AC motors are widely used in industrial production, household appliances, and other fields due to their advantages of simple structure, high reliability, and low cost.The speed is affected by the power frequency, the number of motor poles, and the slip rate (formula: n=60f/p (1-s), where n is the speed, f is the power frequency, p is the number of poles, and s is the slip rate). Based on this principle, common speed control methods can be divided into the following categories:

1、Control method based on power frequency regulation: variable frequency speed regulation

Variable frequency speed regulation is currently the most widely used and high-precision control method for AC motor speed regulation. The core is to achieve precise speed adjustment by changing the power frequency of the input motor.

Working principle: Using a frequency converter to convert AC power (such as 220V/50Hz, 380V/50Hz) into AC power with adjustable frequency, while matching the voltage according to the characteristics of the motor (usually following the principle of “constant voltage/frequency ratio” to avoid saturation of the motor magnetic circuit), thereby changing the synchronous speed of the motor.

Features: Wide speed range (can achieve operation from 0 to rated speed or even beyond rated speed), high accuracy (speed error can be controlled within 0.5%), low energy consumption (motor efficiency remains high even during low-speed operation), and no surge current during startup, effectively protecting the motor and load equipment.

2、 Control method based on motor pole number adjustment: variable pole speed regulation

Variable pole speed regulation is a graded speed regulation method that adjusts the number of magnetic poles (p) of a motor by changing the connection of its stator winding, thereby changing the synchronous speed.
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Working principle: The stator winding of the motor adopts a special tap or switching structure, and the winding connection method is switched by a contactor (such as star/triangle transformation, double star/triangle transformation), so that the number of magnetic poles changes exponentially (such as from 2 poles to 4 poles), and the synchronous speed decreases by half accordingly (such as from 3000r/min to 1500r/min at 50Hz power frequency).
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Features: Simple structure, low cost, easy operation, basically unchanged motor efficiency during speed regulation, but limited speed regulation levels (usually only 2-3 levels of speed regulation, such as 2-pole/4-pole/6-pole switching), unable to achieve continuous speed regulation, and may cause speed shock at the moment of switching.

3、 Control method based on slip adjustment

The slip rate (s) is the ratio of the difference between the actual speed of the motor and the synchronous speed to the synchronous speed. By changing the slip rate, AC motor speed regulation can be achieved. Common methods include series resistance speed regulation, series stage speed regulation, and voltage regulation speed regulation.
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Series resistance speed regulation (only applicable to wound rotor asynchronous motors)
Working principle: In the rotor circuit of a wound rotor asynchronous motor, an adjustable resistor is connected in series. By increasing the resistance value, the slip rate is increased and the actual motor speed is reduced (the higher the resistance, the lower the speed).
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Features: Simple structure, low cost, but high energy consumption (series resistance generates a large amount of Joule heat, resulting in severe energy loss), low speed regulation accuracy (speed fluctuates greatly with load changes), and significant decrease in motor efficiency during low-speed operation.