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Why is the starting current of a three-phase asynchronous motor so large?

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  • Time of issue:2020-11-18
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(Summary description)The so-called start of a three-phase asynchronous motor refers to the process from when the motor is connected to the power source and starts to rotate until it reaches the rated speed.

Why is the starting current of a three-phase asynchronous motor so large?

(Summary description)The so-called start of a three-phase asynchronous motor refers to the process from when the motor is connected to the power source and starts to rotate until it reaches the rated speed.

  • Categories:Company News
  • Author:
  • Origin:
  • Time of issue:2020-11-18
  • Views:0
Information

The so-called start of a three-phase asynchronous motor refers to the process from when the motor is connected to the power source and starts to rotate until it reaches the rated speed. According to theoretical analysis and actual measurement, the three-phase asynchronous motor starts at the moment; the starting current of the stator winding is very large, which can reach 4 to 7 times the rated value.

Why does a three-phase asynchronous motor have such a large starting current? Because the rotor of the asynchronous motor cannot immediately rotate at the moment of starting, the back EMF of the rotor electromagnetic induction has not been established yet, so the external voltage is all added to the back EMF. On the stator winding, the circuit current is the external voltage divided by the winding impedance (indicated by the symbol Z, it is a kind of ability to hinder the current, and the unit is expressed by the ohm formula Z=V/I), so the stator winding current is very large .

Such a large starting current will bring about the following undesirable consequences: the grid voltage will fluctuate (especially when a motor with a larger capacity is started), which will affect the normal operation of other equipment connected to the grid. The motor windings are heated and the insulation is aged, thereby shortening the service life of the motor. In particular, it has a greater impact on motors that often need to be started. At the moment of starting, due to the low power factor of the rotor circuit of the three-phase asynchronous motor, the starting torque is not very large. If the starting torque is less than the load torque, the motor will not start.

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