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3. INSPECT HEATER SIZE - Determine that the specified heater is used in thermal
overload relays. Too often, oversized heaters are arbitrarily
installed to eliminate
unexplained trips.
Actually, the original heaters may have oxidized over a period
of
time, becoming smaller
in
cross section. Then, the heat required to operate the
relay is provided by a
smaller amount
of
current than that intended
by
the original
design. This may make the relay trip prematurely and the heater appear
undersized.
4.
INSPECT SETTINGS (Where applicable) - Most magnetic overload relays have
adjustable settings for minimum operating current and time delay characteristics.
These should be adjusted to the specified settings.
5.
TEST - The motor overload relay should be subjected to a simulated overload and
the tripping time measured. This time should be compared to the manufacturer's
specifications
of
the relay's time-current curves to make certain that the relay is
operating properly. A tolerance
of
± 15% is usually acceptable.
If
the
relay's curves
or
specifications are not available, it is suggested that the Heat Damage Curve of
the motor be used as a guide for maximum trip time at
300%
of
motor full load
current.
28
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