The starting of the motor refers to the process that the motor reaches the rated speed from a standstill after it is energized. Although the start-up time of the motor is short, due to the large current of the motor during this process, it is easy to cause the problem of winding heating. Therefore, it is necessary to adjust the necessary parameters and suitable starting methods to ensure the normal starting and running of the motor.

When the starting current is too large, a large voltage drop will be generated on the line, which will affect the normal operation of other loads on the same line. In severe cases, the starting torque of the motor may be too small and cannot be started. For motors that need to be started frequently, the windings are often heated and the insulation is aged, thereby shortening the service life of the motor.

During the starting process, the motor should have a large enough starting torque to shorten the starting time as much as possible; on the premise of ensuring sufficient starting torque, the starting current should be as small as possible. About how to realize the starting measures of high torque and low current, we have also talked about it in the previous article, so I won't go into details here. Considering the simplification, economy and reliability of the equipment, it should be selected in combination with the characteristics of the motor and the actual working conditions.

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Direct start is to apply a rated voltage to the stator winding of the motor, and it can be started directly when the motor capacity is not too large and the power grid capacity is large. When the motor power is below 7.5kW, or when the motor starts, it will not cause too much fluctuation of the grid voltage (not more than 10%), and direct starting can be used.

When starting directly, no additional starting equipment is required, and the cost is lower and more convenient, but this is not the case for high-power motors.

Commonly used step-down starting includes auto-transformer step-down starting, star-delta step-down starting, Yanbian delta starting and step-down starting with stator series resistance.

The starting method of star connection and delta connection conversion is suitable for no-load or light-load starting. The advantage of this starting method is that the equipment is simple and the cost is low. For convenience, the automatic switching method is adopted, which is widely used.

The delta starting method of extending the side is to connect part of the stator winding by star connection and the other part by delta connection method when the motor starts. Therefore, the stator winding has many taps and the operability is poor.

Stator winding series resistance starting is to connect a resistor in series on the stator winding to generate a voltage drop and share the stator winding voltage when starting. When the motor starts, the resistor is short-circuited.

There are two types of winding rotor starting methods: series resistance and frequency sensitive resistor. Connecting resistance in series in the rotor circuit not only reduces the starting current, but also has a larger starting torque, which can realize the starting of the motor under heavy load conditions. This starting method is widely used in lifting equipment and hoisting equipment. However, the investment of this starting method is relatively large, the starting equipment is also more complicated, and the starting process is not smooth.

When the frequency-sensitive resistor is used for starting, at the beginning of the start, the motor speed is low, the rotor frequency is high, and the resistance of the frequency-sensitive resistor is large, which effectively restricts the starting current and improves the starting torque; with the increase of the motor speed, the rotor frequency increases, The resistance of the frequency-sensitive resistor becomes smaller and the change is relatively smooth. When the rated speed of the motor is reached, the frequency-sensitive resistor is cut off. This starting method is simple in structure, more convenient to use, and has a long service life, but the power factor is relatively low, and the starting torque is not very large, so it is not suitable for heavy-load starting.