Motor, Generator and Exciter Question & Answer

Motor, Generator and Exciter

1. What are the main classifications of alternator?

a. Salient pole.
b. Non – salient pole.


2. What is the emf equation of alternator?

Flux cut per second by each conductor = 2 φm f
Average emf generated in each conductor = 2 φm f Z
Average emf generated per phase = kd kc 2 φm f Z
r.m.s emf generated per phase = kf kd kc 2 φm f Z
For sinusoidal waveform when kf is 1.11 then emf generated
= 1.11 *2 kd kc φm f Z
= 2.22 kd kc φm f Z
kf 􀃆form factor
f 􀃆 Frequency
φm 􀃆 Flux maximum
Z 􀃆 Turns per phase
kd 􀃆 Breadth factor or distribution factor
kc 􀃆 Coil span

3. Why conductors in alternator are transposed?

To reduce eddy current losses.

4. What is the effect of frequency and high voltage at the start of motor.

For a constant load if frequency decreases motor current will increase and at the start
if voltage is more motor current also increases.

5. What is the minimum voltage required for starting of 6.6 kV motors?

Minimum 80% of rated voltage.

6. What are the limits of vibration measurement for motors?

50 microns for displacement and 5 mm/second for velocity.

7. What you mean by SPDP?

Screen protected drip proof.

8. What is the current in single phasing?

2 times of rated current.

9. What is the impedance per phase of delta connected motor?

1.5 times the total impedance.

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10. What is the slip of an induction motor during normal running?
More than zero.


11. What is the effect of increased load on power factor of induction motor?

Power factor of an induction motor increases with loading.

12. Explain the behavior of generator when operating alone and operating parallel with
grid.

Generator operating alone.
a) The power factor of generator depends on load power factor.
b) The terminal voltage decreases when generator is loaded.
c) Governor decides the frequency of generator.
d) Increase in excitation increases the terminal voltage of the generator.

Generator parallel with grid.

a) If we increase the steam input to the generator the frequency of the generator will
not change. It will remain practically constant as same as grid frequency. That is
grid decides the frequency of the generator.
b) Increase in the excitation will not increase the terminal voltage. Instead the
reactive power out put of the generator increases. This reactive power supplies for
the magnetizing current of motors, transformers and etc.
c) Increase in the steam input increases the active power of the generator.
d) If generator is under excited it will draw leading reactive current from the grid.

13. Draw and explain following.

a) Load current Vs terminal voltage at different power factors.

1. At leading power factor as the load current increases the terminal voltage also
increases.
2. At lagging power factor as the load current increases the terminal voltage
drops.
3. At unity power factor as the load current increases there is slight drop in
terminal voltage.
Leading power factor
Unity power factor
Lagging power factor
Ter. Vol.
Load current

b) Torque Vs slip characteristics of induction motor.
Torque (T) = φ I2 cosφ2
Where φ - main flux.
I2 – rotor current Pull out torque (R = XL) I
Cosφ2 – rotor power factor.
Starting current
Starting torque (1.5 times)
Torque Full load current
1 Slip 0
When motor is started from rest the slip is 1 (one) at time of starting and starting
torque is 1.5 times of rated torque. As the motor accelerates slip reduces and torque
increases. Because the power factor of rotor is improving due to the decrease in rotor
frequency and at certain slip the rotor reactance is equal to rotor resistance. At that
time torque is maximum (pull out torque). When the motor accelerates to the rated
speed the torque comes to the rated value, which is less than the starting torque.
Torque is zero when slip is zero, because there will be no relative motion between
stator magnetic field and rotor.
c) Generator capability curve
This curve gives the operating limits of the turbine generator at different power
factor what should the power output of the generator to avoid the heating of
generator stator winding, rotor parts and end parts.

14. What is the cooling medium for different parts of the turbo generator?

Stator – DM water.
Rotor – Hydrogen.
Bearing – Oil.

15. What are the effects of unbalanced voltage on induction motor?

There will be negative sequence current, which will heat up the stator winding and
weakens the insulation. These currents will induce emf in rotor and heat up the rotor
bars and cause breakage in them. Also due to high leakage fluxes due to negative
phase sequence current the end parts heating will be more.

16. What are the types of bearings are adopted for small motors and large motors?

Small motors (LT motors)
Horizontal mounted – deep groove ball bearing at both ends.
Medium motors
Roller bearing at DE and deep groove bearing at NDE.
Large motors (HT motors above 750 kW)
Horizontal mounted – sleeve bearing (pedestal) cooled by lub oil.
Vertical mounted – face to face angular contact ball bearing at NDE and roller or ball
bearing at DE.

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