Electromagnetic torque from event report data — A measure of machine performance

Power system events (such as starting a motor), a fault on the motor supply, or a switching event (such as the transfer of a motor from a primary to an alternate source) all expose a motor to transients. The electromagnetic torque response can be used as a measure of the impact of an event on the motor. A very large torque during a switching event can help identify improper switching controls or even the need to inspect a motor for damage. Motor torque calculated during normal motor starts and transfers can be trended to identify developing problems. While measuring the mechanical torque on the shaft of the machine can be challenging, it is possible to calculate the electromagnetic torque or air-gap torque directly from the motor terminal voltage and stator current. This paper discusses how electromagnetic torque can be calculated from oscillographic event report data obtained from digital motor protection. Several example cases are shared.