# Rotating Principle of Single Phase Alternator

Oct. 28, 2022

The single-phase alternator has only one winding and the rotor is squirrel cage type. When the single-phase sinusoidal current passes through the stator winding, the generator will produce an alternating magnetic field. The strength and direction of this magnetic field change sinusoidally with time, but it is fixed in space, so it is also called an alternating pulsating magnetic field.

This alternating pulsating magnetic field can be divided into two rotating magnetic fields with the same speed and opposite rotating directions. When the rotor is stationary, these two rotating magnetic fields generate two torques with the same size and opposite directions in the rotor, making the resultant torque zero, so the generator cannot rotate.

When we use an external force to make the generator rotate in a certain direction (such as clockwise rotation), the motion of the cutting magnetic force line between the rotor and the rotating magnetic field in the clockwise direction becomes smaller; The motion of the cutting magnetic line of force between the rotor and the rotating magnetic field in the counterclockwise rotation direction becomes larger. In this way, the balance is broken, the total electromagnetic torque generated by the rotor will no longer be zero, and the rotor will rotate in the direction of pushing.

To enable the single-phase generator to automatically rotate, we can add a starting winding in the stator. The starting winding and the main winding have a space difference of 90 degrees. The starting winding should be connected with a suitable capacitor in series to make the current of the main winding have a phase difference of approximately 90 degrees, which is called the phase separation principle.

In this way, two currents with a difference of 90 degrees in time pass through two windings with a difference of 90 degrees in space, which will generate a (two-phase) rotating magnetic field in space. Under the action of this rotating magnetic field, the rotor can start automatically. After starting, when the speed rises to a certain value, the starting winding can be disconnected by means of a centrifugal switch or other automatic control devices installed on the rotor. In normal operation, only the main winding works. Therefore, the starting winding can be made into a short-time working mode. However, in many cases, the starting winding is not disconnected. We call this generator a capacitive single-phase generator. To change the direction of the generator, we can change the position of the capacitor in series.

In single-phase generator, another method of generating rotating magnetic field is called shaded pole method, which is also called single-phase shaded pole generator. The generator stator is made of salient pole type, which has two poles and four poles. Each magnetic pole has a small slot at the 1/3-1/4 full pole surface, which divides the magnetic pole into two parts. A short circuit copper ring is sheathed on the small part, as if this part of the magnetic pole is covered, so it is called a covered pole generator.

The single-phase winding is sleeved on the whole magnetic pole, and the coils of each pole are connected in series. When connecting, the polarity generated must be arranged according to N, S, N, S. When the stator winding is energized, the main magnetic flux is generated in the magnetic pole. According to Lenz's law, the main magnetic flux passing through the short-circuit copper ring generates an induced current in the copper ring that lags 90 degrees in phase. The magnetic flux generated by this current also lags behind the main magnetic flux in phase. Its role is equivalent to that of the starting winding of the capacitor generator, thus generating a rotating magnetic field to make the generator rotate.

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