As we have discussed before, all overload relays have one major limitation - because they operate on line current, they do not directly sense motor temperatures.
However, with proper heater selection and careful attention to factors that affect heater performance, overload relay can work extremely well to protect motors.
Some of these factors that need close attention are:
Motor full load current Motor temperature rise rating Service factor Ambient temperature at motor and at protector Motor locked rotor current ratio Starting time Duty cycle Locked rotor endurance time.
If attention is not given to these factors, 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 operating conditions in plant tends to settle around moving parts in 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 electrical terminals are not loose. Loose electrical connections can create extra heat and cause false tripping of relays.