- Synchronous Motor Definition: A synchronous motor is defined as a constant speed motor that runs at the synchronous speed of the supply, often used for constant speed operations and power factor improvement.
- Speed Control Factors: The speed of a synchronous motor depends on the supply frequency and the number of poles, with frequency adjustment being the practical method for speed control.
- Open Loop Control: Inverter fed open loop synchronous motor drive uses variable frequency without feedback, suitable for less precise speed control needs.
- Closed Loop Operation: Self-synchronous (closed-loop) operation provides accurate speed control by adjusting frequency based on rotor speed feedback, avoiding oscillations.
- Speed Control of Synchronous Motor: Speed control of synchronous motor is achieved through varying the supply frequency using solid-state devices, rectifiers, and inverters.
Synchronous motors are defined as constant speed motors that run at the synchronous speed of the supply. They are typically used for constant speed operations and to improve the power factor under no load conditions. Synchronous motors also have fewer losses compared to induction motors of the same rating.
The speed of a synchronous motor is given by 
Where, f = supply frequency and p = number of poles.
The synchronous speed depends on the supply frequency and the number of poles on the rotor. Since changing the number of poles is difficult, it’s not used. However, with solid-state devices, we can vary the current frequency to the synchronous motor. This allows us to control the motor’s speed by changing the supply frequency.
A combination of rectifiers and inverters can be used to control the speed of synchronous motors. There are two main methods for this:
Inverter Fed Open Loop Synchronous Motor Drive
In this method, the synchronous motor is supplied by a variable frequency inverter in an open loop. Open loop means there is no feedback to the supply, so the inverter doesn’t know the rotor’s position. This method is suitable when precise speed control isn’t necessary. The main supply is fed into a rectifier inverter set to achieve the desired frequency. The motor’s synchronous speed can be varied according to this frequency.
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
Self-synchronous (closed-loop) operation is used for precise speed control. Here, the inverter output frequency is based on the rotor speed. The rotor speed is fed back to a differentiator, and the difference between preset and actual speed is sent to the rectifier. The inverter then adjusts the frequency to control the motor speed accurately. For example, if the speed drops due to increased load, the stator supply frequency reduces to keep the rotor in sync with the stator magnetic field. This method prevents spontaneous oscillation or hunting.
