Synchronous motors are constant speed motors. They run at the synchronous speed of the supply. They are generally used for constant speed operation under no load conditions such as to improve the power factor. Synchronous motors have fewer losses than induction motors at a given rating.
The speed of a synchronous motor is given by 
Where, f = supply frequency and p = number of poles.
As you can see, the synchronous speed depends on the frequency of the supply and the number of poles of the rotor. Changing the number of poles is not easy, so we do not use that method. However, with the invention of solid-state devices, the frequency of the current fed to the synchronous motor can be varied. We can control the speed of the synchronous motor by changing the frequency of the supply to the motor.
We can use a combination of rectifiers and inverters to control the speed of synchronous motors. They can be used in two ways:
Inverter Fed Open Loop Synchronous Motor Drive
In this method, the synchronous motor is supplied by variable frequency inverter in an open loop. By open loop, we mean that there is no feedback given to the supply. The inverter has no information about the current position of the rotor. This method is preferable when highly accurate speed control is not required. Supply from the mains is fed into the rectifier inverter set where desired frequency can be attained. Depending on the frequency, the synchronous speed of the motor can be varied.
In above figure, you can see the block diagram of the speed control drive. The three-phase supply from the mains is converted to dc by using rectifiers. Then the rippled dc is smoothened by using LC filters. The DC is fed to the inverters. These inverters can be either voltage source inverters or current source inverter. The frequency of the supply fed to the motor can be varied and accordingly speed control of synchronous motor can be done. Open loop operation is useful when a number of motors need to be run at exactly the same speed. This method has a disadvantage though. This method causes spontaneous oscillation or hunting.
Self Synchronous (Closed – Loop) Operation
We use self-synchronous (closed-loop) operation when highly accurate speed control is required. In this method, the inverter output frequency is determined by the speed of the rotor. The speed of the rotor is fed back to the differentiator. The difference between the preset speed and the actual speed is fed to the rectifier. Accordingly, the inverter changes the frequency and adjusts the speed of the motor. We get more accurate control over the motor speed with the closed loop operation. For example, if speed gets reduced (due to increase in load), the stator supply frequency gets reduced so that the rotor stays in synchronism with the stator magnetic field. No spontaneous oscillation or hunting occurs in this method.
