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The current changes on Motor starting

Date:2023-09-19   Author:XINDA MOTOR
Understand from the perspective of motor starting principle and motor rotation principle:

When the induction motor is in a stopped state, from an electromagnetic point of view, it is like a transformer. The stator winding connected to the power supply is equivalent to the primary coil of the transformer, and the closed-circuit rotor winding is equivalent to the short-circuited secondary coil of the transformer; the stator winding There is no electrical connection with the rotor winding, only magnetic connection. The magnetic flux forms a closed circuit through the stator, air gap and rotor core.

At the moment of closing, the rotor has not yet started to rotate due to inertia. The rotating magnetic field cuts the rotor winding at the maximum cutting speed - synchronous speed, causing the rotor winding to induce the highest potential possible. Therefore, a large electric potential flows through the rotor conductor. Current, this current generates magnetic energy that cancels the stator magnetic field, just like the secondary magnetic flux of the transformer counteracts the primary magnetic flux.

In order to maintain the original magnetic flux that is compatible with the power supply voltage at that time, the stator automatically increases the current. Because the rotor current is very large at this time, the stator current also increases greatly, even up to 4 to 7 times the rated current. This is the reason for the large starting current. Why is the current small after starting: As the motor speed increases, the speed at which the stator magnetic field cuts the rotor conductor decreases, the induced potential in the rotor conductor decreases, and the current in the rotor conductor also decreases, so the stator current is used to offset the rotor current. The part of the current affected by the magnetic flux also decreases, so the stator current changes from large to small until normal.

The starting current of a three-phase squirrel cage asynchronous motor is generally 4 to 7 times, but it is not absolute. However, it is generally required that the starting current of the motor cannot exceed 2 to 5 times its rated current. Motors with motor power exceeding 30kw are not suitable for frequent starts, because the starting current of motors above 30kw is generally 6-7 times the rated current. Frequent starts will increase the temperature rise of the motor, causing the possibility of burning the motor.

Generally, AC motors are asynchronous motors, and their direct starting current is about 4-7 times the rated current. For small motors, it is 7 times, and for large motors, it is 5-4 times. Because AC motors have impedance, unlike DC motors which only have resistance, the starting current cannot be more than ten times higher. For the same motor, its starting current is the same regardless of heavy load or light load.

It just looks a little different when measured with a meter. Because at light load, the motor starts quickly. When the meter pointer has not risen to the maximum, the motor has already started and the current begins to decrease, so the current appears to be small. When the load is heavy, the motor starts slowly, and the meter can basically keep up with the changes in current, and the current seems to be larger. The reality is the same.