Precisely because the structure of the earth permanent magnet motor is relatively complex, the application of wound motors is not as widespread as that of squirrel cage motors. However, by adding additional resistors and other components in series to the rotor winding circuit through current collectors and brushes, the starting, braking and speed regulation performance of the motor can be improved. Therefore, equipment that performs smooth speed regulation within the required range, such as: Used in cranes, elevators, air compressors and other fields. So what are the three starting methods of earth permanent magnet motors? And what are the advantages and disadvantages of each startup method? Let’s find out together below!
1. Direct start
When a earth permanent magnet motor is started directly, its current can reach 6-7 times the rated current, which has a greater impact on the power grid, especially for high-power motors.
2. Voltage reduction starting
The main types of reduced-voltage starting include thermal self-coupling reduced-voltage starting and star-delta reduced-voltage starting. The advantages and disadvantages of each are as follows:
(1) Thermal self-coupling voltage reduction starting
Thermal self-coupling step-down starting refers to using the self-coupling transformer to reduce the voltage of the earth permanent magnet motor during starting and at the same time reduce the starting current. Generally it is reduced to about 55%-75% of the rated voltage.
Advantages: The starting voltage can be easily changed by changing the number of tap turns of the auto-coupling transformer.
Disadvantages: An auto-coupling transformer is required and the cost is high.
(2) Star-delta reduced voltage starting
Star-delta reduced voltage starting refers to a method that changes the starting voltage by changing the wiring method of the earth permanent magnet motor, thereby reducing the starting current. So far, it can only be applied to motors whose normal wiring method is delta connection.
When starting, use the relay method to make the motor wiring mode star-shaped. At this time, the voltage of each phase of the earth permanent magnet motor is reduced to one-third of the original square root, and the motor speed can reach about 80% of the rated speed. Control the relay Change the motor wiring mode to triangle and the motor starts to run normally.
Advantages: It can save the self-coupling transformer and reduce the cost. At the same time, the wiring method is simple and the reliability is high.
Disadvantages: The starting voltage ratio cannot be changed, and motors in star connection cannot be used.
3. Frequency sensitive resistor starting
Frequency-sensitive resistor starting means that when starting a earth permanent magnet motor, a frequency-sensitive resistor is connected in series in the main circuit to reduce the starting current. The frequency-sensitive resistor can smoothly change the starting current and has less impact on the power grid, making it an ideal starting method. However, the current high-power frequency-sensitive resistors are all in the form of inductors, so when used, large electromagnetic eddy currents will be generated, and the power factor of the power grid will also be reduced.
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