To illustrate the importance of rotor resistance in determining rotor heating, recall that balanced three-phase current in the stator winding of an induction motor creates a field that rotates at synchronous speed. With the motor at rest, the field induces a voltage that drives short-circuit current in the rotor bars. The current, in turn, reacts with the stator field to produce torque that accelerates the rotor. At the same time, the flux forces the current to flow in 54 percent of the cross section of the rotor bars at the periphery of the rotor when the rotor is at rest, as illustrated in Figure 1.
Protection of electric machinery has been paramount to the power system since the advent of electric power at the turn of the twentieth century. Certainly, many advances in both the design of motors and the devices that protect them have occurred since that time to make the overall processes more reliable and help make those that work around machines and maintain them safer. Through microprocessor-based relay technology, we can not only improve the monitoring and control capabilities but also improve the protection of the machine as well. The following examples highlight cases where the application of microprocessor-based motor protection relays improved the protection, control, and/or monitoring of a medium-voltage motor.