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For reliable operation, overload relay must be located at same temperature environment as motor. The heating effect of bimetallic strip mechanism is supposed to represent heating of motor windings.
Melting alloy overload relays calibrated by manufacturer are considered most reliable of all thermal overload protective devices. However, more commonly used is bimetallic overload relays because tripping current setting can be adjusted.
All overload relays have one major limitation - because they operate on line current, they do not directly sense motor temperatures. For normal steady running conditions, this poses no problem at all.
However, when a motor starts and stops frequently, relay may not completely protect motor. Why is that so?
During motor running, relay temperature follows motor temperature closely. When motor is off, relay tends to cool off at a faster rate because of its lower mass. After a number of starts and stops, temperatures of relay and motor may drift further and further apart. Eventually motor becomes hot, and yet 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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