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Starting Methods for Three-Phase Induction Motors
Frequency-sensitive resistor starting involves connecting a frequency-sensitive resistor in series with the main circuit during motor startup, thereby reducing the inrush current.
1. Direct Start
However, when a three-phase induction motor starts directly, the current can reach 6 to 7 times the rated current, causing a significant impact on the power grid—especially in the case of high-power motors.
2. Frequency-sensitive resistor starting
Frequency-sensitive resistor starting involves connecting a frequency-sensitive resistor in series with the main circuit during motor startup, thereby reducing the inrush current. This type of resistor can smoothly modulate the starting current, minimizing the impact on the power grid and making it a highly desirable starting method. However, high-power frequency-sensitive resistors typically use an inductive design, which generates significant eddy currents during operation, ultimately leading to a lower power factor in the grid.
3. Reduced-voltage starting
Voltage-reduction starting primarily includes autotransformer voltage reduction starting and star-delta voltage reduction starting.
Thermal autotransformer reduced-voltage starting refers to lowering the motor voltage—and consequently reducing the starting current—during startup by using an autotransformer. Typically, the voltage is reduced to around 55%–75% of the rated voltage. The advantage is that the starting voltage can be easily adjusted by changing the number of taps on the autotransformer. However, the main drawback is that it requires the use of an autotransformer, which increases the overall cost.
Star-delta voltage reduction starting refers to a method that lowers the starting current by altering the motor's wiring configuration to reduce the starting voltage—this technique is applicable only to motors whose normal connection method is delta. During startup, a relay switches the motor wiring into a star configuration, reducing the phase voltage to approximately one-third of its original value. Once the motor reaches about 80% of its rated speed, a control relay reverses the wiring back to the delta configuration, allowing the motor to resume normal operation. The advantages include saving on autotransformers, thereby reducing costs, while also offering a simple wiring setup and relatively high reliability. However, a key limitation is that the ratio of the reduced starting voltage cannot be adjusted, and this method cannot be used for motors already connected in a star configuration.
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