Question 1

How do VFDs work?

Accepted Answer

Standard AC motors run constantly at full speed. Variable frequency drives (VFDs) allow you to change the speed of a motor. VFDs convert incoming AC power into DC power, then use that DC power to create a simulated sine wave (PWM waveform) using switches called IGBTs. By changing the switching frequency of the IGBTs, a VFD can make the frequency of the waveform appear different to an AC electric motor, changing the speed at which it runs.

Question 2

How do VFDs save energy?

Accepted Answer

Variable frequency drives allow a motor to run at speeds slower than full speed. Mechanical brakes and other throttling methods let the motor consume its fully rated power and then slow the process in other ways. In contrast, a VFD allows a motor to consume less electricity when it runs slower. Both VFDs and mechanical slowing have their applications, but only VFDs allow you to slow a motor while saving energy.

Question 3

What applications benefit the most from a VFD?

Accepted Answer

Variable frequency drives offer the most energy savings in systems that operate based on affinity laws. Centrifugal loads such as pumps or fans gain extra efficiency compared to the amount of slowing seen. For this reason, VFDs are very common in HVAC, pumping, water/wastewater, and any other industry heavily utilizing fans and pumps.

Question 4

What is the ROI of my VFD?

Accepted Answer

Determining the return on investment (ROI) and payback period on a VFD can be a large factor for or against its purchase. Purchase price, utility rates, how often the system is used, and the range of speeds you operate at can all change the result.

The three calculations you would need to determine your ROI and payback period are:

Annual Energy Savings = Total New Energy Savings - Total New Energy Losses
Payback period = Initial Cost / Annual Energy Savings
Annual ROI = Annual Energy Savings / Initial Cost

Learn more from our article Complete Guide to VFD Efficiency, Energy Savings, and ROI.

Question 5

Do VFDs increase production?

Accepted Answer

Variable frequency drives are meant to use an electrical system to control motor speed and increase energy efficiency, but in some cases an increase in production may be a byproduct.

For example, in some situations a mechanical brake to control speed may require periodic shutdowns to cool and a VFD can help bypass those shutdowns. In other cases they might help reduce costs. For example, some systems choose to recirculate pumped water or other liquids if they run at full speed, and holding areas and treatment chemicals may need to be increased. By reducing the speed and amount of liquid pumped, some costs could be reduced.

Question 6

What hidden costs should I expect with VFDs?

Accepted Answer

Like all electrical equipment, there are losses for electricity to pass through it. In the case of VFDs, those losses are often overcome with other efficiency gains. Also remember that VFDs, like all equipment, have costs to be shipped or delivered, installed, commissioned, and then properly maintained. You may also want to plan for costs for spares or end of lifecycle replacement.

Question 7

When do VFDs make sense in my system?

Accepted Answer

Variable frequency drives makes sense when you have an AC electric motor that needs one or more of the following things:

Motor or process speed control
Complete reduction of motor inrush current (limiting it to 100% of the nameplate rating)

There are some other cases where a VFD may make sense, such as phase conversion or power factor correction, but there are often better ways to address those issues unless you also need one of the main benefits of a VFD.

Question 8

When should I NOT use a VFD?

Accepted Answer

Although there may be special circumstances, the biggest misapplication of a VFD is adding it into a system where you’re always going to run the motor at full speed. By running the power through a VFD in these situations, you experience some efficiency losses and have overpaid for a motor starter. Unless you have a major concern over inrush or another niche issue, a soft starter or motor starter would work better in these situations.

Question 9

Do VFDs work on DC motors?

Accepted Answer

There are VFDs that work on DC motors, but the term “VFD” or “Variable Frequency Drive” typically refers only to AC drives. Any drive to control a DC motor is referenced as a “DC Drive” or something that similarly shows its for a DC system.

Question 10

Should I use a soft starter or a VFD?

Accepted Answer

A soft starter reduces your inrush current to about 300-400% of the motor’s full rating and then runs the motor only at full speed. A VFD reduces your inrush to 100% of the motor’s rating and then allows speed control. If you need either speed control or a full inrush reduction due to utility requirements or other concerns, a VFD makes sense. If you don’t need either of those, then a soft starter is likely a better option for you.

Question 11

What information do I need to size a VFD?

Accepted Answer

There are many things that can affect a VFD system but having some basic information ready helps to at least get the base sizing completely. Information such as:

Voltage (supply and motor)
Motor size (amps preferred)
Phase (input and motor)
Motor application (for overload calculations)
Environment (to protect the VFD)
Control method (to make sure the VFD can be properly used)
Are there any other special requirements that are known (harmonic requirements, dV/dt concerns, etc.)

With this information, you can typically find a VFD that will work for basic applications. For more details see article.

Question 12

Should I size a VFD based on horsepower, kilowatts, or amperage?

Accepted Answer

When sizing electrical equipment like VFDs, the current (measured in amps) is the most important measurement. This is what the ratings and overloads of a VFD are designed around. Mechanical measures like horsepower or watts are good approximations but might lead to under- or oversizing based on assumptions.

Question 13

What are the most common mistakes in VFD sizing?

Accepted Answer

One of the things that gets missed the most when sizing a VFD is the motor application. Although two motor systems may have the same full load amp (FLA) rating, one may be designed for very little overload (like a fan) while another is meant to handle overload frequency (like a crusher). Most VFDs are split into series or overload ratings for common applications so that you don’t overpay for capability you don’t need in the case of light applications or have an undersized drive in the case of heavy applications.

Question 14

Can a VFD convert phase?

Accepted Answer

A variable frequency drive can convert phase, but a phase converter is often a better option. A VFD is usually only appropriate if:

You also are trying to control the speed of a motor
You have no other devices or controls after the VFD

VFD phase conversion only works on inductive loads. Sizing a VFD correctly for phase conversion can also be tricky. Learn more about it in our phase conversion article.

Question 15

When should VFDs be installed in an enclosure or panel?

Accepted Answer

For safety and arc flash protection, our engineers often recommend that every VFD should be installed in a metal enclosure. This becomes especially important when VFDs are installed in a harsh environment or in areas where restricting access to live electrical components is extra important, such as high-traffic areas of a plant. It’s also important when integrating other components such as filters, breakers, SPDs, or other devices.

Question 16

What should I know about installing a VFD?

Accepted Answer

Like all equipment, VFD installations should comply with all relevant electrical and facility codes and best practices. Consult an electrician with the appropriate license and experience for more details.

In general, VFDs are like many pieces of electrical equipment where the installation will include mounting, grounding, power connections, and control connections. VFDs are typically different in that they may need more attention to programming, connections to PLC/SCADA/BMS, and there may be extra concerns around long cable runs and other power quality concerns.

Question 17

What should I know about programming or commissioning a VFD?

Accepted Answer

Always make sure to follow all proper safety procedures when programming a VFD. Commissioning or startup typically includes verifying secure connections and programming the VFD.

Basic programming on simple applications is possible for most with some electrical knowledge. It includes setting basic run parameters, motor specifications, and input and feedback devices.

Programming can get complicated quickly when you start to use a VFD for more than basic speed control. If you have questions, reach out to a VFD expert or whoever you bought the VFD from.

You can learn more about basic programming in our article How to Program a VFD: A Basic Guide.

Question 18

Can I add a VFD into an existing system?

Accepted Answer

Adding a variable frequency drive into an existing motor system is often referred to as a retrofit. This could be adding a VFD into a system that has never had a VFD or replacing only the VFD where one already exists. Both are possible and can be very beneficial to your system, but there’s a few things to consider.

A VFD needs physical space to be installed and a way for the equipment to be transported to the correct place. VFDs also can introduce power quality issues or stress on motors, especially ones that are not inverter duty.

Learn more in our article How to Plan a VFD Retrofit.

Question 19

What are the most common VFD failures?

Accepted Answer

VFDs experience faults and alarms. Often these are misunderstood and are not actually drive failures but are the drive protecting the motor and stopping the process.

When a VFD does experience an issue, it’s most commonly due to:

Improper initial sizing
Contamination (water, dirt, etc.)
Overriding protections

Maintenance at an appropriate timeframe, especially one that complies with the NFPA 70B standards, combined with proper troubleshooting of faults and alarms and properly protecting the VFD from contamination will prevent most VFD failures.

Question 20

Will a VFD affect the rest of my electrical system?

Accepted Answer

Variable frequency drives cause power quality issues that can affect the rest of your system. Issues like harmonic distortion, reflective wave phenomenon (or dV/dt), and EDM (or shaft bearing currents) are all things to consider when adding VFDs. These issues have methods to mitigate and address the problem, but the problems will still exist even at a low level.

Question 21

Do I need harmonic filters in my VFD system?

Accepted Answer

It’s always important to consider harmonic filtration in a VFD system. Some advanced VFDs, such as active front end (AFE) or medium voltage VFDs have relatively low harmonic distortion and don’t need filters. Other VFDs are a small part of a large system, such as a 5HP VFD in a large manufacturing plant. In these cases we often recommend a basic level of harmonic mitigation, such as a line reactor.

For most VFD systems that are at 25HP or more, we recommend evaluating harmonic filtration options. Common filters include passive (or hybrid) harmonic filters and active harmonic filters, or AHFs.

Learn more from our Harmonics article.

Question 22

Do I need output or motor filters in my VFD system?

Accepted Answer

The two main considerations for whether you should have an output filter are the length of the cable from the VFD to the motor and the cost and difficulty to repair or replace the motor. If the motor is down a well, difficult to access, or very custom, it’s worth adding extra protection from VFD-caused issues like dV/dt or reflective wave phenomenon. When the cable between a VFD and motor starts to exceed 100 feet, it’s good to begin considering a VFD output filter.

Learn more from our dV/dt article.

Question 23

Should I use a low or medium voltage VFD?

Accepted Answer

There’s not a hard line where it makes sense to transition from low to medium voltage VFDs. As a rule of thumb, motor systems under 500HP likely should be low voltage, systems over 1000HP likely should be medium voltage, and in between is a gray area. If you find yourself in that middle area, talk to a VFD expert.

For more details, see our article When to Use Low Voltage vs. Medium Voltage VFDs Complete Guide.

Question 24

What infrastructure is required for medium voltage VFDs?

Accepted Answer

Medium voltage variable frequency drives are big enough that they usually require accompanying equipment. Switchgear might be included in the VFD or might be an external piece of equipment. In some cases, a transformer may also be needed.

Medium voltage drives are also larger and have larger cables, so supporting infrastructure such as concrete pads, cable trays, or other needs may be required based on your setup.

Question 25

Do I need a transformer for my VFD?

Accepted Answer

Low voltage VFDs are rarely capable of transforming voltage, so the need to go from something like a 240V input up to a 480V output for a motor would require a transformer or adjustment to a new motor.

Medium voltage drives may require a transformer but are often also capable of transforming voltage within the cabinet. Situations where a medium voltage drive is fed the higher voltage a facility receives (such as 13.8kV or 12.47kV) and then outputs a lower voltage that is common among motors (such as 4160V or 6.6kV) are common for industrial sites.

Question 26

Can the same technicians work on both low and medium voltage VFDs?

Accepted Answer

Safety is always the number one priority for work on electrical equipment. All those who work with variable frequency drives and other electrical equipment should be trained, licensed, and following all relevant codes and policies.

In general, medium voltage VFD technicians have also been trained on low voltage VFDs, although they may not work on them daily and will be less familiar with operation. It is much rarer that a low voltage VFD technician is trained to work on medium voltage equipment.

Many assume that medium voltage is far more dangerous but remember that low voltage is often more prevalent in a facility and may not have as many safeguards to access. All electricity is dangerous and technicians should always work safely.

Question 27

Can I replace a medium voltage starter or rheostat with a VFD?

Accepted Answer

A VFD is not a drop-in replacement for medium voltage starters or liquid rheostats, but it can be a good upgrade in many situations. VFDs can improve control, reduce mechanical stress, and increase uptime in tough applications like shredders, mills, and conveyors. VFDs introduce some complication and power quality issues to address and may come at a higher initial cost. Speak to a VFD expert to see if your situation is a good candidate for an upgrade to a VFD.

Variable Frequency Drive FAQ - Selection, Sizing, Installation, and Troubleshooting

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