Date Code 20010719 701 Motor Protection Relay
Motor Thermal Element
Purpose of Motor Thermal Protection
E.3
Purpose of Motor Thermal Protection
A typical induction motor draws six times its rated full load current when starting.
This high stator current induces a comparably high current in the rotor. The rotor
resistance at zero speed typically is three times the rotor resistance when the motor is at
rated speed. Thus, the I
2
r heating in the rotor is approximately 6
2
• 3 or 108 times the
I
2
r heating when the motor is running normally. Consequently, the motor must tolerate
an extreme temperature for a limited time in order to start. Manufacturers communicate
the motor tolerance through the maximum locked rotor time and locked rotor amps
specifications for each motor. In a similar manner, the motor manufacturer
communicates the motor’s ability to operate under continuous heavy load through the
service factor specification.
The purpose of motor thermal protection is to allow the motor to start and run
within the manufacturer’s published guidelines, but trip if the motor heat energy
exceeds those ratings due to overloads, negative-sequence current, or locked rotor
starting.
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