Fuses & Circuit Breakers vs. Overload Heaters for Industrial Motor Protection

A key component of any motor control circuit is some kind of device to detect a condition of excessive overload and interrupt the power to the motor before thermal damage occurs to it. An overload heater is a simple and common overload protective device that consists of resistive elements connected in series with the three lines of a 3-phase AC motor. They are designed to heat and cool at rates modelling the thermal characteristics of the motor itself. Fuses and circuit breakers are also used to protect against overcurrent, but they are employed for different reasons and for different parts of an electrical system.

Both fuses and circuit breakers tend to be fast-acting devices, intended to interrupt overcurrent resulting from an electrical fault such as a phase-to-ground short circuit. They are sized to protect the wiring delivering power to a load, not certainly the load itself. On the other hand, overload heaters, are specifically designed to protect an electric motor from damage resulting from mild overcurrent conditions, such as what might be experienced if the motor is mechanically overloaded. The sizing of overload heaters is unrelated to the wire ampacity, and thus unrelated to the ratings of the fuses or circuit breakers delivering line power to the motor.

Whereas circuit breakers or fuses are sized to protect the power wiring from overcurrent heating, the overload heater elements are sized specifically to protect the motor. As such they act as thermal models of the motor itself, heating to the “trip” point as just as fast as the motor itself will heat to the point of maximum rated temperature and taking just as long to cool to a safe temperature as the motor will.

Additionally, overload heaters are not designed to directly interrupt current by opening, as fuses or circuit breakers do. Rather, each overload heater serves the simple purpose of warming proportionately to the magnitude and time duration of motor overcurrent, causing a different electrical contact to open, which in turn triggers the contactor open.

Note we have more effective technologies that are better than overload heaters that are used for motor protection. For example, we can detect overloading conditions by monitoring the temperature of the stator windings directly, using thermocouples or resistance temperature detectors (RTDs), which report winding temperatures to an electronic ‘trip’ unit with the same control responsibilities as an overload heater assembly. This advanced technique is used on large electric motors, and/or in critical process applications where motor reliability is vital.

Also read:

• Power MOSFET Motor Control
• Industrial Motor Control Using Contactors
• DC Motor Power Op-amp Speed Controller
• Features of Stepper Motors & How they are applied in Industrial Control