Successful Motor Protection Formulae

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


Until next time…

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