Motor Thermal Overload Protection

For large motors, the manufacturer will provide thermal damage curves for use in setting protective relays. Most motor protection relays have settings for the thermal damage curves, and include timers that accumulate heating time during starting and decrement it during cool-down. If you try to start the motor too many times without adequate time for cooling, the relay should block starting. Basler provides the following explanation in their Motor Protection Application Guide:

3.2 Thermal Damage

When a motor stator winding is energized with the rotor at rest or "locked," stator winding currents can range from 3 to 7 times rated full load value depending on motor design and supply system impedance. Heating in the stator winding, proportional to I2t, is 9 to 49 times rated conditions and the winding is without benefit of the ventilation normally produced by motor rotation. Actual values of locked rotor amps are part of the motor data supplied by the motor manufacturer

Depending on the design, a motor can be thermally stator limited or rotor limited during locked rotor conditions. The motor manufacturer can furnish the allowable locked-rotor time only after the motor design is completed. This time is given as time at rated locked-rotor current starting from either rated ambient temperature (cold stall time) or rated operating temperature (hot stall time). These parameters are given as part of the motor time-current curve defined by IEEE Standard 620.

Starting times depend on motor design and load torque characteristics and must be determined for each application. Although starting times of 2 to 20 seconds are common, high-inertia loads might take several minutes to attain full speed. Starting time increases if bus voltage is less than nominal. The starting current of a motor falls between the locked rotor value and the full load amps when the rotor begins to turn. Therefore, stator heating decreases as the motor accelerates.

The motor thermal limits curve has three separate pieces and is based on the three running conditions of the motor (see Figure 5, upper curve):

• Locked rotor or stall condition

• Motor acceleration

• Motor running overload

For most motors the motor thermal limit curve is one smooth curve. In most cases manufacturers provide curves for both hot and cold motor conditions. Overloads must be below this motor thermal damage curve. However, especially in long starting, high-inertia motors, the starting thermal damage curve can have similar current as does the running (overload) mode thermal curve; leading to possible nuisance trip during starting. In this case the starting thermal damage curve (locked rotor and acceleration) and the running thermal damage curves do not fit smoothly together. Modern motor protection relays use custom locked rotor and running (overload) curves to fit motor protection to these type of motor thermal limits.

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