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.

Motor Horsepower

Perhaps the biggest misunderstanding when looking at a motor nameplate concerns the motor's horsepower rating. You should consider this the same as looking into a bucket. Each has a capacity. The motor's horsepower is an expression of the amount of work a motor can do WHEN THE LOAD ASKS FOR IT, not how much work it will shove into the load. Therefore the load current, and consequently the horsepower the motor develops, is determined by the load, not by the motor.

For example, if a pump's impeller is turned at 1750 rpm, it will require a specific amount of horsepower as shown on the pump curves. Let's assume those curves call for a 10HP motor. The load on the motor will be the same whether a 10HP or a 20HP motor is supplied. This, of course, ignores things like efficiency and no-load current which will have minor effects on the motor current. But, you won't get any more output from the pump or draw twice the current by using the larger motor.

So, consider this when you size your overload protection.