Synchronous Motors Short Question And Answers 2024

What is a Synchronous Motor?

A synchronous Motor is a type of motor that operates at Synchronous speed.

Why Synchronous Motor is Not a Self-start?

When we increase the load and load cross the limiting load value it simply comes to rest and the average torque developed by it is zero that's why the synchronous motor is not a self-start.

What are the two Main Parts of a Synchronous Motor?

A Synchronous Motor has Two Main Parts

Stator: A Stator is a part which receives power from the supply.
Rotor: A Rotor has a set of salient poles excited by direct current to form alternate N and S poles.

The exciting coils are connected in series to two slip rings and direct current is fed into the winding from an external exciter mounted on the rotor shaft.

What is the Operating Principle of a synchronous Motor?

The Motor Has Direct voltage applied to the rotor winding and a voltage applied to the stator winding. the stator winding produces a rotating field which revolves around the stator at a synchronous speed.

The direct current setup two-pole field is stationary so long as the rotor is not turning. Thus, we have a situation in which a pair of revolving armature poles (Ns-Ss) and a couple of stationary rotor poles (Nr-Sr) exist.

What Happens When Load is Increase in Synchronous Motor?

If The load on the motor increase the torque angle 𝛅 also increases but the motor continues to run at synchronous speed. the increase in torque angle 𝛅 causes a greater phase shift of back emf Eь w.r.t supply voltage V .

This increases the net voltage Er in the stator winding, consequently armature current increases to meet the load demand.

What Happens When Load is Decreased in Synchronous Motor?

If the load on the motor decrease the torque angle 𝛅 also decreases. this causes a smaller phase shift of Eь W.r.t voltage V. Consequently, the net voltage Er In the stator Winding decreases and so does the armature current Ia.

What is a Pull-out Torque?

The Load torque at which the motor pulls out of synchronism is called pull-out torque. its value varies from 1.5 to 3.5 times the full load torque.

What is a Pull-in Torque?

The amount of torque that a Synchronous motor can exert when pulling into synchronism is called pull-in torque. The value of pull-in torque varies from 0.5-1.5 times the full load torque.

What is under Excitation in Synchronous Motor?

The Motor is said to be under-excited if the field excitation is such that Eь<V. Under such conditions, the current Ia lags behind V so the motor power factor is lagging.

What is Normal Excitation in Synchronous Motor?

The Motor is said to be under-excited if the field excitation is such that Eь=V. Under such conditions, the current 'Ia' phasor has come closer to phasor V so the motor power factor is still lagging.

What is Over Excitation in Synchronous Motor?

The Motor is said to be under-excited if the field excitation is such that Eь>V. Under such conditions, the current Ia Lead V so the motor power factor is leading.

What is the application of Synchronous Motor?

Synchronous motors are particularly attractive for low speeds <300RPM because the power factor can always be adjusted to unity and efficiency is high.
over-excited synchronous motor can be used to improve the power factor of a plant while carrying its rated load.
They are used to improve the voltage regulation of transmission lines.
High power electronic converter generating very low frequency enables us to run a synchronous motor at ultra-low speed.

What is the origin of the name Synchronous Motor?

A Synchronous Motor moves in Synchronism with the rotating magnetic field.in other words, the motor runs at the speed of rotating flux.

What is the Procedure for Starting a Synchronous Motor?

The Procedure for Starting a Synchronous Motor is

Short the Field Winding Through A suitable resistance.

Apply A 3-phase supply to the stator winding. since the rotor is equipped with squirrel cage winding, the motor will start as a 3-phase induction motor.

When the motor approaches synchronous speed, remove the resistance and excite the field winding from the DC supply. the rotor and stator poles will be locked, forcing the motor to run at a synchronous speed.

What happens when the mechanical load on a synchronous motor exceed its pull-out torque?

if the mechanical load exceeds the pull-out torque of a synchronous motor the rotor is pull-out of synchronism.

What is the Hunting of a Synchronous Motor?

Although the speed of a Synchronous motor is constant, it often tends to oscillate or hunt around its synchronous speed is called hunting.

What is the effect of hunting on the Synchronous motor?

Hunting Results in the rapid change of mechanical angle between rotor and stator poles and the corresponding variation of stator current.if hunting becomes too severe either the circuit of the motor will be opened by a circuit breaker or the motor will pull out of synchronism and come to a halt.

What are the Causes of hunting in the Synchronous motor?

The Hunting is caused to

Change in load
Change in Excitatipon
Change in other conditions in the system

How Does the Damper Winding of a Synchronous motor reduce hunting?

The purpose of damper winding is

To start the motor
To reduce the hunting effect

When the motor operates at exactly synchronous speed, there is no relative motion between the revolving flux and damper winding and therefore no emf is induced in the damper winding. the damper winding carries no current and has no effect on the operation of the machines.
if the speed of the motor goes below synchronous speed, there is a relative motion between the revolving flux and the damper winding and a current is induced in the winding. consequently, the developed torque is increased, tending to pull the motor back into step at synchronous speed.

What are the disadvantages of synchronous motors?

The main disadvantages of synchronous motors are

The cost per horsepower is generally higher than that of a 3-phase induction motor.
A DC supply is required for motor excitation.
Some arrangements must be provided for starting and synchronizing the motor.
Since the motor can run at one speed only, it cannot be used for variable speed.