Synchronous Asynchronous Motor

Synchronous Asynchronous Motor

Synchronous Motor

• electric motor which has a rotor that is designed to rotate at the speed of rotational magnetic field of the stator (synchronous speed)
• The operating principle of the synchronous motor is based on the principle of magnetic interlocking between the magnetic fields of stator and rotor.
• The stator generates a rotational magnetic field upon supply alternating current. The rotor can be designed to generate its own magnetic field using an external DC supply through slip rings or using a permanent magnet.
• The rotor is designed to generate magnetic poles that are equal or integral multiple of the stator poles. When the stator and rotor is energized, the rotor magnetic field gets locked with the stator rotational magnetic field, and it rotates at the exact speed of stator field.
• Due to inertia, the synchronous motor does not immediately starts at the synchronous speed (rotational magnetic field). Therefore, an additional winding called “damper winding” is used to provide starting torque. It acts as an induction motor during the startup. So, it suggests, the synchronous motors are not self-start, they need extra starting mechanism.
• The synchronous motor’s rotor rotates with synchronous speed that depends on the supply frequency and the poles of stator windings. Therefore, the motor speed does not vary with the load. In order to vary the speed of a synchronous motor, the supply frequency must be varied.
• Example: Brushless motor, Variable Reluctance Motor, Switched Reluctance Motor and Hysteresis motor.

Asynchronous Motor

• electric motor which is designed in such a way that its rotor does not synchronize with the speed of the rotating magnetic field
• the rotor of the asynchronous motor rotates with a relatively lesser speed than the speed of the rotating magnetic field (or synchronous speed)
• rotor used in the asynchronous motor
  • squirrel cage type rotor : only one input supply to the motor (singly-excited machine)
  • wound (or slip ring) type rotor: two input supplies to the machine (doubly-excited machine)
• The asynchronous motor works on the principle of electromagnetic induction between the stator magnetic field and rotor circuit.
• The stator rotational magnetic field causes an induced current in the rotor. This induced current flows inside the rotor generating its own magnetic field. According to Lenz’s law, this rotor field opposes the cause that’s generating it and tries to eliminate it by catching up to the speed of the stator RMF (the synchronous speed). In doing so, the rotor rotates in the direction of the stator RMF. Since it operates on the principle induction,  therefore, asynchronous motor is also known as Induction motor.
• The asynchronous motor speed depends on the slip of the motor that varies with the load and the rotor resistance. In other words, the induction motor speed can vary with the load or by using varying rotor resistance.
• Example: AC Induction Motor

References

• Difference between Synchronous and Asynchronous Motor
• Difference between Synchronous and Asynchronous Motor
• Difference Between Synchronous and Asynchronous Motor (with Comparison chart) - Circuit Globe