In the name of ALLAH, the Most Merciful, the Most Kind

In the name of ALLAH, the Most Merciful, the Most Kind

SYNCRONOUS MOTOR

Synchronous motorSynchronous motor

Basic principle. Starting of synchronous motor. Why dc excitation is used?

what is synchronous motor?what is synchronous motor?

It is a synchronous machine use to convert electrical power to mechanical power.

A synchronous motor is identical with a an alternator or ac generator or synchronous generator.

Some features of synchronous motorSome features of synchronous motor

It runs as synchronous speed the only to change its speed can be change only by changing supply frequency.

It is not self starting, it has to be run up to synchronous speed by some means before it can be synchronized to the supply.

It can used for both lagging and leading power factor.

BASIC PRINCIPLE

It should be noted:It should be noted:

The rotating part is called rotor and field winding is mounted on it .it is also called rotor winding

Armature winding mounted on stationary part called stator so also called stator winding.

Basic principleBasic principle

Consider a two pole synchronous motor A 3-Φ supply is applied to the stator of the motor which

produces a 3-Φ current flow in the winding. This current results a magnetic flux of constant

magnitude rotating at synchronous speed. The field winding is excited by dc current which

produces steady state magnetic field

Basic principle cont..Basic principle cont..

Now there two magnetic field present in the motor : Rotor filed Stator field Both field will tend to line up each other just like two bar

magnets will tend to line up if place near each other.

Basic principle cont.. Basic principle cont..

The stator magnetic field is rotating so the rotor magnetic field will try to catch up.

Due to inertia of the rotor and also inertia of the load connected to the rotor it can never catch up the stator field but always lags behind it .

The greater the angle between the two magnetic fields greater will be the torque on the rotor.

Basic principle cont..Basic principle cont..

We can say the rotor chases the rotating stator magnetic filed around in a circle never quit catching up with it.

Starting of synchronous

motor

Why synchronous motor is not self starting?Why synchronous motor is not self starting?

A synchronous motor is not a self starting motor because torque is only developed when running at synchronous speed, therefore ,the motor needs some type of device to bring the rotor to synchronous speed.

Methods of starting synchronous motorMethods of starting synchronous motor

By reducing supply frequency. By using external prime mover. By using damper winding.

Reducing the frequency Reducing the frequency

When we reduce the supply frequency speed of the stator magnetic field to a low enough value that the rotor can accelerate and lock in with it during one half of the magnetic field rotation.

Then the frequency is gradually increase up to normal value.

Disadvantage It requires an independent generator for frequency

control.

By using external prime moverBy using external prime mover

The speed of the rotor is made equal to the synchronous speed.

when the both speed are synchronized the prime mover is detached and motor is connected to 3-Φ supply and field is excited the motor will continue to rotate.

Disadvantage It needs the external prime mover.

By using damper winding By using damper winding The pole shoes of most f

the machines are punched with slots having copper bar which are short-circuited at both ends by heavy copper rings. These bars are known as damper winding or amortissuer winding.

By using damper winding cont..By using damper winding cont..

First main field winding is short circuited. Reduce voltage with the help of auto transformer,

applied across stator terminals. The motor starts up. When it reaches a steady speed a weak d.c excitation is

applied by removing the short circuit on the main field winding. If excitations sufficient then the machine will be pulled into synchronism.

Full supply voltage is applied across stator terminals by cutting out the auto transformer.

The motor may be operated at any desired power factor by changing the d.c excitation.

By using damper winding cont..

The torque developed by the synchronous motor as an induction motor with its field un excited) is called pull in torque.

Pull in torquePull in torque

Pull out torquePull out torque

Pull-out torque is the maximum sustained torque the electric motor develops at synchronous speed for one minute with rated frequency and normal excitation. Normal pull-out torque is usually 150% of full-load torque for unity-power-factor electric motors, and 175 to 200% for 0.8-leading-power-factor electric motors.

Torque developed in a motor isunidirectional only when Directions of both the field and armature currents

remains unchanged OR

Both change simultaneously

Why d.c excitation?

So in synchronous motor the armature current and field flux will change simultaneously only if D.C is given to the field because it to rotate at synchronous speed.

Why d.c excitation?

Pole

DC excitationwinding

Fan

Sliprings