VFD Buying Guide

By Paul Rossiter on 17th Jan 2014

Finding the Best Variable Frequency Drive

Finding the perfect VFD or motor controller isn’t easy. A lot depends on the unique needs of your application and system. There isn’t a catch-all model or brand to fall back on for every application.

That’s why we’re here to help, providing the essential information to make a good decision for your operation.

A good VFD will be reliable and easy to operate. Ultimately, it will save money on utility payments, equipment repairs and replacements.

The more you know about what your application needs from a VFD, the easier it is to pick the right one.

The Basics: Why We Need VFDs

Variable frequency drives (VFDs) control the speed of AC induction motors and often save energy, especially while running applications like pumps and fans. When sized correctly, VFDs can also be used for phase conversion if you need to run a three-phase motor but are limited to single-phase supply.

VFDs modify the electricity from the utility to run the motor precisely and provide the right speed and torque to optimally run the application. Drives dictate motor speed and torque by controlling the ratio of frequency and voltage — commonly referred to as the volts/hertz curve.

Motors without VFDs often wear out sooner and consume significantly more energy than may be needed for the application. This is especially important for applications with fluctuating load or speed requirements.

For example, to maintain a set PSI or flow rate on a pumping system, a VFD can be used to automatically accelerate or decelerate the pump to meet the immediate demands of the system. Or on Hammer Mills and large cone crushers, like those found in a Metso HP4, the VFD can be used to increase torque when a load surge requires more output from the motor for a short amount of time.

The Big Picture

Pairing the right drive with an existing motor is fairly simple. Most of the basic motor and system information is found on the motor nameplate.

  • Full load amps (FLA)
  • Horsepower
  • Voltage
  • RPM
  • Service factor
  • Inverter-duty rated (not on nameplate)

Other information will be specific to the needs of your system and application.

  • Type of load (What is the application and its loading characteristics)
  • Speed range and control method (PLC communication protocol required, 4-20mA signal, etc.)
  • Special enclosure needs (where will the VFD be mounted, indoor/outdoor/etc.)

Drive specifications

Full Load Amps (FLA)

The process of sizing a VFD starts with the motor’s full load amps.

Match your motor’s FLA with the amp ratings of each VFD you consider. Or play it safe and get a VFD with a higher amp rating than your motor requires to give yourself a bit of cushion for constant torque loads and/or applications that require more strength during start up. If you don’t get a big enough drive, it will trip out every time you try to power up.

Horsepower (HP)

Load or motor horsepower is a great way to refine your search for drives fit for your application, but it shouldn’t be used as a direct reference for drive requirements. Due to variables in load requirements like RPM (a 900 RPM motor has a much different amp requirement than a 3600 RPM motor), sizing a VFD on HP alone will likely get you in trouble. We strongly recommend you use HP to narrow your selection but that you use amps (FLA) to determine the right VFD for your motor.

Voltage and Phase

You have to match the voltage of the VFD and motor to your available voltage on site. For low-voltage applications in the U.S., this is usually 208, 230 or 460 VAC. For medium voltage (1000 volts to 35kV) or other applications, it’s wise to call application specialists or engineers for assistance.

VFDs are primarily used in industrial facilities with three-phase power supply. A VFD can act as a phase converter if you have a three-phase motor but are limited to one-phase supply.

If your load is 3-horsepower or below (approx. <10 amps FLA), there are several single-phase input drives to consider.  If your motor is greater than 3 HP, you could use a drive built for three-phase input so long as it has been de-rated correctly.

To properly de-rate your VFD to act as a phase converter for one-phase incoming power, start with your motors FLA. Multiply the motor FLA by two and select a VFD rated for twice the motor’s FLA. For example, if you have a 10 HP motor with a FLA of 28 amps, you’ll need a VFD rated more than 56 amps and around 20 HP.

For small shops or home use, keep in mind that VFDs are the number one power polluters on the planet. They pollute power quality even more when used as a phase converter. Talk to your sales engineer to see if using a line reactor is the right option for you.

Application (Constant or Variable Torque)

Now let’s consider the work you’re doing. Do you need to run a pump, rotary kiln or an extruder? The answer will determine whether you need a variable or constant torque drive.

Variable torque drives are for simple centrifugal equipment like fans and pumps. These drives allow the motor to apply only the torque necessary to run the application at slower speeds. Centrifugal applications rarely exceed the rated current, so variable torque drives only need a one-minute overload current capacity of 120%.

Constant torque VFDs are necessary for heavier applications that require continuous torque at all speeds like conveyors, positive displacement pumps, punch presses and extruders. For example, a conveyor runs constantly, but it needs more power as weight is added to the belt, so your drive has to be able to handle the difference. That’s why constant torque drives need at least 150% one-minute overload current capacity to protect against load spikes.

You may think, let’s play it safe and go with constant torque even for a basic fan application. And if you absolutely can’t live without that fan, it could be a worthwhile precaution. But that’s like buying a sports car for your grandma — you’re spending a lot of money on performance that will never get used.

Speed Range

VFDs can under and over speed motors. A VFD can run your motor slow enough that its internal cooling fan isn’t moving enough air to keep the motor operational. Proper precautions should be implemented to protect your motor, such as use of a separate auxiliary cooling fan, if you plan to under speed your motor.

A VFD also can operate the motor faster than its nameplated RPM value. However, be advised that in doing so you will lose torque. We recommend you don’t go more than 20% above a motor‘s rated speed and that you check with your motor manufacturer before doing this to make sure over-speeding won’t void any warranties.

Control Method

When buying a VFD, you’ll want to think through your method of control. Will you be controlling the VFD from a door mounted keypad or from a PLC?

Many manufacturers require Ethernet communications to get the right information from their drives to the PLC and production automation systems. More operations are moving to these advanced communication systems, but some inexpensive drives don’t include these options. So if you’re looking to advance your systems in the future, make sure you’re getting drives that won’t hold you back. Your sales engineer will be able to help you select the correct communication protocol based on your needs and preferences.

Speed Reference Alternatives

  • Speed potentiometer – Allows the operator to set motor speed.
  • Digital programming/display unit – Allows the operator to program and troubleshoot the drive by inputting values through a keypad with an LED or LCD display unit. Drive operation also can be monitored through this display.
  • Analog signal follower – 4-20mA or 0-10VDC; must provide variable frequency drives with an isolated input; must use a twisted/shielded pair and keep wire away from three-phase AC.
  • Selector switch speed selection – Allows the operator to select from several preset speeds. Also can be used if the speed is being set via a PLC and an analog output is not available.
  • Serial communications – Allow VFDs to communicate on a network, such as MODBUS, PROFIBUS, DEVICENET or METASYS, enabling drive operation to be coordinated and monitored from a PC.

Special Enclosure Needs

You’ll want to ensure your VFD will function in its work environment.

Heat, moisture, dust and other factors can damage your VFDs and possibly cause other equipment malfunctions. These malfunctions could hurt someone. We’ve seen metallic dust cause arc flashes that burn like lightning bolts.

Enclosures keep drives clean, cool and running for a long time. We’ve seen properly enclosed and maintained drives operating in challenging environments for 30 years and counting.

You can get a stand-alone drive with the proper enclosure rating or put your VFD inside of another enclosure. Drive enclosures are rated in a few certification types, including ingress protection (IP), NEMA and UL-type enclosures.

If you’re having a difficult time choosing the right enclosure system for your environment, your sales engineer will be able to help guide you in the right direction.

Inverter-duty Rated Motors

VFDs are the best way to control your motor, but they come with challenges. PWM drives have a digital output that stresses the motor windings and bearings.

Newer inverter-rated motors use wire designed to handle the high voltages that drives can create. You also can help protect your motors with  grounding rings, isolated bearings and special cooling features like a separate fan.

We carry a complete line of MDI inverter-grade motors as well as Aegis shaft grounding rings should you need to upgrade your existing motor to be better prepared to handle the stresses of running off of a VFD.

Custom Builds and Accessories

Like any fashionable piece of equipment, there are enough drive accessories and add-ons to make your head spin. But aside from the lights and door-mounted devices detailed below, you may want to think about methods for VFD bypass and harmonic mitigation as you think about the VFD system.

For bypass, we recommend you use a true three-contactor bypass arrangement instead of alternates designed for cost savings not reliability (two-contactor and electronic bypass). A three-contactor bypass will allow you to run across the line in the event you have a VFD failure. In addition, this arraignment allows you to replace the VFD with very limited disruption to your system.

For harmonic mitigation, we recommend using a fully integrated solution built within the same cabinet as the VFD itself. There are several methods for harmonic mitigation. We just recommend that you avoid the added complexity of putting them in separate enclosures that are wired into the VFD panel.

Other options and  VFD accessories include but are not limited to:

  • Disconnect or circuit breaker
  • HOA (Hand/Off/Auto switch)
  • Pilot lights
  • Bypass
  • Line reactor
  • Harmonic mitigation
  • TVSS
  • dV/dt filter

The perfect combination of drives and accessories can be difficult to determine since so much depends on the environment, application and regulatory considerations.

There are many off-the-shelf drives and accessories to choose from, and all our offerings on VFDs.com are produced by well-vetted, quality manufacturers that we stand behind.

Check out the  custom VFD buildup page to see how you can benefit from a system custom designed and built for you by our engineers, application specialists and certified UL 508a panel shop.

Final Recommendations

As you’ve hopefully ascertained by now, you can’t just hop online and buy any old drive. Don’t risk purchasing anything that the manufacturer won’t back up with a warranty.

But no matter the brand, if the drive isn’t set up correctly, it isn’t going to work right. We’ve seen people install drives backwards and ruin them immediately. This and many other common mistakes are easier to do than you may think. Work with qualified installers that get into the weeds of your system to integrate your equipment and properly mitigate safety risks.

Reach out to our tech support if you already have a drive that isn’t performing.

For questions about your specific needs, call 1-855-207-1721 and talk to an application specialist or send us an email.