The motor-protected plastic case circuit breaker (MCCB) is one of the protection devices that are indispensable for the squirrel-cage asynchronous motor to be disconnected during starting, running and running. For motor protection, MCCB is used for rated currents up to 630A and below (ie protected motor power is 315kW and below), infrequently starting and breaking the motor during operation, when the motor is overloaded, short circuited, under voltage, etc. Can be protected (automatically cut off the circuit).
According to the use situation, there are 3 types of circuit breakers used as motor protection:
(1) The AC contactor is used for starting and running of the motor, the thermal relay is used for overload protection, and the circuit breaker is used for short-circuit protection. Since the contactor is used to divide and close the circuit, it can be operated relatively frequently;
(2) Using the contactor as the starting and stopping of the motor, the faults such as overload, short circuit and under voltage of the motor are borne by the circuit breaker;
(3) The starting and running of the motor and the overload, short circuit and under voltage are all operated and protected by one circuit breaker, but it cannot be used frequently.
1 Starting current of the motor and starting impulse current of the motor
1.1 Starting current of squirrel cage asynchronous motor
Stator current of squirrel cage asynchronous motor: 
Where I 0 - excitation current  I 1 - stator current 
I 2 - rotor current 
I 0 is small and can be ignored, so there are:
I 1 ≈I 2 
f 1 —— power frequency, f1=50Hz
R 2 - rotor resistance
X 2 - rotor inductance
S——the slip of the motor
Where n 1 - stator rotating magnetic field speed n - motor rotor speed
S——slip, S changes in S≈0～S≈1 
It can be seen from equations (1) and (2) that the current and power factor (cosφ) of the motor are related to the slip ratio S. At start-up, S≈1 and I2 are large, and cosφ is small. Generally, the starting current can reach 5 to 7 times the rated current of the motor, and at the start of the fixed load, the cosφ is 0.2.
1.2 Inrush current when starting the motor
The inrush current of the motor at start-up is manifested by: direct start of the motor, Y-â–³ (star-delta) start change, instantaneous restart and reverse (reverse), braking, etc., all of which will generate a large inrush current, despite the impact. The duration of the current is only a few cycles, but will cause the action of a protective device (such as a circuit breaker).
The reason for the inrush current is:
(1) The power factor of the starting current is low, and the DC component of the transition process is superimposed.
Although the amplitude of the AC component is stable, an inrush current flows due to the influence of the DC component. When the power factor of the starting current is 0.3, the inrush current (peak) is about twice the starting current (effective value).
(2) The residual (remaining) voltage will generate an inrush current during instantaneous reclosing
Therefore, when MCCB is used as the short-circuit protection (momentary trip) of the motor, the instantaneous current value must be correctly selected according to the starting mode of the motor.
2 Select the circuit breaker instantaneous current setting value according to different motor starting methods
Assume that the motor starting current is 6I n and the power factor at startup is 0.3.
2.1 When starting directly
Since the power factor is 0.3 at start-up, the starting current is 6I m (I m is the rated current of the motor, and the rated current of the selected circuit breaker is I n ≥I m , generally I n =I m ).
Cosφ=0.3 means that the relative resistance value is small and the inductance L is large.
DC component of starting current: 
Large, DC component i2 is large. When the short-circuit current occurs at 1/2 cycle, when ψ-φ=-π/2, i=i1 i2 is the maximum value, that is, the impulse current ich=(1) 
) is the impact coefficient Kch, then ich 
It is the peak coefficient η. Check the relevant information, when cosφ = 0.3, the peak coefficient η reaches 1.978≈2. I is the starting current of the motor. It is assumed that the starting current of the motor is: I = 6I n (I M ), the inrush current ich = 2 × 6I n = 12I n , and if the motor starting current is 7I n , the inrush current is 14I n . The instantaneous current of the MCCB is expressed as an effective value. 
, that is, 8.485 ~ 9.89I n .
2.2 Y-â–³ (star-triangle) start
Since the Y-shaped wiring is used, the starting current is reduced to 1/3, that is, 6I n /3 = 2I n . However, the moment when the motor is connected from the Y (start) to the Δ connection (operation) due to a short pause, the residual voltage of the motor's inertia rotation and the vector voltage of the grid voltage when it is switched to Δ, a pulse current of nearly 10I n is generated. When the winding is turned to △ wiring, it is equal to starting again, and the motor speed at this time is not completely equal to the normal speed. Therefore, the inrush current is 
. The instantaneous setting of the circuit breaker must be at 19/ 
(effective value), which is 14 times or more.
Cosφ=0.3, the inductance in the circuit is large, the resistance value is small, and the inrush current of the motor is 2 times the starting current. When the motor is started by the auto-decompression starter, if the voltage entering the motor is 80% tap of the autotransformer, the starting current is 6×0.82=3î€84I n , and the power supply voltage is connected to the 65% tap of the autotransformer; The current is 6 × (0.65) 2 = 2.535In, connected to the 50% tap of the autotransformer, and the starting current is 1.5I n . The inrush currents were 2 × 3.84 ≈ 7.7 I n , 2 × 2.535 ≈ 5I n and 2 × 1.5 = 3I n , respectively . The instantaneous current setting values ​​(effective values) of MCCB are 5.43I n , 3.6I n and 2.12I n , respectively .
It should be noted that the transition from the different values ​​of the transformer tap to the operation of the full-voltage motor at the start of the auto-decompression starter, if open-circuit switching, also produces a 10In and pulse current, which is required for the motor. Causes "jitter", which has an impact on the dimensional accuracy of machined parts. This is not the case if it is a closed circuit conversion.
2.4 Instantaneous restart
When the running motor stops supplying power for a short time, the mechanical inertia of the motor makes it still rotate. If it has not stopped rotating, it suddenly restarts it. Because of the residual voltage, the applied voltage plus residual voltage Its effect will maximize its inrush current to 6 × 2 × 2 = 24In (peak). MCCB's instantaneous current setting is up to
3 Conclusion
According to the above four starting methods, it can be considered that the circuit breaker for motor protection has a theoretical value based on the standard value of the instantaneous current setting value of 8 to 15 In (effective value). As for the motor-protected MCCB, how much of the instantaneous setting current is selected depends on the type of squirrel-cage motor (including the number of poles), the capacity, and the starting method. Source: Transmission and Distribution Equipment Network
4 Points to note when starting the motor with the starter
(1) The setting value of the starting time must be moderate. Otherwise, the overload will be caused and the protective device will be tripped to stop the motor. According to the actual measurement, the light load starting time of the motor is less than 1 s, and the starting time is about 2.5 s. The starting time is only greater than 5 s. For this reason, if the time relay is used to automatically control the starting time, the time setting should be reasonable and correct. Otherwise, the instantaneous current envelope heating area (the product of all starting time and starting current) exceeds a certain value, which will cause thermal relay and open circuit. The overload release action of the device.
(2) In order to avoid the instantaneous 10In pulse current generated by the Y-â–³ starter when performing Y-direction conversion, when starting with the manual star-delta starter, when the motor speed reaches 80% of the rated speed, switch to â–³ (triangle) connection.
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