Circuit breakers are designed to trip in case of overloads in the circuit which they are protecting, and for short circuits. They will trip on small overloads after a long time, large overloads after a short time and instantaneously for a short circuit. The exact times for different levels of current are shown on circuit breaker tripping curves. Induction motors have high inrush currents on start-up with high currents for short times. Unless these load currents are coordinated with the breaker tripping curves, the breaker supplying the motor will trip when it sees the starting current even though there is no danger to the motor or the circuit.
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Obtain the starting current from the current speed curve of the motor. If the curve is not available, get the full load current from the motor nameplate and multiply by six. Estimate the time the motor takes to reach rated speed. A typical time might be five seconds. Find the point corresponding to the starting current and the starting time on the breaker tripping curve. The point must be just to the left of the upper, curved part of the curve, showing that the breaker will not trip under the motor starting current. If the point is to the right of the curve, the breaker will have to be replaced.
Start the motor under normal operating conditions and read the current five or six times during start-up to verify the estimated start-up and the full load currents. Find each of the time/current points on the breaker tripping curve to make sure they fall to the left of the curve. If some of the points fall into the shaded area of the curve, adjust the tripping unit to make the curve move up and to the right so that all measured points are to the left of the curve.
Adjust the motor overload relay to coordinate with the breaker tripping curve. Use the overload relay time/current curve and compare to the breaker tripping curve. Adjust the motor overload relay curve so that it lies to the left of the breaker tripping curve but to the right of the measured points. Normal operation, represented by the measured points, must lie to the left of both curves so that neither the overload relay or the breaker trip. When the motor is overloaded, the overload relay will see the overload first and open the circuit to protect the motor. If the overload relay doesn't operate and the overload persists, the breaker will trip, acting as a backup.
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