In brief
A motor protection circuit breaker is a switching device that disconnects motors on overload, phase failure and short circuit and can also be switched manually.
Contents
What is Motor protection circuit breaker?
The motor protection circuit breaker combines several functions: it switches a motor on and off, protects it against overheating with an adjustable thermal or electronic overload release and interrupts short-circuit currents with a magnetic instantaneous release. Many devices also detect phase failure. It therefore replaces the combination of fuse and overload relay in many motor feeders.
The overload release is set to the rated current of the motor; each device has a limited setting range for this. The short-circuit release responds at a fixed multiple of the set current. Key parameters are setting range, rated voltage, short-circuit breaking capacity and trip class. Accessories such as auxiliary contacts, signalling contacts, undervoltage releases and shunt releases are mounted at the side or front. The motor protection circuit breaker is frequently combined with a contactor to form a load feeder. The requirements are described in the IEC/EN 60947 series of standards.
The motor protection circuit breaker is standard in the control cabinets of pumps, fans and conveyor systems. Tripping usually points to a cause at the motor or the driven machine, such as stiff bearings, a blockage or phase failure.
In practice: maintenance and procurement
For a replacement, what matters is a setting range that matches the motor current, the short-circuit breaking capacity at the installation location, the frame size and existing accessories such as auxiliary contacts. If a motor protection circuit breaker trips repeatedly, the cause at the drive should be clarified before replacement.
Synonyms and abbreviations: MPCB · motor circuit breaker · manual motor starter · motor starter protector · motor protective switch
Frequently asked questions
What is the difference between a motor protection circuit breaker and a miniature circuit breaker?
The miniature circuit breaker protects cables and wiring against overload and short circuit and has fixed tripping characteristics. The motor protection circuit breaker is designed for motors: its overload release is set precisely to the motor current, it tolerates starting currents and often detects phase failure. It is therefore the right choice for motor feeders.
How is a motor protection circuit breaker set correctly?
The overload release is generally set to the rated current on the motor nameplate, matching the voltage and connection actually used. Deviations only make sense in accordance with the manufacturer’s instructions. Too high a value endangers the motor, too low a value leads to nuisance tripping. The motor current must lie within the setting range of the device.
What information is needed to replace a motor protection circuit breaker?
Important are the type designation, setting range, rated voltage, required short-circuit breaking capacity and frame size for mounting. Also relevant are fitted accessories such as auxiliary contacts or undervoltage releases and any link modules to the contactor. A photo of the front usually provides all the data.
Why does the motor protection circuit breaker keep tripping?
Possible reasons are overload from the driven machine, stiff bearings, starts that are too long or too frequent, phase failure, voltage imbalance, an incorrect setting or winding damage. High ambient temperatures in the control cabinet also affect thermal releases. Current measurement on all three phases and insulation testing of the motor help to find the cause.