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Analysis of starting current of compressor motor

Author:HU Date:2019/3/15 19:18:01
 How big is the motor starting current?
 
The starting current of the motor is different from the rated current. Many of them are different according to the specific situation. For example, ten times, 6 to 8 times, 5 to 8 times, 5 to 7 times, etc.
 
One is to say that at the moment of starting (ie, the initial moment of the starting process), the motor's speed is zero, and the current value at this time should be its stall current value.
 
 
For the most frequently used Y series three-phase asynchronous motor, there are clear provisions in the JB/T 10391-2002 "Y series three-phase asynchronous motor" standard. The specified values ​​of the ratio of the stall current to the rated current of the 5.5 kW motor are as follows:
 
When the synchronous speed is 3000, the ratio of the stall current to the rated current is 7.0;
 
When the synchronous speed is 1500, the ratio of the stall current to the rated current is 7.0;
 
When the synchronous speed is 1000, the ratio of the locked current to the rated current is 6.5;
 
At a synchronous speed of 750, the ratio of locked rotor current to rated current is 6.0.
 
The power of 5.5kW motor is relatively large, and the ratio of starting current and rated current of motor is smaller. Therefore, electrician teaching materials and many places say that the starting current of asynchronous motor is 4~7 times of rated working current.
 
Why is the motor starting current large and small after starting?
 
Here we need to understand from the perspective of the motor motor starting principle and the motor rotation principle:
 
When the induction motor is in the stop state, from the electromagnetic point of view, like a transformer, the stator winding connected to the power supply is equivalent to the primary coil of the transformer, and the closed rotor winding is equivalent to the secondary coil of the transformer being short-circuited; the stator winding There is no connection between the rotor winding and the rotor. Only the magnetic connection is made. The magnetic flux is closed by the stator, the air gap and the rotor core. At the moment of closing, the rotor has not turned due to inertia, and the rotating magnetic field cuts the rotor winding at the maximum cutting speed-synchronous speed, so that the rotor winding senses the highest potential that can be reached, and thus, a large flow in the rotor conductor Current, this current produces magnetic energy that cancels the stator's magnetic field, just as the transformer's secondary flux cancels the action of a 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 current of the rotor is very large at this time, the stator current also increases greatly, even up to 4 to 7 times the rated current, which is the reason for the large starting current. Why is the current small after starting: as the motor speed increases, the stator magnetic field cuts the rotor conductor at a reduced speed, the induced potential in the rotor conductor decreases, and the current in the rotor conductor also decreases, so that the stator current is used to cancel the rotor current. The portion of the current affected by the flux is also reduced, so the stator current is from large to small until normal.

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