Solution to Heavy-load Starting of Three-phase Asynchronous Motor

Solution to Heavy-load Starting of Three-phase Asynchronous Motor
I. Heavy-duty starting of low-power three-phase asynchronous motor
The main problem is that the starting torque is insufficient, and the low-power three-phase asynchronous motor is generally squirrel cage type. The solution is to obtain high starting torque with special motors, mainly high slip rate motors, deep slot motors and double cage motors. From the starting current formula and the starting torque formula, it can be seen that increasing the rotor resistance can not only limit the starting current but also improve the starting torque.
The rotor guide bar of high slip asynchronous motor is not ordinary aluminium bar, but aluminium alloy with high resistivity. This kind of motor has strong overload capacity, but low power factor, high loss in normal operation and low efficiency. So it is only suitable for frequent starting, mainly for lifting and transportation machinery.
Deep slot asynchronous motor uses skin effect in the rotor guide bar to increase the resistance of the rotor during starting, improve the starting performance without reducing the normal operation efficiency, but reduce the power factor and overload capacity. It is suitable for the occasion where overload starting is needed and overload capacity is not high.
Double cage asynchronous motor uses skin effect to improve the starting performance and ensure the basic operation performance, but the price of the motor is higher. It is generally used in situations requiring higher starting torque.
2. Heavy-load starting of high-power three-phase asynchronous motor
In this case, both high starting torque and limited starting current should be required. If the cage asynchronous motor with high starting torque can not meet the requirements, winding asynchronous motor can be used in series with suitable resistance in the rotor circuit, which can not only improve the starting torque but also reduce the starting current. Therefore, winding asynchronous motor is often used in production machinery requiring large starting torque or frequent starting. Stepper motor drive, high-power motor is generally winding type.
The common starting methods of three-phase winding asynchronous motor are rotor series solid resistance, frequency sensitive resistance or liquid resistance. The solid resistance starting of high-power winding asynchronous motor rotor series can not be steplessly adjusted and the starting is not smooth enough. In order to reduce the impact, the multi-stage symmetrical resistance should be added in the rotor circuit and the starting resistance should be removed gradually with the increase of rotational speed. Therefore, the investment of the equipment is large, the operation and maintenance are inconvenient, the starting of the series frequency sensitive resistor is simple and easy to maintain, and it can be steplessly adjusted. However, the starting current is large and the power factor is low, which limits the starting torque, and the motors with different capacities need to be operated and maintained. With different specifications of frequency-sensitive rheostat and series liquid resistance of rotor circuit, the motor can start smoothly with continuous stepless regulation. It has the characteristics of large starting torque, small starting current, short starting time, high power factor, low noise, low temperature rise and simple structure. It can also change the resistance by adjusting liquid concentration, so that a starter can adapt to different power motors. It is the preferred scheme for heavy-duty starting of high-power motors.