GE Multilin: Motor Protection Principles

Unbalanced load in the case of AC motors is mainly the result of an unbalance of the power supply voltages. The negative-sequence reactance of the three-phase motor is 5 to 7 times smaller than positive-sequence reactance, and even a small unbalance in the power supply will cause high negative sequence currents. For example for an induction motor with a staring current six times the full load current, a negative sequence voltage component of 1% corresponds to a negative sequence current component of 6%. The negative-sequence current induces a field in the rotor, which rotates in the opposite direction to the mechanical direction and causes additional temperature rise. Main causes of current unbalance are: system voltage distortion and unbalance, stator turn-to-turn faults, blown fuses, loose connections, as well as faults.

Short Circuit

Phase to Ground Fault

The short circuit element provides protection for excessively high overcurrent faults. When a motor starts, the starting current (which is typically 6 times the Full Load Current) has asymmetrical components. These asymmetrical currents may cause one phase to see as much as 1.7 times the RMS starting current. As a result the pickup of the short circuit element must be set higher than the maximum asymmetrical starting currents seen by the phase CTs to avoid nuisance tripping. The breaker or contactor that the relay is to control under such conditions must have an interrupting capacity equal to or greater then the maximum available fault current.

Undervoltage
If an induction motor operating at full load is subjected to an under voltage condition, full load speed and efficiency will decrease and the power factor, full load current and temperature will increase. The undervoltage element can be considered as backup protection for the thermal overload element. If the voltage decreases, the current will increase, causing an overload trip. In some cases, if an undervoltage condition exists it may be desirable to trip the motor faster than the overload element.

The overall result of an undervoltage condition is an increase in current and motor heating and a reduction in overall motor performance.

Overvoltage
When the motor is running in an overvoltage condition, slip will decrease as it is inversely proportional to the square of the voltage and efficiency will increase slightly. The power factor will decrease because the current being drawn by the motor will decrease and temperature rise will decrease because the current has decreased (based on I2t). As most new motors are designed close to the saturation point , increasing the V/HZ ratio could cause saturation of air gap flux causing heating.

The overall result of an overvoltage condition is an increase in current and motor heating and a reduction in overall motor performance.

Mechanical Jam
The mechanical jam element is designed to operate for running load jams due to worn motor bearings, load mechanical breakage and driven load process failure. This element is used to disconnect the motor on abnormal overload conditions before motor stalls. In terms of relay operation, the Mechanical Jam element prevents the motor from reaching 100% of its thermal capacity while a Mechanical Jam is detected. It helps to avoid mechanical breakage of the driven load and reduce start inhibit waiting time.

Load Loss Detection
Undercurrent protection is useful for indicating the loss of suction in a pump application, or a broken belt in a conveyor application. The second method of load loss detection is to use of the underpower protection element.