Difference between Synchronous and Asynchronous Motor

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In this article, we will see what a synchronous motor is and how it differs from an asynchronous motor. We will compare and contrast the various features of synchronous and asynchronous motors.

What is an Electric Motor?

The electrical machines that convert electrical energy into mechanical energy are known as electric motors. Based on the electric supply input to the motor, they are of two types viz.: AC motor and DC motor. The AC motors are further classified into two types viz. ' synchronous motors and asynchronous motors. These motors have some similarities and some differences in their construction, operation and performance.

What is a Synchronous Motor?

The type of AC electric motor which has a rotor that is designed to rotate at the speed of rotational magnetic field of the stator (synchronous speed) is called the synchronous motor.

In the synchronous motor, the stator winding produces a rotational magnetic field when AC supply is connected. The rotor is also designed such that it produces its own magnetic field either by using a permanent magnet or an external DC supply through slip rings. As it can be seen that the synchronous is supplied by two supplies, i.e. one for stator and other for rotor, for this reason it is known as doubly-excited machine.

What is an Asynchronous Motor?

An electric motor which is designed in such a way that its rotor does not synchronize with the speed of the rotating magnetic field is called the asynchronous motor. In actual practice, the rotor of the asynchronous motor rotates with a relatively lesser speed than the speed of the rotating magnetic field (or synchronous speed).

The rotor used in the asynchronous motor is of two types viz.: squirrel cage type rotor and wound (or slip ring) type rotor. If the squirrel cage type rotor is used, then there is only one input supply to the motor and hence the motor is called singly-excited machine. When wound type rotor is used in the asynchronous motor, there two input supplies to the machine, therefore, it is called the doubly-excited machine.

Differences between Synchronous Motor and Asynchronous Motor

The following table highlights the major differences between synchronous and asynchronous motors '

Point of ComparisonSynchronous MotorAsynchronous MotorDefinitionThe AC motor which runs at synchronous speed is known as synchronous motor.The type of AC motor which runs at speed less than the synchronous speed is known as asynchronous motor.Principle of operationThe operating principle of the synchronous motor is based on the principle of magnetic interlocking between the magnetic fields of stator and rotor.The asynchronous motor works on the principle of electromagnetic induction between the stator magnetic field and rotor circuit.Expression of speed (Rotor)$$\mathrm{Synchronous\: speed,\mathit{N_{s}\mathrm{\: =\: }\frac{\mathrm{120}f}{P}}}$$
Where, f is the supply frequency and P is the number of stator poles.
$$\mathrm{Rotor\: speed,\mathit{N_{r}\mathrm{\: =\: }N_{s}\left ( \mathrm{1}-s \right )}}$$Where, s is the slip and $\mathit{N_{r}< N_{s}}$
Dependency of speedSpeed of the synchronous motor depends upon the frequency of input AC supply and the number of poles on stator.Speed of an asynchronous motor depends upon the mechanical load, rotor circuit resistance and slip in the motor. In actual practice, the speed of the asynchronous motor is always less than synchronous speed.SlipIn case of synchronous motor, there is no slip, i.e., both stator and rotor rotate at the same speed.There is slip in an asynchronous motor and it is always greater than 0 and less than 1.Effect of load on speedThe speed of the synchronous motor does not vary with the variation in the load.The speed of an asynchronous varies with the change in the load.StartingThe synchronous motors are not self-starting. They require some external mean of starting.The asynchronous motors are self-starting.Rotor supplyThe rotor of the synchronous motor requires an extra input supply to produce rotor magnetic field.In case of asynchronous motor, if the rotor is squirrel cage type, then there is no need of rotor supply but the wound type rotor needs extra supply input.Type of rotor supplyDC supply is given to the rotor of the synchronous motor.AC supply (in wound type rotor) is fed to the rotor circuit.Need of slip ring and brushesSynchronous motor requires slip rings and brushes to supply DC to its rotor circuit.Asynchronous motor with squirrel cage rotor does not require slip rings. However, with the wound type rotor, slip rings are used.Speed controlThe speed of the synchronous motor is controlled by changing the supply frequency using VFD (variable frequency drive).The speed of an asynchronous motor can be controlled either by using variable rotor resistance or VFD.Effect of supply voltage on speed and torqueThe supply voltage does not affect the speed and torque of a synchronous motor.By changing the supply voltage, the torque and speed of an asynchronous motor can be changed.Capital costThe capital cost of synchronous motor is higher.The initial cost of an asynchronous motor is comparatively lower.EfficiencyA synchronous motor has high efficiency.Operating speedThe synchronous motors are best suited for low speed (constant) applications, below 300 RPM.Asynchronous motors are best suited for high speeds, more than 600 RPM.Power factorSynchronous motor can be operated at lagging leading or unity power factor by varying the excitation.The asynchronous motors always operate at lagging power factor.ApplicationsSynchronous motor can be used for driving mechanical loads as well as power factor correction (by operating at leading power factor).Asynchronous motor can only be used for driving mechanical loads.

Conclusion

From the above discussion, it is clear that synchronous motors are more efficient than asynchronous motors, but the cost of a synchronous motor is comparatively higher than that of an asynchronous motor of the same rating.

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In practice, synchronous motors are used in specialised applications like power factor correction, driving a load at low and constant speed, while asynchronous motors are used in general purposes like driving the mechanical loads. Also, the operation of an asynchronous motor is easier than that of a synchronous motor.

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