VFD and Motor Controls

Tech Support

"I don't know if it's the motor or the drive!"

If we only had a quarter for every time we heard this. 

The fact is, variable frequency drives are appearing in nearly every application in every industry; they are no longer primarily held to HVAC. And with a potential for 25% to 70% energy savings, it's easy to see why.

VFD drives are used in residential, commercial, industrial, government, and anywhere else requiring controlled motor rotation output.

Benefits of using a VFD


  • Cost Savings! The most popular reason by far, especially beneficial in centrifugal pump and fan applications. VFDs provide energy savings for lower operating costs (cheaper electric bill!)


  • Improved Production Efficiency: VFDs provide precision control without affecting critical operation components, such as power consumption, impedance, and torque.


  • Controlled Starting Current and Demand: Electric motors started "across the line" take as much as seven-to-eight times the motor full-load current to get going. A VFD starts operations at zero frequency and voltage, then energizes the motor's windings as frequency and voltage increase.


  • Controlled Stopping: This is important to reduce equipment wear and tear. And VFDs have regenerative braking.


  • Application Flexibility: Take control and optimize your system. VFDs allow you to easily program changes in speed and run times to meet process needs. Also enjoy remote speed adjustment via controllers.


  • Speed control can also be used to replace a valve or damper-type flow control.


  • Adjustable Torque Limit: Ever see a machine get jammed but the motor keeps trying to rotate until someone shuts it down? Take too long, or if the initial lockup was too forceful, and you are in for an expensive motor and/or machinery repair. VFDs will sense the extra torque and shut the system down, hopefully saving you expensive repair costs.


  • Updated Systems: AC motors and drives are being used more than ever to replace outdated DC motors and other expensive mechanical drive components, like gearboxes. An AC motor / drive combo is not only cheaper but is usually available off the shelf with most manufacturers. Say goodbye to expensive repairs and long lead times!


  • Longer Equipment Life: VFDs provide smooth and controlled starting and stopping  that equates to reduced mechanical wear. They also buffer the high starting currents that will damage your machinery. Your motors (should) last longer, as long as you take the recommended motor safe guards when using a drive. 


  • Less Power Line Disturbance: Starting motors fully open is a drain on the power system. Depending on the facilities' set up, you may have even noticed lights or computers flicker (or even go out!) on a large start up due to low voltage. VFDs start motors at zero voltage and ramps up.

Problems will occur, it's inevitable. Improper sizing, tripped circuit breaker, corrosion, power surge, mishandling, and any other number of situations that will cause equipment failure.

Downsides of using a VFD


  • Higher Initial Cost: Drives aren't always cheap. But take in consideration energy savings over the long run and whether you are replacing outdated, inefficient machinery.


  • Inverter Duty Motors: Yes, you should use inverter duty motors for longer equipment life. These electrical motors cost more than a standard general purpose motor, but they are designed to handle incoming currents from the drive. Luckily, more and more standard motors are being manufactured with inverter-duty wire these days.


  • Harmonic Distortion: VFDs create voltage pulses unlike the typical sinusoidal waves provided from power utilities. These pulses put repetitive stress on the motor and its insulation, leading to deterioration and premature failure. Depending on the preventative measures taken, failure can occur quickly or over an extended period of time. 


  • Expert Installation: You need to fully understand the equipment and your application in order to properly set it up. Each drive must be tuned to the motor for the specific application; and system operating parameters must be checked, including voltage at the motor.


  • Accessories: When you spend the money on this equipment you want it to last as long as possible, right? Part of proper set up will include necessary protection devices for extended machine life, such as load and line reactor filters.


  • Fluting: From the minute you hit start, a VFD induces voltages that build up on the motor shaft until they discharge to the frame (ground) through the path of least resistance, which is typically the bearings. The results are pits and craters in the bearing race wall and balls. Fortunately, there are a number of options to help prevent  premature bearing failure, like motors with Aegis Rings.


  • Hot Hot Hot: Drives operate at less than 100% efficiency, which results in heat generation. As part of the setup, you need to consider the drive enclosure, cabinet, and air flow.

But when the VFD is down and production is stopped, you need an expert to call. 


Square One Electric Motors is a controls service company for industrial and commercial VFDs. We have an experience crew to service all brands of VFDs in Delaware and surrounding parts of Maryland, Pennsylvania, and New Jersey.

Many engineers pick the drive that they are comfortable with. Some facilities have one brand in use, while others have a mix of brands.


Either way, our crew can handle the VFD brand that you are using, whether it's Weg, ABB, Toshiba, Siemens, Allen Bradley, or something else.

How to choose a VFD - Five Things to Know

Replacing a variable frequency drive is usually straightforward; people tend to go with the same brand and model that was in place.


However, if the VFD was not sized properly, or you are implementing a control into an application that previously had none, there are a few things to consider before you purchase.


  • Can your motor handle a VFD? Not all electric motors are created equally. Check that your motor is inverter capable, or you will be shopping for a new motor soon enough.


  • AMPS are more important than horsepower. Size a drive so that the motor maximum current does not exceed the drive. Look at the inverter amps and make sure that the amps on the motor is not higher. Sometimes, you may need to choose an inverter higher in HP than the motor to meet this requirement.


  • Is your application constant torque or variable torque? In constant torque loads, torque is independent of speed (ie – conveyors); versus variable torque loads, where torque varies as a function of speed (ie – pumps and fans). Know your application before sizing the drive.


  • What are the acceleration and deceleration requirements? How fast does the load need to come up to speed, if it matters? Applications that practically require full torque from near zero speed will require a sensorless vector control. With deceleration, consider brake resistors for energy / heat dissipation and control in constant torque applications. Also, for very low speed applications, an encoder may be needed.


  • Do you need or want to improve energy usage? Regenerative drives store unused energy in internal capacitors to apply to the next operation, reducing energy use.


It will be easier to find the right VFD once you establish these parameters. Unless you already have a brand preference, other considerations include price, availability, and ease of use.


Square One Electric Motors is a distributor for many brands – more than we have on our website. Feel free to Contact Us if you have any questions.

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