Synchronous Motor Drive

As the name suggests, synchronous motors rotate at synchronous speed. The main advantage of synchronous motors is that they are powered by 3-phase AC power supply and when running at synchronous speed, DC power supply is given to the rotor which has very low losses. It can be said that if synchronous motors are only designed to work at synchronous speeds, what is the point of using a drive in them. The answer is very simple, synchronous motor drives make the process of starting, traction and braking smoother and without any problems

As the name suggests, synchronous motors rotate at synchronous speed. The main advantage of synchronous motors is that they are powered by 3-phase AC power supply and when running at synchronous speed, DC power supply is given to the rotor which has very low losses. It can be said that if synchronous motors are only designed to work at synchronous speeds, what is the point of using a drive in them. The answer is very simple, synchronous motor drives make the process of starting, traction and braking smoother and without any problems

The problem with synchronous motors is that they are not self-starters. Before discussing the starting method of this motor, we must briefly know the type of power source and motor rotor and stator. The stator of synchronous motors is similar to induction motors and the only difference is in the rotor of these types of motors, the rotor of synchronous motors has a DC source

Starting the synchronous motor

Now before we know how synchronous motors are started, we need to know why they can’t start by themselves?, when 3 phase supply is given to the stator, there is a rotating magnetic flux that rotates at synchronous speed, and If the rotor is also supplied with DC power, the rotor acts as a magnetic flux rotating at synchronous speed. And if the rotor is also supplied with DC power, the rotor acts as a magnet with two salient poles

Since the rotor is stationary, it cannot follow the magnetic field which is rotating at a synchronous speed. The rotor is stuck in position because the opposite poles are moving so fast that the rotor is locked, which is why synchronous motors don’t start. Now we come to the point of how synchronous motors are started. At first, synchronous motors are started as normal induction motors, and no DC source is given to the rotor of the motor, when the rotor speed reaches the synchronous speed, the DC source is connected and tension and torque are made in it, which will be discussed later

Another way to start the synchronous motor is by an external motor. In this method, the rotor of the synchronous motor is rotated by an external motor and when the speed of the rotor is close to the synchronous speed, the DC field is connected and traction takes place. In this method, the starting torque is very low and it is not a popular method

Torque of synchronous motors

When the rotor of the synchronous motors is close to synchronous speed, then the DC power supply is turned on and torque generation begins. When turning on the DC power supply, due to the phase angle and torque angle, various disturbances are observed in the motor and several pole slips are also observed in the air gap. At the end of the working process, the rotor acquires the synchronous speed. The best time to connect the DC source is when the rotor speed is very close to the synchronous speed, this will be the best moment because the speed difference will be minimum at that point in time