The frequency converter is a power control device that controls the AC motor by changing the working frequency of the motor by applying the frequency conversion technology and the microelectronic technology. The frequency converter is mainly composed of rectification (AC to DC), filtering, inverter (DC to AC), braking unit, drive unit, and detection unit micro processing unit. The inverter adjusts the voltage and frequency of the output power supply by the internal IGBT breaking, and supplies the required power supply voltage according to the actual needs of the motor, thereby achieving the purpose of energy saving and speed regulation. In addition, the frequency converter has many protection functions. Such as overcurrent, overvoltage, overload protection and so on. With the continuous improvement of industrial automation, inverters have also been widely used.
How the frequency converter worksOverview
The main circuit is a power conversion part that supplies a voltage-regulating and frequency-modulated power supply to an asynchronous motor. The main circuit of the frequency converter can be roughly divided into two types.
The voltage type is a frequency converter that converts the direct current of the voltage source into an alternating current, and the filtering of the direct current circuit is a capacitor. The current type is a frequency converter that converts the direct current of the current source into an alternating current, and the direct current loop filtering is an inductance. It consists of three parts, which converts the power frequency power supply into a "rectifier" of DC power, and absorbs the "pulse-wave loop" generated by the voltage ripple generated by the converter and the inverter.
Rectifier
A large number of diode converters are used, which convert the commercial power supply into a DC power supply. Two sets of transistor converters can also be used to form the inverter, and the regenerative operation can be performed because the power direction is reversible.
Flat wave loop
The DC voltage rectified by the rectifier contains a ripple voltage of 6 times the frequency of the power supply, and the ripple current generated by the inverter also causes the DC voltage to fluctuate. In order to suppress voltage fluctuations, the ripple voltage (current) is absorbed by the inductor and the capacitor. When the device capacity is small, if the power supply and the main circuit constitute a device with a margin, the inductor can be omitted and a simple smoothing circuit can be used.
Inverter
In contrast to the rectifier, the inverter converts the DC power to the AC power of the required frequency, and the three switching devices are turned on and off at the determined time to obtain the 3-phase AC output. The switching time and voltage waveforms are shown by taking a voltage type pwm inverter as an example.
The control circuit is a circuit that provides a control signal to the main circuit of the asynchronous motor (voltage and frequency adjustable). It has an "arithmetic circuit" of frequency and voltage, a "voltage and current detecting circuit" of the main circuit, and a "speed detecting" of the motor. The circuit" is composed of a "drive circuit" that amplifies the control signal of the arithmetic circuit, and a "protection circuit" of the inverter and the motor.
1. Operation circuit: The external speed, torque and other commands are compared 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.
3. Drive circuit: A circuit that drives a main circuit device. It is isolated from the control circuit to turn the main circuit device on and off.
4. Speed ​​detection circuit: The signal of the speed detector (tg, plg, etc.) mounted on the asynchronous motor shaft machine is the speed signal, and is sent to the calculation circuit. According to the command and operation, the motor can be operated at the command speed.
5, protection circuit: detecting the voltage, current, etc. of the main circuit, in case of abnormalities such as overload or overvoltage, in order to prevent damage to the inverter and asynchronous motor
Nine problems when using the inverter1. The signal line and control line should be shielded, which is good for preventing interference. When the line is long, for example, the distance jumps 100m, the wire cross section should be enlarged. Do not place the signal wires and control wires in the same cable trench or bridge as the power cables to avoid mutual interference. It is best to place them in a tube. This is more appropriate.
2. The transmission signal is mainly based on the selection of the current signal, because the current signal is not easy to be attenuated and is not susceptible to interference. In practical applications, the signal output by the sensor is a voltage signal, and the voltage signal can be converted into a current signal by the converter.
3. The closed-loop control of the inverter is generally positive, that is, the input signal is large and the output is also large (for example, when the central air-conditioning is working and the general pressure, flow, temperature, etc. are controlled). However, it is also counterproductive, that is, the input signal is large, and the output is relatively small (for example, the central air conditioner is used for heating and the heating water pump of the heating station). Closed loop control is shown in Figure 1.
4. When the pressure signal can be selected during closed-loop control, do not select the flow signal. This is because the pressure signal sensor is low in price, easy to install, small in workload, and easy to debug. However, if the process has flow ratio requirements and the requirements are accurate, then the flow controller must be selected, and the appropriate flow meter (such as electromagnetic type and target type) should be selected according to the actual pressure, flow, temperature, medium, speed, etc. , vortex street, orifice plate, etc.).
5. The built-in PLC and PID functions of the inverter are suitable for systems with small signal fluctuation and stable. However, since the built-in PLC and PID functions only adjust the time constant during operation, it is difficult to obtain a satisfactory over-process requirement, and the debugging is time consuming.
In addition, this adjustment is not intelligent, so it is not often used, but an external intelligent PID regulator is used. For example, Japan Fuji PXD series, Xiamen Anton, etc., is very convenient. When using SV (upper limit value), there is PV (running value) indication during operation, and it is intelligent, ensuring the best transition process conditions, and it is ideal to use. Regarding PLC, according to the nature of control quantity, number of points, digital quantity, analog quantity, signal processing and other requirements, various brands of external PLC, such as Siemens S7-400, S7-300, S7-200, etc., are selected.
6. The signal converter is also frequently used in the peripheral circuit of the inverter, and is generally composed of a Hall element plus an electronic circuit. According to signal transformation and processing, it can be divided into voltage variable current, current variable voltage, DC to AC, AC to DC, voltage to variable frequency, current to variable frequency, one input and multiple output, multiple input and output, signal superposition, signal splitting, etc. Various converters. For example, the Shenzhen St. CE-T series of power isolation sensors / transmitters, the application is very convenient. There are quite a few similar products in China, and users can choose their own applications as needed.
7. Inverter often needs to be equipped with peripheral circuits when it is applied. The methods are often:
(1) A logic function circuit composed of control elements such as self-made relays;
(2) buy off-the-shelf unit external circuits (such as Mitsubishi Corporation of Japan);
(3) Select simple programmable controller LOGO (both foreign and domestic);
(4) When using different functions of the inverter, a function card (such as Japan Sancha inverter) can be selected;
(5) Select small and medium programmable controllers.
8. There are two common types of frequency conversion technology transformation schemes for multiple pumps connected in parallel with constant pressure water supply (for example, clean water pumps, medium and large water pump stations, hot water supply center stations, etc.).
According to the experience of use, the scheme (1) saves the initial investment, but the energy saving effect is poor. Start the inverter to 50Hz after starting, then start the power frequency, and then switch to energy-saving control. In the water supply system, only the water pump driven by the frequency converter is used, the pressure is slightly smaller, and the system has turbulence and loss.
The scheme (2) has a large investment, but it is 20% more energy-efficient than the scheme (1). The pressure of the pump is the same, and there is no turbulence loss, and the effect is better.
9. When multiple pumps are connected in parallel with constant pressure water supply, only one sensor is used in the signal series mode, and the advantages are as follows.
(1) Cost savings. Just a set of sensors and PIDs, as shown in Figure 4.
(2) Since there is only one control signal, the output frequency is the same, that is, the same frequency, so the pressure is also consistent, and there is no turbulence loss.
(3) During constant pressure water supply, when the flow rate changes, the number of pumps that are started is changed by PLC control. At least one set at a time, two at a moderate amount, and three at a larger amount. When the inverter is not working, the circuit (current) signal is in the path (the signal flows in, there is no output voltage, frequency).
(4) More beneficially, because the system has only one control signal, even if the three pumps are different, the operating frequency is the same (ie, synchronous), and the pressure is also the same, so that the turbulence loss is zero, that is, the loss is the smallest, so the power is saved. The best results.
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