What control method is used for AC induction motors?

1. Traditional control method
a. Step-down starting method
The step-down starting method is a method of reducing the starting current by reducing the voltage on the stator winding of the motor. In the early stage of motor starting, since the rotor has not yet rotated, the relative speed between the stator rotating magnetic field and the rotor winding is the largest, resulting in large induced electromotive force and current. By reducing the voltage, the starting current can be effectively reduced to avoid excessive impact on the power grid and the motor. When the motor speed is close to stable, the voltage is restored to the rated value. Although this method is simple, it requires additional step-down equipment and control circuits, and the cost is relatively high.
b. Direct starting
Direct starting is to connect the motor directly to the power grid and start it at full voltage. This method is simple to operate, but the starting current is large, the impact on the power grid and the motor is large, and the equipment life and power grid stability are affected. Therefore, in large motors or places with small power grid capacity, direct starting is generally not used.

2. Modern control method
a. Vector control (VC)
Vector control was proposed by German scholar Blaschke et al. in 1971. It successfully solved the problem of effective control of electromagnetic torque of AC motors. Vector control decomposes the stator current into excitation component and torque component through coordinate transformation, and controls them separately to achieve high-performance speed regulation of the motor. This method, like the DC speed regulation system, realizes the separate measurement control of the magnetic flux and torque of the Induction AC Motors, so that the AC motor variable frequency speed regulation system has all the advantages of the DC speed regulation system.
b. Direct Torque Control (DTC)
Direct torque control technology is another high-performance AC variable frequency speed regulation technology after vector control. This technology abandons the control idea of ​​current decoupling in vector control, removes the PWM pulse width modulator and current feedback link, and instead directly calculates the motor's flux and torque by detecting the bus voltage and stator current, and uses two hysteresis comparators to directly realize the decoupling control of the stator flux and torque. Direct torque control has the advantages of simple control structure, fast dynamic response, and insensitivity to changes in motor parameters, but the steady-state accuracy is relatively low, and there is a large torque pulsation at low speed.
c. Intelligent control method
With the development of control theory, intelligent control methods such as fuzzy control and neural network control have also been applied to the control of AC induction motors. These methods use the powerful self-learning and adaptive capabilities of fuzzy mathematics theory or neural networks to achieve intelligent control of motors. They do not rely on precise mathematical models and have strong robustness and adaptive capabilities, but may require a large amount of training data and computing resources.

3. Braking control
The braking control of AC induction motors is also an important part of their control method. Motor braking refers to a control method that applies electromagnetic torque in the reverse direction when the motor is running to achieve the purpose of braking. Common motor braking methods are divided into direct braking and indirect braking. Direct braking directly applies reverse electromagnetic torque, decelerates to zero speed, and then cuts off the power supply; while indirect braking converts electrical energy into mechanical energy and achieves braking through brakes or reducers. The key to braking control lies in the control of braking torque, including parameters such as the size of the braking torque, braking time, and braking curve, to ensure that the motor can stop smoothly during braking.