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
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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