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How Big Is The Starting Current Of The Motor? Why Does The Current Decrease Again After Starting?

1. How big is the starting current of the motor?

In fact, there are different opinions about how many times the starting current of the motor is the rated current, and many of them are determined according to specific conditions.
One way of saying: when the speed of the motor 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 commonly used Y series three-phase asynchronous motors, there are clear relevant regulations.
The power of a 5.5 kW motor is relatively large, and the smaller the power, the smaller the ratio of the starting current to the rated current of the motor. Therefore, many places say that the starting current of an asynchronous motor is 4 to 7 times the rated current.

2. Why is the starting current of the motor large, but the current drops after starting?

Here we need to understand the perspective of the principle of motor starting and the principle of motor rotation:
When an induction motor is at a standstill, it acts like a transformer from an electromagnetic point of view. 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. There is no electrical connection between the stator winding and the rotor winding, only a magnetic connection, and the magnetism is closed through the stator, air gap, and rotor core.

How Big Is The Starting Current Of The Motor? Why Does The Current Decrease Again After Starting?插图

①Why is the starting current of the motor large?

When switched on, the rotor does not rotate due to inertia, and the rotating magnetic field cuts the rotor winding at the maximum cutting speed—synchronous speed, and the rotor winding induces the highest potential possible so that a large current flows through the rotor conductor, and the generated magnetic energy cancels the stator magnetic field. Just like the secondary flux of the transformer cancels the effect of the primary flux.
In order to maintain the original flux corresponding to the supply voltage, the stator automatically increases the current. Since 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, which is also the reason for the large starting current.

② 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 part of the stator current used to counteract the magnetic flux effect generated by the rotor current also decreases, so the stator current varies from large to small until normal.

3. What are the methods to reduce the starting current of the motor?

Common starting methods to reduce the starting current of the motor include direct starting, series resistance starting, autotransformer starting, star-delta decompression starting, and inverter starting to reduce the impact on the power grid.

① Direct Start

Direct start is to connect the stator winding of the motor directly to the power supply and start it under the rated voltage. It has the characteristics of large starting torque and short starting time, and it is also the simplest, most economical, and most reliable starting method. The current is large during full-voltage starting, but the starting torque is not large, the operation is convenient, and the starting is fast. However, this starting method requires a relatively large grid capacity and load and is mainly suitable for starting motors below 1W.

② String Resistor Start

Motor series resistance start, which is a method of step-down start. During the starting process, a resistor is connected in series in the stator winding circuit. When the starting current passes, a voltage drop is generated on the resistance, which reduces the voltage applied to the stator winding, so that the purpose of reducing the starting current can be achieved.

③ Autotransformer start

The use of multi-tap decompression of autotransformer can not only meet the needs of starting with different loads, but also obtain greater starting torque. It is a decompression starting method that is often used to start large-capacity motors. Its biggest advantage is that the starting torque is relatively large. When its winding tap is at 80%, the starting torque can reach 64% of that of direct starting, and the starting torque can be adjusted through the tap.

④ Star delta decompression start

For a squirrel-cage asynchronous motor whose stator windings are connected in the delta in normal operation, if the stator windings are connected in a star shape when starting, and then connected in a delta shape after starting, the starting current can be reduced and its impact on the power grid can be reduced. shock. Such a starting method is called a star-delta decompression start, or simply a star-delta start. When using star-delta starting, the starting current is only 1/3 of the original direct starting according to the delta connection. When starting in the star delta, the starting current is only 2-2.3 times. That is to say, when the star-delta connection is used, the starting torque is also reduced to 1/3 of the original direct start according to the delta connection. Suitable for no-load or light-load starting occasions. And compared with any other decompression starter, its structure is the simplest and the price is the cheapest. In addition, the star-delta starting method has another advantage, that is, when the load is light, the motor can be operated in a star connection. At this time, the rated torque can be matched with the load, which can improve the efficiency of the motor and thus save power consumption.

⑤ Inverter Start

Inverter is a motor control device with the highest technical content, the most complete control functions, and the best control effect in the field of modern motor control. It adjusts the speed and torque of the motor by changing the frequency of the power grid. Because it involves power electronic technology and microcomputer technology, the cost is high and the requirements for maintenance technicians are also high, so it is mainly used in fields that require speed regulation and have high requirements for speed control.

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