A synchronous motor is a device that converts electrical energy into mechanical energy by rotating at a constant speed, known as the synchronous speed. This speed is determined by the frequency of the power supply and the number of poles of the motor. The synchronous motor is a doubly excited machine because its field winding, which determines the magnetic field in the motor, is excited by a separate DC power supply.
One of the main limitations of a synchronous motor is that it is not self-starting, meaning that it requires an external force to bring it up to the synchronous speed. This is because the average torque of a synchronous motor is zero at the start, which means that there is no net force acting on the rotor to cause it to rotate. In order to generate a net average torque, it is necessary to rotate the rotor at a speed very close to the synchronous speed.
There are various methods that can be used to start a synchronous motor, including
1. Using a pony motor,
2. A damper winding,
3. A slip ring induction motor (synchronous induction motor),
4. A small DC machine.
These methods are used to bring the rotor of the synchronous motor up to the synchronous speed, at which point the motor is able to maintain its rotation on its own as long as it is supplied with a constant DC excitation current.
Using pony Motors:
The pony motor, also known as a starter motor, is a small induction motor that is used to bring the rotor of the synchronous motor up to the synchronous speed. To do this, the pony motor is coupled to the rotor of the synchronous motor and is started up. As the pony motor rotates, it rotates the rotor of the synchronous motor as well, causing the rotor to increase in speed.
Once the rotor of the synchronous motor reaches the synchronous speed, the pony motor is disconnected from the rotor. At this point, the synchronous motor is able to maintain its rotation at the synchronous speed by itself, as long as it is supplied with a constant DC excitation current through the slip rings.
It is important to note that the pony motor must have fewer poles than the synchronous motor, as having a pony motor with the same number of poles as the synchronous motor would not be able to bring the rotor up to the synchronous speed.
Using Damper Winding:
In a synchronous motor, the damper winding is an additional winding consisting of copper bars placed in the slots in the pole faces. These bars are short-circuited using end rings, which allows the damper winding to act like the squirrel cage rotor winding of an induction motor.
When the stator of the synchronous motor is excited by a three-phase supply, the motor starts rotating as an induction motor at a sub-synchronous speed. At this point, a DC supply is applied to the field winding. The motor then gets "pulled" into synchronism, meaning that it starts rotating at the synchronous speed.
As the rotor of the synchronous motor rotates at the synchronous speed, there is no relative motion between the damper winding and the rotating magnetic field, which means that there is no induced emf in the damper winding. As a result, the damper winding is active only during the start-up process, when the motor is operating as an induction motor. After the motor has reached the synchronous speed, the damper winding is disconnected from the circuit.
Since the damper winding is short-circuited and the motor starts as an induction motor, it draws a high current at the start. To handle this high starting current, induction motor starters such as a star-delta starter or an autotransformer starter are often used.
As a Slip Ring Induction Motor:
In this method, an external rheostat is connected to the rotor of the synchronous motor through slip rings. The ends of the damper winding are also brought out of the motor and connected either in a star or delta configuration. The rheostat is then connected in series with the rotor.
At the start of the motor, a high resistance is connected with the rotor to limit the current drawn by the motor. As a result, the motor starts as a slip ring induction motor and draws large currents.
As the motor picks up speed, the resistance is gradually cut off from the rotor circuit. As the speed of the motor approaches the synchronous speed, a DC excitation is applied to the rotor, which "pulls" the motor into synchronism. At this point, the motor is able to maintain its rotation at the synchronous speed on its own, as long as it is supplied with a constant DC excitation current through the slip rings.
Using Small DC Machine:
In this method, a small DC machine is used to start a higher rating synchronous motor. The DC machine initially acts as a motor, driving the synchronous motor from a zero speed to its rated speed. As the synchronous motor accelerates, it is able to establish synchronism and rotate at its synchronous speed.
At the same time, the DC machine also acts as a DC exciter, supplying a DC excitation current to the rotor of the synchronous motor through the slip rings. This DC excitation current is necessary to maintain the rotation of the synchronous motor at the synchronous speed.
To start the synchronous motor using this method, the small DC machine is first energized, causing it to start rotating. As the DC machine rotates, it drives the synchronous motor, which begins to accelerate. As the synchronous motor reaches the synchronous speed, the DC excitation current is applied to the rotor, allowing the synchronous motor to maintain its rotation at the synchronous speed.