Motor Overload Protection

Many people plan their motor protection around a fuse or breaker. Fuses and breakers look at short-circuit current that happens after the motor has failed. Their job is to protect the power system from damage due to the level of short-circuit current. When they go, it's already too late for your motor. For motor protection, you should look to the overload relay in your starter.

Overload relays are defined by their protection Class. Protection is based on the amount of time it takes the overload to trip at locked rotor current. A Class 10 overload is faster that a Class 20 or Class 30 overload.

Another important feature is the overload's ability to recognize a single-phase condition and trip faster. Many overloads have two trip curves covering symetrical or single-phase tripping; most replaceable heater overload relays do not.

The biggest mistake I see is the application of overload protection at the motor nameplate current. That assumes the motor is completely loaded. Many motors run at 75% to 90% load. Overcurrent protection should be applied slightly above RUNNING load current, not full load current. This allows the overload relay to tell you something is wrong when you exceed normal running load current but before you exceed full load current. Take care not to get too close to the running load current in order to reduce nuisance tripping.