# Question: Why does the synchronous motor run at synchronous speed?

Contents

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.

## Why the rotor of synchronous machines are bound to run on synchronous speed only?

As stated in Chapter 1, this is because synchronous motors can develop a torque only when running at the synchronous speed. However, the synchronous speed for the utility frequency is too fast for the rotor to synchronize for starting as shown in Fig. 3.63.

## When the motor runs at synchronous speed slip is?

If the rotor run at synchronous speed then there will be no difference between the synchronous speed and rotor speed, so the Slip will be zero, hence no torque will be developed and the rotor will slow down.

## How does a synchronous motor achieve synchronous speed?

The frequency of the power supply and the number of poles of the machine determine the synchronous speed. A synchronous motor is one in which the rotor turns at the same speed as the rotating magnetic field in the stator.

## What is the main disadvantage of synchronous motors?

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 are the advantages of synchronous motor?

The advantage of using synchronous motor is the ability to control the power factor. An over excited synchronous-motor has leading power factor and is operated in parallel to induction motors thereby improving the system power factor. Speed remains constant irrespective of the loads in synchronous motors.

## What is synchronous speed and slip?

Slip is the difference between the synchronous speed of a motor and its actual speed. Mathematically it is determined by. Slip = Synchronous Speed − Actual Speed. Using the example above for synchronous speed of a motor with four poles operating at 60 hertz, the value was calculated to be 1800 RPM.

## How is slip speed calculated?

Slip speed is the speed difference between the Synchronous speed and Rotor speed. Slip speed = Synchronous speed – Rotor speed = Ns -N. Slip, s = (Ns – N) / Ns.

## What is the working principle of synchronous motor?

Working of synchronous motors depends on the interaction of the magnetic field of the stator with the magnetic field of the rotor. The stator contains 3 phase windings and is supplied with 3 phase power. Thus, stator winding produces a 3 phased rotating Magnetic- Field. 