Common starting methods and benefits of soft starting for 6-10kV medium and high voltage motors

2. Benefits of motor soft start

⑴ It can reduce the impact on the power grid and lower the capacity of transformers

When a regular squirrel cage motor is started directly under no-load full voltage, the starting current will reach 5-7 times the rated current. When the motor capacity is relatively large, the starting current will cause a sharp drop in the grid voltage. After adopting soft start, the starting current can be reduced to 1.5 to 3 times the rated current, which can greatly reduce the fluctuation rate of the power grid voltage.

⑵ It can reduce damage to the motor and extend its lifespan

The high current generated during direct full voltage starting of the electric motor causes significant impact force on the stator coils and rotor squirrel cage bars, leading to motor failure. The electric power during soft start is one fourth of it. The visible effect is very obvious.

The high current during direct full voltage starting of an electric motor will cause a large amount of Joule heat to be generated in the stator and rotor windings. Burning the insulation of the winding reduces the lifespan of the motor. Soft start can greatly reduce heat generation. Improve the lifespan of the motor.

When the motor is directly started at full voltage, it will generate operating overvoltage. Under unfavorable conditions, the overvoltage will reach 5 times the rated voltage, which will cause great damage to the motor insulation.

⑶ Can reduce damage to machinery and extend its lifespan

During soft start, the motor accelerates slowly and the torque gradually increases, which is beneficial for the sufficient lubrication of the grease and avoids dry grinding. These greatly reduce the damage to the motor and are beneficial for improving the lifespan of mechanical equipment.

3. Voltage resistance issue of thyristors

The thyristors used in the soft start of high-voltage motors also need to be operated in series due to insufficient voltage resistance of a single thyristor, similar to the thyristor valve group in the TCR mentioned above. When the rated voltage of the thyristor is lower than the actual requirement, two or more devices of the same model can be connected in series. Ideal series connection hopes that each device can withstand equal voltage, but in reality, due to differences in device characteristics, there is generally an issue of uneven voltage distribution. The leakage current flowing through the devices in series is always the same, but due to the dispersion of static volt ampere characteristics, the voltage borne by each device is unequal. The device that bears a high voltage will first reach the transition voltage and conduct, causing the other device to bear all the voltage and also conduct, and both devices will lose control. Similarly, in reverse, uneven voltage due to different volt ampere characteristics may cause one device to reverse breakdown first, followed by the other. This type of voltage equalization problem caused by different static characteristics of devices is called static uneven voltage problem.

To achieve static voltage equalization, the first step is to select devices with parameters and characteristics that are as consistent as possible. In addition, resistor voltage equalization can be used.

The uneven voltage caused by differences in dynamic parameters and characteristics of devices is called dynamic uneven voltage. To achieve dynamic voltage equalization, it is also necessary to first select devices with dynamic parameters and characteristics that are as consistent as possible. In addition, RC parallel branches can also be used for dynamic voltage equalization. For thyristors, using gate strong pulse triggering can significantly reduce the difference in device turn-on time.