# Electrical Drives and Controls Important Questions: Set-3

#### Electrical Drives and Controls Important Questions

In this post we had collected various important interview questions from electrical drives. We are sure that it will help you to attend interviews.

Permanent Magnet Synchronous Motor related interview Questions:

[1] What is the difference between Synchronous Reluctance Motor and PM Synchronous Motor?

 SRM PMSM Rotor has no permanent magnet Rotor has permanent magnet Less cost High cost Low efficiency High efficiency

[2] What are the differences between PMBLDC and PMSM?

 PMBLDC motor PMSM Rectangular distribution of magnetic flux in the air gap Sinusoidal or quasi-sinusoidal distribution of magnetic flux in the air gap Rectangular current waveforms sinusoidal or quasi-sinusoidal current waveforms Concentrated stator winding Quasi-sinusoidal distribution of stator conductors, ie, short-pitched and distributed or concentric stator windings

[3] Distinguish between self-control and vector control of PMSM

 Self control Vector control Dynamic performance is poor Dynamic performance is better Control circuit is simple Control circuit is complex

[4] What is meant by slotless motor?
In slotless motor, the stator teeth are removed and resulting space is partially filled with addition of copper

[5] State the advantages of PMSM?

1. Improved performance
2. Reliability increases
3. Reduced components
4. Versatility of the controller
5. Less cost

1. It runs at constant speed.
2. No field winding, no field loss, better efficiency.
3. No sliding contacts, so it requires less maintenance.

1. Power factor of operation can not be controlled as field winding cannot be controlled.
2. It leads to losses and decreases efficiency

[7] Write the EMF equation of PMSM

Eph = 4.44 f. Φm. Kp. Kb. Nph volts

This is the RMS value of induced EMF per phase
where
f - frequency in Hz
Nph - Turns per phase
Φm - Flux per pole
Kw - Winding factor

[8] What are the assumptions made in derivation of EMF equation for PMSM?

1. Flux density distribution in the air gap is sinusoidal.
2. Rotor rotates with a uniform angular velocity of ωm(r/sec).
3. Armature winding consists of full pitched, concentrated similarly located coils of equal number of turns.

[9] Why PMSM operating in self-controlled mode is known as Commutatorless DC Motor?
Load side controller performs somewhat similar function as commutator in a DC machine. The load side converter and synchronous motor combination functions similar to a DC machine.
First it is fed from a DC supply and secondly like a DC machine. The stator and rotor field remain stationary with respect to each other at all speeds. Consequently, the drive consisting of load side converter and synchronous motor is known as commutator less DC motor.

[10] What is Pulse Mode?

• For speeds below 10% of base speed, the commutation of load side converter SCRs is done by forcing the current through the conducting SCRs to zero.
• This is realised by making source side converter to work as inverter each time load side converter SCRs are to be turned off.
• Since the frequency of operation of load side converter is very low compared to the source frequency, such kind of operation can be realised.
• The operation of inverter is termed as Pulsed Mode.

Commutation of thyristors by induced voltages of load is known as Load Commutation. Here, frequency of operation is higher and it does not require commutation circuits.

[12] What is meant by self-control?

• As the rotor speed changes the armature supply frequency is also changes proportionally so that the armature field always moves (rotates) at the same speed as the motor.
• The armature and rotor field move in synchronism for all operating points.
• Here accurate tracking of speed by frequency is realised with the help of rotor position sensor.