In recent years, with the ever-decreasing energy sources, the continuous upgrading of new energy-saving equipment and the rapid development of science and technology, rationalized design and the increasingly widespread use of energy-saving equipment have brought more convenience to people's work and life. This paper first introduces the development process of AC motor speed control system, and then introduces the demonstration of AC motor speed control system. Finally, the main circuit design of AC motor speed control system is described.
First, the development process of AC motor speed control system1, AC motor excitation speed
In the early stage, a prime mover was used to drive a generator, and the output voltage of the generator was adjusted by controlling the excitation of the generator to adjust the rotational speed of the driven motor and the active power output of the motor, and the motor could also be turned off and started.
2, current motor controllable rectifier speed
With the continuous development of science and technology, invented the thyristor conduction time to control the voltage (controlled rectifier technology). The first is that the response speed of the governor system has been greatly improved, and the problem of current interruption at low speeds has been well solved. SCR speed regulation is to change the thyristor conduction angle method to change the voltage waveform of the motor terminal, thereby changing the motor terminal voltage effective value, to achieve the purpose of speed control.
3, content summary
1) Frequency control method
Variable frequency speed regulation is to change the frequency of the motor stator power, thus changing the speed of its synchronous speed control method. The frequency conversion speed control system mainly carries on through the frequency changer, the frequency changer may divide into the exchange-DC-AC frequency changer frequency changer and the exchange-exchange frequency changer two big kinds, most at present domestically uses the exchange-DC-AC frequency changer.
The basic principle of frequency conversion speed regulation technology is based on the relationship between the motor speed and the input frequency of the operating power supply: (where n, f, s, and p respectively represent the speed, input frequency, motor slip ratio, and number of motor pole pairs). Change the frequency of motor power supply to achieve the purpose of changing the motor speed.
2) Variable pole logarithm speed control method
This kind of speed regulation method is to change the stator pole pairs of the squirrel-cage motor to achieve the purpose of speed regulation by changing the red connection of the stator windings. This method is applicable to production machines that do not require stepless speed regulation, such as metal cutting machine tools, lifts, and Heavy equipment, fans, pumps, etc.
Inverter speed regulation is characterized by a smooth, adjustable speed according to need, is the future direction of the development of AC motors.
1, single-chip speed
With the development of digital control systems worldwide, people's dependence on digital information is also increasing. Realizing the full digital control of the governor system not only enables the AC speed governor system and the information system to be closely integrated, but also improves the function of the AC governor system itself.
As the AC motor control theory continues to evolve, control strategies and control algorithms have become increasingly complex. Expansion cards, filters, FFTs, state observers, adaptive controls, artificial neural networks, etc. are all applied to various AC motor vector control or direct torque control. Therefore, the DSP chip finds its place in the all-digital high performance AC speed control system. In the all-digital process of AC speed regulation, various buses also play a very important role. STD bus, industrial PC bus, field bus and CAN bus have played an important role in the automatic application of AC speed control system.
2, PWM speed
PWM control is the control core of the AC speed control system, which can complete the final realization of any control algorithm.
The PWM control scheme has been applied in various fields in various fields. Especially after the application of the microprocessor technology in the PWM technology, there are always new technological innovations, from the quest to pursue the sine of the voltage waveform to the sine of the current waveform. And then the sine of the magnetic flux; from the initial efficiency maximization, torque ripple is less to later to eliminate noise as the main subject, these are the constant upgrade and perfect PWM control technology. At present, more and more new programs are constantly being proposed and applied, indicating that this technology has a wide range of applications. Among them, space vector PWM technology has been applied more and more in the AC speed control system because of its high voltage utilization, simple control algorithm, and small current harmonics. V/f constant, speed open-loop control of the general variable frequency speed control system and slip frequency and speed closed-loop control system, basically solved the problem of asynchronous motor speed control. However, when the production machinery imposes higher requirements on the dynamic and static performance of the governor system, the above system is still slightly inferior to the DC governor system. The reason is that the law of system control is derived from the steady-state equivalent circuit of the asynchronous motor and the steady-state torque formula, and the steady-state value control is derived. The transient process is not considered at all, and the system dynamic response to the stability, start-up and low-speed torque Other aspects of performance are not yet satisfactory.
The asynchronous motor is a multivariable, strongly coupled, nonlinear, time-varying parameter system. It is difficult to directly control the electromagnetic torque directly through the applied signal. However, if the rotor flux is used as the reference for the rotating space vector, the slave stationary coordinate is used. In the transformation between the rotating coordinate systems, the excitation current component and the torque current component in the stator current become independent of the scalar quantity and are controlled separately. In this way, the motor model reconstructed by the coordinate transformation can be equivalent to a DC motor, and thus can perform fast torque and flux control, ie, vector control, like a DC motor.
In addition to computers, other equipment, such as transmissions, machine tools, robots, and transmissions of robots, electric vehicles, and train transmissions, are all inseparable from the speed control system. After a long period of development, AC speed-regulating electric drives have risen to become the dominant trend of electric speed control, replacing conventional DC variable speed drives step by step.
Inverter to convert the inverter power source into the form of variable frequency power supply, the main use of the motor speed control frequency converter, so that the ordinary AC voltage regulator power supply form changes, 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 required frequency, the ideal AC power supply is characterized by the stability of the frequency. The asynchronous motor is connected with the synchronous generator coaxially, and the frequency of the generator output current can be precisely controlled by the frequency converter controlling the rotation speed of the motor, and the output voltage amplitude E of the synchronous generator is related to the magnetic flux Φ, so the adjustment of the excitation unit provides Excitation current can control the amplitude of the generator output voltage, and ultimately reach the voltage and frequency are adjustable, the waveform is a sine wave.
PLC is the core of the entire control system. It can give the inverter output speed signal, control the motor speed; according to the proposed control strategy to give the excitation unit control signal, to achieve synchronization with the generator frequency adjustable. In this way, the output frequency and amplitude of the entire variable frequency power supply can be continuously adjusted. At the same time, real-time communication with the touch screen provides data for the display of the touch screen, and it is more humanized to process the information input by the touch screen.
Frequency conversion constant pressure water supply control system through the measured pipe network pressure, after the inverter's built-in PID regulator operation, adjust the output frequency, to achieve constant pressure water supply pipe network. The frequency limit signal of the frequency converter (usually used as the pressure limit signal of the pipeline network) can timely report to the logic switching of the inverter pump. In order to prevent the emergence of water hammer signs, the start and stop of the pump will link its outlet valve.
Assume that the system consists of four pumps, one inverter, one PLC, one pressure transmitter, and several auxiliary devices. The pressure transmitter installed on the water supply pipe converts the pipe network pressure into 0-5V or 4-20mA electrical signals; the frequency converter is used to adjust the pump speed; the PLC is used for logic switching. If 1# water pump reaches 50Hz but the actual value does not reach the set value, 1# pump goes to the power frequency, after about 10 seconds, the frequency converter takes 2# water pump to accelerate, if it has not reached the set value, 2# pump goes To industrial frequency, about 10 seconds after the inverter with 3 # pump acceleration, added to a certain frequency value, the actual value to get the set value, the frequency converter starts to deceleration to 0Hz, 1# frequency pump stop, 2# frequency When the pump stops, the actual value remains unchanged. When there is little nighttime water consumption, one frequency conversion pump remains in the pipe network and the frequency drops to about 35Hz. When the actual value ≥ the set value, about 2 minutes later, the small pump starts to start. P2 (small pump upper limit pressure) value when the small pump stops. At this time, the pump is completely off. The pressure of the pipe network is supplied by the pressure tank. When the P2 value drops to the value of the small pump starting pressure, the P1 pump starts. When the water consumption is low, the pressure is When P2 stops, if a large amount of water is used, a small pump is started, and when the value of Hz is lower than P1, the inverter starts throwing the small pump away when the large pump accelerates to 35 Hz. This cycle of work, greatly saving the increasingly tense electricity.
In addition, the above-mentioned system is also equipped with a peripheral auxiliary circuit to ensure that when its active control system fails, it can maintain the system operation by manually adjusting the physical structure, guarantee continuous production, and make the entire system operate normally.
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