Gingoog Electric Trading (G.E.T.) ™

March 5, 2009

What is a VFD? Part 1

Filed under: GET to know VFD's — Ralf @ 8:09 AM

What is a VFD and how does it work? Part 1

In order to provide our readers here the best possible information we are using a variety of resources such as manufacturer’s support materials, our extensive experience in this field but also various online researches, etc, etc, etc.

When we want to go more into applications in order to achieve the ultimate goal of enjoying all the benefits that come along with a Variable Frequency Drive we should also know what is a VFD and how does it work?

And to understand the VFD’s much better we found for you also the following very interesting material for your further review.

It explains in a very comprehensive way what a VFD actually is and how it works, it confirms what we have posted earlier in our category GET Savings that we can really safe a lot, it gives us an initial idea how we select a Drive and very important also how to size it up. We learn about the relevance of torque and that at times we do have to oversize the VFD for one step in order to reach the required torque. It touches also special applications where we have to watch out or should have at least a closer eye on and it tackles some important corresponding considerations.

It answers a lot of our questions and I would rate this article as another MUST READ, it is not the usual shop talk you might read on other blogs, we make it all easy digestible.

GET it easy, GET is easy, GET more today, with GET you GET it, GET it here and GET it now:

However, this is a larger entry which doesn’t upload in one piece, so we will do it in a few parts.

Here is Part 1:

Understand VFD’s:

A thorough understanding of how to match the VFD to the driven load is the key to a successful application.

When applied properly, the Variable Frequency Drive (VFD) is the most effective motor controller in the industry today. Modern VFD’s are affordable and reliable, have flexibility of control, and offer significant electrical energy savings through greatly reduced electric bills.

They are used in a wide variety of applications for various reasons. For example, they are the most effective energy savers in pump and fan applications; they enhance process operations, particularly where flow control is involved. VFD’s provide soft-start capabilities, which decrease electrical stresses and line voltage sags associated with full voltage motor start-ups, especially when driving high-inertia loads.

To obtain a clear understanding of the proper and most effective application of VFD’s, you first should gain a working knowledge of VFD basic theory as well as a strong familiarity with practical know-how.

Basic VFD theory:

Applying a VFD to a specific application is no mystery when you understand the requirements of the load. Simply put, the VFD must have ample current capability for the motor so that the motor can produce the required torque for the load. You must remember that machine torque is independent of motor speed and that load horsepower increases linearly with RPM.

VFD applications can be divided into the following individual load types.

Constant torque loads:

These loads represent 90% of all general industrial machines (other than pumps and fans). Examples of these load types include general machinery, hoists, conveyors, printing presses, positive displacement pumps, some mixers and extruders, reciprocating compressors, as well as rotary compressors.

Constant horsepower loads:

These loads are most often found in the machine-tool industry and center driven winder applications. Examples of constant horsepower loads include winders, core-driven reels, wheel grinders, large driller machines, lathes, planers, boring machines, and core extruders.

Traditionally, these loads were considered DC Drive applications only. Having available high-performance flux vector VFD’s, many DC Drive applications of this type can be now handled by VFD’s.

Variable torque loads:

Variable torque loads are most often found in variable flow applications, such as fans and pumps. Examples of applications include fans, centrifugal blowers, centrifugal pumps, propeller pumps, turbine pumps, agitators, and axial compressors. VFD’s offer the greatest opportunity for energy savings when driving these loads because horse power varies as the cube of speed and torque varies as square of speed for these loads. For example, if the motor speed is reduced 20%, motor horsepower is reduced by a cubic relationship (.8 X .8 X .8), or 51%. As such, utilities often offer subsidies to customers investing in VFD technology for their applications. Many VFD manufactures have free software programs available for customers to calculate and document potential energy savings by using VFDs.

Sizing VFD’s for the load:

How do you size a VFD Drive for an application and feel confident it’s going to work? First, you must understand the requirements of the load. It helps also if you understand the difference between horse power and torque. As electrical people, we tend to think of loads in horse power ratings instead of torque ratings. When was the last time you sized something based on torque? Thus, both torque and horse power must be carefully examined.


Torque is an applied force that tends to produce rotation and is measured in lb-ft or lb-in or in the metric system nowadays also with NM (Newton Meter). All loads have a torque requirement that must be met by the motor. The purpose of the motor is to develop enough torque to meet the requirements of the load.

Actually, torque can be thought of as “OOUMPH”. The motor has to develop enough “OOUMPH” to get the load moving and keep it moving under all the conditions that may apply.

Horse power:

Horse power (HP) is the time rate at which work is being done. One HP is the force required to lift 33,000 lbs 1 ft in 1 min. If you want to get the work done in less time, get yourself more horses!

Here are some basic equations that will help you understand the relationship between HP, torque, and speed.

HP = (Torque x Speed)/5250 (eq. 1)

Torque = (HP x 5250)/Speed (eq. 2)

As an example, a 1HP motor operating at 1800 rpm will develop 2.92 lb-ft of torque.

Know your load torque requirements every load has distinct torque requirements that vary with the load’s operation; This torques must be supplied by the motor via the VFD.

You should have a good understanding of this torques:

* Break-away torque:

Torque required to start a load in motion (typically greater than the torque required to maintain motion).

* Accelerating torque:

Torque required to bring the load to operating speed within a given time.

* Running torque:

Torque required to keep the load moving at all speeds.

* Peak torque:

Occasional peak torque required by the load, such as a load being dropped on a conveyor.

* Holding torque:

Torque required by the motor when operating as a brake, such as downhill loads and high inertia machines.

Practical knowhow guidelines

The following guidelines will help ensure a correct match of VFD and motor.

1. Define the operating profile of the load to which the VFD is to be applied.

Include any or all of the “torques” discussed above. Using a recording true RMS Ampere meter to record the motor’s current draw under all operating conditions will help in doing this. Obtain the highest “peak” current readings under the worst conditions. Also, see if the motor has been working in an overloaded condition by checking the motor full-load amps (FLA). An overloaded motor operating at reduced speeds may not survive the increased temperatures as a result of the reduced cooling effects of the motor at these lower speeds.

2. Determine why the load operation needs to be changed.

Very often VFD’s have been applied to applications where all that was required was a “soft start” reduced voltage controller. The need for the VFD should be based on the ability to change the load’s speed as required. In those applications where only one speed change is required, a VFD may not be necessary or practical.

3. Size the VFD to the motor based on the maximum current requirements under peak torque demands.

Do not size the VFD based on horse power ratings. Many applications have failed because of this. Remember, the maximum demands placed on the motor by the load must also be met by the VFD.

4. Evaluate the possibility of required oversizing of the VFD.

Be aware that motor performance (break-away torque, for example) is based upon the capability of the VFD used and the amount of current it can produce. Depending on the type of load and duty cycle expected, oversizing of the VFD may be required.

Ralf Wabersich

Gingoog Electric Trading (G.E.T.)


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