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What is VFD (Variable Frequency Drive)? Types, Benefits & Working Principle

VFD

There is a number of industries and plants around the world which use a different type of high power motors. But due to the consumption of high power, the industries and plants had to pay a high amount of electricity bills. To overcome this problem of high power consumption and to increase efficiency, VFD was introduced. In earlier days circuitry was not strong enough.VFD stands for Variable Frequency Drive or adjustable frequency drive. Here the frequency defines the motor RPM and by controlling the AC frequency motor RPM can be controlled. Different varieties of VFD are present in the electrical and electronics market ranging from small applications to high power induction motors.

VFD Circuit and Its Operation


Fig 1 shows the block diagram of the VFD circuit. It consists of the major 3 sections.

1. The rectifier section

2. The filter section

3. The switching or inverter section.

The switching section or inverter section has shown in figure 4.This section inverts the DCto AC. This section uses different types of electronic switches that range from high power transistors, IGBT or MOSFETs. The switches are quickly turned on or off and the load obtains a pulsating voltage that is very similar to AC. The output frequency is directly proportional to the switching rate. A high switching rate offers high-frequency output whereas a low switching rate offers a low-frequency output.

1) Rectifier Section of VFD Circuit

What is VFD (Variable Frequency Drive)? Types, Benefits & Working Principle

Fig. 2
Fig 2 shows the rectifier section. In this section, there are 6 diodes used.D1, D2, and D3 diodes are connected with positive rail and D4, D5 and D6 diodes are connected with the negative rail. All 6 diodes work as a diode bridge to converts the three-phase AC signal into a single DC rail. Three-phase R, B, and Y are connected diagonally the diode. Depending on the polarity of the sinusoidal wave diodes conduct either forward bias or reverse biased thus providing a +ve pulse or a -ve pulse in both positive and negative rail.

2) Filter Section of VFD Circuit

What is VFD (Variable Frequency Drive)? Types, Benefits & Working Principle

Fig. 3
It is known that typically rectifier diodes convert the AC signal to DC, but this output DC signal is not smooth enough because here the frequency-dependent AC ripples also associated with it. Hence to rectify AC ripples and for a smooth DC output, there is a necessity of some kind of ripple rejection filters shown in fig.3. The standard component for any filter is large capacitors and inductors. In this section, mainly the capacitor filters out the AC ripple and gives a smooth DC output. In some cases, other types of filters are also used to reduce the input AC noises and harmonics.

3) Switching or Inverter section of VFD Circuit

What is VFD (Variable Frequency Drive)? Types, Benefits & Working Principle

The switching section or inverter section has shown in figure 4.This section inverts the DC to AC. This section uses different types of electronic switches that range from high power transistors, IGBT or MOSFETs. The switches are quickly turned on or off and the load obtains a pulsating voltage that is very similar to AC. The output frequency is directly proportional to the switching rate. A high switching rate offers high-frequency output whereas a low switching rate offers a low-frequency output.

Advantage and Benefits of VFDs

As use of VFDs in HVAC (heating, ventilation, and air conditioning) applications has increased so pumps, fans, air handlers, and chillers can take benefits from speed control.

There are following advantages discussed

• Helps in energy savings

• While using VFDs require low motor starting current

• Reduces thermal and mechanical stresses on motors and belts during starts

• Easy to install

• VFDs high power factor

• Lower KVA ratings.

• It has a good speed range.

• Multiple motor control facility.

• Less design complexity

• It is cost-effective from the production and installation side.

• Good regeneration capabilities.

• Support higher horsepower induction motors.

Disadvantages of VFD

• There are also some disadvantages associated with the use of a VFD system. The prime drawback of the VFD system is the initial setup investment cost. For any factory or a plant where a number of high horsepower motors need to be controlled using VFDs, it needs high investments.

• Using VFDs can because of motor heating and need distinct motors construction. Also, the construction requires special kinds of motor insulations.

• Another big disadvantage of VFD is that the main source power line is extremely disturbed with distortion and line notching harmonics. Because of this, the other devices connected in the same power line also get hampered during the operating condition.

• Because of the cogging effect, the load motor face jerking during starts and stops the situation.

How to select a VFD for any Application?

The selection of VFD for a particular application requires a very good understanding of the load. Every motor produces a different type of torque. In some applications constant torque is required whereas in some other applications the torque needs to be controlled. Also, the load value across the motor is determining factor of the motor specification, essentially the Power rating.

So for the selection of appropriate VFD for the particular application we need to consider the following points.

Horsepower of the given motor.

The working conditions of the VFD and motors.

Single-phase or three-phase supply.

Single VFD with single motor characteristics or single VFD with multiple motors.

Control features requirements

Global VFD suppliers

ABB VFDs –

Because of its stable performance, a combination of extraordinary features, high-performance vector control technology, low speed with high torque output, dynamic and superior overload capacity, ABBdrives occupy a significant position in the global VFD market.

Allen Bradley –

They offer a wide range of AC and DC variable frequency drives. That is all designed for productivity, flexibility, and ease of use. Their VFDs design to handle both low voltage and medium voltage uses, with a wide range of power ratings.

Danfoss –

Global market leader in VFDs, focusing on drives only. Provider of a high-quality drive that can match to our exact requirements. Each and every single drive is properly tested before leaving the factory. Experts are focusing on every possible detail of drive optimization, and are always up-to-date on the newest technological developments. Allow customers to access their application know-how and a range of maintenance services to keep the system running optimally throughout the drive lifecycle. It offers the best possible components and flexibility so one can fine-tune system performance suitable for application exactly.

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Dewakar Pandey

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