Why the speed of synchronous motor is constant?

When you supply 60 Hz (or 50 Hz), the motor will spin at one speed, which is dependent on the number of poles. This rotational speed will be constant with different mechanical loads, up to the point that the motor (or coupling) fails, hence it is a “constant speed” motor.

Why the speed of the motor is less than the synchronous speed?

Actual speed of the induction motor will be less than the synchronous speed. The reason for this is if the armature turns exactly at synchronous speed, then the magnetic field of the stator windings no longer cuts the rotor windings. Under this condition, no current would be induced to the rotor windings.

Why do we keep the speed of the synchronous generator constant at no load?

We actually keep the speed of alternator (i.e. synchronous generator) constant and at rated values of field voltage so as to get rated output voltage at rated frequency. If you vary the speed of prime mover or external loading, then the output voltage frequency will also be varied.

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How does the synchronous motor maintain its speed?

In the synchronous AC motor, the speed of rotation is synchronized to the frequency of the AC input current. Also, produced torque can be regulated with the phase difference between them. … The torque of a synchronous motor is produced by the phase difference between the rotating magnetic field and the rotor.

How can synchronous motor be stopped?

The methods that are employed for Braking of Synchronous Motor are:

  1. Regenerative braking while operating on a variable frequency supply.
  2. Rheostatic braking.
  3. Plugging.

What is relation between synchronous speed and frequency?

The synchronous speed of the motor can be changed by changing the number of poles or by changing the frequency. The machine which runs at synchronous speed is called synchronous machine.



Synchronous speed Formula.

No. of Poles Frequency (50 Hz) Frequency(60 Hz)
2 3000 3600
4 1500 1800
6 1000 1200
8 750 900

Can induction motor run at synchronous speed?

So, theoretically, the Induction motor can never run at synchronous speed. … But due to the reduction of speed, again lag influx will be achieved, and the motor will continue to move at this speed due to flux difference between rotor and stator. In short, an induction motor cannot run at induction speed.

Why the synchronous motor is not self starting?

Above a certain size, synchronous motors are not self-starting motors. This property is due to the inertia of the rotor; it cannot instantly follow the rotation of the magnetic field of the stator. … Once the rotor nears the synchronous speed, the field winding is excited, and the motor pulls into synchronization.

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What is synchronous speed of turbine?

The synchronous speed is the speed of the revolution of the magnetic field in the stator winding of the motor. It is the speed at which the electromotive force is produced by the alternating machine. … The table below gives the number of poles and synchronous speeds for a power frequency of 50 hertz.

What is the main disadvantage of synchronous motors?

Disadvantages or Demerits:



Synchronous motors requires dc excitation which must be supplied from external sources. Synchronous motors are inherently not self starting motors and needs some arrangement for its starting and synchronizing. The cost per kW output is generally higher than that of induction motors.

What is the difference between synchronous speed and rotor speed?

Induction motor with squirrel cage rotor has two different speeds. The synchronous speed refers to the stator rotating magnetic field, which depends on the number of poles and frequency. The other speed is the rotor’s. The rotor speed will be always slower than the stator speed, we call it slip.

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