Question

What is the most common method of controlling the speed of an alternating-current motor?

Answer 1

The most common methods for controlling the speed of AC motors mainly include the following:

1. Variable frequency speed regulation method

Principle: The speed of the motor is adjusted by changing the frequency of the power supply. 

The variable frequency speed regulator is the key device to achieve this function. 

It uses electronic technology to change the power supply frequency to accurately control the speed of the motor.

Advantages:

* Wide adjustment range, which can achieve smooth stepless speed regulation.

* High control accuracy, suitable for occasions with high speed regulation accuracy requirements.

* Significant energy saving effect, because the motor is most efficient when it is close to the rated speed, and variable frequency speed regulation can make the motor run in the high efficiency area as much as possible.

Disadvantages:

* The equipment cost is high and a special variable frequency speed regulator is required.

* The technology is complex and requires certain professional knowledge for debugging and maintenance.

2. Voltage modulation method

Principle: Speed ​​regulation is achieved by changing the size and frequency of the motor input voltage. This method is one of the simplest and most commonly used methods for AC motor speed regulation.

Advantages:

* Simple structure and easy to implement.

* Relatively low cost, no additional speed regulation equipment is required.

Disadvantages:

* Low speed regulation accuracy, especially when the load changes greatly.

* It has a certain impact on the mechanical characteristics of the motor, which may cause the motor to heat up or reduce efficiency.

3. Vector control method and direct torque control method

Principle:

* Vector control method: The current and torque of the AC motor are controlled separately, and the speed regulation is achieved by controlling the current and torque of the motor. 

This method is suitable for occasions with large load changes and high requirements for speed regulation accuracy.

* Direct torque control method: Similar to the vector control method, speed regulation is also achieved by controlling the current and torque of the motor. 

However, it is different in control strategy and focuses more on directly controlling the torque of the motor.

Advantages:

* High speed regulation accuracy and fast response speed.

* Suitable for industrial applications with high requirements for speed regulation performance.

Disadvantages:

* High hardware and software requirements and high cost.

* Relatively complex debugging and maintenance.

Summary

The most common method to control the speed of AC motors is variable frequency speed regulation. This method has the advantages of wide adjustment range, high control accuracy, and significant energy saving effect. 

Although the equipment cost is high and the technology is complex, it has been widely used in many industrial applications.

The voltage modulation law is also used in some occasions where the speed regulation accuracy is not high due to its simplicity and low cost. 

The vector control method and direct torque control law are more suitable for specific applications with high requirements for speed regulation performance.

Answer 2

The speed control of AC motors is an important topic in the field of motor control. 

Its control methods are diverse and continue to improve with the development of technology. 

The following is a detailed analysis of the speed control of AC motors:

Speed ​​control principle

The speed control of AC motors is mainly achieved by adjusting the power supply voltage of the motor, changing the power supply frequency and controlling the armature current of the motor. Specifically:

1. Adjust the power supply voltage: The speed of the AC motor is proportional to the power supply voltage, so the speed of the motor can be adjusted by changing the power supply voltage. Reducing the power supply voltage will reduce the motor speed, while increasing the power supply voltage will increase the motor speed. 

This method is simple and direct, but for high-power motors, the economy and effect are not ideal.

2. Change the power supply frequency: The speed of the AC motor is also proportional to the power supply frequency, so the speed of the motor can be controlled by changing the power supply frequency. 

In the industrial field, frequency converters are usually used to change the power supply frequency to achieve motor speed control. 

The frequency converter can convert the fixed-frequency power supply voltage into an adjustable-frequency power supply voltage and can accurately control the speed of the motor.

3. Control the armature current: The armature current is another important parameter for controlling the speed of the AC motor. The motor speed can be adjusted by controlling the armature current. 

Usually, an electronic speed regulator or an electronic controller is used to control the armature current. It is suitable for large AC motors and can achieve precise speed regulation and load adjustment of the motor.

Main speed regulation methods

1. Variable frequency speed regulation:

- Principle: The frequency and voltage of the power supply are adjusted by the frequency converter to change the frequency and size of the rotating magnetic field, thereby changing the speed of the motor.

- Advantages: wide speed regulation range, high efficiency, high precision, and easy to achieve closed-loop automatic control.

- Application: widely used in occasions requiring precise speed regulation, such as industrial automation, robot control, etc.

2. Speed ​​regulation by changing the number of magnetic poles:

- Principle: By changing the number of magnetic poles of the motor to control the synchronous speed of the magnetic field, the rotor speed is controlled. It is a step-by-step speed regulation with limited application occasions.

- Features: The slip rate remains unchanged, the slip loss is small, but the number of speed regulation levels is limited.

3. Speed ​​regulation by changing the slip rate:

- Principle: When the synchronous speed remains unchanged, the slip rate is changed by changing the excitation current of the rotor circuit to achieve speed regulation.

- Disadvantages: The slip power of the motor cannot be used, the power factor is low, and the slip power is consumed in the form of heat energy.

4. Speed ​​regulation by voltage regulation:

- Principle: The AC speed regulation circuit is composed of power electronic devices such as thyristors, and the terminal voltage of the asynchronous motor is changed by changing the trigger angle to achieve speed regulation.

- Features: The equipment is simple and the control is convenient, but the speed regulation range is small and the torque drops more.

Other speed regulation methods

In addition to the above main speed regulation methods, there are some other speed regulation methods, such as rotor series resistance speed regulation, cascade speed regulation, etc. 

These methods have their own advantages and disadvantages and are suitable for different applications.

Control practice

When controlling the speed of an AC motor, the following points should be noted:

1. Choose a suitable speed regulation method: Choose a suitable speed regulation method according to actual needs.

2. Select appropriate control equipment: such as frequency converter, electronic speed regulator, etc., according to the power, speed and load of the motor.

3. Debugging and testing: Careful debugging and testing are required before speed regulation to ensure the stability and reliability of the control system.

In summary, the speed control of AC motors is a complex and important process. It is necessary to select appropriate speed regulation methods and control equipment according to actual needs, and conduct sufficient debugging and testing.