Monday

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.

24 comments:

Bee Keeper said...

It is true to say that better overload protection is achieved by setting the overload trip level closer to the running current of the motor, no arguement about that however I believe it to be incorrect to advise industry to be setting overloads below the motor name-plate rating as this is defeating the object of motor protection. An overload is installed to allow maximum usage of the installed motor and protect only when operating outside the safe working parameter for which the motor was designed. Meaning, run the motor as close as possible to rated full load current. It should be clearly understood that an overload protection device is designed for protection not signalling, there are some overloads which have alarm features that is different. Should a specific load application prove to be to challenging for a specific overload device, one should rather invest in a better type/designed overload, i.e. different trip class or even explore the merits of installing an ellectronic overload relay which could offer multiple trip class selections and other features which could delivery better dynamic dimension to your motor protection and production. There are many very sophicated overload protection devices on the market which cater for all application. The issue of motors running at loaded currents which are lower than motor name plate rated current is another subject for another discussion.

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