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Successful Motor Protection Formulae

By: Thomas Yoon

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.

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Until next time...

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