Reactive Power Flow of Sync. Generators and Motors

Synchronous machine is a form of the conventional machines which has an operation principle based on Faraday's' law of electromagnetic induction. In practical life, it is found as either a synchronous generator or a synchronous motor. One of the interesting characteristics of this machine is the reactive power flow in either case; a generator or a motor. Understanding and differentiating this characteristic in the generator and the motor cases is very basic and very crucial as well. ??

There is a very basic information about the synchronous motors which is that the field current of the motor can be controlled to make the motor operate at a leading power factor and supply the system with a reactive power instead of absorbing it. This is not an intuitive phenomenon, that is, how a motor which is normally consuming reactive and active powers could be controlled to provide a reactive power. However, because of this phenomenon, synchronous motors were broadly used in electrical systems for improving the power factor of the system.

There is a curve which has a shape of letter “V” so that it is called the V-curve, this curve shows the relation between the field current and the armature current of the synchronous motor, see the below curve [1]. From this curve, it can be seen that by increasing the field current, the power factor starts to be less and less lagging until it reaches a point where the power factor equals unity. Further increase in the field current above the unity power factor point causes the motor to operate in the leading power factor region, that is, now it is supplying reactive power not absorbing it.

Synchronous generators can also either supply or absorb reactive power by controlling the field current of the generator. If the generator is operating in parallel with the grid and is supplying a reactive power, so it is operating with a lagging power factor. Whereas, if the generator is absorbing a reactive power, it is operating with a leading power factor.

Furthermore, the operating modes of synchronous machines are termed based on the level of excitation as either under-excited or over-excited. A synchronous generator supplying a reactive power is an over-excited generator, whereas it is called as an under-excited generator if it is absorbing a reactive power. However, if the synchronous motor is absorbing a reactive power, it is called as an under-excited motor. But, when the synchronous motor is supplying a reactive power, it is termed as an over-excited motor.?

Finally, the reactive power flow in the synchronous machines can be summarised in the below table. Whenever we say “supplying reactive power”, this means that the machine is in the over-excited mode of operation. This mode of operation indicates that the generator has a lagging power factor but the motor has a leading power factor. Similarly, if we say “consuming reactive power”, this means that the machine is in the under-excited operation mode. This operation mode shows that the generator has a leading power factor but the motor has a lagging power factor.

References:

[1] Electric Machinery Fundamentals (4th Edition), Stephen J. Chapman