What is an Asynchronous Motor?
Asynchronous motors, also known as induction motors, operate based on the principle of electromagnetic induction. The stator, which is the stationary part of the motor, creates a rotating magnetic field, while the rotor, which is the rotating part of the motor, is induced with a current that creates its own magnetic field. The interaction between the stator's magnetic field and the rotor's magnetic field causes the rotor to rotate, but at a slower speed than the stator's field.
As you mentioned, the rotor of an asynchronous motor can be either a squirrel cage or a wound type. The squirrel cage rotor is made up of heavy copper or aluminum bars that are shorted together at the ends, forming a cage-like structure. This type of rotor is simpler and more rugged, making it suitable for use in harsh environments. The wound type rotor, on the other hand, consists of wire windings on a steel laminated core, which allows for more control over the rotor's magnetic field.
What is Synchronous Motor?
Synchronous motors are designed to rotate at the same speed as the stator's rotating magnetic field, also known as the synchronous speed. The stator generates a rotational magnetic field when supplied with alternating current, and the rotor can be designed to generate its own magnetic field using either an external DC supply through slip rings or a permanent magnet. The rotor's magnetic field must have an equal or integral multiple of the stator's poles in order to lock with the stator's field and rotate at the same speed.
As you mentioned, synchronous motors do not have the ability to self-start and require an additional starting mechanism. One common method is to use a damper winding, which acts as an induction motor during startup and provides starting torque. Once the motor reaches the synchronous speed, the damper winding is no longer needed and the motor operates as a synchronous motor.
Difference Between Synchronous and Asynchronous Motor
| Synchronous Motor | Asynchronous Motor | |
|---|---|---|
| Definition | Rotor speed equals stator magnetic field speed | Rotor speed is slower than stator magnetic field speed |
| Types | Brushless, variable reluctance, switched reluctance, hysteresis | AC induction |
| Slip | 0 | Not equal to 0 |
| Additional power source | Requires additional DC power source to start | Does not require additional starting source |
| Slip ring and brushes | Required | Not required |
| Cost | More expensive | Less expensive |
| Efficiency | Higher | Lower |
| Power factor | Can be adjusted (lagging, leading, unity) | Only operates at lagging power factor |
| Current supply | Current is supplied to the rotor | No current supplied to the rotor |
| Speed | Constant, regardless of load | Decreases with increasing load |
| Self-starting | Not self-starting | Self-starting |
| Torque | Unaffected by applied voltage | Affected by applied voltage |
| Operational speed | Smooth operation at low speeds (below 300 rpm) | Excellent operation at high speeds (above 600 rpm) |
| Applications | Power stations, manufacturing industries, voltage control | Centrifugal pumps, fans, blowers, paper and textile mills, compressors, lifts |
Key Difference Between Synchronous and Asynchronous Motor
- Synchronous motors operate at a constant speed known as the synchronous speed, which is determined by the frequency of the power supply and the number of poles on the stator. Asynchronous motors, on the other hand, operate at a speed less than the synchronous speed.
- Synchronous motors operate based on the principle of magnetic interlocking between the rotor and stator field, while asynchronous motors operate based on the principle of electromagnetic induction between the stator and rotor.
- Synchronous motors have a slip of 0, while asynchronous motors have a slip that is always greater than 0.
- The speed of a synchronous motor depends on the supply frequency and the number of stator poles, while the speed of an asynchronous motor depends on the load, rotor resistance, and slip.
- Synchronous motors are not self-starting and require extra windings or an external excitation source to start, while asynchronous motors are self-starting and do not require an external source.
- Synchronous motors require an extra current supply for the rotor, while asynchronous motors do not.
- Synchronous motors can be controlled by varying the supply frequency using a variable frequency drive (VFD), while asynchronous motors can be controlled using a VFD or by varying the rotor resistance.
- Synchronous motors have a higher initial cost than asynchronous motors and are more complicated to operate, but they offer greater efficiency and precision. Asynchronous motors are simpler to operate and are cheaper, but they are not as efficient as synchronous motors.
- Synchronous motors can operate at very low speeds using a VFD, while asynchronous motors are more difficult to operate at low speeds.
- Synchronous motors are best suited for operation at speeds below 300 rpm, while asynchronous motors are best suited for operation at speeds above 600 rpm.
- Synchronous motors can operate at a lagging, leading, or unity power factor by adjusting their excitation, while asynchronous motors always operate at a lagging power factor.
- Synchronous motors can be used for power factor correction by operating in a leading power factor, while asynchronous motors cannot be used for power factor correction.
- Fluctuations in the main supply voltage do not affect the operation of synchronous motors, while they can affect the operation of asynchronous motors.
- Sudden variations in load will cause fluctuations in the current drawn by synchronous motors, while this is not a phenomenon in asynchronous motors.