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Inverter control cabinet and the selection of applications

1, introduction

    stepless speed regulation of frequency conversion technology is AC motor born out of need. In the 1960 of the 20th century yihou, power electronic devices experience has SCR (Crystal gate tube), and GTO (door very can shut broken Crystal gate tube), and BJT (double very type power transistor), and MOSFET (metal oxide field effect tube), and SIT (induction transistor), and SITH (induction Crystal gate tube), and MGT (MOS control transistor), and MCT (MOS control Crystal gate tube), and IGBT (insulation gate double very type transistor), and HVIGBT (high voltage insulated-gate bipolar transistor) development process, device update promoting the development of power electronics technology. In the 1970 of the 20th century, the pulse width modulation variable voltage variable frequency (PWM-VVVF) speed have attracted great attention. In the 1980 of the 20th century, as the core PWM mode optimization of frequency conversion technology to attract people's interest and draw a lot of optimization models, with saddle-shaped wave PWM mode works best. In the 1980 of the 20th century began in the second half, the United States, Japan, Germany, Britain and other developed countries of the VVVF inverter application has been put into market and access to a wide range.

    2, frequency converter control modes

    low pressure of general frequency converter output voltage is 380~650V, 0.75~400kW output power, frequency of 0~400Hz, its main circuit using cross ª² ª² pay Lu. Its control mode has gone through four generations.

    2.1, U/f=C sine pulse width modulation (SPWM) control

    characterized by a control circuit of simple structure, low cost, good mechanical properties hardness, to meet the General smooth speed transmission, has found wide application in various areas of industry. However, this form of control frequency, due to the lower output voltages, torque the influence of stator resistance pressure drop significantly, reduce the maximum torque output. In addition, no DC hard after all its mechanical characteristics, static and dynamic torque capacity speed performance is still not satisfactory, and the performance is not high, control curve will change as changes in load, slow speed and motor torque torque utilization is not high, low speed due to stator resistance and inverter dead time effects the existence of performance degradation, poor stability. So people developed vector control variable frequency adjusting speed.

    2.2, vector control (VC)

    vector control variable frequency adjusting speed to put in three-phase asynchronous motor stator current coordinates Ia, Ib, Ic, through three-phase-phase transformation, equivalent to AC current Ia1Ib1 under the phase coordinate, again based on rotor field-oriented rotation transformation Equivalent to DC current of synchronous rotating reference frame Im1, It1 (Im1 is equivalent to DC motor field current It1; equivalent to the armature current is proportional to the torque), and imitation of DC motor control method, obtained control of DC motors, via the corresponding coordinates of the inverse transform, realize the control of asynchronous motors. Its essence is the equivalent AC motors DC motors, respectively, for speed, independent control of two components of the magnetic field. By controlling the flux and decomposition of the stator current and obtain the torque and the two components of the magnetic field, coordinate transformations, quadrature or decoupling control. Vector control method is of epoch-making significance. In practice, however, as the rotor flux linkage is difficult to make accurate observations, greatly influenced by parameters of the motor system characteristics, and in the equivalent DC motor control using vector rotation transformation complex, make practical analysis of control effect to achieve the desired results.

    2.3, voltage space vector modulation (SVPWM) control

    It was premised on three-phase Waveform generated overall effect, approximating the ideal circular rotating magnetic field of the motor Air gaps path for the purpose, generates three-phase waveform, polygonal approximation approach to control. After practice and improvements, the introduction of frequency compensation, speed control can eliminate errors; feedback estimated flux magnitude, eliminate the influence of stator resistance speed; closed-loop output voltage and current in order to improve the accuracy and stability of. But more control circuit without introducing regulation of torque, so the system performance has not been fundamentally improved.

    2.4, direct torque control (DTC)

    in 1985, Germany Ruhr DePenbrock, Professor at the University of direct torque control technology for the first time. This technology to a large extent the lack of vector control and novel control system of thinking, concise structure, excellent dynamic and static performance has been developing rapidly. At present, the technology has been successfully used in high power AC drive electric locomotive traction on.

    analysis of direct torque control directly on the stator AC motor model, control the flux and torque of the motor. It does not require equivalent AC motors DC motors, thus eliminating the rotation transformation complex vector calculation; it does not need to imitate DC motor control, do not need AC motor simplified model for decoupling.

    2.5, the matrix AC-AC control

    VVVF inverter, vector control inverter, direct torque control frequency is one of AC-DC-AC frequency converter. The common drawback is the low power factor, harmonic current, DC needs a large capacitor, renewable energies cannot be fed back into the grid, that is, four-quadrant operation is not possible. To this end, the matrix AC-AC frequency conversion came into being. Because the matrix AC-AC frequency eliminates the DC link, thus eliminating the need for bulky, expensive electrolytic capacitors. It can achieve power factor l, input current is sinusoidal, and four quadrants, the power density of the system. Although the technology is not yet mature, but still attracts scholars studied. Substance not indirectly control the current, flux equivalent, instead of direct torque control to true. The specific method is:

   --controlled introduction of stator flux of stator flux observer without speed sensors;
   --automatic identification (ID) relies on precise mathematical models of electric machines, automatic identification of motor parameters;
   --to calculate the actual values correspond to stator impedance, inductance, magnetic saturation, Inertia calculate the actual torque, stator flux and the rotor speed control in real time; Band-Band control
   --flux and Band-Band of torque control to generate the PWM signal, control of inverter switching States.

    matrix AC-AC frequency converter with fast torque response (<2ms), the very high rate of accuracy (± 2%, no PG feedback), high torque accuracy (<+3%) and also have a high starting torque and high torque accuracy, especially at low speeds (including 0 speed), you can 150%~200% output torque.

    3, Inverter control mode selection

    control are inverter performance of key. Many low voltage inverter brands currently on the market, including Europe, the United States, Japan and China about more than 50 species. When choosing inverters do not think the level the higher the better, but according to the load characteristics to meet the requirements for the associate, to use, and affordable. Parameters listed in table 1 of reference for the selection.

    4, torque control Inverter selection and related problems

    based on speed, energy saving, reliable operation of the advantages of frequency converter has gradually replaced traditional pole-changing adjustable-speed, electromagnetic speed regulation and variable voltage speed control mode. Launched the flux vector control inverter PWM a few years later, in late 1998 and appeared with DTC control technology of inverters. ACS600 series is the first generation of uses the DTC of ABB company technology inverter, it could open the way to precisely control the speed and torque, and static and dynamic indicators is better than closed-loop PWM control indicators.

    direct torque control to measure the motor current and DC voltage as input to the model of Adaptive motor.

the model every 25 μ s produces a set of precise torque and flux actuals torque magnetic flux comparator comparator and the torque and the actual value of magnetic flux and torque and flux compares the given value to determine the optimum switch position. From this we can see that it is based on the measurement of torque and flux, instant adjustment of inverter switching States, thereby adjusting the motor's torque and flux in order to achieve the aim of precise control.
    4.1, selection principles of

    first of all depends on the mechanical speed (maximum and minimum) and torque (starting, overload and continuous) requirement, determine the maximum input power of mechanical requirements (that is, the nominal power of the motor minimum). Empirical formula

    P=nT/9950 (kW)-
   : P--mechanical requirements power input (kW);
    n--mechanical rotational speed (r/min),
    T--machine maximum torque (N • m).

    then choose the number of poles of the motor and the power rating. Motor pole number synchronous speed, requiring motor synchronous speed as much as possible to cover the entire speed range, high continuous load capacity. In order to use their full potential and avoid waste, short-time exceeds permissible motor synchronous speed, but must be less than the motor the maximum allowed speed. Torque equipment start-up and continuous operation, overload or high speed conditions of maximum torque. Finally, according to the inverter output power and slightly larger than the current rating of motor power and principles to determine the parameters of the inverter rated current model.

    is in need of attention, the rated capacity and parameters of the inverter is aimed at a certain altitude and marked by ambient temperature, generally refers to 1000m above sea level, temperature is 40 c or 25 deg. If use of the environment beyond the provisions in determining the inverter parameters, the model takes into account environmental capacity-reducing factor.

    4.2, and frequency device of external configuration and the should note of problem

    1) select right of external fuse device, to avoid for internal short-circuit on rectifier devices of damaged frequency device of model determine Hou, if frequency device internal rectifier circuit Qian no protection silicon devices of fast fuse device, frequency device and power Zhijian should configuration meet requirements of fuse device and isolation switch, Cannot use air circuit breakers replace fuses and disconnect switch.

    2) on the input side-mounted AC reactor or EMC filters according to the converter the installation site's other equipment on power quality requirements, if the inverter has affected the normal operation of the equipment, can be installed at the inverter input side AC reactor or EMC filters, suppression of electromagnetic interference caused by power switching. If connected to the inverters power network neutral point grounding, you cannot choose the EMC filter. When the voltage of 500V or more when the drive motor for frequency converter, need to configure du/DT at the output side filter, to inhibit the inverter output voltage spikes and voltage changes, to protect the motor, while also reducing the capacitive high frequency radiation leakage and motor cable and motor and loss at high frequency bearing currents. Note to use the du/DT filter filter on the slightly lower voltage as long as this over-production caused by motor torque; between the inverter and filter cable length must not exceed 3M.

    3) select converter introduction and selection according to the inverter's power led cable conductor cross section suitable for three-core or quad-core shielded power cable. Especially from the inverter to the motor power cables must be shielded structure between cable and as short as possible, so as to reduce electromagnetic and capacitive leakage currents. When the cable is longer than the inverter when the output cable length allowed by, stray capacitance of the cable will affect the normal operation of the inverter, and to configure the output inductor. Control cables, especially I/0 signal cable shielding structure. Inverter for peripheral components connecting cable between inverter and its length shall not exceed 10m.

    5, conclusion

    selection of transducer is a need for serious work, low-voltage inverter on the market many varieties and specifications, selection should be based on the actual load characteristics to meet the requirements for the associate in order to use, and affordable.

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