The rotor of a synchronous motor moves at the same speed as the changing, rotating field in its stator. The rotor’s magnetic field is “synchronized” with the stator’s field. When you supply 60 Hz (or 50 Hz), the motor will spin at one speed, which is dependent on the number of poles.
How does synchronous motor change speed?
Changing the number of poles is not easy, so we do not use that method. However, with the invention of solid-state devices, the frequency of the current fed to the synchronous motor can be varied. We can control the speed of the synchronous motor by changing the frequency of the supply to the motor.
What determines the speed of a synchronous motor?
The synchronous speed of an AC motor is determined by the frequency of the source and the number of poles. The RPM is calculated by multiplying the frequency times 60 and dividing by the number of pairs of poles.
How do you control the speed of a synchronous motor?
Speed of synchronous motors can be controlled using two methods called open loop and close loop control. Open loop contol is the simplest scalar control method where motor speed is controlled by independent frequency control of the converter.
Why is synchronous speed constant?
In the synchronous motor, rotor too have a DC excitation, so it produces a constant flux forming the electromagnetic poles on rotor which initially are stationary. … Even on loading, the speed is always constant but the torque angle (between stator and rotor magnetic poles) may vary.
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 advantage of synchronous motor?
The advantages of the synchronous motor are the ease with which the power factor can be controlled and the constant rotational speed of the machine, irrespective of the applied load. Synchronous motors, however, are generally more expensive and a d.c. supply is a necessary feature of the rotor excitation.
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.
What two factors determine synchronous speed?
What two factors determine synchronous speed? 1. The number of stator poles (per phase). 2.
- The strength of the magnetic field of the stator.
- The number of turns of wire cut by the magnectic field (in the case of a squirrel-cage rotor, this will be the number of bars in the rotor).
- The speed of the cutting action.
What is asynchronous speed?
Synchronous motor is a machine whose rotor speed and the speed of the stator magnetic field is equal. Asynchronous motor is a machine whose rotor rotates at the speed less than the synchronous speed.
What is self control of synchronous motor?
In self controlled mode, the supply frequency is changed so that the synchronous speed is same as that of the rotor speed. Hence, rotor cannot pull-out of slip and hunting eliminations are eliminated. For such a mode of operation the motor does not require a damper winding. Fig (5.4 )Self Controlled Mode.
What is true synchronous mode?
In true synchronous mode, the stator supply frequency is controlled from an independent oscillator. Frequency from its initial to the desired value is changed gradually so that the difference between synchronous speed and rotor speed is always small. This allows rotor speed to track the changes in synchronous speed.
How do you start a synchronous motor?
Starting a Synchronous Motor Using Slip Ring Induction Motor
The resistance is gradually cut-off as the motor gains speed. When it achieves near synchronous speed, DC excitation is given to the rotor, and it is pulled into synchronism. Then it starts rotating as a synchronous motor.
What is the value of synchronous speed?
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.
|Number of Poles||Synchronous Speed NS in r.p.m|
Why does alternator run at synchronous speed?
A synchronous motor runs at synchronous speed only because it is meant to run like that . It cannot run on a speed other than synchronous speed. Stator(N) links with Rotor(S). … For many other applications you can use Induction motor which is simple robust and efficient.
What is the slip speed?
The speed at which the induction motor work is known as the slip speed. The difference between the synchronous speed and the actual speed of the rotor is known as the slip speed. In other words, the slip speed shows the relative speed of the rotor concerning the speed of the field.