Motor protection using Z thermal
overload relays
Overload relays are included in the group of
current-dependent protective devices. They monitor the
temperature of the motor winding indirectly via the current
flowing in the supply cables, and offer proven and
cost-efficient protection from destruction as a result of:
- Non starting
- Overload
- Phase-failure
Overload relays operate by using the characteristic
changes of shape and state of the bimetal when subjected to
heating. When a specific temperature is reached, they
operate an auxiliary contact. The heating is caused by
resistances through which the motor current flows. The
equilibrium between the reference and actual value occurs at
various temperatures depending on the magnitude of the
current.
Tripping occurs when the reference temperature is reached.
The tripping delay depends on the magnitude of the current
and preloading of the relay. Whatever the current, the relay
must trip out before the motor insulation is endangered,
which is why EN 60947 states maximum response times. To
prevent nuisance tripping, minimum times are also given for
the limit current and locked-rotor current.
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