The so-called AC speed regulation system is based onAC motorAs a conversion device of "electric energy → mechanical energy", it can generate the required torque and speed through the control of electric energy (voltage, current, frequency).AC motor andDC motorThe biggest difference is that AC motors do not have the mechanical commutator of DC motors, the commutator.When the motor is rotating, the related winding coil will pass through the magnetic field in the same direction twice per revolution, which will change the direction of the induced current in the winding. The role of the commutator is to keep the direction of the output current unchanged.[1]
Development and characteristics of AC speed regulation
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DC speed regulation system and AC speed regulation system are electrical speed regulationautomatic control systemThe two important components of are similar. AC motors are used for AC speed regulation. About 90% of the total number of motors in operation are AC motors.
In the past quite a long time, due to the difficulty in realizing AC frequency conversion, the price is expensive, andAC motorInsynchronous motorThe speed regulation of is almost completely dependent on the frequency conversion method, so the AC speed regulation is limited toasynchronous motorSimple speed regulation modes such as pole changing, rotor circuit series resistance, etc. cannot compete with DC speed regulation with good starting, braking and speed regulation performance.In the past 20 years, with the development of power electronics technologylarge-scale integrated circuitWith the rapid development and application of computer control technology, vector control technology and various frequency conversion devices, the AC speed regulation system has a wide speed regulation range, high steady-state accuracy, fast dynamic response, and can operate in four quadrants, which is comparable to DC speed regulation.In addition, the AC speed control system has the advantages of reduced floor area and low maintenance costs, especially in large capacity occasions.Therefore, the AC speed control system has been more and more widely used. Up to now, the AC speed control system has developed to the level with the DC speed control system.[2]
AC speed regulation method
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Single phase motors generally have two different starting modes: starting capacitor phase-shifting starting and commutator commutation starting. The motors using this starting mode are generally four pole (two pairs of magnetic poles) asynchronous motors, and the speed formula is:
Where f is the grid frequency and p is the number of motor poles.
According to different working principles,AC motorAnd divided into exchangesasynchronous motorandAC synchronous motorTwo.The number of AC asynchronous motors in all sectors of the national economy accounts for more than 80% of AC motors, and AC asynchronous motors in industrial production alone account for more than 90%,.
Fig. 1 Mechanical characteristics of different speed regulation methods
Asynchronous motor is called asynchronous, which means that the rotating magnetic field speed and rotor speed are different.The speed of stator rotating magnetic field strictly corresponds to the grid frequency, which is called synchronous speed, so the slip rate is the difference between the speed of stator rotating magnetic field and the speed of rotor divided by the speed of stator rotating magnetic field (synchronous speed).For asynchronous machines, the motor science does not use the ideal no-load speed and speed drop to express the speed as DC machines do. The speed is characterized by the synchronous speed n1 and slip rate S.However, as a kind of motor, the speed of asynchronous motor is actually also composed of ideal no-load speed n0 and speed drop △ n, which is determined by the general law of motor mechanical characteristics, and is also a general expression of motor speed.
Where, n1 is synchronous speed, and n is motor speed.
Principle: stator winding inputThree phase alternating currentGenerates a rotating magnetic field that cuts the rotor conductorInduced electromotive forceThe induced electromotive force generates current in the conductor, and the interaction between the rotor current and the stator magnetic field generates electromagnetic force, which drives the rotor to rotate in the same direction as the rotating magnetic field of the stator.Without the influence of external force, the speed of rotor rotation is lower than that of stator magnetic field rotation.The ratio of the difference between the rotating speed of the stator magnetic field and the rotating speed of the rotor and the rotating speed of the stator magnetic field is the slip rate.
In actual use, it is often necessary to change the speed of asynchronous motor, that is, speed regulation,Three phase AC asynchronous motorThe speed formula is:
Where, n is the motor speed (r/min), p is the motor pole pair, f is the power supply frequency (Hz), and S isasynchronous motorThe slip rate of.
It can be seen from the speed formula that the speed regulation of AC asynchronous motor can be achieved by changing the stator voltage frequency f, pole pair p and slip rate S. Specifically, it can be divided into three categories: variable pole speed regulation, variable slip rate speed regulation and variable frequency speed regulation. Variable slip rate speed regulation includes voltage regulation, speed regulationRotor series resistance speed regulation, cascade speed regulation, etc. They are slip power consumption speed regulation methods.[3]
Variable pole speed regulation
Pole changing speed regulation is realized by changing the number of magnetic poles pAC motorSpeed of.This kind of speed regulation belongs to stepwise speed regulation, and the speed cannot be continuously adjusted. Its mechanical characteristics are shown in Figure 1a.
Pole changing speed regulation is only applicable to pole changing motors. Multiple sets of windings are installed when the motor is manufactured. During operation, the number of pole pairs is changed by controlling the connection mode of windings with external switchgear, thus changing the speed of the motor.Its advantages are: at each speed level, it has hard mechanical characteristics and good stability.Its disadvantages are: the speed can only be changed at several speed levels, and the smoothness of speed regulation is poor;In some connection modes, the maximum torque is reduced, which is only applicable to the load speed regulation of fans and pumps;The motor has large volume and high manufacturing cost.
Variable slip speed regulation
Variable slip speed regulation is to achieve the purpose of speed regulation by changing slip s.This method can be realized in the following ways:
1. Voltage regulation and speed regulation: change the terminal voltage of asynchronous motor for speed regulation, and its mechanical characteristics are shown in Figure 1b.The slip power loss in the process of voltage regulation and speed regulation is in the rotor or on the external resistance, which has low efficiency and is only used in the speed regulation system of small capacity motors such as special cage type and wound rotor.It can be seen from the characteristic curve that when the stator voltage of the motor changes, the working point can be placed on different working curves, thus changing the working speed of the motor.The characteristics of step-down speed regulation are narrow speed regulation range;Soft mechanical properties;The scope of application is narrow.In order to improve the speed regulation characteristics, the closed-loop working mode is generally used, and the system structure is complex.
2. Rotor series resistance speed regulation: a variable resistance is connected to the external circuit of the rotor to change the slip rate of the motor to achieve speed regulation. Its mechanical characteristics are shown in Figure 1c.
Series resistance speed regulation is applicable to wound rotorasynchronous motorThe hardness of the mechanical characteristics of the motor is artificially changed by inserting resistors of different resistance values in the rotor circuit of the motor, thus changing the speed under certain load characteristics.Its advantages are: simple equipment, cheap price, easy realization and convenient operation, which can realize both stepwise speed regulation and stepless speed regulation.Its disadvantage is that the slip power loss is on the resistance, the efficiency decreases proportionally with the slip increase, the mechanical characteristics are soft at low speed, and the static slip is large.
3. Rotor additional electromotive force speed regulation (cascade speed regulation): it is a method of speed regulation by adding electromotive force in the rotor circuit of asynchronous motor to change the slip rate. Its mechanical characteristics are shown in Figure d1:
Cascade speed regulation mode is an improvement of the rotor circuit series resistance mode, and the basic working mode is to change the working characteristics of the motor by changing the equivalent impedance of the rotor circuit, so as to achieve the purpose of speed regulation.The realization method is: a variable DC electromotive force E is connected in series in the rotor circuitadSo as to change the circuit current of the rotor circuit, thereby changing the motor speed.Compared with other speed regulation modes, cascade speed regulation has the following advantages: the energy of the rotor circuit can be fed back to the power grid through a certain control mode, thus improving efficiency;Under proper control mode, continuous speed regulation of low synchronization or high synchronization can be realized.Disadvantages: only applicable to wound rotorasynchronous motorAnd the control system is relatively complex.
4. ApplicationElectromagnetic clutchSpeed regulation (slip motor): in cage typeasynchronous motorThe electromagnetic slip clutch is connected in series with the load, and the speed is adjusted by adjusting the excitation current of the electromagnetic slip clutch.
The advantages of this speed regulating system are simple structure and low price.Disadvantages: During speed regulation, differential energy is lost on the coupler, which is low in efficiency. It is only applicable to small capacity drive control systems with low speed regulation performance requirements.
frequency control
Variable frequency speed regulation is a method of speed regulation by changing the power supply frequency of the motor by using the characteristic that the synchronous speed of the motor changes with frequency. Its mechanical characteristics are shown in Figure 1e.
It can be seen from the characteristic curve that if the power frequency of the motor can be continuously changed, its synchronous speed can be continuously changed, and the speed of the motor can be continuously changed in a wide range.From the perspective of speed regulation characteristics, the hardness of any speed segment of variable frequency speed regulation is close to the natural mechanical characteristics, and the speed regulation characteristics are good;If there is a variable frequency AC power supply, continuous speed regulation can be achieved with good smoothness.This speed regulation method can be applied to cage motors, so it has a wide range of applications.
communicationasynchronous motorAs the equipment for conversion of electrical energy and mechanical energy, one of the most important performances is the mechanical characteristics. Since frequency conversion speed regulation has obvious advantages in the aspects of operating economy, speed regulation smoothness, and speed regulation mechanical characteristics, it is an ideal speed regulation method for AC asynchronous motors, and also the preferred method for AC speed regulation.[4]
Basic Types of AC Asynchronous Motor Speed Regulating System
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withAC motorThe device used to complete various production and processing processes as the control object is called the AC speed regulating system. The AC speed regulating system using the above different speed regulating methods can be specifically divided into pole changing speed regulating system, AC voltage regulating speed regulating system, wound rotor asynchronous motor rotor series resistance speed regulating system, cascade speed regulating system, electromagnetic slip clutch speed regulating system, frequency conversion speed regulating system, etc.By investigating how these systems deal with slip power P during speed regulations=sPm。(It is known from the working principle of AC asynchronous motor that the electromagnetic power P of the motor transferred from the stator to the rotormIt can be divided into two parts, one part Pd=(1-s)Pm, is the effective power of the driving load;Another part Ps=sPmIs slip power), whether it is consumed or fed back to the power grid, which can measure the efficiency of this system.Generally, AC speed regulating systems are divided into three categories according to whether the slip power is consumed:
Slip power consumption speed regulating system
The slip power consuming speed governing system converts all the slip power into heat energy during the energy transmission process and consumes it, such as the reduced voltage speed governing system, the electromagnetic slip clutch speed governing system, and the wound rotor asynchronous motorRotor series resistance speed regulationSystem.This kind of speed regulation system has the lowest efficiency, which is to increase the consumption of slip power in exchange for reducing the speed.However, this kind of speed regulation system has the simplest structure and can be used in small capacity occasions with low performance requirements.
Slip power feedback speed control system
A part of slip power is consumed, and most of it is fed back to the power grid through converter or converted into mechanical energy for use. The lower the speed is, the more power will be recovered. For example, the cascade speed regulation of wound rotor asynchronous motor belongs to this category.The system composed by this speed regulation method has the highest efficiency, the most complex structure and is not easy to realize.
Slip power invariant speed control system
In this type of system, the slip power consumption is basically unchanged regardless of the speed, such as variable frequency speed regulation and pole change speed regulation.Among them, the speed regulation method of pole changing logarithm p can only achieve stepwise speed regulation, and its application is limited.The frequency conversion speed regulation method has high efficiency, the best performance, the most widely used, can replace the DC motor speed regulation, and has the most promising future. It is the main development direction of AC speed regulation and the mainstream of the 21st century.
By comparing the efficiency, performance and structure of the above AC speed regulating system, it can be seen thatPower electronic devicesLarge scale application and communication of single-chip microcomputerasynchronous motorVariable frequency speed regulation system has become the preferred system to drive AC asynchronous motor.[4]
Main performance indexes of AC speed regulating system
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The main performance indicators of the AC speed regulating system are the basis for evaluating the performance of the AC speed regulating system. When measuring the performance of the AC speed regulating system, it should be comprehensively evaluated from the technical and economic perspectives according to the needs of production practice. The following describes the technical performance indicators for evaluating the AC speed regulating system:
Speed regulation efficiency
No matter what application, it is hoped that the higher the speed regulation efficiency is, the better. Especially for the speed regulation adopted for energy saving, the speed regulation efficiency is more strict.The efficiency of speed control system should be divided intoAdjustable speed motorThe efficiency of itself and the efficiency of the speed control device are two parts. The efficiency of the speed control device, which is usually composed of power semiconductors, is more than 95%. Therefore, the efficiency of the system focuses on the asynchronous motor.
Speed regulation smoothness
Within the speed regulation range, the difference between the speeds of two adjacent gears is taken as a sign. The smaller the difference, the smoother the speed regulation will be.Speed regulation smoothness is an indicator of the accuracy of speed that can be obtained by the system, which is usually measured by stepless and stepless.Stepped speed regulation is stepped, and the speed regulation is discontinuous;The stepless speed regulation is linear. Within the speed regulation range, the speed points are continuous. Most production practices require smooth speed regulation, which can meet the needs of various production conditions.
Speed regulation range
The speed range is defined as the ratio of the highest speed to the lowest speed, but it can also be defined in reverse.The speed range should be scientifically determined according to the actual production needs, and it is not necessary to blindly pursue too large a range, because expanding the speed range usually requires technical and economic costs.At the same time, the speed regulation range is also restricted by the speed regulation method. Some speed regulation methods, such as the speed regulation method of changing the number of magnetic poles or the speed regulation method of slip power consumption, can not expand the speed regulation range greatly in any case.The speed regulation range (theoretically achievable) mentioned here is the larger the better.
power factor
The power factor of speed control system includes the power factor of asynchronous motor and speed control system, and the power factor is expected to be close to 1.
aboutasynchronous motor, when the speed drops, the output mechanical power Pd=(1-s)PmDecrease, input active power PoneThe power factor of asynchronous motor at this time depends on the excitation reactive power QoneWhether to reduce, if QoneIf the power factor of speed regulation is unchanged, the power factor of speed regulation must be reduced. If QoneIt will also be reduced, and the power factor will be improved.
The power factor of the speed regulation system is mainly related to the structure of the power frequency converter (main frequency converter circuit) and the control mode of the control circuit.If thyristor frequency converter is used for power frequency converter (main frequency converter circuit), the power factor is low;If SPWM frequency converter is used, the power factor is high.
harmonic content
In speed control systemPower electronic deviceAre all distortedNonlinear circuitTherefore, it is inevitable that the speed control system will generate current harmonics.Since harmonics will have adverse effects on the motor and power supply, it is required that the harmonics of the speed regulating system should be small.
Operating characteristics of speed regulation
The working characteristics of speed regulation have two aspects: static characteristics and dynamic characteristics.Static characteristics mainly reflect the hardness of mechanical characteristics during speed regulation.For most loads, the harder the mechanical characteristics, the smaller the speed change when the load changes, and the more stable the operation.So I hope the harder the mechanical properties are, the better.The dynamic characteristics are the characteristics shown in the transient process. The main indicators include two aspects: first, whether the speed increase (including starting) and speed decrease (including braking) process is fast and stable; second, whether the speed of the system can recover quickly when the load suddenly increases, decreases or the voltage suddenly changes.
The above performance indicators are too scientific, rigorous and professional. For most users and production technicians aiming at energy conservation, simple and popular evaluation standards will be more practical. Therefore, the above contents can be condensed into the following three characteristics: ① energy conservation.Energy saving mainly assesses the efficiency of the speed governing system, with an average of no less than 85%. ②Reliability.The requirements of high reliability are low failure rate and strong overload capacity of motor and control device. ③Economy.High economy is characterized by relatively low price, small maintenance cost and short return period of investment.[4]
