Variable-frequency drive (VFD) is a power control device that controls the AC motor by changing the working frequency of the motor by applying variable frequency technology and 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.

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 and torque 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 the 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 sent to the calculation circuit as a speed signal, and the motor can be operated at the command speed according to the command and calculation.

(5) Protection circuit: Detect the voltage, current, etc. of the main circuit, in order to prevent damage to the inverter and asynchronous motor when an abnormality such as overload or overvoltage occurs.

function

Frequency conversion energy saving

The energy saving of the inverter is mainly reflected in the application of fans and pumps. In order to ensure the reliability of production, all kinds of production machinery have a certain margin when designing with power drive. When the motor can't run under full load, in addition to meeting the power drive requirements, the excess torque increases the consumption of active power, resulting in waste of electrical energy. Fans, pumps and other equipment traditional speed control method is to adjust the air volume and water supply by adjusting the baffle and valve opening of the inlet or outlet. The input power is large, and a large amount of energy is consumed in the interception process of the baffle and the valve. in. When using variable frequency speed regulation, if the flow demand is reduced, the speed can be met by reducing the speed of the pump or fan.

The role of the motor in the frequency converter is to speed up and reduce the starting current. In order to generate a variable voltage and frequency, the device first converts the alternating current of the power source to direct current (DC), a process called rectification. A device that converts direct current (DC) to alternating current (AC) is scientifically termed "inverter". The general inverter is an inverter power source that inverts the DC power supply to a certain fixed frequency and a certain voltage. For the inverter whose inverter is frequency adjustable and voltage adjustable, we call it the inverter. The waveform outputted by the inverter is an analog sine wave, which is mainly used for speed regulation of three-phase asynchronous motor, also called frequency converter. For variable frequency inverters with high waveform requirements mainly used in instrumentation detection equipment, the waveform should be sorted to output a standard sine wave, called a variable frequency power supply. The general variable frequency power supply is 15-20 times the price of the inverter. Since the main device that generates a varying voltage or frequency in the inverter device is called "inverter", the product itself is named "inverter", ie: the inverter.

Frequency conversion is not everywhere to save power, there are many occasions to use frequency conversion does not necessarily save power. As an electronic circuit, the inverter itself consumes power (about 3-5% of rated power). A 1.5-hp air conditioner has a power consumption of 20-30W, which is equivalent to a long-light lamp. It is a fact that the inverter runs at the power frequency and has a power-saving function. But his preconditions are:

First, high power and is a fan/pump type load;

Second, the device itself has a power saving function (software support);

This is the three conditions that reflect the power saving effect. Other than that, it doesn't matter if the festival doesn't save electricity. If you do not add preconditions to say that the inverter power frequency operation is energy-saving, it is exaggerated or commercial speculation. Know the original, you will use him skillfully for you. Be sure to pay attention to the use of the occasion and the conditions of use in order to apply correctly, otherwise it is blindly obedient, credulous and "deceived."

Power factor compensation

Reactive power not only increases the line loss and heat of the equipment, but more importantly, the power factor decreases, which leads to the reduction of the active power of the grid. A large amount of reactive energy is consumed in the line, the equipment is inefficient and wasteful, and the frequency converter is used. After that, due to the internal filter capacitor of the inverter, the reactive power loss is reduced, and the active power of the power grid is increased.

Soft start energy saving

1: The hard start of the motor has a serious impact on the power grid, and it also requires too much power grid capacity. The large current and vibration generated during startup have great damage to the baffle and the valve, and the service life of the equipment and pipeline is extremely high. unfavorable. After using the variable frequency energy-saving device, the soft start function of the inverter will start the starting current from zero, and the maximum value will not exceed the rated current, which will reduce the impact on the power grid and the power supply capacity, and prolong the use of equipment and valves. life. Saves on equipment maintenance costs.

2: In theory, the inverter can be used in all mechanical equipment with electric motor. When the motor starts, the current will be 5-6 times higher than the rated value, which will not only affect the service life of the motor but also consume more power. The system will have a certain margin on the motor selection during the design, the speed of the motor is fixed, but in actual use, sometimes it is to run at a lower or higher speed, so the frequency conversion transformation is Very necessary. The inverter can realize the soft start of the motor and compensate the power factor.

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