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. For example, more than ten times, 6~8 times, 5~8 times, 5~7 times, etc.
One is to say that 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 stall current value.For the most frequently used Y series three-phase asynchronous motors, there are clear regulations in the JB/T 10391-2002 "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:When the synchronous speed is 3000, the ratio of locked rotor current to rated current is 7.0;When the synchronous speed is 1500, the ratio of locked rotor current to rated current is 7.0;When the synchronous speed is 1000, the ratio of locked rotor current to rated current is 6.5;When the synchronous speed is 750, the ratio of locked rotor current to rated current is 6.0.The power of a 5.5kW motor is relatively large, and the ratio of the starting current to the rated current of a motor with a smaller power is smaller, so electrical textbooks and many places say that the starting current of an asynchronous motor is 4 to 7 times the rated operating current
Why is the starting current of the motor large? Is the current low after starting?Here we need to understand from the perspective of motor motor starting principle and motor rotation principle: When the induction motor is in a stopped state, it looks 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 secondary coil of the transformer short-circuited; the stator winding There is no electrical connection with the rotor winding, only a 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 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 induces the highest possible potential, so a large current flows in the rotor conductor. Current, this current produces magnetic energy that cancels the stator magnetic field, just as the secondary magnetic flux of the transformer needs to cancel 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 very large at this time, the stator current also increases greatly, even as high as 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 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 it is normal.
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, self-coupling transformer starting, star-delta decompression starting and inverter starting to reduce the impact on the power grid. direct boot 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 relatively large grid capacity and load, and is mainly suitable for starting motors below 1W.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.Self-coupling transformer 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.Star delta decompression start For a squirrel-cage asynchronous motor whose stator windings are connected in 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 (y-& 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 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 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.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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