Several starting methods of three-phase asynchronous motor

The motor starting method has the risk of burning the motor. The motor starting method mainly considers the motor power, wiring method and whether the motor needs frequency conversion requirements. The common motor starting methods are 1. Full voltage direct start; 2. Auto-coupling step-down start; 3. Y-Δ start; 4. Soft starter; 5. Frequency converter start.

The different startup methods are mainly reflected in their application scenarios and wiring methods. The application scenarios of different startup methods end as follows:

1. Full pressure direct start

Full-voltage direct start, as the name implies, it connects the stator winding of the motor directly to the power supply and starts at the rated voltage. If the power grid capacity and load allow full-voltage direct start, full-voltage direct start can be considered. The general application scenario is that the power of the motor is less than 11kW. This wiring method is low in cost, widely used, and easy to operate and maintain. Disadvantages: large starting torque and large starting current, resulting in large voltage fluctuations.

2. Auto-coupling step-down start

Autotransformer step-down starting refers to the use of autotransformer to reduce the starting voltage applied to the motor stator winding when the motor is started. After the motor is started, the motor is separated from the autotransformer so that it can move normally under full voltage. This step-down start is divided into manual control and automatic control. The high-voltage side of the autotransformer is put into the grid, and the low-voltage side is connected to the motor . There are several taps with different voltage ratios for selection. The high-voltage side of the autotransformer is put into the grid, and the low-voltage side is connected to the motor . There are several taps with different voltage ratios for selection. Advantages: Different taps of the autotransformer can be selected according to the allowable starting current and the required starting torque to achieve step-down starting , and it can be used no matter whether the stator winding of the motor adopts Y or Δ connection. The equipment is bulky and the investment is expensive.

3. Y-Δ start

Y-Δ start is also called star-delta start. Star-delta step-down starting is to control the starting process according to time. The difference is that the stator winding of the motor is connected into a star shape when starting, and the voltage that each phase winding bears is the phase voltage of the power supply, which reduces the impact of the starting current on the power grid. After it is started, it is changed to a delta connection according to the pre-set time. The voltage that each phase winding bears is the line voltage of the power supply, and the motor enters normal operation. This circuit can be used for any squirrel-cage asynchronous motor whose stator winding is connected in a triangle during normal operation. The starting current and voltage are the original delta starting          . This starting method is simple, low in cost, and widely used.

4. Soft starter

The soft starter uses three-phase parallel thyristors as voltage regulators, which are connected between the power supply and the motor stator. Such a circuit is a three-phase fully controlled bridge rectifier circuit. When using a soft starter to start the motor, the output voltage of the thyristor gradually increases, and the motor gradually accelerates until the thyristor is fully turned on, and the motor works on the mechanical characteristics of the rated voltage to achieve smooth start, reduce the starting current, and avoid starting overcurrent tripping. When the motor reaches the rated speed, the starting process is over, and the soft starter automatically replaces the thyristor that has completed the task with a bypass contactor to provide the rated voltage for the normal operation of the motor, so as to reduce the heat loss of the thyristor and prolong the service life of the soft starter. Improve its work efficiency, and make the power grid avoid harmonic pollution. The soft starter also provides a soft stop function. The soft stop is the opposite of the soft start process. The voltage gradually decreases and the speed drops to zero to avoid torque impact caused by free stop. Soft start generally has the following starting methods: (1) ramp boost soft start; (2) ramp constant current soft start; (3) step start; (4) pulse impact start. It is more cost-effective to use this method in motors larger than 132kW.

5. Start the frequency converter

The principle of inverter start-up is that the inverter controls the motor, and keeps the motor at constant torque during speed regulation. When the frequency is adjusted upward, the voltage is also adjusted upward. When the highest frequency is reached, the voltage is similar to the supply voltage. The variable frequency starting device includes a control drive circuit part and a main circuit part. Using the variable frequency starting method and device can make the load current carried by the control inverter system increase slowly and slightly until it reaches the rated voltage and rated frequency when the load is running smoothly, and limit the increase of the load current during starting within a safe range to protect circuit components. The stable operation of the control inverter is guaranteed, and the energy waste at startup is reduced at the same time, which has good economic value.