Research on AC Motor Speed ​​Control System

In recent years, with the decreasing energy, the continuous updating of new energy-saving equipment and the rapid development of science and technology, the rational design and the increasingly widespread application of power-saving equipment have brought more convenience to people's work and life.

1 Development process of AC motor speed control system

1.1 AC motor excitation speed regulation

In the early stage, the prime mover was used to drive a generator, and the output voltage of the generator was adjusted by controlling the excitation of the generator, thereby adjusting the rotational speed of the driven motor and the active power output of the motor, and also turning off and starting the motor.

1.2 Current motor controllable rectification speed regulation

With the continuous development of science and technology, the voltage (controlled rectifier technology) was controlled by the conduction time of the thyristor. First of all, the response speed of the speed control system has been greatly improved, and the current interruption problem at low speed is well solved. The thyristor speed regulation is to change the waveform of the motor terminal voltage by changing the conduction angle of the thyristor, thereby changing the effective value of the motor terminal voltage and achieving the purpose of speed regulation.

1.3 Summary of contents

1.3.1 Frequency control method

Frequency control is a speed control method that changes the frequency of the stator power of the motor and changes its synchronous speed. The frequency conversion speed control system is mainly carried out by frequency converter. The frequency converter can be divided into two categories: AC-DC-AC inverter and AC-AC inverter. At present, most of the domestic use AC-DC-AC inverter.

The basic principle of the frequency conversion speed regulation technology is based on the relationship between the motor speed and the input frequency of the working power supply: (where n, f, s, p respectively represent the speed, input frequency, motor slip, motor pole pairs), Change the working frequency of the motor to achieve the purpose of changing the motor speed.

1.3.2 Variable pole logarithmic speed control method

The speed control method is to change the stator pole number of the cage motor to change the number of stator pole pairs of the cage motor to achieve the purpose of speed regulation. The method is applicable to a production machine that does not require stepless speed regulation, such as a metal cutting machine tool, an elevator, and a lifting machine. Heavy equipment, fans, pumps, etc.

The characteristics of frequency converter speed regulation are stable, and the speed can be adjusted according to needs. It is the development direction of AC motor in the future.

2 AC motor speed control system program demonstration

2.1 Single-chip speed regulation

With the development of digital control systems worldwide, people are increasingly dependent on digital information. Realizing the full digital control of the speed control system not only enables the AC speed control system to be closely integrated with the information system, but also improves the functions of the AC speed control system itself.

As AC motor control theory continues to evolve, control strategies and control algorithms are becoming increasingly complex. Expansion cards, filters, FFTs, state observers, adaptive controls, artificial neural networks, etc. are all applied to vector control or direct torque control of various AC motors. Therefore, the DSP chip finds a stage for exerting its talents in a fully digital high-performance AC speed control system. In the process of full digitalization of AC speed regulation, various buses also play a very important role. STD bus, industrial PC bus, field bus and CAN bus play an important role in the automation application of AC speed control system.

2.2 PWM speed regulation

PWM control is the control core of the AC speed control system, which can complete the final implementation of any control algorithm.

There are several versions of the PWM control scheme in various fields. Especially after the application of the microprocessor technology, there are always new technical updates, starting from the sine of the voltage waveform and the sine of the current waveform. And then to the sine of the magnetic flux; from the initial efficiency maximization, torque ripple less to the later to eliminate noise as the main topic, these are the continuous upgrade and improvement of PWM control technology. At present, more and more new programs are constantly being proposed and applied, indicating that the application space of this technology is very extensive. Among them, space vector PWM technology has been applied more and more in AC speed control system due to its high voltage utilization rate, simple control algorithm and small current harmonics. The universal variable frequency speed control system with V/f constant and speed open loop control and the closed loop frequency control system of slip frequency speed basically solve the problem of smooth speed regulation of asynchronous motor. However, when the production machinery puts higher requirements on the dynamic and static performance of the speed control system, the above system is still slightly inferior to the DC speed control system. The reason is that the law of system control is to derive the steady-state value control from the steady-state equivalent circuit and the steady-state torque formula of the asynchronous motor, without considering the transition process, the dynamic response of the system during stability, starting and low speed. The performance of the other aspects is not satisfactory.

Asynchronous motor is a multivariable, strongly coupled, nonlinear time-varying parameter system. It is difficult to accurately control the electromagnetic torque directly through the applied signal. However, if the rotational space vector of the rotor flux is used as the reference coordinate, the stationary coordinate is used. When the transformation is made between the rotating coordinate systems, the excitation current component and the torque current component of the stator current can be separated into scalars and controlled separately. Thus, the motor model reconstructed by the coordinate transformation can be equivalent to a DC motor, so that rapid torque and flux control, that is, vector control, can be performed like a DC motor.

3 AC motor speed control system main circuit design

In addition to computers, the integrated equipment, the transmission of other equipment, the transmission of machine tools, robots and automatic devices, electric vehicles and train transmissions are all inseparable from the speed control system. After a long period of development, the AC speed control electric drive has risen to become the dominant trend of electric speed regulation, replacing the traditional DC speed control drive step by step.

The inverter converts the inverter power supply into the form of variable frequency power supply, which mainly utilizes the frequency converter for motor speed regulation, so that the common AC stabilized power supply form has a change effect, so the main function of the variable frequency power supply is The effect of converting an existing AC grid power supply into a stable, pure sine wave power supply of the desired frequency is ideal for AC power. The asynchronous motor is coaxially connected with the synchronous generator. The frequency of the output current of the generator can be accurately controlled by controlling the speed of the motor through the frequency converter, and the amplitude E of the synchronous generator output voltage is related to the magnetic flux Φ, so the adjustment of the excitation unit is provided. The excitation current can control the amplitude of the generator output voltage, and finally the voltage and frequency are respectively adjustable, and the waveform is a sine wave.

PLC is the core of the whole control system. It can output the speed signal to the inverter and control the speed of the motor. The excitation unit control signal can be given according to the proposed control strategy to realize the synchronous frequency adjustment with the generator. Thereby, the output frequency and amplitude of the entire variable frequency power supply are continuously adjustable. At the same time, real-time communication with the touch screen, providing data for the display of the touch screen, and processing the information input by the touch screen, etc., is more humanized.

The variable frequency constant pressure water supply control system realizes the constant pressure water supply of the pipe network by measuring the pressure of the pipe network and calculating the output frequency through the built-in PID regulator of the frequency converter. The frequency overrun signal of the inverter (usually can be used as the pressure limit signal of the pipe network) can be timely reported to the PLC to hold the logic switch of the variable frequency pump. In order to prevent the occurrence of water hammer signs, the start and stop of the pump will link its outlet valve.

It is assumed that the system consists of four pumps, one inverter, one PLC and one pressure transmitter and several auxiliary devices. The pressure transmitter installed on the water supply pipe converts the pipe network pressure into an electric signal of 0 to 5V or 4 to 20 mA; the frequency converter is used to adjust the water pump speed; and the PLC is used for logic switching. If the 1# pump reaches 50Hz but the actual value does not reach the set value, the 1# pump turns to the power frequency. After about 10 seconds, the inverter accelerates with the 2# pump. If the set value has not been reached, the 2# pump turns. To the power frequency, after about 10 seconds, the inverter is accelerated with 3# water pump and added to a certain frequency value. When the actual value is set, the inverter starts to decelerate to 0Hz, 1# power frequency pump stops, 2# power frequency When the pump stops, keep the actual value unchanged. When the water consumption is low at night, a variable frequency pump is left in the pipe network and the frequency drops to about 35 Hz. When the actual value ≥ the set value, after about 2 minutes, the small pump starts to start. P2 (small pump upper pressure) value when the small pump stops, then the pump is completely rested, the pipe network pressure is supplied by the pressure tank, the P2 value is reduced to the small pump starting pressure value, the P1 small pump is started, and when the water consumption is small, the pressure is up. When P2 stops, if the water consumption is large, the small pump starts, and when the Hz value is lower than the P1 value, the inverter starts to take the large pump to accelerate to 35Hz, and the small pump is opened. This cycle of work, greatly saving the increasingly tense electricity.