Synchronous And Asynchronous Motors – Where To Use Them?

The following information deals with the general working principles of Synchronous and Asynchronous motors, their advantages and where are they normally used and what can be achieved using each of these motors.

Synchronous and Asynchronous Motors – Working Principles

Synchronous Motors

This is a typical AC electrical motor that is capable of producing synchronous speeds. In these motors, both the stator and the rotor rotate at the same speed thus achieving synchronization. The basic working principle is, when the motor is connected to the mains, electricity flows into the stator windings, producing a rotating electromagnetic field. This is in turn induced on to the windings in the rotor which then starts rotating.

An external D.C source is required to lock the rotor’s rotating direction and position with that of the stator. As a result of this interlocking, the motor has either to run synchronously or not run at all.

Asynchronous Motors

The working principle of asynchronous motors is almost same as to that of synchronous motors except that it has no external exciter connected to it. Simply put, asynchronous motors also known as induction motors, also run by the principle of electromagnetic induction, in which the rotor does not receive any electric power by conduction as in the case of D.C motors.

The only catch here is that there is no external device connected to excite the rotor in asynchronous motors and hence, the rotor speed depends on the varying magnetic induction. This varying electromagnetic field causes the rotor to rotate at a speed lower than that of the stator’s magnetic field. Since the speed of rotor and the speed of stator’s magnetic field vary these motors are known as asynchronous motors. The difference in the speed is known as “slip.”

Synchronous and Asynchronous Motors – Advantages and Disadvantages

1. Synchronous motor runs at a constant speed at a given frequency irrespective of the load. But, the speed of an asynchronous motor reduces with the increase in the load.
2. Synchronous motor can be operated at a wide range of power factors, both lagging and leading, whereas asynchronous motor always runs with a lagging p.f which can be very low at diminishing loads.
3. Synchronous motor is not self-starting, where as an asynchronous motor can be self-started.
4. Synchronous motor’s torque does not get affected by the changes applied in the voltage as much as an asynchronous motor’s gets affected.
5. An external D.C excitation is required for synchronous motor to start, but an asynchronous motor does not require any external excitation to run.
6. Synchronous motors are usually expensive and complicated when compared to the asynchronous motors which less expensive and user friendly.
7. Synchronous motors are good for especially good for low-speed drives (below 300 rpm) because their power factor can always be adjusted to 1.0 and are very efficient. On the other hand, asynchronous motors are excellent for speeds above 600 rpm.
8. Unlike asynchronous motors, synchronous motors can be run at ultra-low speeds by using high power electronic converters which generate very low frequencies. They can be used to drive crushers, rotary kilns and variable speed ball mills.Synchronous and Asynchronous Motors – Applications

   Synchronous Motor Applications

1. They are generally used in power stations to attain appropriate power factor. They operate in parallel to the bus bars and are often over-excited, externally, to reach the desired power factor.
2. They are also used in manufacturing industries where a large number of asynchronous motors and transformers are used to overcome the lagging p.f.
3. Used in power stations to generate electricity at a desired frequency.
4.  Used to control voltage by changing its excitation in the transmission lines.

Asynchronous Motor Applications

Over 90% of the motors used in the world are asynchronous motors and they have huge applications all around, in a wide variety of domains. Some of them are:
1. Centrifugal fans, blowers and pumps
2. Compressors
3. Conveyors
4. Lifts as well as heavy duty cranes
5. Lathe machines
6. Oil, textile and paper mills, etc.

Conclusion

In conclusion, synchronous motors are only used when low or ultra-low speed performance is required by a machine and that too at desired power factors (both lagging and leading). Whereas, asynchronous motors are predominantly used in most of the rotating or moving machines, such as fans, lifts, grinders, etc.