As we have discussed before, all overload relays have one major
limitation – because they operate on line current, they do not
directly sense the motor temperatures.
However, with proper heater selection and careful attention to
the factors that affect heater performance, the overload relay
can work extremely well to protect the motors.
Some of these factors that need close attention are:
Motor full load current
Motor temperature rise rating
Service factor
Ambient temperature at the motor and at the protector
Motor locked rotor current ratio
Starting time
Duty cycle
Locked rotor endurance time.
If attention is not given to these factors, the overload relay
will perform poorly. The other factor to be considered is
maintenance. Below are some maintenance routines:
Cleaning – The overload relays should be cleaned periodically.
Dirt or dust created by the operating conditions in the plant
tends to settle around the moving parts in the device and
prevent it from operating properly.
Tightening connections – Because most overload relays make use
of current flowing through heater elements to sense overloaded
conditions, it is important that the electrical terminals are
not loose. Loose electrical connections can create extra heat
and cause false tripping of the relays.
Inspecting heater size – Heaters can oxidize over a period of
time and become smaller in cross section. The smaller current
flowing in the smaller heater generates the same amount of
heat to cause unnecessary tripping of the relay.
With careful selection and maintenance, overload relays are
excellent for motor protection.
The wide use of thermal overload relays in industry attests to
their acceptance as the most practical means of protecting
motors from overloads. And they are the most economical solution
available.
Until next time…
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