What is the purpose of under voltage protection of generators and motors?

Answer 1

An AC electric motor, powered directly from the AC grid, operating in traction of a given mechanical load, which presents characteristic of resistant torque constant, this means that it will need to develop an air-gap flux, also constant.

In order that such electric motor to produce a flow of air gap constant, this means that both electrical current (i) that it demands, as the V / F (voltage / frequency) that it must operate, are both constant, too.

If, by chance, the mechanical load is disconnected from the motor shaft, the flow is drastically reduced to a minimum, just enough to produce a minimal torque necessary to keep rotating, the rotor mass.

In this case, both voltage and frequency of the AC power grid that feeds the motor, its values ​​are maintained, whereas the amount of current that the motor consumes falls proportionally to the torque that is produced by the motor, so that, by the electric motor windings, will flow only magnetizing current.

Thus, the AC electric motors are manufactured, specified to operate powered directly from the power grid with a voltage and frequency, both predetermined.

In this case, if there is any load variation (variation of the resistant torque), only the current it demands will vary, while the V / f ratio remains constant.

However, if there is a need to vary the rotational speed of the electric motor shaft, this may be obtained by the purposeful variation of the frequency of its AC power. But necessarily concurrently, the voltage also must be varied in order to maintain the V / f constant. This is the working principle of the inverter frequency control scalar type.

However, when operated fed directly from the AC mains, if for any failure of the electric grid, the voltage starts to fall in value (undervoltage), whereas the frequency value is maintained, it causes the air-gap flux motor falls , impairing thereby the torque produced.

However, because of autoregulation inherent to the electric motor, it will seek, in contrast, automatically compensate for the loss of torque draining a higher current, trying thus recover the nominal flow and the ability to produce mechanical energy.

The problem that occurs is as follows: the extent that decrease the supply voltage to the motor, the more increase the current flowing through the motor, causing it to overheat.

Thus, the overall result of an undervoltage condition is an increase in current and motor heating and a reduction in overall motor performance, so that the motor must be protected against this.

The undervoltage protection element can be thought of as backup protection for the thermal overload element. In some cases, if an undervoltage condition exists it may be desirable to trip the motor faster than thermal overload element.

For this purpose can be used an undervoltage relay which removes the motor from service when a low-voltage condition develops, so that the motor will not draw excessive current and become itself overheated.

The undervoltage trip should be set to 80-90% of nameplate unless otherwise stated on the motor data sheets.

Motors that are connected to the same power grid may experience a temporary undervoltage, when one of motors starts and the other motor is ahead operating.

To override this temporary voltage sags, a time delay setpoint should be set greater than the motor starting time.

I believe that it should respond satisfactorily to your question, at least with respect to the occurrence of undervoltage, in the case of supply lines for AC motors.