Synchronous Motor Drives
As the name suggests the synchronous motors rotates at synchronous speed. The main advantage of synchronous motors are that they run on 3 AC supply and DC supply is given to the rotor when they run on synchronous speed the loss is very minimal. We can say that if the synchronous motors are designed to run only at rated synchronous speeds, then what is the use of introducing drives to them. The answer is pretty simple, synchronous motor drives makes the starting, pull in and braking process smooth and without any problems. We shall discuss about them one by one.
Starting Synchronous MotorsThe problem with synchronous motors are that they are not self-starting. Before discussing the starting method of this motor, we should know about the type of supply and the rotor and the stator of the motor briefly. 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.
Now before knowing, how the synchronous motors are started, we should know why they are not self started? The answer can be given as, when 3 phase supply is given to the stator, there is a rotating magnetic flux which rotates at synchronous speed, and if the rotor is also given DC supply, rotor acts as a magnetic flux which rotates at synchronous speed, and if the rotor is also given DC supply, rotor acts as a magnet having two salient poles.
As the rotor is at standstill position, it cannot follow the magnetic field which is rotating at synchronous speed. The rotor stacks at its position because the opposite poles move so rapidly that the rotor locks, this is the reason why synchronous motors are not self starting. Now coming to the point how synchronous motors are started. At first the synchronous motors are started as a normal induction motors, the rotor of the motor is not given DC supply, when the motor reaches the rotor and pull in takes place, which is discussed later.
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.