• Starting methods of synchronous motor?
Synchronous motor is widely used in industries. This motor is not self starting it requires some additional devices to start the motor. Synchronous motor can started by the following three methods:
1. External prime mover
2. Damper winding
3. Starting using variable frequency
External prime mover
In this method, an external motor is coupled mechanically to the synchronous motor. The external motor may be a DC shunt motor or an induction motor. The purpose of using external motor is to rotate the rotor of synchronous motor and bring it to synchronous speed or near synchronous speed.
Once the speed reaches synchronous speed, three phase supply is switched on to the armature and excite the DC field winding from DC supply. The external motor is then decoupled and synchronous machine will work as a motor. At present this methods of starting is hardly used.
Synchronous motor is not self starting but the damper winding makes it self starting. Damper winding is provided on the rotor of the synchronous motor.
First step is to de-energize the field winding by shorting it through a suitable resistor and then connect stator of synchronous motor to three phase supply.
When three phase supply is given to stator of synchronous motor it produce rotating magnetic field. This rotating magnetic field links with the damper windings and motor will start as a three phase induction motor.
When the speed comes close to synchronous speed, remove the resistor and excite the field winding from DC supply. As a result of it rotor gets locked to the stator magnetic field and the machine runs continuously at a synchronous speed.
Starting using variable frequency
In this method, synchronous motor must have electronic control. When a reduced frequency is supply to the stator winding, this generates a slowly rotating magnetic field in the stator. Hence the rotor of synchronous machine is able to follow this magnetic field.
Once the rotor gets locked to stator magnetic field, the frequency is gradually raised to the normal frequency. As a result of it rotor speed increases gradually with frequency and once frequency reaches normal frequency, the rotor will continuously run at synchronous speed.
• What are differences between synchronous motor and induction motor?
AC Motors are mainly of two types, one is synchronous motor and other is asynchronous motor. Asynchronous motor is also known as induction motor.
Although, there is a big list of differences between these two motors but here we will discuss some basic and major differences as given below :
1. synchronous motor always run at a constant speed i.e. synchronous speed.
2. Synchronous motor requires DC excitation.
3. Synchronous motor is not self starting.
4. Power factor of synchronous motor can be changed from lagging to leading and vice versa.
5. Speed of synchronous motor is constants at all load.
6. Synchronous motor rotate because of magnetic locking between rotor pole and stator pole.
7. Synchronous motor cost is high and required regular maintenance.
1. Induction motor always less than synchronous speed.
2. Induction motor does not required any extra DC excitation source.
3. Induction motor is self starting.
4. Induction motor operates at legging power factor.
5. When the load on induction motor increases, its speed decreases.
6. Three phase induction motor operates on induction principle.
7. It is less costly and requires less maintenance.