When the general motor is started, the current that the stator of the motor takes from the power grid is about 5 ~ of the rated current of the motor ~ 7 times. After starting the small motor for one or two seconds, the current decreases rapidly as the rotating speed of the rotor gradually rises; while the large motor needs more than ten seconds, even dozens of seconds before the rotor reaches the stable rotating speed. That is, until the start-up ends, the current drops to the rated value.
If the starting current of asynchronous motor is high, what problems will it bring to the use? If this motor starts frequently when it is used, the motor will heat severely due to the influence of starting current, which will affect the normal service life of the motor. In addition, if the starting times of the motor used are not frequent, when its capacity exceeds 30% of the power transformer capacity, due to the large starting current, it will cause the overvoltage drop on the power transmission line of the transformer, which will affect the work of other electrical equipments connected to the same transformer. Therefore, certain measures must be taken when starting to limit the starting current from being too large. Due to different types of motors used and different production conditions, the starting methods of motors are also different.
As for squirrel cage motor, as long as the power grid permits and the starting times are not very frequent, direct starting should be adopted as far as possible. After the constant dry winding is connected, it is directly connected to the rated voltage. Using direct start is the simplest and most economical, and there is no need to start the equipment. If the capacity of squirrel cage motor is relatively large, in order to limit its starting current, it generally adopts step-down starting. Step-down start is to add a lower voltage rather than a rated voltage to the motor when the motor starts. This can greatly reduce the starting current. The commonly used decompression method is Y-△ start. This method can be applied to motors with low starting load such as fans and water pumps. There are many methods to start the step-down, but the squirrel cage motor uses Y-△ to start. The equipment used is simple, small in size, light in weight, easy to maintain and low in price, so it is most commonly used.
Winding motor often has heavy load when starting, in order to limit the starting current, the stator is connected with rated voltage, and the rotor circuit is connected with resistance or frequency sensitive rheostat. This method can not only reduce the starting current, but also increase the starting torque.
