What are the speed regulation methods of MILL AC Motor?

1. Frequency conversion speed regulation
1. Principle
According to the speed formula of AC motor (where is the speed, is the power frequency, is the number of magnetic pole pairs), the motor speed is regulated by changing the power frequency. When the inverter inputs AC power of different frequencies to MILL AC Motor, the synchronous speed of the motor will change accordingly, thereby achieving speed regulation.
At the same time, the inverter can also control the size of the output voltage to ensure that the magnetic flux of the motor remains basically constant during the speed regulation process, avoiding the degradation of torque, efficiency and other performance caused by the change of magnetic flux.
2. Advantages
Wide speed regulation range: A wider speed regulation range can be achieved, generally reaching 1:10 or even wider, which can meet the different speed requirements of the rolling mill under different rolling processes (such as biting, stable rolling, throwing, etc.).
High speed control accuracy: Modern high-performance inverters can increase the speed control accuracy to ±0.1% - ±0.5%, which is very important for ensuring the quality of rolled products (such as plate thickness, pipe diameter, etc.).
Smooth speed regulation: It can achieve smooth speed regulation. The acceleration and deceleration of the motor during the speed regulation process can be set according to the process requirements, avoiding the adverse effects of sudden speed changes on rolling equipment and product quality.
3. Application
Whether it is an AC asynchronous motor or an AC synchronous motor as a rolling mill motor, variable frequency speed regulation is currently the most widely used speed regulation method. In the actual rolling mill production line, the frequency converter can accurately adjust the speed of the motor according to the control system instructions of the rolling mill to meet the rolling requirements of products of different specifications.
2. Pole-changing speed regulation
1. Principle
The synchronous speed of the motor is changed by changing the number of pole pairs of the stator winding of the AC motor. By changing the connection method of the winding, such as changing the triangle connection to the double star connection, or using a special multi-speed motor winding design, the number of pole pairs can be changed. According to the speed formula, when the number of pole pairs changes, the speed of the motor will also change accordingly.
2. Advantages
Simple structure: No complex external equipment is required, and speed regulation can be achieved by only changing the connection method of the internal winding of the motor. This speed regulation method was once used in early rolling mill equipment, especially for some rolling mills with limited speed regulation requirements and limited speed regulation gears.
High reliability: Since there is no complex electronic equipment involved in speed regulation, the reliability of pole-changing speed regulation is relatively high and is not easily affected by electronic component failures.
3. Limitations
Limited speed regulation levels: Usually only a limited number of speed changes can be achieved, such as two-speed and three-speed, which cannot meet the requirements of modern rolling mills for continuous and wide-range speed regulation.
Discontinuous speed regulation: Since it is a discrete speed regulation achieved by changing the number of magnetic pole pairs, it cannot achieve smooth speed regulation like variable frequency speed regulation, and may have a certain impact on the rolling process when switching speeds.
Application scenarios: In some small rolling mills or old equipment renovations that do not require high speed regulation accuracy and have relatively simple rolling processes, pole-changing speed regulation may still have certain application value, but it is rarely used in modern large-scale, high-precision rolling mill equipment.
III. Vector control speed regulation
1. Principle
The stator current of the AC motor is decomposed into an excitation component and a torque component. Through coordinate transformation (such as Clarke transformation and Park transformation), the current in the three-phase stationary coordinate system is converted into the current in the two-phase rotating coordinate system, and then the excitation current and torque current are independently controlled. This control method is similar to the control of DC motors, and can achieve precise control of the torque and flux of AC motors.
In vector control speed regulation, the torque component and excitation component are precisely adjusted according to the load demand and speed instruction of the rolling mill, so as to achieve the adjustment of the motor speed. For example, when it is necessary to increase the rolling speed of the rolling mill, the output torque of the motor can be increased by increasing the torque component, thereby increasing the speed.
2. Advantages
Improve dynamic performance: Vector control speed regulation greatly improves the dynamic response capability of AC motors. During the operation of the rolling mill, when the load changes suddenly (such as changes in the hardness of the rolled material, the roll bites and throws out the metal billet, etc.), the output torque and speed of the motor can be quickly adjusted to keep the rolling mill running stably.
Achieve high-precision speed regulation: By precisely controlling the torque and flux, the speed regulation accuracy comparable to that of the DC motor can be achieved, meeting the speed control requirements of the rolling mill under the strict requirements of product quality.
3. Applications
It is widely used in high-performance AC rolling mill motor control, especially for rolling mill equipment with high requirements for speed regulation accuracy and dynamic response capabilities, such as high-precision plate rolling production lines, high-speed pipe rolling production lines, etc.
4. Direct torque control speed regulation
1. Principle
Direct torque control directly controls the torque and flux of the motor, and does not require complex coordinate transformation like vector control. It detects the stator voltage and current of the motor, calculates the torque and flux using space vector theory, and then directly controls the switching state of the inverter according to the given torque and flux values, thereby achieving control of the motor torque and speed.
In rolling mill applications, the torque output of the motor is quickly adjusted according to the torque requirements of the rolling process, thereby achieving speed regulation. For example, when the rolling mill bites into the metal billet, a large torque is required, and the required torque can be quickly provided by direct torque control while maintaining the stability of the speed.
2. Advantages
Fast response: It has an extremely fast response speed and can adjust the torque and speed in a very short time. This is very important for the rolling mill to maintain stable operation under complex rolling conditions (such as frequent load changes, rapid speed adjustment, etc.).
Simple and efficient control: Since no complex coordinate transformation is required, the control algorithm is relatively simple, which reduces the amount of calculation and control links and improves the control efficiency.
3. Application
It is used in some rolling mills with extremely high torque response speed requirements, especially in some special rolling processes that require rapid adjustment of rolling speed and torque, such as the rolling production line of some special metal materials.