By: Thomas Yoon

The starting current of motor can become 3 to 6 times the normal running current! That’s normal for motors. The motor can experience this high current for a period ranging from 20 to 50 seconds.

A fuse will not be very useful in protecting a motor from this type of overload condition. A selected fuse large enough to permit passage of the necessary starting current would give little or no protection against overheating of the windings under normal running load conditions. The same case happens for the circuit breaker.

Overload relays offer protection of motors that is both compatible with the starting current of the circuit, and the requirement of protection in the event of overload running condition.

These conditions need to be adjustable. A particular motor requiring 56 seconds starting time under normal load will suffer serious damage if the rotor locks and the motor is not tripped in 20 seconds. Another motor may be able to withstand 25 percent overload for 30 minutes. A hermetically sealed compressor motor may burn in 3 minutes at 25 percent overload!

Good protection against overheating of the motor windings can be obtained from temperature monitoring protector devices that are embedded into the motor windings. These do not protect the motor itself, but act as sensors to trigger a temperature control circuit to stop the motor.

A thermal overload relay, however, uses a heating element to heat up a bimetallic strip so that it can trip a latch that will open the motor control circuit. This action will disconnect the motor from the line.

For reliable operation, the overload relay must be located at the same temperature environment as the motor. The heating effect of the bimetallic strip mechanism is supposed to represent the heating of the motor windings.

Melting alloy overload relays calibrated by the manufacturer are considered the most reliable of all the thermal overload protective devices. However, more commonly used is the bimetallic overload relays because the tripping current setting can be adjusted.

All the overload relays have one major limitation – because they operate on line current, they do not directly sense the motor temperatures. For normal steady running conditions, this poses no problem at all.

However, when a motor starts and stops frequently, the relay may not completely protect the motor. Why is that so?

During the motor running, the relay temperature follows the motor temperature closely. When the motor is off, the relay tends to cool off at a faster rate because of its lower mass. After a number of starts and stops, the temperatures of the relay and the motor may drift further and further apart. Eventually the motor becomes hot, and yet the relay does not trip because it is still cool. The motor burns.

Frequent starting and stopping of motors is no good both electrically and mechanically.

Until next time…