Intro to VFD Motor Controls
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Intro to VFD Motor Controls

So many aspects of life involve the flick of a switch, a turn of the knob, a press of a button, or a swipe across the screen. Most of us take for granted automated and mechanical operations, without a thought to how it works.

Take, for example, VFD motor controls. These deceptively simple-looking boxes do a lot more than meets the eye. Without drives, automation, production levels, and methods for that production in many industries would be drastically different.

Why don’t we look at these wonderful machines and how they make our lives easier?

What Is a VFD?
A VFD, or variable frequency drive, is a type of motor drive used to control speed and torque by manipulating the input frequency and voltage that it receives. It has a wide variety of uses from commercial air conditioning (like at your bank) to assembly lines for glass bottles.

A VFD makes an electric motor (the muscle in the operation) more efficient by using only the necessary amount of power to make it run. In simple terms, it acts as a controller, or the brain for the muscle.

VFDs convert AC (alternating current) power to DC (direct current) using diodes. Diodes allow the current to flow cleanly in one direction while transistors regulate current, turning it on and off based on the frequency required from the motor. This is how VFDs control frequency and voltage.

VFDs Are Also Known As:
  • AC Drives
  • Adjustable frequency drives
  • Inverter drives
  • Microdrives
  • Variable speed drives
  • Variable frequency drives

Keep in mind that not all applications call for a drive. AC and DC motors are either on or off, and when they are on, they run full blast. A drive is connected to a motor when you want to control the motor’s speed, such as to start slowly, or slow down intermittently, etc.

A Brief VFD History

Before the 1950’s, an electric motor’s (the workhorses of industry) speed was controlled by mechanical devices. VFDs used rotary drives before they were replaced with solid-state drives; this allowed VFDs to grow in functionality.

Then, with the advent of AC variable frequency drives, electricity was being used to control the motors.

The introduction of oversized transistors ensured that VFDs became a mainstay in industrial and commercial environments due to their ability to drive larger motors. However, VFDs didn’t just become bigger, they were also condensed for smaller applications.

In the early years of development, AC Drives were big, bulky, and expensive. Linear amplifiers and resistors eventually gave way to circuits with capacitors or microprocessors which opened a world of possibilities for VFDs. Almost overnight, its practical applications multiplied.

It took decades of research and development to improve VFDs’ into the small, efficient, and cost-effective devices that they are today.

In 1992, the Energy Policy Act was passed to reduce America’s dependence on petroleum and improve air quality by exploring and expanding alternative energy methods. Legislation was passed on types of electric motor efficiencies, and companies began to focus more on energy conservation to lower production costs and meet regulations.  

In the 20th century, many industrial facilities critically viewed VFDs as a costly and unnecessary product. After the turn of the century, however, VFDs became a standard, cost saving component in most systems.

Parts of a VFD
A VFD has a main drive controller assembly and drive/operator interface. Here’s how those parts work together:

Controller Assembly
This is the heart of the system where complex processes happen out of sight. A VFD controller is usually made up of a rectifier, a DC link, and an inverter.

The rectifier (or rectifier bridge converter) is what converts AC input into DC.

The DC link on the other hand is what acts as a capacitor buffer for the two power conversion stages. This is usually placed in between the rectifier and the VSI (voltage source inverter). It regulates the DC output from the rectifier and delivers it as stable energy to the inverter.

Lastly, there’s the inverter which is most often called the VSI. This is what runs the motor.   

Drive/Operator Interface
The drive/operator interface is what we interact with. This has the same function as a computer’s GUI (graphical user interface) except it has physical buttons that start or stop the motor as well as control the power output.

Most VFDs use an LED (light-emitting diode) display with a simple layout. This enhances ease of use while avoiding confusion especially in environments where two or more operators use the same VFD.

More recent improvements to this mechanism include remote operation. You can also hook it up to a computer for a more detailed report on what’s happening and for additional controls.

How Does a VFD Drive Work?

Chances are that you’ve already used a VFD.

You see, VFDs are commonly used in home appliances. They can be used to control fans, pumps, and compressors. HVAC systems also use VFDs.

But how does it really work?

Well, the entire system is hooked up to a power source (or in the case of pumps and compressors, water, and air sources, respectively) and turned on so that it can do its work.

Without a VFD, you would control these devices simply by turning them on or off. This would result in delivering full power to the device when you switch it on and zero power in the off position. The cooling, pumping, or compression will only be felt the moment it is on with diminishing effects as the energy tapers off before coming to a complete stop.

The more ideal setup would be to deliver energy at a consistent pace to get the desired results. This is called pulse width modulation. It makes power delivery more efficient.

A VFD captures the power from the source and allow the bridge rectifier converter to turn AC into DC. The DC link then delivers this power to the VSI and it turns the motor based on the amount of energy it receives.

All you must do is configure the drive through the user interface and allow the device to complete the operation as desired.

What Are the Benefits of Using a VFD?


A VFD regulates the amount of power coming out of it to drive the motor. This means that you can set it to whatever power level you need depending on the type of operation, time, or availability of power when using it.

VFDs have a distinct advantage over fixed-speed motors not just because of their operational flexibility but also for their significant energy savings. With a VFD, you’ll only need a large amount of energy to start your motor before it tapers off as it reaches optimal operation.

With a fixed-speed motor, you would be running at full speed the entire time, wasting a lot of energy in the process.

For example:

An industrial ceiling fan to keep your premises cool will require a certain amount of power to keep it running. The power requirement changes when the temperature becomes hotter or cooler. A VFD allows you to set it accordingly to ensure that proper power is delivered based on the fan settings.

Another example would be a water pump with an erratic source of water and/or a limited reservoir space. To ensure you don’t end up with a dry pump or an overflown reservoir, you will need a VFD. The VFD regulates the amount of water it allows to enter the pump before it is delivered to the reservoir.

It also helps monitor the amount of water in the reservoir and adjusts water delivery according to the level of water present.

Now all of this can be done by pressing a few buttons and setting the VFD up so you can leave it alone while you attend to other things.

With that said, VFDs are usually found in commercial and industrial settings where automation benefits the company by relieving the need for additional manpower or constant supervision of the device.

Better Control
VFDs provide full process control. You can monitor every small detail that affects your operations like speed, temperature, torque, etc., and make the necessary adjustments just by pressing a few buttons.

VFDs can also be hooked up to a computer so you can monitor everything from a larger screen and make the necessary adjustments with your keyboard. You can also hook it up to a network and allow remote access and control for safety purposes.

One desirable aspect of running a fully automated establishment is it frees your resources for more productive activities. Freeing up manpower will help you allocate essential personnel to handle other tasks that can increase productivity. In turn, this helps increase profit margins or generate income.

VFDs Come in All Shapes and Sizes

VFDs are built to withstand a lot of abuse and can handle the wear and tear of daily use.

Multiple enclosures are available to suit the application. Drives can be built with an open chassis for clean environments, a standard enclosure for indoor use, washdown units for food and pharmaceutical locations, or NEMA 4x for outside mounting to withstand the elements.

Although drives have the same function, there are small differences in operations and programming. For instance, the programming layout may differ between brands; or the help menu may be accessed through the user interface or through a separate book.

VFDs give alarms and fault codes to assist with troubleshooting, but you may need to contact a motor shop like Square One Electric for technical assistance. VFDs are robust pieces of equipment that can last for decades if sufficient preventive maintenance is performed.

Need help selecting a drive? Shop our online store or Contact Us for help.

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