What is a Variable Frequency Drive? VFD Diagram and FAQ

In this blog, we explain what VFD stands for and what it can do for you.

We talk about what VFD is and why it’s important, how it works, and the most common questions people have about it.

What is a VFD?

Variable Frequency Drive is what VFD stands for. It is a variable frequency drive, which means that the frequency of the AC supply can be changed. Since an induction motor’s speed depends on the frequency of the power supply, the VFD can be used to change its speed.

You should know that the voltage frequency used to run an AC motor changes its speed. It means that when you give a 50Hz voltage to an AC motor, the motor runs at a rated speed. But if you use an input voltage that is higher than 50 Hz, the engine will run faster than the rated speed. But if the frequency of the voltage you give it is less than 50 Hz, the motor moves slowly.

The history of the VFD

Nikola Tesla came up with three-phase alternating current (AC) motors in the late 1800s, which was a big step forward. This is especially true if you compare it to Thomas Edison’s DC motor in terms of how well it works and how reliable it is.

To control the speed of an AC motor, you must either change the magnetic flux or the number of poles on the motor. Even after the induction motor had been used for many years, it was still very hard to change the frequency to control the speed, and the way the motor was built made it impossible to make motors with more than two speeds.

In the 1960s, variable frequency drives (VFDs) had small solid-state parts that limited how much current the VFD could supply to the motor. This usually meant that a frequency could only control a motor of a certain size, and they were not used very often.

In the 1980s, AC motor drive technology became reliable and cheap enough to compete with traditional DC motor control. The speed of standard AC induction or synchronous motors can be accurately controlled by these variable-frequency drives (VFDs). With VFDs, you can control the speed with full torque from 0 rpm to the maximum rated speed, and if you need to, you can control the speed above the rated speed with less torque. VFDs change the frequency of their output by turning an AC voltage current that comes in into DC and then using voltage pulse-width modulation to make the output waveform look like an AC current and voltage.

Today’s control hardware, software, and microprocessors make it possible for VFDs to work in a wide range of ways. One of the most important parts of saving energy is being able to limit the amount of force a motor needs to start.

Why is VFD used in motors?

Getting things done better. VFD is used in industries to improve motor efficiency and save a lot of energy. The variable voltage method is the most common way to control speed, but it wastes a lot of energy compared to the variable frequency method.

How many sections are in a VFD?

VFD circuit has three parts, and each part has its own job. Rectifier, DC filter/DC intermediate circuit, and Inverter are the three parts of a VFD.

vfd diagram
What is a Variable Frequency Drive? VFD Diagram and FAQ 1

Rectifier in a VFD

What is a VFD’s rectifier?

The rectifier is a circuit that controls the three-phase AC power that comes in and changes it to DC power. Low-power single-phase VFDs have single-phase rectifier circuits that use diodes. Still, three-phase VFDs use SCRs in their three-phase rectifier circuits because SCRs work well with high positive voltages and high power.

Intermediate circuit

What is a circuit in the middle?

The DC-bus or DC-link, which is part of the intermediate circuit, can be thought of as a battery from which the motor can get power through the inverter.

Inverter in a VFD

What is the VFD’s inverter?

With the help of an inverter, the DC is turned on and off, giving the motor a voltage that pulses like AC. By changing the alternation rate, the frequency of the simulated AC that is sent to the engine can be changed. It is important because the main way a VFD circuit works depends on this part, and a VFD uses a full-controlled inverter circuit. In short, the waveform of the output AC supply from the controlled inverter circuit lets the motor work.

block diagram of vfd
What is a Variable Frequency Drive? VFD Diagram and FAQ 2

Video Explanation about VFD

Why is VFD so popular?

VFD is popular because it uses less energy and costs less to set up. VFD use less energy than any other controller device in the same segment. Because of this, VFD is good for large factories and plants that need motors with more horsepower because it uses less power. This lowers the energy bill and helps save money.

How does a VFD work?

With a standard AC across-the-line motor starter, the line voltage and frequency are sent to the motor, and the speed depends only on the number of motor stator poles. A VFD, on the other hand, sends different amounts of voltage and frequency to the motor, which controls its speed.

If you give the motor a higher frequency, it will run faster. When power is sent to the motor through the VFD, the working speed of the motor can be lower than the nameplate base speed or higher than synchronous speed. The maximum speed at which a motor can be used safely is listed by the company that makes it.

VFD applications

VFDs are used in a lot of different industrial and commercial settings.

  • VFDs are used to control everything from extruders and electric cranes to roller coasters and mechanical bulls in industrial settings.
  • VFDs are often used in commercial settings to control the flow of water and even the amount of water in a tank. They are also used in the HVAC industry and are considered “green technology.”

In the end, a VFD controls the speed of an AC motor by changing the frequency of the power it gets. This lets the motor start up smoothly and lets the speed be changed based on what the application needs.

What type of motor is used for variable frequency?

Most of the time, three-phase motors are better, but single-phase variable frequency drives can be used with some types of single-phase motors. People often use motors that are made to run at a fixed speed on main voltage, but some improvements to how a standard motor works make it more reliable and give it better performance.

Below is a simplified diagram of how a VFD works. Here are the three most important parts of the controller: Converter-Reverses the three-phase AC power that comes in and changes it to DC.

vfd wiring diagram
What is a Variable Frequency Drive? VFD Diagram and FAQ 3

What does VFD do to a motor?

When power is sent to the motor through the VFD, the working speed of the motor can be lower than the base speed listed on the nameplate, or it can be raised to synchronous speed or higher. The maximum speed at which a motor can be used safely is listed by the company that makes it.

Why is voltage reduced with frequency in VFD?

In VFD, the voltage going to the motor is also lowered as the frequency goes up. This is done to keep the motor from being damaged by too much current.

Why use a VFD for a motor?

Because of how well the VFD controls the frequency and voltage of the voltagemotor, it will protect your motor better from problems like electrothermal overloads, phase protection, voltage, overvoltage, and so on.

What is a PWM VFD?

A PWM VFD makes a sinusoidal wave by sending out a series of voltage pulses of different lengths. Pulse-width modulation (PWM) VFDs are the most common type used in industry because the DC bus voltage stays the same, there is no motor cogging, they are more efficient, and they cost less.

Why do we convert AC to DC in VFD?

AC signals can’t be stored, but DC signals or power can. So, to store the energy, we need to change it from AC to DC. Also, digital devices need constant voltages, so we need to use Rectifiers to turn AC into DC to get those constant voltage levels.

Does VFD need starter?

Electrical engineers recommend soft starters for motors that only need a way to start and VFDs for motors that also need to be able to control their speed while they are running. A soft starter can’t save energy by ramping down a motor with a variable load, and when used only as a starter, a VFD wastes its ability to control speed.

Can a VFD step up voltage?

One of the many benefits of using a variable frequency drive (VFD) to control an electric motor is that the voltage output to the motor can be changed to make it work as efficiently as possible. A VFD turns the AC voltage that comes in across the DC bus capacitors into a DC voltage.