- Synchronous Motor Overview: A synchronous motor is designed to operate at the same speed as the AC power supply’s frequency, ensuring consistent performance.
- Starting Process: Synchronous motors are not self-starting; they initially operate like induction motors or use an external motor to reach near synchronous speeds before activating the DC field.
- Synchronous Motor Working Principle: The working principle involves a DC-powered rotor creating a magnetic field that synchronizes with the stator’s rotating field to achieve synchronous speed.
- Dynamic Braking: This method involves disconnecting the motor from its power supply and using it as a generator to dissipate kinetic energy through resistors, effectively slowing the motor.
- Pull-In Technique: Proper timing of the DC field activation is crucial for minimizing speed difference and ensuring smooth acceleration to synchronous speed.
As the name implies, synchronous motors rotate at synchronous speed. The main advantage of these motors is their efficiency—they operate on a three-phase AC supply, and when a DC supply is given to the rotor at synchronous speed, power loss is minimal. Although synchronous motors are designed to operate at specific speeds, drives are crucial. They simplify the starting, pull-in, and braking processes, which we will explore in detail.
Starting Synchronous Motors
Synchronous motors are not self-starting, which presents a unique challenge. To understand their starting methods, it’s essential to briefly know about the types of supply and the components of the motor, specifically the rotor and the stator.
The stator of synchronous motors are similar to that of an induction motors but the only difference lies in the rotor, the rotor of the synchronous motors are given DC supply.
Before exploring how synchronous motors are started, it’s crucial to understand why they aren’t self-starting. When a three-phase supply energizes the stator, it generates a rotating magnetic flux at synchronous speed. If the rotor, supplied with DC power, acts as a magnet with two salient poles, it struggles to align and rotate with this rapidly moving field.
The rotor, being stationary initially, cannot match the synchronous speed of the magnetic field. It remains stuck due to rapid movement of opposite poles, leading to locking—this explains why synchronous motors don’t start on their own. To start, they initially operate like induction motors without a DC supply to the rotor, until it achieves enough speed to engage, or pull in, which will be detailed further.
Another method of starting the synchronous motor drives is by external motor. In this method the rotor of the synchronous motor is rotated by an external motor and when the speed of the rotor reaches near synchronous speed, the DC-field is switched on and pull in takes place. In this method, the starting torque is very low and it is not very popular method also.
Pull in of Synchronous Motors
When the rotor of the synchronous motors reaches near synchronous speed, then the DC field supply is switched on and the pull in process begins. As during switching on the DC supply due to the phase angle and torque angle there are various disturbances seen in the motor and there are several slip of poles of air-gap flux is also seen. As the pull in process is completed the rotor acquires synchronous speed. The complete pull in as fast as possible the DC supply should be switched on at the most favorable angle. Like when the synchronous motor is running as induction motor, the DC supply should be fed when the induction motor is at top speed, this will be the best moment because the speed difference will be least at that point of time.
Braking of Synchronous Motors
Three common types of braking are regenerative, dynamic braking, and plugging. However, only dynamic braking is suitable for synchronous motors—plugging current is theoretical but not practical due to its potential for causing severe disturbances. During dynamic braking, the motor is disconnected from its power supply and connected to a three-phase resistor, transforming it into a synchronous generator that dissipates energy safely through the resistors.
