Relevant knowledge of the hottest frequency conver

2022-10-20
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Relevant knowledge of frequency converter

variable frequency drive (VFD) is a power control device that uses frequency conversion technology and microelectronics technology to control AC motors by changing the frequency mode of motor working power supply. The frequency converter is mainly composed of rectifier (AC to DC), filter, inverter (DC to AC), braking unit, driving unit, detection unit, microprocessor unit, etc. The frequency converter adjusts the voltage and frequency of the output power supply by switching off the internal IGBT, and provides the required power supply voltage according to the actual needs of the motor, so as to achieve the purpose of energy saving and speed regulation. In addition, the frequency converter has many protection functions, such as overcurrent, overvoltage, overload protection, etc. With the continuous improvement of industrial automation, frequency converters have also been widely used

2. Working principle of frequency converter

the main circuit is the power conversion part that provides voltage and frequency regulation power supply for asynchronous motor. The main circuit of frequency converter can be generally divided into two categories: voltage type is the frequency converter that converts the DC of voltage source into AC, and the filtering of DC circuit is capacitance. Current mode is a frequency converter that converts the DC of current source into AC, and its DC circuit filter is inductance. It consists of three parts, a "rectifier" that converts power frequency power supply into DC power, Absorbing voltage pulsations generated in converters and inverters "Smoothing circuit

a diode converter is widely used, which converts the power frequency power supply into DC power supply. Two groups of transistor converters can also be used to form a reversible converter, which can regenerate because its power direction is reversible.

smoothing circuit

in the DC voltage rectified by the rectifier, there is a pulsating voltage of 6 times the frequency of the power supply. In addition, the pulsating current generated by the inverter also changes the DC voltage. In order to To suppress voltage fluctuation, inductors and capacitors are used to absorb fluctuating voltage (current). The capacity of the device is small. Not long ago, if the components of the power supply and the main circuit have margin, the inductance can be omitted and a simple smoothing circuit can be used

in contrast to the rectifier, the inverter converts the DC power into the AC power of the required frequency, and the 3-phase AC output can be obtained by turning on and off the 6 switching devices in the determined time. Taking the voltage source PWM inverter as an example, the switching time and voltage waveform are shown

the control circuit is a circuit that provides control signals to the main circuit that supplies power to the asynchronous motor (voltage and frequency are adjustable). It consists of "operation circuit" of frequency and voltage, "voltage and current detection circuit" of the main circuit, "speed detection circuit" of the motor, "drive circuit" that amplifies the control signals of the operation circuit ", and" protection circuit "of the inverter and motor

(1) operation circuit: compare the external speed, torque and other commands with the current and voltage signals of the detection circuit to determine the output voltage and frequency of the inverter

(2) voltage and current detection circuit: it is isolated from the main circuit potential to detect voltage and current, etc

(3) drive circuit: the circuit that drives the main circuit device. It is isolated from the control circuit to make the main circuit devices turn on and off

(4) speed detection circuit: take the signal of the speed detector (TG, PLG, etc.) installed on the shaft machine of the asynchronous motor as the speed signal, send it to the operation circuit, and make the motor run at the commanded speed according to the command and operation

(5) protection circuit: detect the voltage, current, etc. of the main circuit. In case of overload or overvoltage and other abnormalities, in order to prevent the inverter and asynchronous motor from damage

3. The function of the frequency converter

frequency conversion energy saving: the energy saving of the frequency converter is mainly reflected in the application of fans and water pumps. In order to ensure the reliability of production, all kinds of production machinery have a certain margin when they are designed to be equipped with power drive. When the motor cannot run under full load, in addition to meeting the requirements of power drive, the excess torque increases the consumption of active power, resulting in a waste of electric energy. The traditional speed regulation method of fans, pumps and other equipment is to adjust the air and water supply volume by adjusting the opening of baffles and valves at the inlet or outlet. Its input power is large, and a large amount of energy is consumed in the closure process of baffles and valves. When using frequency conversion speed regulation, if the flow requirement is reduced, by reducing, when the contactor is checked, it is found that a group of normally closed contacts have been arc welded, the contactor coil is energized, and the normally closed contact cannot be disconnected. Finally, the real fault point is found, and the speed of the pump or fan can meet the requirements

the purpose of using frequency converter for motor is to regulate speed and reduce starting current. In order to produce variable voltage and frequency, the equipment must first convert the alternating current of the power supply into direct current (DC). This process is called rectification. The scientific term for a device that converts direct current (DC) into alternating current (AC) is "inverter". Generally, the inverter is an inverter that inverts the DC power supply into a certain fixed frequency and a certain voltage. The inverter with adjustable frequency and voltage is called frequency converter. The waveform output by the frequency converter is an analog sine wave, which is mainly used for speed regulation of three-phase asynchronous motor, also known as frequency converter. For the variable frequency inverter that is mainly used in the detection equipment of instruments and meters with high waveform requirements, the waveform should be sorted out, and the standard sine wave can be output, which is called variable frequency power supply. Generally, the frequency conversion power supply is 15-20 times the price of the frequency converter. Since the main device that produces variable voltage or frequency in frequency converter equipment is called "inverter", the product itself is named "inverter", that is, frequency converter

frequency conversion can not save power everywhere, and there are many field shared frequency conversion may not save power. As an electronic circuit, the frequency converter itself also consumes electricity (about% of the rated power). A 1.5-horse air conditioner also consumes W, which is equivalent to a continuous light It is a fact that the frequency converter operates at power frequency and has power saving function. But its preconditions are: first, high power and load for fans/pumps; Second, the device itself has power-saving function (software support); These are the three conditions that reflect the effect of power saving. Besides, it doesn't matter whether you save electricity or not, which makes no sense. It is exaggeration or commercial hype to say that the frequency converter can save energy without preconditions. Knowing the reason, you will skillfully use him to serve you. We must pay attention to the use occasions and conditions before we can correctly apply them. Otherwise, we will be blindly obedient, credulous and "deceived"

power factor compensation energy saving: reactive power not only increases line loss and equipment heating, but also the reduction of power factor leads to the reduction of electric active power. Among a large number of reactive electric energy consumption circuits, the equipment is inefficient and wasteful. After using the frequency conversion speed regulation device, due to the function of the internal filter capacitor of the frequency converter, the reactive power loss is reduced and the electric active power is increased

soft start energy saving: 1: the hard start of the motor will cause serious impact on the electricity, and will also have too high requirements on the capacitance. The large current and vibration generated during the start will cause great damage to the baffle and valve, and will be extremely detrimental to the service life of equipment and pipelines. After using the frequency conversion energy-saving device, the soft start function of the frequency converter will make the starting current start from zero, and the maximum value will not exceed the rated current, reducing the impact on electricity and the requirements for power supply capacity, and extending the service life of equipment and valves. The maintenance cost of the equipment is saved. 2: Theoretically, the frequency converter can be used in all mechanical equipment with motor. When the motor is started, the current will be times higher than the rated value, which will not only affect the service life of the motor, but also consume more power During the design of the system, a certain margin will be left in the motor selection, and the speed of the motor is fixed, but in the actual use process, sometimes it is necessary to run at a lower or higher speed, so it is very necessary to carry out frequency conversion transformation. The frequency converter can realize the soft start of the motor and compensate the power factor

4. The basic composition of the frequency converter

the frequency converter is usually divided into four parts: rectifier unit, high-capacity capacitor, inverter and controller

rectifier unit: convert AC with fixed working frequency into DC

high capacity capacitor: store the converted electric energy

inverter: an electronic switch composed of a high-power switching transistor array converts DC into square waves of different frequencies, widths and amplitudes

controller: work according to the set program, control the amplitude and pulse width of the output square wave, make the superposition of AC wave similar to sine wave, and drive the AC motor

5. Setting mode of frequency converter

the common frequency setting modes of frequency converter mainly include: setting of operator keyboard, setting of contact signal, setting of analog signal, setting of pulse signal and setting of communication mode. These frequency setting methods have their own advantages and disadvantages. They must be selected and set according to the actual needs

6. The control mode of the frequency converter

the low-voltage general frequency conversion output voltage is 380-650v, the output power is 0.75-400kw, and the working frequency is 0-400hz. Its main circuit adopts AC-DC-AC circuit. Its control mode has experienced the following four generations

The sinusoidal pulse width modulation (SPWM) control mode with

1u/f=c:

it is characterized by simple control circuit structure, low cost and good mechanical hardness. It can meet the smooth speed regulation requirements of general transmission, and has been widely used in various fields of the industry. However, at low frequency, due to the low output voltage, the torque is significantly affected by the stator resistance voltage drop, which reduces the maximum output torque. In addition, its mechanical characteristics are not as hard as DC motor after all, and its dynamic torque capacity and static speed regulation performance are not satisfactory. In addition, the system performance is not high, the control curve will change with the change of load, the torque response is slow, the motor torque utilization is not high, and the performance will decline and the stability will deteriorate due to the existence of stator resistance and inverter dead time effect at low speed. Therefore, people have developed vector control variable frequency speed regulation

voltage space vector (SVPWM) control mode:

it is based on the overall generation effect of three-phase waveform, with the purpose of approaching the ideal circular rotating magnetic field trajectory of the motor air gap, generating three-phase modulation waveform at one time, and controlling in the way of inscribed polygon approaching circle. After practical use, it has been improved, that is, the introduction of frequency compensation can eliminate the error of speed control; The amplitude of flux linkage is estimated by feedback to eliminate the influence of stator resistance at low speed; The output voltage and current are closed-loop to improve the dynamic accuracy and stability. However, there are many control circuits and no torque regulation is introduced, so the system performance has not been fundamentally improved

vector control (VC) mode:

the method of vector control variable frequency speed regulation is to equivalent the stator currents IA, IB, IC of asynchronous motor in the three-phase coordinate system to the AC current ia1ib1 in the two-phase static coordinate system through three-phase two-phase transformation, and then equivalent to the DC current IM1 in the synchronous rotating coordinate system through the directional rotation transformation according to the rotor magnetic field It1 (IM1 is equivalent to the excitation current of the DC motor; it1 is equivalent to the armature current proportional to the torque), and then imitate the control method of the DC motor to obtain the control quantity of the DC motor, and realize the control of the asynchronous motor through the corresponding inverse transformation of coordinates. Its essence is to equivalent AC motor to DC motor

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