Current and Load Monitoring Relays

Monitoring the mechanical load on a motor is important to protect the motor against prolonged overloads. It can also be used to monitor under load as well as in the case where a fan were to have a broken belt or if a pump was running dry.

In most cases, load monitoring on a motor is done by monitoring the motor current. As the mechanical load increases, the current increases and as the mechanical load decreases, the current decreases. Usually, the current changes enough to reliably detect the load change.

Thermistors are frequently placed in the windings of a motor and are used in conjunction with a Thermistor Relay to to protect motors against over-temperature caused by environmental and load influenced temperature rises.

In some cases on lightly loaded motors or when detecting smaller changes in mechanical load, current monitoring is not reliable. In these cases, there are devices called load guards that monitor the displacement power factor (cos phi). As the load on the motor increases, the power factor approaches 1 and when the load decreases the power factor approaches 0. Over the working range of the motor, this is more linear than current monitoring and is more reliable to detect small changes in load.