical type rotor are connected in series to the slip rings through which they are excited by the DC exciter.
- The top portion of the slot is covered with the help of steel or manganese wedges and the unslotted portion of the cylinder acts as the poles of an alternator.
- The field windings are arranged in such a way that its flux density is maximum on the polar central line.
- In the cylindrical rotor, the pole doesn’t project out from the smooth surface of the rotor hence they maintain the uniform air gap between stator and rotor.
- Since the steam turbine runs at very high speed, therefore, they required less number of poles hence the diameter of the rotor is small and axial or rotor length is large.
67.The steady-state fault current during a 3 phase terminal fault on a generator is determined by:
- Sub-transient (xd”). Subtransient reactance is a value used to determine the short circuit current during the first few cycles after a short-circuit occurs. This is the short-circuit current value to be used in all short-circuit studies.
- Transient reactance (Xd’). Transient reactance is a value used to determine the short-circuit current from the first few cycles up to about 30 cycles after the short-circuit occurs (depending upon the design of the machine). This value is often used in voltage regulation studies.
- Synchronous reactance (Xd). Synchronous reactance is a value used to determine the short-circuit current when the steady-state condition has been reached. Steady-state is reached several seconds afier the shortcircuit occurs. This value is often used to determine the setting of the generator backup overcurrent relays.
68.To reduce the peripheral speed of an alternator, the diameter of the rotor is
Peripheral speed refers to the actual linear speed of a tooth or point of the circumference of an alternator when it is revolving under power. Peripheral speed is thus the product of the circumference of the alternator and the number of revolutions per minute. The peripheral speed of the alternator Ps = π × D × RPM The peripheral speed depends on the speed as well as the diameter of the rotor. So to reduce the peripheral speed of the alternator the diameter of the rotor should be decreased.
69.The positive, negative and zero sequence impedances of 3-phase synchronous generator are j 0.5 pu, j 0.3 pu, and j 0.2 pu respectively. When the symmetrical fault occurs on the machine terminals. Find the fault current. The generator neutral is grounded through reactance of j0.1 pu
For symmetrical fault, the fault current is given as If = E/(Zi +Zn) Where E = Pre fault voltage Which is equal to 1 Zi = poisitive impedance Zn = Neutral Impedance Zi = 0.5j & Zn = 0.1 j If = 1/(0.5j + 0.1j) If = -j 1.67
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70.The emf induced per phase in a three-phase star connected synchronous generator having the following data: - Distribution factor = 0.955, Coil-span factor = 0.966, Frequency = 50 Hz, Flux per pole = 25 mwb, Turns per phase = 240, then emf per phase is
E.M.F equation of an alternator is given as E = Kc Kd √2π f Φ Np Or E = 4.44 Kc Kd f Φ Np ………….. (since √2π = 4.44) Where Kc = Coil span factor Kd = Distribution factor Φ = Flux per pole f = frequency Np = Turns per phase E = 4.44 × 0.955 × 0.966 × 50 × 25 × 10-3 × 240 E = 1228.1 volts
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