Synchronous motors are among the most common types of AC motors used in a variety of applications today. They are unique in their ability to run at a constant speed (also known as synchronous speed) regardless of load variations.

However, the operation of synchronous motors isn’t as simple as it appears. There are several things operators need to understand about these motors’ working principles and starting methods. In addition, just because synchronous motors typically run at synchronous speeds, it doesn’t mean it is impossible to control their speeds to meet different application needs.

Synchronous motors working principle

Like most motors, a synchronous motor is made of two essential parts: the stator and rotor. The stator is the stationary part of the motor that carries the stator winding, while the rotor is the rotating part of the motor that carries the armature winding.

Figure 2. Synchronous motor animation. Source: Michele Pozzi/CC BY-SA 4.0

The stator requires a three-phase supply for its excitation, while the rotor requires a DC source. In this case, excitation means electromagnetism. For instance, the three-phase supply connected to the stator creates a rotating magnetic field in the air gap between the stator and rotor. Likewise, the DC source connected to the motor’s field winding causes the field winding to become a permanent magnet and interact with the stator winding.

The interaction of the stator’s rotating magnetic field and the rotor’s permanent magnet causes the rotor to rotate at a synchronous speed, as shown below.

synchronous_EQ1

Where:

f = The frequency of the AC supply system connected to the motor

p = number of stator poles

However, synchronous motors are not self-starting, so there must be an additional help to break these motors’ inertia of rest.

Two starting methods for synchronous motors

Synchronous motors are usually started using:

• An external prime mover
• Damper windings in the motor

No. 1: External prime mover

External prime movers are simply coupled with the synchronous motors to bring the motors’ rotors to synchronous speed before the prime movers are disconnected. This operation causes the motor’s rotor to synchronize with the rotating magnetic field. As a result, magnetic locking occurs, allowing the motor to remain synchronized.

DC motors and induction motors are two commonly used external prime movers for starting synchronous motors. DC motors and induction motors have similar modes of operation when used to start synchronous motors. However, with induction motors, engineers must ensure that the induction motor has a lesser number of poles than the synchronous motors to achieve synchronous speeds.

No. 2: Damper windings in the motor

In the damper winding method, an additional winding (typically made of copper bars) is placed on the rotor. These copper bars are short-circuited at both ends, similar to an induction motor with a squirrel cage rotor winding.

As a result, when a 3-phase supply is given to the synchronous motor, the rotor starts rotating at speeds less than the synchronous speed. The motor attains synchronous speed once DC excitation is given to the field winding.

However, just because synchronous motors are designed to be fixed-speed motors that do not alter with load variation, it doesn’t mean engineers cannot achieve speed control with these motors. There are several ways engineers can set up their synchronous motors to achieve speed control.

Speed control for synchronous motors

The synchronous speed formula shows that the speed of synchronous motors varies with the frequency of the AC supply and the number of poles. However, synchronous motors generally have a fixed number of poles, so it might be impractical to control motor speed by varying the number of poles. Instead, speed control can be achieved by varying the AC supply frequency using any of these two methods:

• The open-loop control method
• The closed-loop control method

The open-loop control method

Figure 3 shows the block diagram of the open-loop control method of four synchronous motors. This system features a rectifier, a combination of inductor and capacitor, and an inverter.

Figure 3. Block diagram of a typical open-loop speed control system of synchronous AC motors.

The three-phase supply is connected directly to the rectifier, which converts the incoming AC into pulsating direct current. The inductor and capacitor then filter and refine this pulsating DC before it is converted to AC by the inverter. The resulting alternating current has a different frequency, which changes the motor speed accordingly.

However, keep in mind that this system doesn’t give feedback to the rectifier and inverter, so they have no information about the rotor’s position. The open-loop speed control method is ideal in applications where changes in speed control don’t affect the load. Engineers can also use them in applications where there is a need to control several motors simultaneously.

The closed-loop control method

Figure 4 shows the block diagram of a closed-loop control system featuring a rectifier, inverter, inductor and capacitor filters (like the open-loop system). However, unlike the open-loop control method, the closed-loop system features a sensor that continuously monitors the motor’s speed and compares it to a preset speed. This sensor sends a feedback signal to the rectifier, adjusting the pulsating DC and AC to control the speed of the motor.

Figure 4. Block diagram of a typical closed-loop speed control system of synchronous AC motors.

Conclusion

AC synchronous motors are unique motors capable of meeting application requirements as long as they are correctly sized for the application. Therefore, engineers are advised to reach out to synchronous motor manufacturers to discuss their application needs.