The Motor’s Best Friend

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


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


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


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…

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