What is VFD (inverter)?

VFD is an acronym for “Variable Frequency Drive”, commonly known as inverter. VFD can also be called alternating current regulator (AC drive), frequency regulator, speed regulator, speed regulator (VSD – variable speed drive), frequency converter (VFI – variable speed drive). frequency inverter),… Regardless of the name, a VFD is a type of motor controller that drives and controls electric motors. The VFD controls the speed and torque of the motor to meet the requirements of the application by varying the frequency and voltage source. Frequency is directly linked to the engine RPM; The higher the frequency, the greater the number of RPMs.

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VFD inverter structure

The two main features of the inverter are adjustable speed and soft start/stop capability. These two features make the VFD a powerful controller for controlling AC motors. VFD is mainly composed of four parts: rectifier, intermediate DC link (DC Bus), inverter and control circuit.


It converts AC power supplied from mains power into DC power. This part can be unidirectional or bidirectional based on the application used such as four-quarter motor operation. It uses diodes, SCRs, transistors or other electronic switches.

If it uses diodes, the switching output DC power is not controllable while using the SCR, the DC output will change when driving the gate. A minimum of six diodes is required for three-phase conversion, so the rectifier is considered a six-pulse converter.

DC Bus

The DC power from the rectifier is fed into the DC link (DC Bus). This section includes capacitors and inductors to smooth out ripples and store DC power. The main function of a DC link is to receive, store, and supply DC power.


This section (inverter – inverter – inverter) covers electronic switching devices such as transistors, thyristors, IGBTs, etc. It receives DC power from the DC Bus and converts it to AC and then feeds it to the motor. It uses modulation techniques such as pulse width modulation to vary the output frequency to control the speed of the motor.

Control circuit

It consists of a microprocessor and performs various functions such as control, configuration settings, fault alarms and setting of communication protocols. It receives the feedback signal from the motor as the current speed reference and accordingly adjusts the voltage to frequency ratio (V/Hz) to control the motor speed.

Operating principle of inverter

The first stage AC inverter (VFD) acts as a converter (rectifier). The converter consists of six diodes, similar to the one-way valve used in plumbing. They allow current to flow in only one direction; direction is shown as the arrow in the diode symbol. For example, whenever the phase A voltage (a voltage similar to that in plumbing) is more positive than the phase B or C voltage, that diode will open and allow current to flow. When phase B becomes more active than phase A, phase B diode will open and phase A diode will close. The same is true for the 3 diodes on the negative positive side of the Bus. Thus, getting six “pulses” of current as each diode opens and closes. This is called “six-pulse VFD”, which is the standard configuration for current inverters.

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Assume that the inverter is operating on a 480V electrical system. The 480V rating is “rms” (root mean squared). The peaks on a 480V system are 679V. As you can see, the VFD bus has a DC voltage with AC ripple. Voltage runs between 580V and 680V.

We can eliminate AC ripple on the DC bus by adding a capacitor. Capacitors work in a similar fashion to a reservoir or accumulator in plumbing. This capacitor absorbs AC ripple and provides a flat DC voltage. The AC ripple on a DC bus is usually less than 3 Volts. Thus, the voltage on the DC bus becomes “approximately” 650VDC. The actual voltage will depend on the voltage level of the AC line supplied to the inverter, the degree of voltage imbalance on the electrical system, the motor load, the impedance of the electrical system and any reactors or wave filters. harmonics on the inverter.

AC-to-DC converter diode bridge (rectifier). A converter that converts DC to AC (the inverter), however to distinguish it from a diode converter, is often referred to as an “inverter”. It has become common in the industry to treat any DC-to-AC converter as an inverter or AC drive.

When we close one of the top switches in the converter, that phase of the motor is connected to the positive dc bus and the voltage across that phase becomes positive. When we close one of the bottom switches in the converter, that phase is connected to the negative DC bus and becomes negative. Thus we can make any phase on the motor as positive or negative as we want and thus can generate any frequency we want. So we can make any phase anode, cathode or zero.

Note that the output from the VFD is a “shaped” waveform Japan”. The VFD does not produce a sinusoidal output. This rectangular waveform will not be a good choice for general control systems, but is perfectly suitable for motors.

If we want to reduce the motor frequency to 30 Hz, then we just need to switch the output transistors of the inverter to work slower. But, if we reduce the frequency to 30 Hz, then we also have to reduce the voltage to 240V to maintain the V/Hz ratio (see the presentation of Motor Control Theory by VFD for more on this) . How will we drop the voltage if the only voltage we have is 650VDC?

This is called pulse width modulation (PWM). Imagine that we can control the pressure in the water line by turning the valve on and off at high speed. While this would not be practical for plumbing systems, it is nevertheless very doable with VFDs. Notice that during the first half cycle, the voltage is ON half the time and OFF half the time. Thus, the average voltage is half of 480V or 240V. By pulsed the output we can achieve any average voltage across the output of the VFD.

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Advantages of using inverter

1- Reduce energy consumption and energy costs

In cases where the application does not need to run at full speed, you can cut energy costs by controlling the motor with a VFD, allowing you to adjust the speed of the motor driver to the load requirements. . Electric motor systems consume more than 65% of the electricity consumed in industry today. Optimizing the motor control system with a VFD can reduce energy consumption in the facility by up to 70%. In addition, using VFDs can improve product quality and reduce production costs. These benefits, when combined, provide a return on investment for VFDs.

2- Soft and efficient motor control

Operating your engine at the most efficient speed for your application can reduce errors, thereby increasing production and increasing revenue. For example, smoother operation of VFDs allows conveyors and belts to eliminate kick-starting, allowing for higher throughput.

3- Extend equipment life and reduce maintenance

Ensuring optimal engine control will keep equipment running longer and reduce downtime due to maintenance. As VFD optimizes motor voltage and frequency control, VFD provides better protection for your motor from problems like thermoelectric overload, phase protection, under voltage, over pressure, etc. Starting the load with a VFD protects the motor or motor load from “instant shock” on the starting line; Smooth start eliminates heavy wear on belts, gears and bearings. And the added benefit is reducing and/or eliminating water hammer through smooth acceleration and deceleration cycles.

Inverter Classification (VFD)


The VFD market segment is divided into five size ranges defined by the capacity (KW or HP) of the VFD. While the global VFD market research covers micro, small and mid range VFDs, the high power VFD market research covers mid range, large and super large VFDs.

Many VFDs have a very wide operating power range. For example, some manufacturers offer a variation of the same VFD model that operates from range to high. Manufacturers can offer a wide range of products divided into several segments. Since such models can span different kilowatt segments, we classified these VFDs into their most appropriate segment to avoid repeatability.

Size KW HP Super small 600 > 750

Control mode

Volts/Hertz (Hz), the traditional motor control mode, prevents saturation of the motor’s magnetic field by maintaining a constant V/Hz ratio. At low speeds, the low voltage provides less magnetizing current, resulting in a loss of torque. Another limitation is that the V/Hz control is not correct with respect to the variation of slip between the rotor and stator with frequency and load. Suppliers of electromechanical transmission products with inverters typically supply gears and belts to meet low-speed torque requirements.

The control modes that overcome the limitations of V/Hz control are closed-loop flux vector control and open-loop sensorless vector control. With the former, a sensor, such as an encoder on a motor shaft, provides a slip gauge to control speed. With the latter, a built-in current sensor (no external sensor) provides information for control. In either case, the VFD compensates for the magnetization and slip current needs based on an internal software model of the motor characteristics, but the feedback is from an external encoder or a built-in current sensor. These modes require adjustment for effects, such as time delay of highly inertial loads. Auto-tuning VFDs address this requirement without manual intervention.

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Input voltage

The input voltage is the distinguishing feature of a VFD because the power supplied varies considerably with voltage. Therefore, the classification of VFDs according to The voltages in this study look at the main ranges: 115V to 240V, 380V to 400V, 460V to 480V and 560V to 690V.

Bus Configuration

The VFD converts the AC input to DC to produce the required output voltage and frequency. For applications involving a large number of inverters (such as conveyor belts or synthetic fiber manufacturing), there are inverters designed for a single large DC power supply, collectively known as the DC Bus. or DC Bus. This reduces costs and makes better use of regenerative braking energy compared to a stand-alone drive. This segmentation helps to better understand the bus configuration usage graph.

Hardware, Software and Services

Segment the VFD market in hardware (VFD hardware and hardware peripherals), software and services (including project support and MRO/parts and other services) to better compete against other products on the market. “Hardware VFDs” are VFDs as standalone devices. It includes pre-programmed features and functions, as well as programmable options.

Many vendors don’t sell pre-programmed VFDs but just do the hardware. Other products sold with a VFD are called peripheral hardware and accessories when their value is small compared to the value of the VFD. Peripheral hardware and accessories may include control system components, enclosures, cabling, and items, such as harmonic filters, not found in the inverter. However, peripheral hardware and accessories do not include items sold separately, such as switchgear, PACs, PLCs, motors, or mechanical components, such as gears, clutches and brake.

As users use VFDs, they focus on core functionality, they increasingly trust VFD vendors that offer bundled services. In addition to training, installation and commissioning services, users expect responsibility for warranty, single maintenance or project key management, application engineering and custom software development to tailor the VFD to their own needs. VFD users also expect vendor-supported maintenance and upgrade programs to protect their investment.

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Popular VFD inverter manufacturers

The manufacturers and suppliers of VFD inverters are known in the Vietnamese market such as:

Germany: Siemens, Rexroth, LenzeDenmark: Danfoss Japan: Mitsubishi, Yaskawa, Fuji, Omron, Panasonic, Hitachi, Toshiba, Nidec Switzerland: ABBFrance: SchneiderUSA: Allen Bradley (rockwell), Emerson, Parker, Sew-EurodriveIsrael: UnitronicsKorea: LSTaiwan: Delta, Shihlin, China: INVT, Kinco, Sinovo, Goodwe, Growatt, Sungrow, Inovance, Veichi, Gtake, Frecon, V&T, Powtran, Sinee, Delixi, Senlan, Enc, Zoncn, Sunfar, Sumo,..