# Reluctance Motor MCQ (Multiple Choice Questions) :: Single Phase Induction Motor

1. Reluctance motor is basically a

1.  DC shunt motor
2. Servo motor
3. DC series motor
4. Single-phase synchronous motor

Explanation:  A single-phase reluctance motor is basically the same as the single-phase salient pole synchronous cage type induction motor, hence it is also called ​a self-starting type synchronous motor.

2. The stator of reluctance motor resembles

1. Induction Motor
2. DC motor
3. Synchronous Motor
4. Compound Motor

Explanation: The stator of the reluctance motor has the main and auxiliary winding. The stator of the single-phase reluctance motor is the same as the induction motor. The rotor of a reluctance motor is a squirrel cage with some rotor teeth removed in certain places to provide the desired number of salient rotor poles.

3. The excitation required to start the reluctance motor is

1. DC excitation
2. Variable DC excitation
3. No DC excitation Required
4. None of the above

Explanation: Single-phase induction motors built with a variable air-gap reluctance and with having no dc supply on the rotor are called reluctance motors. Such motors start as induction motors but are pulled into synchronous speed because of the variation in air-gap reluctance.

4. The types of reluctance motor are

1. Switched Reluctance Motor
2. Synchronous Reluctance Motor
3. Both 1 & 2
4. None of the above

Explanation: There are two main designs of the reluctance motor.

(a) Synchronous reluctance motor is an electrical rotating machine that converts the electrical power into mechanical power.

(b) Switched reluctance motor is an electric motor that runs by reluctance torque. Unlike common brushed DC motor types, power is delivered to windings in the stator (case) rather than the rotor.

5. How to extract maximum torque in a single phase repulsion type motor?

1. The angle between brush axis and stator field axis must be equal to 180°.
2. The angle between brush axis and stator field axis must be equal to 0°.
3. The angle between brush axis and stator field axis must be equal to 90°.
4. The angle between brush axis and stator field axis must be equal to 45°.

Answer.4. The angle between brush axis and stator field axis must be equal to 45°.

Explanation: The torque in the repulsion motor is given as

Te = 0.5k IsNs2 sin 2α

Where Is and Ns are the stator filed-current and an effective number of stator turns.

α is the angle between the brush axis or rotor field axis to the stator field axis.

Since a constant stator field motor extract maximum torque when

sin 2α = 1 ⇒ 2α = 90°  ⇒ α = 45°

Maximum torque in the repulsion motor is achieved when the stator and rotor field axis is 45° apart.

6.  The switched Reluctance motor is also called as

1. Variable Synchronous Motor
2. Variable Reluctance Motor
3. Variable Induction motor
4. None of the above

Explanation: Switched Reluctance Motor (SRM) is also known as Variable Reluctance Motor. This motor works on the principle of variable reluctance. This means, the rotor always tries to align along the lowest reluctance path.

7. A 8-kW, 4-pole, 220-V, 50-Hz reluctance motor has a torque angle of 30° when operating under rated load conditions. Calculate the load torque.

1. 51 Nm
2. 4.3 Nm
3. 43 Nm
4. 5.1 Nm

Explanation:

Speed of Reluctance Motor (Ns) = 120 f / P

f = frequency, P = number of poles

Angular speed (ω) = (2 π Ns) / 60

Calculation:

Given

P = 4, f = 50 Hz, power = 8000 W

Ns = 120 × 50 / 4 = 1500 RPM

ω = (2 × π × 1500) / 60

Torque = Power / ω

Torque = 8000 / 157

Torque = 51 N-m

8. Which of the following is NOT a characteristic of an unexcited single phase synchronous motor?

1. Operates from a single-phase ac supply
2. Requires DC excitation for the rotor
3. Self-starting
4. Runs at constant speed

Answer.2. Requires DC excitation for the rotor

Explanation: Single-phase induction motors built with a variable air-gap reluctance and with having no dc supply on the rotor are called reluctance motors. Such motors start as induction motors but are pulled into synchronous speed because of the variation in air-gap reluctance.

9. The direction of rotation of Switch reluctance Motor can be reversed by

1. Changing the supply terminal
2. Changing the Rotor terminal wire
3. Changing the Stator terminal wire
4. Rotation can’t be reversed

Explanation: In a reluctance motor when the rotor starts rotating in the same direction, it will continue to rotate like other types of single-phase inductor motors. It is evident that the direction of rotation of these motors is fixed by the construction (i.e., stator poles) and cannot be reversed.

10. The starting torque of the reluctance motor can be achieved by creating

1. Uniform air gap
2. Non-uniform air gap
3. DC excitation
4. Any of the above

Explanation: The stator of a reluctance start motor is constructed with salient poles. The starting torque is achieved by creating a non-uniform air gap of the salient poles. Each pole is excited by its own winding carrying the same current.

11. A reluctance motor is usually preferred in

1. Electric shavers
2. Refrigerators
3. Lifts and hoists
4. Recording instruments

Explanation: Reluctance motors can deliver very high power density at low cost, making them ideal for timing devices and control apparatus. The other applications of reluctance motor are

• Signaling devices
• Recording instruments
• Phonographs
• Analog electric meters

12. The main advantage of reluctance Motor is

1. No DC supply required
2. Low maintenance
3. Self-starting
4. All of the above

Explanation:

• It doesn’t require DC supply.
• Stable characteristics
• Maintenance is less
• Less heat
• No magnets
• It is self-starting.

13. The main disadvantage of reluctance motor is

1. Low power factor
2. Low torque
3. Low efficiency
4. All of the above

Explanation: Reluctance start motors have very small starting torque, low efficiency, and poor power factor, and therefore, their applications are limited. For most of small power applications, shaded pole motors are preferred.

14. The starting winding of the reluctance motor gets disconnect when the motor reaches

1. 25% of rated speed
2. 75% of rated speed
3. 50% of rated speed
4. 10% of rated speed

Explanation: The reluctance motor starts as a single-phase induction motor. The starting winding gets disconnected at a speed of about 75 percent of the synchronous speed, with the help of a centrifugal switch or overcurrent relay. The motor continues to develop torque through its main winding. As the rotor speed approaches synchronous speed, the reluctance torque developed is sufficient to pull the rotor to synchronous speed.

15. The main winding and auxiliary winding of the reluctance motor is connected in

1. Series
2. Parallel
3. Either series or parallel
4. Series-Parallel