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Difference Between Synchronous and Asynchronous Motor

Synchronous vs Asynchronous Motor

The synchronous speed of an AC motor is the rotation rate of the rotating magnetic field created by the stator. The synchronous speed is always an integer fraction of power source frequency. The synchronous speed (ns) of an asynchronous motor in revolutions per minute (RPM) is given by, where f is the frequency of the AC source, and p is the number of magnetic poles per phase.

For example, a general 3-phase motor has 6 magnetic poles organized as three opposing pairs, kept 120° apart around the perimeter of the stator, each powered by a single phase of the source. In this case p = 2, and for a line frequency of 50 Hz (frequency of the power main), the synchronous speed is 3000 RPM.

Slip (s) is the change in rotation rate of the magnetic field, with respect to the rotor, divided by the absolute rotation rate of the stator magnetic field, and it is given by, where nr  is the rotor rotation speed in RPM.

A synchronous motor is an AC motor in which the rotor normally rotates at the same RPM as the revolving field (stator field) in the machine. Another way of saying this is that the motor does not have a “slip” under usual operating conditions, that is s=0, and as a result, produces torque at synchronous speed. The speed of the synchronous motor is directly dependent on the number of magnetic poles and the source frequency.

Basic structural components of a synchronous motor are the stator winding connected to the AC supply that creates a rotating magnetic field and the rotor placed within the stator field supplied by DC current from the slip rings, to form an electromagnet.

The rotor is a solid cylindrical steel casting, in case of a non-excited machine. In permanent magnet motors, permanent magnets are in the rotor. The synchronous motors should be accelerated with a starting mechanism, to acquire the sync speed. Once at synchronous speed, motor runs without a change in the RPM.

There are three types of synchronous motors; they are, Reluctance Motors, Hysteresis motors, and Permanent Magnet motors.

The rotational speed of the sync motor is independent of the load, if sufficient field current is applied. This allows the accurate control in speed and position using open loop controls; they do not change the position when a DC current is applied to both the stator and the rotor windings. Construction of sync motor allows for increased electrical efficiency at low speed, and more torque is required.

If the slip of the motor in not zero (), then the motor is known as an asynchronous motor. The rate of rotation of the rotor is different from that of the stator field. In asynchronous motors, slip determines the torque produced. An induction motor is a good example of an asynchronous motor, in which the main components are the squirrel cage rotor and the stator. In contrast to the synchronous motors, the rotor is not fed with any supply electricity.

Synchronous Motor vs Asynchronous Motor

• The rotor of the asynchronous and the synchronous linear motors are different, where current is supplied to the rotor in sync motors, but asynchronous motor rotor is not supplied with any current.
• The slip of the asynchronous motor is not zero, ant the torque is dependent on the slip, whereas synchronous motors have no, i.e. slip (s) =0
• Sync motors have a constant RPM at varying loads, but asynchronous motor RPM changes with the load.

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