Synchronous A.C. Motors Explained

We have learnt about the various types of electric motors in our previous article. Now we will start to learn about these motors individually. In this article we will take a look at the synchronous motor theory of operation and its working.

The main principle is same as applicable for all motors. It is the mutual induction between the stator & rotor windings which make any motor operational. Also when a 3 phase winding is fed with a 3 phase supply, then a magnetic flux of constant magnitude but rotating at Synchronous speed is produced.

 To understand the working of a Synchronous Motor easily, let us consider only two poles in the stator and rotor. With reference to the figure, the stator has two poles Ns & Ss. These poles when energised, produces a rotating magnetic field, which can be assumed that the poles themselves are rotating in a circular manner. They rotate at a synchronous speed and let us assume the direction of rotation to be clockwise. If the rotor poles are at the position shown in the figure, we all know that “Like poles repel each other”. So, the North Pole in the stator repels the north pole of the rotor. Also the south pole of the stator repels the south of the rotor. This makes the rotor to rotate in anti-clockwise direction thus, half a period later, the stator poles interchange themselves, thus making them get aligned with “unlike poles” which attracts each other. I.e. the South Pole of the stator & the North Pole of the rotor gets attracted and get magnetically interlocked.

At this position the poles Ns attracts S and poles Ss attracts N. These opposite unlike poles of the rotor & stator start rotating in the same direction as the stator poles. This makes the rotor to rotate unidirectional and at a synchronous speed which is the same speed of rotation of the stator poles. Thus as the position of the stator poles keep changing with a rapid speed and reversal, the rotor poles also rotate and reverse as same as the stator thus causing the rotor to rotate at a constant, synchronous speed and in the same direction.

When the motor is supplied with a.c. power supply, the stator poles get energised. This in turn attracts (opposite) the rotor poles, thus both the stator and rotor poles get magnetically interlocked. It is this interlock which makes the rotor to rotate at the same synchronous speed with the stator poles. The synchronous speed of rotation is given by the expression Ns=120f/P.

When the load on the motor is increased gradually, the rotor even though runs at same speed, tends to progressively fall back in phase by some angle, “β”, called the Load Angle or the Coupling Angle. This Load angle is dependent on the amount of load that the motor is designed to handle. In other words, we can interpret as the torque developed by the motor depends on the load angle, “β”.

 The electrical working of a Synchronous Motor can be compared to the transmission of power by a mechanical shaft. In the figure are shown two pulleys, “A” & “B”. Pulley “A” and the pulley “B” are assumed to be keyed on the same shaft. Pulley “A” transfers the power from the drive through the shaft, in turn making the pulley “B” to rotate, thus transferring power to the load.

The two pulleys which are keyed to the same shaft can be compared to the interlock between the stator & rotor poles.

If the load increases, the pulley “B” transfers the increase in load to the shaft, which is exhibited by the twisting of the shaft.

Thus the twist of the shaft can be compared to the rotor falling back in phase with the stator.

The twist angle can be compared to the load angle “β”. Also when the load increases, the twisting force and the twist angle increases, thus the load angle “β” also increases.

If the load on the pulley “B” is increased to such an extent that it makes the shaft to twist and break, then the transmission of power through the shaft stops as the shaft is broken. This can be compared with the rotor going out of synchronism with the stator poles.

Thus Synchronous motors can run either at synchronous speed or they stop running.