How big is the starting current of the motor?

There are different opinions on how many times the starting current of the motor is the rated current, and many of them are based on specific conditions. Such as ten times, 6 to 8 times, 5 to 8 times, 5 to 7 times and so on.

One is to say that when the motor’s speed is zero at the moment of starting (that is, the initial moment of the starting process), the current value at this time should be its locked-rotor current value. For the most frequently used Y series three-phase asynchronous motors, there are clear regulations in the JB/T10391-2002 “Y series three-phase asynchronous motors” standard. Among them, the specified value of the ratio of the locked-rotor current to the rated current of the 5.5kW motor is as follows: at the synchronous speed of 3000, the ratio of the locked-rotor current to the rated current is 7.0; at the synchronous speed of 1500, the ratio of the locked-rotor current to the rated current is 7.0; When the synchronous speed is 1000, the ratio of the locked-rotor current to the rated current is 6.5; when the synchronous speed is 750, the ratio of the locked-rotor current to the rated current is 6.0. The motor power of 5.5kW is relatively large, and the motor with a smaller power is the ratio of the starting current to the rated current. It should be smaller, so the electrician textbooks and many places say that the starting current of the asynchronous motor is 4~7 times the rated working current.

Why is the motor starting current high? After starting the current is small?

Here we need to understand from the perspective of the motor starting principle and the motor rotation principle: when the induction motor is in a stopped state, from the electromagnetic point of view, it is like a transformer, and the stator winding connected to the power supply is equivalent to the primary coil of the transformer , The closed-circuit rotor winding is equivalent to the short-circuited secondary coil of the transformer; the non-electric connection between the stator winding and the rotor winding is only the magnetic connection, and 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 turned due to inertia, and the rotating magnetic field cuts the rotor windings at the maximum cutting speed—synchronous speed, so that the rotor windings induce the highest possible electric potential. Therefore, a large amount of electricity flows in the rotor conductor. Electric current, this current produces magnetic energy that cancels out the stator’s magnetic field, just like the secondary magnetic flux of a transformer cancels out the primary magnetic flux. In order to maintain the original magnetic flux compatible with the power supply voltage at that time, the stator automatically increases the current. Because the rotor current is large at this time, the stator current also increases greatly, even as high as 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 electric 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 generated The part of the current affected by the magnetic flux is also reduced, so the stator current changes from large to small until normal.

By Jessica