control mode of three-phase asynchronous motor:
1. direct start - the method is simple, the cost is low, and most motors with small capacity are started directly.
2. step down starting of self coupling transformer - the advantage is that it can be directly controlled manually or automatically controlled by ac contactor. it is durable and low maintenance cost. it is suitable for all no-load and light load starting asynchronous motors.
3. star delta step-down starting - small starting current and small starting torque. the advantage is that it can be realized without adding starting equipment and control equipment such as starting switch or ac contactor. the disadvantage is that it can only be used for motors connected with △ and large asynchronous motors cannot be started with heavy load.
4. rotor series resistance starting - in this starting mode, since the resistance is constant, the starting resistance is divided into several stages, which are cut off step by step in the starting process, so as to obtain a smoother starting process.
5. variable frequency starting - the advantage of variable frequency starting is that the frequency is changed in proportion to the voltage, that is, while changing the frequency, the output voltage of the frequency converter is controlled to keep the magnetic flux of the motor constant, which is similar to the constant power speed regulation mode, so as to avoid the occurrence of magnetic weakening and magnetic saturation. although the frequency converter is expensive, it has good performance, complex structure and simple use.
6. soft start - it has the advantages of low voltage start and low starting current, which is suitable for all no-load and light load asynchronous motors; the disadvantage is that the starting torque is small, which is not suitable for large motors with heavy load starting.
three phase asynchronous motor needs soft start to help start. the purpose is to make the motor start smoothly and effectively reduce the damage to the power grid and motor caused by large instantaneous starting current. next, hengda motor will explain the reasons in detail:
1. for three-phase asynchronous squirrel cage motors, soft starters can be used as long as the speed is not regulated, including 380v 5.5kw-630kw and 10kv 200kw-10000kw;
2. generally, soft starter is required for low-voltage 380v and above 75kw or high-voltage 10kv and above 200kw;
3. the power grid capacity is limited, and there is a short-term voltage drop during startup;
4. especially when the loads of pumps and fans require the combination of soft start and soft stop;
5. it is necessary to reduce the starting current and reduce the impact on the power grid; as well as the occasions where there are requirements for the equipment process to avoid the impact of startup on the equipment and machinery.
classification of soft start:
1. soft starting voltage cabinet and high voltage cabinet
2. according to the load, fan soft start cabinet, water pump soft start cabinet, etc
3. according to different load control modes, one driven one soft start cabinet, one driven multiple soft start cabinet, one for use and one for standby soft start cabinet, dual-use and one for standby soft start cabinet, etc.