Electrical Drives and Controls Interview Questions

1). What is Switched Reluctance Motor?

The switched reluctance motor is a double salient and singly excited motor. 

This indicates that it has a salient pole on both the rotor & the stator, although only one member has windings.

The rotor contains no windings, magnets, or cage windings. It works on the variable reluctance principle.

2). What are the two types of control techniques in SRM?

The two types of control techniques in SRM are:

• Hysteresis Type Control
• PWM Type Control

3). What is meant by Energy Ratio?

Energy Ratio = W_m/[W_m + R] = 0.455

Where

W_m – Transformed Mechanical Energy

This energy ratio cannot be described as efficient. As the stored energy R is not lost, it is given back to the source via feedback diodes.

4). What is Phase Windings?

Stator poles with field coils.

The field coils of opposing poles are coupled in series so that mmfs are additive, and they are known as phase wings of SRM. 

5). What is the difference between SRM and Stepper Motor?

Switched Reluctance Motor (SRM)	Stepper Motor
Used for high-speed and variable load.	Utilized for accurate position control.
Closed-loop control (needs position sensor).	Mostly open-loop control.
Non-uniform torque, high torque density.	Uniform step torque, lower torque.

6). What is Hysteresis Current Control?

This form of current controller keeps the current relatively constant during the conduction period in every phase. This controller is known as the hysteresis type controller.

7). When do you go for chopping mode of control in switched reluctance motor?

SRM’s chopping technique of control is used only at low speeds. Here, each phase winding is energized for a suitably long amount of time.

8). What is the working principle of Switching Reluctance Motor?

The switched reluctance motor works based on the principle of Faraday’s law of Electromagnetic Induction.

• When a current-carrying conductor is placed in a magnetic field, it experiences a force. The direction of force determined by Fleming’s left hand rule.
• SRM is a single-excited electric motor with two salient poles. Because of the reluctance variations, the motor is supposed to run. The SRM generates torque due to changing reluctance.
• When the air gap is minimal, the reluctance is minimal, and thus the inductance is highest. Therefore, the rate of change of inductance is 0.
• When the reluctance in the motor varies, the inductance changes, and when a certain stator winding of the SRM is activated, the rotor pole aligns with that stator pole, causing the rotor to rotate.

9). Why rotor position sensor is essential for the operation of Switched Reluctance Motor?

Commutation and speed feedback typically require the installation of a rotor position sensor. The signals collected from the rotor position sensor influence the ON and OFF operations of the various devices in the power semiconductor switching circuit.

10). What are the Disadvantages of a Switched Reluctance Motor?

• Stator phase windings should be able to carry magnetizing current.
• For high-speed operation, produced torque has undesired ripples, which causes undesirable noises (or) acoustic noises.
• To reduce undesired harmonics in high-speed current waveforms, a large capacitor should be attached.
• It needs position sensors.

11). What are the Advantages of Switched Reluctance Motor?

• Construction is simple & durable.
• The rotor has no windings, slide rings, or brushes, requiring minimal maintenance.
• There are no permanent magnets.
• Ventilating systems are simpler as losses occur primarily in the stator.
• Power semiconductor switching circuitry is simple.
• Power short circuits are unlikely to occur due to a shoot-through fault.
• The produced torque is independent of the polarity of the current in phase winding.
• The machine can simply switch between driving and generating modes by adjusting the conduction region.
• It is feasible to achieve high speeds.
• T-w characteristics may be adapted to meet specific requirements.
• This is self-starting machine.
• During the off period, energy stored in phase winding is returned back into the supply via feedback diodes.

12). Whar are the applications of SRM?

• Vacuum cleaners
• Future auto mobile applications
• Washing machines
• Robotics control applications
• Fans

13). What is the difference between Synchronous Reluctance Motor and PM Synchronous Motor?

Synchronous Reluctance Motor	PM Synchronous Motor
No magnets; depends on rotor saliency.	Uses permanent magnets on the rotor.
Lower torque & efficiency.	Higher torque density & better efficiency.
Cost-effective & simple rotor.	More costly due to magnets.

14). What are the differences between PMBLDC and PMSM?

PMBLDC (Permanent Magnet BLDC)	PMSM (Permanent Magnet Synchronous Motor)
Trapezoidal back EMF	Sinusoidal back EMF
Commutation is done electronically in 6 steps.	Uses sinusoidal commutation (often with vector control).
Simpler control, more ripple torque.	Smoother torque with better performance.

15). Difference between self-control and vector control of PMSM

Self-Control	Vector Control
Rotor position sensor utilized to synchronize stator frequency with the rotor speed.	Uses transformation (like Clarke & Park) to control torque & flux independently.
Minimizes DC motor control (commutator less DC motor).	Provides fast dynamic response & accurate control.
Simpler control logic.	More complex & requires DSP/microcontroller.

16). What is meant by Slotless Motor?

In slotless motors, stator teeth are eliminated and partially filled with copper

17). State the advantages of PMSM?

• Increased Reliability.
• Improved Performance.
• Controller Versatility
• Cost-Effectiveness
• Reduced Components.

18). What are the advantages and disadvantages of PMSM?

Advantages of PMSM:

• It maintains constant speed.
• Eliminates field winding and loss, leading to higher efficiency.
• Requires minimal maintenance as there are no sliding connections.

Disadvantages of PMSM:

• Field winding does not allow for adjustment of power factor during operation.
• It causes losses and reduces efficiency

19). Write the EMF equation of PMSM

E_ph = 4.44 f Φ_m K_p K_b N_ph  (Volts)

The RMS value of the induced EMF per phase is calculated using the following parameters:

• Frequency (Hz),
• Number of turns per phase (Nph),
• Flux per pole (Φm), and
• Winding factor (Kw).

20). What are the assumptions made in derivation of EMF equation for PMSM?

• The flux density in the air gap is sinusoidal.
• The rotor revolves at a uniform angular velocity of ω_m (r/sec).
• Armature winding is made up of full-pitch, focused coils with equal number of turns.

21). Why PMSM operating in self-controlled mode is known as Commutatorless DC Motor?

The load side controller serves a similar purpose as the commutator in a DC machine.

The

• Load Side Converter and
• Synchronous Motor

combination performs similarly to a DC machine.

It is initially powered by a DC supply & then operates similarly to a DC machine. At all speeds, the stator & rotor fields are stationary relative to one another. As a result, the drive consisting of a load side converter and a synchronous motor is referred to as a commutatorless DC motor.

22). What is Pulse Mode?

For speeds less than 10% of base speed, load side converter SCRs are commutated by setting the current via the conducting SCRs to 0.

This is accomplished by making the source side converter function as an inverter each time the load side converter SCRs are turned off.

Such an action is possible because the load side converter operates at a much lower frequency than the source frequency.

The operation of an inverter is known as Pulsed Mode.

23). What is Load Commutation?

Load Commutation refers to the commutation of thyristors using induced voltages from the load. Here, the frequency of operation is higher, and no commutation circuits are required.

24). What is meant by Self-Control?

• As the rotor speed increases, the armature supply frequency changes accordingly, ensuring that the armature field moves (rotates) at the same rate as the motor.
• The armature & rotor field move in synchronism at all operational positions.
• The use of a rotor position sensor allows for accurate tracking of the speed by frequency.

25). What is meant by Negative Torque in a SRM?

In a switched reluctance motor, negative torque is the torque that opposes the desired direction of rotation.

It occurs when phase excitation is provided before or after the rotor and stator poles have aligned.

This occurs as a result of improper timing in phase excitation, which causes magnetic forces to pull the rotor in opposite directions.

To avoid negative torque, accurate control of the turn-on & turn-off angles using a rotor position sensor is required.