# Working Principle of Synchronous Motor

## Working Principle of Synchronous motor: Construction & Applications

### Introduction: Working Principle of Synchronous Motor

• Synchronous motors are used to convert AC electric power to mechanical power at constant speed called as Synchronous Speed. Synchronous motors are widely used in the industry for high-precision applications
• Synchronous motors run in synchronism with the revolving field.
• The speed of rotation is tied to the frequency of the source.
• The motor speed stays constant, irrespective of the load or voltage of the 3-phase line
• and it does not depend on the torque acting on it. So it has a constant-speed torque characteristic.
• The efficiency of synchronous motor is around 90%–93%.
• Lets see the Working Principle of Synchronous Motor

### Characteristic Features of a Synchronous Motor

• It runs either at synchronous speed or not at all i.e. while running it maintains a constant speed. The only way to change its speed is to vary the supply frequency (because Ns = 120 f / P).
• It is not inherently self-starting. It has to be run upto synchronous (or near synchronous) speed by some means, before it can be synchronized to the supply.
• It is capable of being operated under a wide range of power factors, both lagging and leading. Hence, it can be used for power correction purposes, in addition to supplying torque to drive loads.

## Construction

The construction of 3-Phase Synchronous motors are identical to AC synchronous generators.

### The Stator:

• The stator has a laminated core with slots to hold the three-phase armature windings and are wound for the same number of poles as the rotor.
• It is composed of a slotted magnetic core, which carries a lap winding which is also identical to a 3-phase induction motor.

### The Rotor:

• Rotor holds the field winding.
• The rotor can be of salient-pole type or cylindrical type.
• Has a set of salient poles excited by a dc current using two slip-rings.
• Synchronous motor is likely to hunt and so damper windings are mounted on the rotor in the slots located in the pole faces and parallel to the shaft.
• It also carry a squirrel-cage winding similar to that in a 3-phase induction motor.
• This damper winding serves to start the motor.
• Damper winding also serve to increase the stability of the motor during load transient.
• The ends of Copper (Cu) bars are short circuited in the same manner as the Cage Rotor of an Induction Motor.
• Modern synchronous motors often employ brushless excitation, similar to that used in synchronous generators

## Working Principle of Synchronous Motor

Working principle of Synchronous motor is Magnetic Locking. When two unlike poles are brought near each other, if magnets are strong there exist a tremendous force of attraction between the two poles. In such condition two magnets are said to be magnetically locked. If one of the magnet is rotated, the other also rotates in the same direction with the same speed due to force of attraction i.e. due to magnetic locking condition.

MUST READ: WHY THE SYNCHRONOUS MOTOR IS NOT SELF STARTING

### Synchronous Motor Working:

• The field current of a synchronous motor produces a steady-state magnetic field BR
• A three-phase set of voltages is applied to the stator windings of the motor, which produces a three-phase current in the windings.
• This three-phase set of currents in the armature winding produces a uniform rotating magnetic field Bs in the air gap.
• The stator rotates at synchronous speed.
• DC supply on the rotor will also produce a flux of constant magnitude.
• Therefore, there are two magnetic fields present in the machine, and the rotor field will
• tend to line up with the stator field, just as two bar magnets will tend to line up if placed near each other.
• Since the stator magnetic field is rotating, the rotor magnetic field (and the rotor itself) will try to catch up with the rotating magnetic field of stator.
• This is possible when the rotor also rotates at synchronous speed.

MUST READ: SYNCHRONOUS MOTOR STARTING METHOD

### Equivalent Circuit for a Synchronous Motor

Figure shows the Equivalent Circuit for a Synchronous Motor for one armature phase of a cylindrical rotor synchronous motor. It is seen from  figure that the phase applied voltage V is the vector sum of reversed back e.m.f. i.e., −Eb and the impedance drop Ia ZS. In other words, V = (−Eb + Ia ZS). The angle α* between the phasor for V and Eb is called the load angle or power angle of the synchronous motor

### Operation of Synchronous Motor

• The basic principle of operation of the synchronous motor is that the rotor chases the stator magnetic field.
• The stator rotating magnetic field tends to drag the rotor along, as if the north pole of the stator locks in with the south pole of the rotor.
• The larger the angle between the two magnetic fields (up to certain maximum), the greater the torque on the rotor of the machine
• A three-phase synchronous motor is not self-starting.
• If the rotor of the synchronous motor is rotated by some external means at the start, there will be a continuous force of attraction between the stator and also the rotor.
• This is called magnetic locking which is the Working principle of Synchronous motor.
• Once this stage is reached, the rotor pole is dragged by the revolving stator field and thus the rotor will continue to rotate.

#### Pull-in torque

• As soon as the motor is running at close to synchronous speed, the rotor is excited with DC current. This produces N and S poles around the circumference of the rotor.
• If the poles on the rotor at the moment the exciting current is facing the poles of opposite polarity on the stator, a strong magnetic attraction is set up between them.
• The mutual attraction locks the rotor and also stator poles together.
• The rotor is pulled with the revolving field. Torque develop at the moment is called pullin torque.

### Application Synchronous Motor

Synchronous motors find extensive application for the following classes of service :

#### Power factor correction

• Overexcited synchronous motors having leading power factor are widely used for improving power factor of those power systems which employ a large number of induction motors  and other devices having lagging p.f. such as welders and fluorescent lights etc.

#### Constant-speed applications

• Because of their high efficiency and high-speed, synchronous motors (above 600 r.p.m.) are well-suited for loads where constant speed is required such as centrifugal pumps, belt-driven reciprocating compressors, blowers, line shafts, rubber and paper mills etc.
• Low-speed synchronous motors (below 600 r.p.m.) are used for drives such as centrifugal and screw-type pumps, ball and tube mills, vacuum pumps, chippers and metal rolling mills etc.

#### Voltage regulation

• The voltage at the end of a long transmission line varies greatly especially when large inductive loads are present. When an inductive load is disconnected suddenly, voltage tends to rise considerably above its normal value because of the line capacitance. By installing a synchronous motor with a field regulator (for varying its excitation), this voltage rise can be controlled.
• When line voltage decreases due to inductive load, motor excitation is increased, thereby raising its p.f. which compensates for the line drop. If, on the other hand, line voltage rises due to line capacitive effect, motor excitation is decreased, thereby making its p.f. lagging which helps to maintain the line voltage at its normal value.

## What is meant by a synchronous motor?

Synchronous motors are used to convert AC electric power to mechanical power at constant speed called as Synchronous Speed.

## What is synchronous motor working principle?

Working principle of Synchronous motor is Magnetic Locking. When two unlike poles are brought near each other, if magnets are strong there exist a tremendous force of attraction between the two poles. In such condition two magnets are said to be magnetically locked. If one of the magnet is rotated, the other also rotates in the same direction with the same speed due to force of attraction i.e. due to magnetic locking condition.

## Is synchronous motor AC or DC?

A synchronous electric motor is an AC electric motor

## Where is synchronous motor used?

• Power factor correction
• Voltage regulation

## What are the two types of synchronous motor?

• Non-Excited Synchronous Motors.
• Direct Current (DC) Excited Synchronous Motors.