Continued from page 1
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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