Reluctance Motor

Reluctance Motor — Key Points

Single-phase synchronous reluctance motor has the same stator winding as a single-phase induction motor: main + auxiliary winding.
The rotor is a squirrel-cage type, but with some teeth removed to form salient poles.
There are short-circuited end rings in the rotor.
Starting: When powered, the motor initially works like a single-phase induction motor.
A centrifugal switch disconnects the auxiliary winding at about 75% of synchronous speed. Torque generation:
When speed is close to synchronous, the rotor aligns to the position of minimum magnetic reluctance.
After synchronism: the induction torque vanishes, but the rotor stays synchronized due to synchronous reluctance torque.
Starting torque depends on rotor position; can be 300-400% of full load torque.
Constant speed operation: Works at constant speed up to a bit over 200% of its full load torque.
If load increases beyond its pull-out torque, the motor can lose synchronism, but it may continue running as an induction motor up to 500% of its rated torque.
Cogging is a problem at start; can be reduced by skewing the rotor bars and adjusting rotor slot design.
Power factor is low because the rotor is unexcited (no DC field).
Size: Since there's no DC excitation, the output for a given size is lower → so reluctance motors are larger compared to synchronous motors of same power.
Advantages: Simple construction (no brushes, no slip rings, no DC windings), low cost, easy maintenance.
Applications: Good for constant-speed devices like electric timers, signalling devices, recording instruments, etc.

Advantages

The reluctance motor has following advantages,

1) No d.c. supply is necessary for rotor

2) Constant speed characteristics

3) Robust construction

4) Less maintenance.

Limitations

The reluctance motor has following limitations,

1) Less efficiency

2) Poor power factor

3) Need of very low inertia rotor

4) Less capacity to drive the loads.

Applications

This motor is used in signalling devices, control apparatus, automatic regulators, recording instruments, clocks and all kinds of timing devices, teleprinters, gramophones etc.

Question: Torque in a reluctance motor is produced primarily due to:
A) Permanent magnet alignment
B) Induction principles
C) Magnetic saliency
D) Eddy current generation

Answer: C) Magnetic saliency

Explanation: In a reluctance motor, the rotor has different reluctance along different axes (direct axis vs quadrature axis). The rotor tends to align itself in the position of minimum reluctance when the stator’s magnetic field rotates, producing torque. This is because of the magnetic saliency (difference in reluctance).

Question: In a single-phase reluctance motor:
A) It is self-starting and requires DC excitation
B) It is constant-speed and needs DC excitation
C) It is self-starting and needs no DC excitation
D) It is not self-starting

Answer: C) It is self-starting and needs no DC excitation

Explanation: A single-phase reluctance motor starts like an induction motor (because of its squirrel-cage rotor), and once near synchronous speed, it transitions to synchronous operation using reluctance torque. There is no need for DC excitation on the rotor because the rotor is just a ferromagnetic core (no field winding).

MCQ 3

The rotor of a reluctance motor is basically:
A) Shaded pole rotor
B) Squirrel-cage rotor with some teeth removed
C) Permanent magnet rotor
D) Wound rotor with slip rings

Answer: B) Squirrel-cage rotor with some teeth removed

Explanation:
According to CircuitGlobe, the rotor is a squirrel-cage type, but some teeth are removed to form salient poles. This saliency creates the reluctance torque.

MCQ 4

A reluctance motor initially starts as:
A) DC motor
B) Universal motor
C) Induction motor
D) Stepper motor

Answer: C) Induction motor

Explanation:
At start, the rotor cage conducts currents due to induction. So it behaves like a single-phase induction motor until it reaches about 75% synchronous speed.

MCQ 5

At what speed does the centrifugal switch disconnect the auxiliary winding?
A) 10% of synchronous speed
B) 40% of synchronous speed
C) 75% of synchronous speed
D) 100% of synchronous speed

Answer: C) 75% of synchronous speed

Explanation:
As per CircuitGlobe, the centrifugal switch opens around 75% synchronous speed, after which the motor continues to accelerate.

MCQ 6

Once the reluctance motor reaches synchronous speed:
A) Induction torque increases
B) Induction torque becomes zero
C) Rotor is supplied DC current
D) Motor stops rotating

Answer: B) Induction torque becomes zero

Explanation:
At synchronism, there is no relative motion between rotor & rotating magnetic field, so induction torque is zero. Only reluctance torque keeps the rotor locked in synchronism.

MCQ 7

Reluctance torque is produced because the rotor always tries to align itself to:
A) Maximum eddy current
B) Maximum torque angle
C) Minimum reluctance path
D) Maximum flux linkage

Answer: C) Minimum reluctance path

Explanation:
Rotor aligns to the position where magnetic resistance (reluctance) is lowest. This alignment produces torque.

MCQ 8

Reluctance motors generally have:
A) High power factor
B) Low power factor
C) Zero power factor
D) Leading power factor

Answer: B) Low power factor

Explanation:
Because the rotor is unexcited (no DC field), the power factor is low. More magnetizing current is required to establish flux.

MCQ 9

What is a major problem during starting of reluctance motors?
A) Voltage drop
B) Cogging
C) Over-fluxing
D) Demagnetization

Answer: B) Cogging

Explanation:
Rotor may lock at certain positions due to magnetic locking, causing starting issues. This is cogging, and is reduced by skewing rotor bars.

MCQ 10

Reluctance motors are ideal for which type of applications?
A) Variable-speed drives
B) Very high starting torque applications
C) Constant-speed applications
D) Battery-operated portable devices

Answer: C) Constant-speed applications

Explanation:
Reluctance motors maintain constant speed once synchronized, making them useful for timers, recorders, clocks, and signaling devices.