What are the common methods for reducing the starting voltage of motors?

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The common methods for reducing the starting voltage of motors are as follows: 

### Star-Delta Reduced-Voltage Starting

- **Principle**: At startup, the stator windings of the motor are connected in a star shape. At this time, the voltage borne by each phase winding is 1/√3 of the line voltage, and both the starting current and the starting torque are reduced to 1/3 of those in the case of direct full-voltage starting. When the rotational speed of the motor rises close to the rated value, the windings are then switched to a delta connection so that the motor can operate normally under the rated voltage.

- **Characteristics**: The equipment has a simple structure and relatively low cost, but the starting torque is relatively low, and it is suitable for occasions where the motor starts under light load or no load conditions. 

### Autotransformer Reduced-Voltage Starting

- **Principle**: Different taps of the autotransformer are used to reduce the starting voltage of the motor. When starting, the power source is connected to the motor through the autotransformer, and the motor starts at a lower voltage, with the starting current and starting torque reduced accordingly. After the startup is completed, the autotransformer is cut off, and the motor is directly connected to the power source for normal operation.

- **Characteristics**: The starting voltage can be flexibly adjusted according to the load requirements, and it can provide a relatively large starting torque. However, the autotransformer is relatively large in size and has a higher cost, and it is suitable for large-capacity motors.

Reduced-Voltage Starting by Connecting Resistors or Reactors in Series with the Stator Windings - **Principle**: Resistors or reactors are connected in series with the stator windings of the motor. Due to the voltage drop across the resistors or reactors, the voltage applied to the motor windings is lower than the power supply voltage, thus reducing the starting current. After the motor starts, the resistors or reactors are short-circuited so that the motor can operate under the rated voltage.

- **Characteristics**: The starting torque is reduced due to the decrease in the starting current, and the starting resistors or reactors will consume a certain amount of power, so it is not suitable for frequent starting.

### Reduced-Voltage Starting with a Soft Starter - **Principle**: The conduction angle of the thyristor modules connected in reverse parallel is controlled to gradually increase the voltage of the motor, thereby smoothly accelerating the motor and achieving a soft start.

- **Characteristics**: The starting process is smooth, which can significantly reduce the impact on the power grid and mechanical equipment. It also has multiple protection functions, enhancing the reliability of the system. However, its price is relatively high, and it is suitable for equipment that is sensitive to mechanical impact and voltage fluctuations.

### Starting with a Frequency Converter - **Principle**: By changing the power supply frequency and voltage supplied to the motor, stepless speed regulation and soft starting of the motor are achieved.

- **Characteristics**: It not only optimizes the starting process but also enables precise speed control during operation, and has a good energy-saving effect. However, the frequency converter has a high cost and is suitable for applications with extremely high requirements for starting and speed regulation performance.