Nowadays, the popular asynchronous motor speed control methods can be divided into two kinds: frequency control and variable voltage speed control, in which the frequency control of asynchronous motor is applied more, and its speed control methods can be divided into two kinds: variable frequency and variable voltage speed control and vector control method, and the control method of the former one is relatively simple, with more than twenty years' development experience. Therefore, the application of more, most of the frequency converters sold on the market are currently using this control method.
Keywords: AC speed control system, asynchronous motor, PWM technology .....
Table of Contents
Excerpt 1
Preface 3
1.1 Purpose and significance of the design 3
1.2 Energy-saving principle of frequency converter speed regulation and operation 3
Chapter 2 Frequency Converter 4
2.1 Selection of frequency converter: 4
2.2 Design of frequency converter control schematic diagram: 4
2.3 Inverter control cabinet design 6
2.4 Inverter wiring specifications 7
2.5 The operation of the inverter and the setting of the relevant parameters 8
2.6 Analysis of common failures 8
Chapter 3 Overview of the AC speed control system 10
3.1 Characteristics of the AC speed control system 10
Chapter 4 Frequency conversion motor Characteristics 14
4.1 Electromagnetic Design 14
4.2 Structural Design 14
Chapter 5 Main Characteristics of Inverter Motors and Principles of Construction of Inverter Motors 15
5.1 Specialized Inverter Motors Have the Following Characteristics 15
5.2 Principles of Construction of Inverter Motors 15
Chapter 6 Alternating-Current Induction Motors 16
6.1 Basic Principles of Frequency Regulation of AC Asynchronous Motors 16
6.2 Mechanical Characteristics of Motors during Variable Frequency Voltage Varying Frequency (VVVF) Regulation 18
6.3 Mechanical Characteristics of the Fractional Discipline during Variable Frequency Varying Frequency (VVVF) Operation 19
CHAPTER 7 PRINCIPLES OF PWM TECHNOLOGY 24
7.1 Sinusoidal Wave Pulse Width Modulation (SPWM) 25
7.2 Unipolar SPWM method ................................................................................................................. .26
Conclusion 31
Acknowledgments 32
References 33
Preface
1.1 Purpose and significance of the design
In recent years, with the rapid development of power electronics technology, computer technology, automatic control technology, AC transmission and control technology has become one of the most rapidly developing technologies. Electrical transmission technology is facing a historical revolution, that is, AC speed control instead of DC speed control and computer digital control technology instead of analog control technology has become the development trend. Motor AC frequency conversion speed control technology is today's power saving, improve the process to improve product quality and improve the environment, to promote technological progress of a major means. Frequency conversion speed control with its excellent speed and braking performance, high efficiency, high power factor and power saving effect, a wide range of applications and many other advantages and is recognized at home and abroad as the most promising way of speed control. In-depth understanding of the direction of AC drive and control technology, has a very positive significance.
1.2 inverter speed operation of energy saving principle
The realization of frequency conversion speed control device called inverter. Frequency converter is generally composed of rectifier, filter, drive circuit, protection circuit and controller (MCU / DSP) and other parts. First of all, the single-phase or three-phase AC power supply through the rectifier and capacitor filtering, the formation of amplitude is basically a fixed DC voltage added to the inverter, the use of the inverter power components of the on-off control, so that the output of the inverter to obtain a certain shape of the rectangular pulse waveform. Here, by changing the width of the rectangular pulse to control its voltage amplitude; by changing the modulation period to control its output frequency, so that the output voltage and frequency control on the inverter at the same time, and to meet the requirements of frequency control on the coordinated control of U/f. The advantage of PWM is that it can eliminate or inhibit the low harmonics, so that the load motor in the nearly sinusoidal wave of the alternating voltage operation, the torque pulse is small, and a wide range of speed control.
The motor speed with PWM control is limited by the upper speed limit. For example, for the compressor, generally not more than 7000r / rain, and the use of PAM control mode of the compressor speed can be increased by about 1.5 times, which greatly improves the rapid growth and deceleration capacity. At the same time, due to the PAM in the adjustment of the voltage has a current waveform shaping effect, so you can get a higher efficiency than PWM. In addition, in terms of anti-interference also has PWM incomparable superiority, can inhibit the generation of high harmonics, reduce the pollution of the power grid. After adopting the control mode of frequency conversion speed control technology, the motor stator current decreased by 64%, the power supply frequency decreased by 30%, the glue pressure decreased by 57%. By the motor theory can be seen, the speed of asynchronous motor can be expressed as: n = 60?f 8 (1-8) / p
Chapter 2 Frequency converter
Frequency converter is the use of power semiconductor devices on and off the role of power frequency power supply will be converted to another frequency of the electrical energy control device. We are now using the frequency converter is mainly used in AC-DC-AC mode (VVVF frequency conversion or vector control frequency conversion), the first frequency AC power through the rectifier into DC power, and then convert the DC power into frequency, voltage can be controlled by the AC power supply to supply the motor. The circuit of inverter is generally composed of four parts: rectifier, intermediate DC link, inverter and control. The rectifier part is a three-phase bridge uncontrollable rectifier, the inverter part is an IGBT three-phase bridge inverter, and the output is a PWM waveform, and the intermediate DC link is filtering, DC energy storage and buffer reactive power.
2.1 Selection of frequency converter:
When selecting the frequency converter, the following points should be determined:
1) The purpose of using frequency conversion; constant voltage control or constant current control, etc..
2) The type of load of the frequency converter; such as vane pumps or positive displacement pumps, etc., pay special attention to the performance curve of the load, the performance curve determines the way and method of application.
3) Frequency converter and load matching problem;
I. Voltage matching; the rated voltage of the frequency converter and the rated voltage of the load match.
II. Current matching; for ordinary centrifugal pumps, the rated current of the inverter matches the rated current of the motor. For special loads such as deep water pumps need to refer to the motor performance parameters to determine the maximum current inverter current and overload capacity.
III. Torque matching; this situation is possible in constant torque loads or when there is a deceleration device.
4) When inverter is used to drive high speed motors, due to small reactance of high speed motors, increase in high harmonics leads to increase in output current value. Therefore, the selection of inverter for high-speed motors, its capacity should be slightly larger than the selection of ordinary motors.
5) If the frequency converter is to be run on long cables, measures should be taken to suppress the influence of the long cable coupling capacitance on the ground to avoid insufficient output of the frequency converter, so in this case, the capacity of the frequency converter should be enlarged by one step or output reactor should be installed on the output side of the frequency converter.
6) For some special applications, such as high temperature, high altitude, which will cause the inverter capacity reduction, the inverter capacity should be enlarged by one gear.
2.2 Inverter control schematic design:
1) First confirm the installation environment of the inverter;
I. Operating temperature. Frequency converter inside is a high-power electronic components, very susceptible to the influence of the operating temperature, the general requirements of the product is 0 ~ 55 ℃, but in order to ensure the safety and reliability of the work, the use of the use should be considered to leave a margin of error, it is best to control the temperature below 40 ℃. In the control box, the frequency converter should generally be installed in the upper part of the box, and strictly comply with the installation requirements of the product specification, absolutely do not allow the heat-generating components or easy to heat the components installed close to the bottom of the frequency converter.
II. Ambient temperature. When the temperature is too high and the temperature change is large, the inverter is prone to internal condensation phenomenon, its insulation performance will be greatly reduced, and may even cause a short-circuit accident. If necessary, desiccant and heater must be added to the box. In the water treatment room, the general water vapor is heavier, if the temperature changes, this problem will be more prominent.
III. Corrosive gases. Use of the environment if the concentration of corrosive gases is large, not only will corrode the component leads, printed circuit boards, etc., but also accelerate the aging of the plastic device, reducing the insulation properties.
IV. Vibration and shock. Control cabinets equipped with inverters are subject to mechanical vibration and shock, which can cause poor electrical contact. Huai'an thermoelectricity on such a problem. In addition to improving the mechanical strength of the control cabinet, away from the source of vibration and shock sources, should also use anti-vibration rubber pads to fix the control cabinet outside and inside the electromagnetic switches and other components that generate vibration. After a period of operation, the equipment should be inspected and maintained.
V. Electromagnetic interference. Frequency converter in the work due to rectification and frequency conversion, around the generation of a lot of interference electromagnetic waves, these high-frequency electromagnetic waves on the nearby instruments, instruments have a certain degree of interference. Therefore, the cabinet instrumentation and electronic systems, should be selected metal shell, shielding the frequency converter on the interference of the instrument. All components should be reliably grounded, in addition to the electrical components, instruments and meters between the connection should be selected shielded control cable, and the shield should be grounded. If the electromagnetic interference is not handled well, often the whole system can not work, resulting in control unit failure or damage.
2) The distance between the inverter and the motor to determine the cable and wiring method;
I. The distance between the inverter and the motor should be as short as possible. This reduces the capacitance of the cable to ground and reduces the source of interference emission.
II. Control cables use shielded cables, power cables use shielded cables or from the inverter to the motor all shielded with a penetrating pipe.
III. motor cable should be independent of other cable routing, the minimum distance of 500mm. at the same time, should avoid the motor cable and other cables long distance parallel routing, so as to reduce the frequency converter output voltage rapid changes in electromagnetic interference. If the control cable and power cable cross, they should be crossed at an angle of 90 degrees as far as possible. The analog signal lines related to the inverter and the main circuit line are routed separately, even in the control cabinet.
IV. The analog signal line related to the frequency converter should preferably use shielded twisted-pair cable, and the power cable should use shielded three-core cable (whose specification should be larger than that of the ordinary motor cable) or follow the user's manual of the frequency converter.
3) Inverter control schematic;
I. Main circuit: the role of the reactor is to prevent the frequency converter generated by the high harmonics through the power supply input circuit to return to the power grid, thereby affecting other powered equipment, need to be based on the size of the capacity of the inverter to decide whether to add the reactor; filter is installed in the output of the inverter, to reduce the output of the frequency converter of the high harmonics when the distance from the frequency converter to the motor is relatively large. The filter should be installed when the distance from the inverter to the motor is far. Although the inverter itself has a variety of protection functions, but the phase loss protection is not perfect, the circuit breaker in the main circuit plays a role in overload, phase loss and other protection, selection can be made in accordance with the capacity of the inverter. The overload protection of the inverter itself can be used to replace the thermal relay.
II. Control circuit: with frequency inverter manual switching, in order to frequency inverter failure can be manually cut frequency operation, because the output can not add voltage, solid frequency and frequency to have interlock.
4) Grounding of the inverter;
Correct grounding of the inverter is an important means of improving system stability and suppressing noise. The smaller the grounding resistance of the grounding terminal of the frequency converter, the better, and the cross-section of the grounding wire is not less than 4mm, and the length is not more than 5m. The grounding of the frequency converter should be separated from the grounding point of the power equipment, and can not be **** ground. One end of the shield of the signal line is connected to the grounding end of the inverter, and the other end is floating. The inverter and the control cabinet are electrically connected to each other.
2.3 Frequency converter control cabinet design
Frequency converter should be installed inside the control cabinet, the control cabinet should pay attention to the following issues in the design
1) Heat dissipation: the heat generation of the frequency converter is generated by the internal loss. In the frequency converter in each part of the loss is mainly in the main circuit, accounting for about 98%, the control circuit accounts for 2%. In order to ensure the normal and reliable operation of the frequency converter, the frequency converter must be dissipated we usually use fan cooling; frequency converter internal fan can be the frequency converter box internal heat dissipation away, if the fan can not work properly, you should immediately stop the frequency converter operation; high-power frequency converter also need to add a fan on the control cabinet, the control cabinet air ducts should be reasonably designed, all the inlet should be set up to prevent dust nets, exhaust smooth, avoid the formation of eddy currents in the cabinet, and avoid the formation of eddy currents in the fixed cabinet. cabinet to form eddy currents, the formation of dust accumulation in a fixed location; according to the inverter manual ventilation to select the matching fan, fan installation should pay attention to the problem of anti-vibration.
2) Electromagnetic interference problems:
I. Inverter in the work due to rectification and frequency conversion, around the generation of a lot of interference electromagnetic waves, these high-frequency electromagnetic waves on the nearby meters, instruments have a certain degree of interference, and will produce high harmonics, this high harmonics will be through the power supply circuit into the entire power supply network, thus affecting other instruments. If the power of the frequency converter is very large accounted for more than 25% of the whole system, you need to consider the control of the power supply anti-interference measures.
II. When the system has high-frequency impact loads such as welding machines, electroplating power supply, the inverter itself will be protected because of interference, then consider the power quality of the entire system.
3) Protection issues need to pay attention to the following:
I. Waterproof condensation: If the inverter is placed in the field, need to pay attention to the inverter cabinet above the pipe flange or other leakage points, in the vicinity of the inverter can not have splash water, in short, the field cabinet protection level should be above IP43.
II. Dustproof: all air inlets should be set up to block the dust net flocculent debris into the dust net should be designed as a removable type, in order to facilitate cleaning, maintenance. The grid of the dust net is determined according to the specific conditions of the site, and the combination of the dust net around the control cabinet should be handled tightly.
III. Anti-corrosive gases: In the chemical industry, this situation is more common, this time you can put the frequency converter cabinet in the control room.
2.4 Frequency converter wiring specification
Signal line and power line must be separated from each other: when using analog signals to remotely control the frequency converter, in order to reduce the analog interference from the frequency converter and other equipment, please separate the signal line of the frequency converter and the strong power circuit (the main circuit and the downstream control circuit). The distance should be more than 30cm. The same wiring specification should be maintained even inside the control cabinet. The maximum length of the control loop line between this signal and the frequency converter shall not exceed 50m.
The signal line and power line must be placed in different metal pipes or inside metal hoses: the signal line connecting PLC and frequency converter is extremely susceptible to interference by the frequency converter and external equipment if it is not placed in a metal pipe; at the same time, since the frequency converter does not have any built-in reactors, the input and output level of the frequency converter Power line to the outside will produce strong interference, so place the signal line of the metal pipe or metal hose has to be extended to the control terminals of the frequency converter, in order to ensure that the signal line and the power line of the complete separation.
1) analog control signal line should use double stranded shielded wire, wire specifications for the 0.75mm2. In the wiring must pay attention to, cable stripping should be as short as possible (5-7mm or so), and at the same time after the stripping of shielding layer should be wrapped with insulating tape to prevent the shielding line from contacting with other equipment to introduce interference.
2) In order to improve the simplicity and reliability of the wiring, it is recommended to use the crimp bar terminal on the signal line.
2.5 The operation of the frequency converter and the setting of the relevant parameters
The frequency converter has many setting parameters, each parameter has a certain selection range, the use of individual parameters are often encountered due to improper settings, resulting in the phenomenon of the frequency converter can not work properly.
Control mode: speed control, pitch control, PID control or other ways. After taking the control mode, generally according to the control accuracy, the need for static or dynamic identification.
Minimum operating frequency: that is, the minimum speed of motor operation, the motor runs at low speed, its heat dissipation performance is very poor, the motor runs for a long time at low speed, will lead to motor burnout. And at low speed, the current in its cable will also increase, which will also lead to cable heating.
Maximum operating frequency: the general frequency converter maximum frequency to 60Hz, some even to 400 Hz, high frequency will make the motor high-speed operation, which is for ordinary motors, its bearings can not be a long time to run over the rated speed, the motor's rotor can withstand such a centrifugal force.
Carrier frequency: the higher the carrier frequency is set, the higher the harmonic component is, which is closely related to the length of the cable, the heat of the motor, the heat of the cable, the heat of the frequency converter and other factors.
Motor parameters: the frequency converter in the parameters set motor power, current, voltage, speed, maximum frequency, these parameters can be obtained directly from the motor nameplate.
Frequency hopping: at a certain frequency point, there is a possibility of *** vibration phenomenon, especially when the whole device is relatively high; in the control of the compressor, to avoid the compressor's wheezing point.
2.6 Common Failure Analysis
1) overcurrent fault: overcurrent fault can be divided into acceleration, deceleration, constant speed overcurrent. It may be due to the frequency converter acceleration and deceleration time is too short, sudden load changes, uneven load distribution, output short circuit and other reasons. This can generally be done by extending the acceleration and deceleration time, reduce the sudden change of load, plus energy braking elements, load distribution design, check the line. If disconnecting the load inverter or overcurrent fault, indicating that the inverter inverter circuit has been ring, need to replace the inverter.
2) overload fault: overload faults include frequency conversion overload and motor overload. It may be acceleration time is too short, the grid voltage is too low, the load is too heavy and other reasons. Generally can be extended by extending the acceleration time, extend the braking time, check the grid voltage and so on. The load is too heavy, the selected motor and inverter can not drag the load, may also be caused by poor mechanical lubrication. Such as the former must be replaced with high-power motors and inverters; such as the latter to overhaul the production machinery.
3) undervoltage: the inverter power input part of the problem, need to check before you can run.
Chapter 3 Overview of AC Speed Control System
3.1 Characteristics of AC Speed Control System
For the adjustable speed of the power drag system, engineering is often divided into two categories: DC speed control system and AC speed control system. This is mainly based on the use of what current system type of motor for the conversion of electrical and mechanical energy and division, the so-called AC speed control system, is to AC motor as electrical energy - mechanical energy conversion device, and its control to produce the required speed.
Throughout the development of power drag, AC and DC two major speed control systems have been coexisting in various industrial fields, although the development of science and technology due to the various periods of time they are in a different position, but they have always been along with the development of industrial technology, especially with the development of power electronic components in competition with each other. In the past a long period of time, due to the DC motor's excellent speed performance, in the reversible, speed and high precision, wide speed range of power drag technology field, almost all are using DC speed control system. However, due to its mechanical commutator of the DC motor this fatal weakness, resulting in high manufacturing cost of DC motors, expensive, maintenance trouble, the use of environmental restrictions, its own structure also restricts the speed of a single motor, the power ceiling, thus bringing a series of limitations to the application of DC transmission. Relative to DC motors, AC motors, especially squirrel cage asynchronous motors have the advantages of simple structure, low manufacturing costs, robustness, reliable operation, easy maintenance, small inertia, good dynamic response, and easy to high-voltage, high-speed and high-power direction. Therefore, in recent decades, many countries are committed to the research of AC speed control system, with no commutator AC motor to achieve speed control to replace the DC motor to break through its limitations.
With the rapid development of power electronics, large-scale integrated circuits and computer control technology, as well as modern control theory to the field of AC electrical transmission penetration, for the development of AC speed control system to further create favorable conditions. Such as series speed control of AC motor, various types of frequency conversion speed control, especially the application of vector control technology, so that the AC speed control system gradually has a wide speed range, high accuracy of speed stabilization, fast dynamic response and reversible operation in four quadrants and other good technical performance. Now from hundreds of watts of servo systems to hundreds of kilowatts of ultra-high-power high-speed drive system, from the general requirements of a small range of speed drive to high precision, fast response, a wide range of speed drive, from single-machine drive to the coordinated operation of multiple machines, has been almost all can be used in the AC speed drive. The objective development trend of AC speed drive has shown that it can be completely comparable to DC drive, counterbalance, and have a tendency to replace.
3.2 AC speed control commonly used speed control program and its performance comparison
By the electrical engineering, AC asynchronous motor speed formula is as follows:
n= 60?1 (1-s) pn (1-1)
Pn - the number of pole pairs of the stator winding resistance of the motor;
f1 - motor stator voltage supply frequency;
s - motor slew rate.
From equation (1-1), it can be seen that there are three main types of schemes for regulating the speed of AC asynchronous motors.
(1) change the number of magnetic pole pairs of the motor
by the synchronous speed of the asynchronous motor
no= 60?1 pn
It can be seen that, under the condition that the frequency of the supply power supply f1 remains unchanged, the number of magnetic pole pairs of the asynchronous motor's stator windings Pn can be changed by reconnecting the stator winding connection to change the synchronous rotational speed n0, which can be changed to achieve speed regulation of asynchronous motor. thus achieving the purpose of speed regulation. This control method is relatively simple, only requires the motor stator winding has more than one tap, and then through the contacts of the magnetic pole pairs to change the motor. With this type of control, the motor speed change is graded, not continuous, generally up to three gears, suitable for automation is not high, and only need to have a level of speed occasions.
(2) Frequency control
From the formula (1-1) can be seen, when the asynchronous motor pole number Pn must be certain, the rate of rotation s - fixed, change the stator winding power supply frequency f1 can be achieved to speed the purpose of motor speed n is basically directly proportional to the frequency of the power supply f1, therefore, smoothly adjusting the frequency of the power supply, it is possible to achieve the purpose of speed control. Therefore, by smoothly adjusting the frequency of power supply, the speed of asynchronous motor can be smoothly and steplessly adjusted. Frequency control speed range is large, low-speed characteristics are hard, the fundamental frequency f = 50Hz or less, belongs to the constant torque speed control mode, above the fundamental frequency, belongs to the constant power speed control mode, and the DC motor is very similar to the reduced voltage and weak magnetic speed control. And the use of variable frequency starting can significantly improve the starting performance of AC motors, significantly reduce the starting current of the motor, increase the starting torque. Therefore, frequency conversion speed control is the ideal speed control program for AC motors.
(3) Variable speed control
There are many ways to change the speed control of the speed control of the differential rate, and the commonly used programs are: asynchronous motor stator voltage control, electromagnetic differential clutch speed control, and winding asynchronous motor rotor circuit series resistance speed control, series speed control, and so on.
The stator voltage regulation system is a constant AC power supply and AC motor access between the thyristor as the AC voltage controller, this voltage regulation system is only applicable to some of the short-time and repeated short-time for deep speed operation of the load. In order to get a good speed control accuracy and stable operation, generally use the control mode with speed negative feedback. The motor used can be a winding type asynchronous motor or a squirrel cage asynchronous motor with a high rate of rotation.
The electromagnetic differential clutch speed control system is a combination of squirrel cage asynchronous motor, electromagnetic differential clutch and control device. The squirrel cage motor as a prime mover to drive the electromagnetic clutch armature rotation at a constant speed, through the control of the electromagnetic clutch excitation current to realize the speed regulation of its magnetic poles. This system also generally adopts closed-loop speed control.
Wire-wound asynchronous motor rotor circuit series resistance speed control is to change the rotor circuit resistance to speed control, this speed control method is simple, but the speed control is graded, the series of large additional resistance, the motor's mechanical characteristics are very soft, low-speed operation loss, poor stability.
Winding asynchronous motor series speed control system is in the motor rotor circuit to introduce and rotor potential of the same frequency of the reverse potential Ef, as long as the change of this additional, with the motor rotor voltage of the same frequency of the reverse potential Ef, can be winding asynchronous motor smooth speed control. the larger the Ef, the lower the motor speed.
The *** same feature of these speed control is that there is no change in the synchronous speed of the motor n0 during the speed control process, so at low speeds, the slew rate s is larger.
In the AC asynchronous motor, from the stator into the rotor electromagnetic power PM can be divided into two parts: a part of the P2 = (1-s) PM is the effective power to drag the load, the other part is the rotational power PS = sPM, and the rotational rate of s is proportional to the direction of its go is the sign of the efficiency of the speed control system. In terms of the direction of the differential power, AC asynchronous motor speed control system can be divided into three kinds:
1) differential power consumption type
This speed control system, all the differential power is consumed, to increase the consumption of differential power in exchange for a reduction in speed, the rate of s increased, the differential power PS = sPM increased, in the form of heat consumed in the rotor circuit, so that the efficiency of the system has also been reduced. The efficiency of the system also decreases. Stator voltage regulation, electromagnetic differential clutch speed control and winding asynchronous motor rotor string resistance speed control of these three methods belong to this category, this type of speed control system exists in the wider the speed range, the greater the differential power PS, the lower the efficiency of the system, so it is not worth advocating.
2) Differential power feedback type
Most of the differential power of this speed control system is fed back to the grid through the converter or to be utilized, the lower the rotational speed of the power back to the more power, but the installation of additional devices to consume a portion of the power. Winding asynchronous motor rotor series speed control belongs to this category, it will rotate the differential power through the rectifier and inverter, through the transformer back to the AC power grid, but there is no form of energy consumption in the form of heat, even at low speeds, the efficiency of the series speed control system is also very high.
3) the rotor power does not change
This speed control system, the rotor power is still consumed in the rotor, but regardless of speed, the rotor power is basically unchanged. Such as variable pole logarithmic speed control, frequency control belongs to this category, because in the speed control process to change the synchronous speed n0, the rate of s is certain, so the system efficiency will not be reduced due to speed control. In the change of n0 in the two speed control program, and because of the variable pole logarithmic speed control for the pole speed control, and the number of poles is very limited, narrow range of speed control, so, at present in the AC speed control program, frequency control is the most ideal, the most promising AC speed control program.
Chapter 4 Characteristics of Inverter Motors
4.1 Electromagnetic Design
For ordinary asynchronous motors, the main performance parameters to be considered in re-design are overload capacity, starting performance, efficiency and power factor. And frequency conversion motor, because the critical transfer rate inversely proportional to the power supply frequency, can be in the critical transfer rate close to 1 when the direct start, therefore, overload capacity and start performance is not in need of too much consideration, and to solve the key problem is how to improve the motor on the non-sinusoidal power supply adaptability. The way is generally as follows: