Selection principles of circuit breakers for motor protection

Circuit breakers for motor protection can be selected and adjusted according to the following principles.

(1) For circuit breakers used for motor protection, the long-delay release is divided into two types: adjustable and non-adjustable. The setting current adjustment range of the adjustable overcurrent release is 0.7 to 1.0 times the rated current of the release. Because the long-delay release characteristic is the same as that of the JR series thermal relay, it is very suitable as an overload protection device for motors. The protection characteristics of the long-delay release are shown in Table 5-1.

Note: The return time refers to the maximum time that the release can return to its original state when the current drops to 90% of the long-delay release setting current within the long-delay and short-delay ranges. The long-delay current setting value is equal to the rated current of the motor.

(2) The return time of the 6-times long delay current setting value shall not be less than the actual starting time of the motor. Depending on the load at the time of starting, the return time can be selected from 1s, 3s, 5s, 8s, and 15s.

(3) Instantaneous setting value: for circuit breakers protecting cage-type asynchronous motors, it is equal to 8 to 15 times the rated current of the motor; for circuit breakers protecting winding-type motors, it is equal to 3 to 6 times the rated current of the motor. It should be pointed out that the life of a circuit breaker is generally only about 10,000 times, which is two orders of magnitude lower than the operating life of a general AC contactor. When starting the motor directly, it is only suitable for occasions where operation is not frequent. If a circuit breaker that can meet the motor overload protection cannot be selected, the distribution circuit breaker can be used in conjunction with a thermal relay to achieve motor overload protection. At this time, the instantaneous tripping action current setting value of the distribution circuit breaker is equal to 14 times the rated current of the motor to avoid false operation due to the motor starting impact current.

Example 1] A power supply system is shown in Figure 5-2. It is known that the calculated currents on each line are I1=900A, I2=540A, I3=Ied1=103A, I4=Ied2=184A; the rated current and starting current of motor M1 are Ied1=103A, Iqd1=670A; the rated current and starting current of motor M2 are Ied2=184A, Iqd2=1200A. The short-circuit current calculation results of each short-circuit point d1~d5 are marked on the figure. Try to select circuit breakers Q1~Q4.

Solution: (1) Select circuit breaker Q3. Select the circuit breaker for motor protection. You can choose DW15-200A circuit breaker for starting and overload and short circuit protection. Because the rated current of the motor Ied1=103A, the long-delay release action current is set at 105A. The return time of 7 times the long-delay action current setting value is 3s (light load start). The instantaneous release action current is set at 1.35Iqd1=1.35×670=904 (A), which is 1000A. This value is less than Id4=6.632kA.

(2) Select circuit breaker Q2. Select according to the circuit breaker for power distribution. Since the line load current is I2=540A and the short-circuit current Id2=15.65kA, the DW15-630A selective circuit breaker can be selected. The long-delay release action current is set at 1.1I2=1.1×540=594 (A), and the return time of 6003 times the long-delay action current setting value is 8s. The short-delay release delay time is 0.2s. The action current is set at 1.2［I2+（Iqd1-Ied1）］=1.2×［540+（670-103）］=1328 (A), only a 630A release can be selected, and the action current is 1800A. The instantaneous operating current is set at 1.1Id3=1.1×8.53=9.4 (kA) and can be adjusted on a 630A electronic release.

(3) Select circuit breaker Q1. Select according to the circuit breaker for power distribution. Since I1=900A, select DW15-1000A selective circuit breaker. The long-delay release action current is set at 1000A. The return time of 3 times the long-delay action current setting value is 1.5s. The short-delay release delay time is 0.4s. The action current is set at 1.2 [I1 + (Iqd2-Ied2)] = 1.2 × [900 + (1200-184)] = 2300 (A), take 3000A. The instantaneous action current is set at 1.1Id2 = 1.1 × 15.65 = 17.215 (kA), which can be adjusted on the 1000A electronic release. ④ Select circuit breaker Q4. Select according to the circuit breaker for motor protection. You can choose DW15-200A current limiting circuit breaker. The instantaneous release action current is set at 1.7Iqd2=1.7×1200=2040 (A). If you choose a 200A release, the action current is 2400A, which is less than Id5=12.51kA.