Principles for selecting a circuit breaker for motor protection - News - Global IC Trade Starts Here.

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When using circuit breakers to protect motors, it's essential to consider two key characteristics of the motor, particularly AC induction motors. First, they have a certain level of overload capacity, and second, their starting current can be several times the rated current—sometimes even up to ten times in cases of reversible operation or phase reversal. To ensure the motor operates reliably and starts smoothly, the following guidelines should be followed when selecting a circuit breaker: (1) Set the long-time delay trip current of the circuit breaker based on the motor’s rated current. This ensures that the breaker will not trip during normal operation but will respond to overloads. (2) The return time for the 6x long-time delay setting should be longer than the actual starting time of the motor. This prevents the circuit breaker from tripping prematurely during startup, which could cause unnecessary downtime. (3) For the instantaneous trip setting, cage motors typically require a setting between 8 to 15 times the rated current of the trip unit, while wound rotor motors should be set between 3 to 6 times the rated current. These settings help prevent damage from high inrush currents during start-up. In addition, for motors that are frequently started, if the risk of phase failure is low or if there is a dedicated phase failure protection system in place, it may be more effective to use a combination of a fuse and a magnetic starter. This setup allows for easier remote control and provides reliable protection without the need for complex coordination between different protective devices.