Excitation of Synchronous motor and AVR

DC Supply given to rotor of synchronous motor to achieve synchronous speed. Induction motor  always draw lagging reactive power from system but in synchronous motor you can change it through excitation.

Brushless Excitation System:

Nowadays Brushless exciter is used in synchronous motor for smooth operation. It include Permenant magnet pilot exciter and main exciter along with rotating diodes.

 

Pilot exciter has magnet as field which is mount on rotor. Stator consist of 3 phase AC winding. Rotor of pilot exciter rotate at speed of Synchronous motor so 3 phase AC Voltage generate in stator, this AC Voltage feed to fully controlled rectifier and converted into DC.

Main exciter consist field winding on stator and 3 phase winding on rotor, Rectified DC Voltage feed to stator of Main exciter so it result into 3 phase high frequency voltage on rotor which is fed to the rotating bridge rectifier.

This rotating diode bridge rectifier convert AC voltage into DC and fed to the rotor of synchronous motor.

Automatic Voltage Regulator:

Voltage is controlled by controlling field current of machine. As EMF equation of synchronous machine is,

E = 4.44 f ⦽ N Kw Volts

Where, f = frequency

            ⦽ = flux

            N = No. of turns

            Kw = Winding factor

As you increase field current, flux increase which increase EMF. 

As you know equation of reactive power consist Voltage so reactive power can be controlled by Voltage.

I have some images of AVR of Synchronous motor. Let's Analyze them one by one.

1. Stator Voltage:- Which is 6.6 kV 3 phase, Star connected.

2. Stator current:- It shows how much current your motor draws. Here motor draws 315A. 

3. DC Field Voltage:- 58V DC is feed to rotor for excitation.

4. DC Excitation current:- When 58V given to rotor winding it draws 6.5 A. 

5. Actual Power factor:- It is very interesting, You can see power factor of motor which is around 0.9 lead which means synchronous motor operating in overexcited condition and supply lagging VAR to system which improve overall power factor of system.

6. Reactive power:- As you can see right side is for leading power factor and left side is for lagging power factor. So from meter motor is supplying around 0.5 MVAR at 0.9 pf leading.

There is two Mode in AVR to control Voltage and power factor. 

1. Auto

2. Manual

In Auto mode you need to set power factor, AVR Automatic calculate required field voltage and feeded to rotor. In Manual mode you need to increase and decrease excitation by push button to achieve desired power factor.