Synchronous motor Construction & Starting methods

Presenting you most complex and my favourite machine in Electrical Engineering.In this post i am going to discuss about construction,starting method and some questions.

Construction:-

Every motor has two part one is Stator and another is Rotor.

Fig-1, Synchronous motor

1. Stator

Stator is used for holding the 3 phase armature winding made of special magnetic iron or steel alloy. The core is laminated to minimise loss due to eddy currents. The laminations are insulated from each other and have spaces between them for allowing the cooling air to pass through.

2. Rotor

Two types of rotor are used in alternators. 1. Salient pole type 2. Smooth cylindrical type

Salient Pole Type rotor

It is used in low and medium speed machine. It has large number of projecting poles, having their core bloted onto a heavy magnetic wheel of cast iron, or steel of good magnetic quality.             

Such machines are characterised by their large diameter and short axial lenghts. The poles and Pole shoes are laminated to minimize heating due to eddy currents. Fig-1 is image of salient pole synchronous motor.

In AC machine, it is desirable for the induced emf to be sinusoidal in waveform, therefore, flux density wave in the machine air gap must be sinusoidal. This is achieved in the salient pole construction (with concentrated field coils) by providing nonuniform air gap above pole shoes; minimum air gap in the middle of  the pole shoe progressively increasing towards outer edge

Cylindrical type rotor

  

It is used in high speed machine. The rotor consists of a smooth solid forged steel cylinder, having a number of slots for accommodating field coils. Such rotors are designed mostly for 2 pole or 4 pole.               

Fig-2, Cylindrical rotor

Rating of synchronous motor:-

Power:- 3970 kW
Stator Voltage:- 6.6 kV
Rotor type:- Salient pole

Why is it called Synchronous machine?

As you all know about induction motor, Two Rotating magnetic fluxes create, one is stator and other is rotor flux. In synchronous motor stator flux speed and rotor speed is same means Rotor speed is in synch with Stator rotating magnetic field. Speed of stator magnetic field is given by equation,

 Ns=120f/P

Ns = Synchronous speed

f= Frequency

P = Pole

Why synchronous motor is not self starting?

The condition for an electric machine to produce steady torque is the stator and rotor field must be relatively stationary.

In Synchronous motor when 3 phase applied to stator it generate RMF, As rotor is stationary, rotor conductor cuts this RMF but rotor has field winding and this field winding is not short circuited so in rotor winding EMF produced but current will not flow in rotor, therefore rotor flux cannot be generated due to stator. So our main condition of production of steady state torque not satisfied.
Therefore Synchronous motor is not self starting.

What is different methods of Starting of Synchronous motor.?

Auxiliary motor starting:-

It is a small dc or induction motor (much smaller in size than the synchronous motor). Because of the universal availability of ac supply induction motor is preferred choice. It should have the same number of poles as the synchronous motor and run from the same supply. Same frequency, same synchronous speed

Synduction Motor:-

A Synchronous motor starting on induction principle by means of damper winding or bar is called a synduction motor. The damper winding act like the squirrel cage rotor producing the starting torque;

In the starting operation of a synduction motor the field is kept shorted while the stator is switched on to 3 phase AC supply. As the motor reaches close to synchronous speed, the field is energized from dc supply. The rotor now gets synchronized automatically as explained in auxiliary motor starting.

What is Damper winding in Synchronous motor?

In Salient pole synchronous machine additional damping is provided by means of damper bars located in the main poles of the machine and short circuited through round rings at both end as shown in Fig,

Fig-3, Damper bars

As the rotor oscillates, the damper bars have a relative movement with respect to the air gap flux pattern which cause induction of emfs and flow of currents in these bars. The torque created by the bar current as per Lenz's law always oppose the relative motion.

This is how a positive damping term is brought into play so that the oscillatory motion of the rotor is quickly return to the steady position. These short circuited bars known as damper winding or Ammortisseur winding.