Buying Guide

So you’d like to pick up a new Variable Frequency Drive (VFD)? Finding the perfect VFD or motor controller can be a fairly daunting task as there are many variables with each application and system. However, below are some helpful tips and suggestions to help you select the right drive. Hopefully, the mystery becomes less complex as you understand what your application needs are.

 

The Big Picture

 

Before you get too far into the process of looking at VFDs, it is a good idea to gather the basic motor and system information that will help you in selecting the proper VFD. Most of this information can be found on your motor nameplate such as:

  • Horsepower
  • Full Load Amps (FLA)
  • Voltage
  • RPM
  • Service Factor
  • It is also helpful to know if the motor is inverter duty rated

Other information will be specific to the needs of your system and application such as:

  • Type of Load (Constant Torque or Variable Torque)
  • Speed Range and Control Method
  • Special Enclosure Needs

Some of this information may seem silly to look for; however, by gathering more information now, it will be easier to find and compare different drives and ultimately easier to pick the best Variable Frequency Drive for your needs.

 

Now that you have that information, it is time to look at some of the important specifications on each drive that will assist you in deciding what will work best for you.

 

Horsepower (HP)

 

Although it is important to size the VFD with the FLA of the motor(s) being controlled, knowing the horsepower of the load is a great way to search for drives that may fit your application to be further narrowed down by other variables.

 

Full Load Amps (FLA)

 

This is perhaps one of the most critical pieces of information to gather. Using the FLA rather than horsepower ratings is the proper way to size a VFD. You will want to compare the FLA of the motor with the amp ratings of each VFD you are considering. This is an area in which being conservative pays off and you will want to give your application a little bit of a cushion with matching the amp rating. This is especially important if you have a constant torque load or a load that is difficult to get started. Although it may cost slightly more to purchase a drive with a higher amp rating in the beginning, it will likely pay off over time in reliability and functionality.

 

Voltage

 

For three-phase input this one is easy. It is crucial to match the voltage of the VFD and motor to your available voltage on site. For the US low voltage application, this is usually either 208 VAC, 230/240 VAC, or 460/480 VAC. For medium voltage or other applications it would be a good idea to call into one of our application specialists or engineers to assist you in establishing your needs.

 

For single-phase input, it is important to take a few more things into consideration. If the load is 3 HP (roughly a 230 VAC motor with an FLA below 10 amps) or below it is likely that we have a drive built for single phase input in stock for you to hook up and run with. If your needs are greater than 3 HP, then it is possible to use a drive built for three phase input, however the drive must be properly sized for your application. This is because the diode bridge is meant to carry current through all three legs. Thus, the rule of thumb for sizing the single phase input on a three-phase drive is to use a VFD rated for 2 times the FLA of the motor. For example if your motor is a 10 HP motor with a FLA of 28 amps, then you would need to select a VFD with an amp rating of 56 amps which ends up being around 20 HP.

 

In addition, if continuous operation is a must, then the following should be specified:

  • +/- 10% voltage fluctuation
  • +/- 3% frequency variation

Type of Load (Constant Torque or Variable Torque)

 

Does your variable frequency drive application require a variable torque or constant torque drive?

 

If the equipment being driven is centrifugal, such as a fan or pump, then a variable torque drive will be more appropriate. Energy savings are usually the primary motivation for installing variable frequency drives for centrifugal applications, and variable torque drives offer the greatest energy savings.

 

For example, a fan needs less torque when running at 50% speed than it does when running at full speed. Variable torque operation allows the motor to apply only the torque needed resulting in reduced energy consumption, which is one of many VFD Benefits. Conveyors, positive displacement pumps, punch presses, extruders, and other similar type applications require constant level of torque at all speeds. In which case, constant torque variable frequency drives would be more appropriate for the job.

 

A constant torque drive should have an overload current capacity of 150% or more for one minute. Variable torque variable frequency drives need only an overload current capacity of 120% for one minute since centrifugal applications rarely exceed the rated current. If tight process control is needed, then you may need to utilize a sensorless vector, or flux vector variable frequency drive, which allow a high level of accuracy in controlling speed, torque, and positioning.

 

Speed Range

 

Generally speaking, a motor should not be run at any less than 20% of its specified maximum speed allowed. If it is run at a speed less than this without auxiliary motor cooling, the motor will overheat. Auxiliary motor cooling should be used if the motor must be operated at slow speeds.

 

In addition you should take care when over speeding a motor. Generally it is not a good idea to run the motor more than 20% above its rated speed. You need to check with the manufacturer of  your motor to ensure what speed range will still be allowed within their warranty. Keep in mind that you also lose torque as you go above the design speed. Additionally, you should make sure that you do not run your motor consistently above the FLA rating.

 

Control Method

 

With 2-wire control, only one switch is used to run the variable frequency drive. An open switch stops the drive, and a closed switch starts the drive. Two-wire control is predominately used in HVAC applications since it is able to maintain the RUN command to the drive during a loss of power, which enables variable frequency drives to automatically restart when power is restored. Plus, 2-wire control allows drives that have "power loss ride-through" to operate during a power drop that is 2 seconds or less in duration.

 

With 3-wire control, two switches are used to run the drive. One switch is needed to stop, and another to start the variable frequency drive. This is typical in an industrial application on a conveyor or process control where multiple stations can start/stop the VFD, but a single safety circuit must be made to allow this operation to start.

 

Speed Reference Alternatives

  • Speed Potentiometer – Allows the operator to set motor speed with pot.
  • 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 can also 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 3-phase AC.
  • Selector switch speed selection – Allows the operator to select from several preset speeds. Can also be used if the speed is being set via a PLC, and an analog output is not available.
  • Serial Communications – Allow variable frequency drives to communicate on a network, such as MODBUS, PROFIBUS, or DEVICENET  enabling drive operation to be coordinated and monitored from a PC.

Special Enclosure Needs

 

An important aspect of selecting the right VFD for your application deals with the environment surrounding the drive. There are a few types of certifications that drives are rated in including, Ingress Protection (IP), NEMA, and UL Type Enclosures. It is important to match your application needs to the right enclosure. This can be done by finding a stand-alone drive with the proper enclosure rating, or you can put your VFD inside of another enclosure.

 

Inverter Duty Rated Motors

 

Although running a VFD is the best way to control your motor, there are some issues that arise in motors as a result of the way a VFD works. PWM VFD’s don’t produce a sinusoidal waveform, but rather a digital output that can put additional stresses on the motor windings and bearings. Newer motors all use wire that has been designed to handle the high voltages that can occur due to a drive usage. Some additional recommended features of inverter rated motors include grounding rings, isolated bearings and special cooling features (such as a separate fan to cool the motor at low speeds, typically on a constant torque application). 

 

Custom VFD Buildups

 

At this time you may also consider other options and VFD Accessories such as:

  • Disconnect or Circuit Breaker
  • HOA (Hand/Off/Auto Switch)
  • Bypass
  • Line Reactor
  • Harmonic Mitigation
  • dv/dt filter

 

Final Recommendations

 

This guide is intended to be of use for general application sizing and is not intended to be a comprehensive guide. There are applications and loads that may require special sizing and consideration. When you are sizing or specifying a VFD for any application it pays to be conservative and leave some buffer room in the FLA and overload ratings. This is especially true if your load is hard to start or sees heavy loading during operation. If you have any questions about your application or in sizing a new VFD call and talk to one of our application specialists before purchasing.