How-To: Choose a VFD for a Motor

With the growing complexity and connected nature of today’s manufacturing and warehousing environments, control systems are being asked to do more and more to improve efficiency and performance.  When adding or upgrading a control system, the proper selection of the right variable frequency drive (VFD) for your motor is crucial. 

While there are many options for VFDs from a variety of manufacturers, there are many things to consider when choosing a new VFD.  These include motor considerations, VFD selection and how the VFD can or should integrate with PLCs.

What Do You Need from the Motor?

Before diving into the VFD itself, it is helpful to understand as much information as possible from the motor itself.  Most of the critical information is included on the nameplate and should be easily accessible.  This information will inform the buyer and provide a foundation for selecting the proper VFD by helping narrow down options for the optimum sized unit.

  • Horsepower – Even though it is not the most important factor, horsepower ratings impact which VFD you select.  VFDs will state the HP range that they can service, allowing you to exclude those over or underpowered for your application.
  • Full Load Amps (FLA) – Full load amps is one of the most important motor considerations in choosing a VFD.  Motor FLA will determine the sizing of the inverter to be used and you will want to compare this number with the amp ratings of any VFD under consideration.  For loads that have a high torque (inertia) to overcome or loads that may be especially hard to start due to the weight being moved on the motor, having the inverter sized a bit larger will contribute to a cooler running VFD.  This will result in a more reliable VFD with a longer lifespan.
  • Voltage – In the US, low voltage applications are usually 208 VAC, 234/240 VAC or 480 VAC for three-phased input motors.  If the input is single phased, there are a few more issues to be aware of.  For one, drives built for 3 -phase input can be used if correctly sized for the application.  Here, you would need to size the single phase for a VFD with twice the FLA of the motor used.  There are also drives designed especially for this situation.  For single phased motors running loads of less than 3HP, there are many options available such as the Allen-Bradley PowerFlex 40 AC drive that provides a single-phase input with a 3-phase output.    
  • Speed Range – While variable speed functionality is common in automation systems, there are both absolute and practical limits.  Motors should not be ran under 20% or over 20% of their rated speed.  Running motors under 20% of rated speed will cause overheating and can lead to failure.  The only way to address this is by using auxiliary cooling.  Likewise, over speeding the motor can result in torque loss and damage over time.

What Do You Need in a VFD?

With choices narrowed down by understanding motor specifics, you can move on to specifying the VFD itself.  VFDs cane be low, medium or high voltage and the type of application will determine which unit is selected.  These are a few things to consider when specifying the VFD.

  1. FLA – As discussed, the FLA of the motor will have led you to a range of VFD options.  It is also important to note that if an AC motor is used with a VFD, then the motor must be three-phased.  This is because the output by a VFD is always three-phased regardless of whether the power for the VFD is single-phased or three-phased.
  2. Overload Capacity – Factory automation often requires brief overloads or high torque starts depending on application.  If the motor must move to full speed quickly, it may require an overload to bring it to the appropriate speed within a short amount of time.  Generally, AC drives are rated for 150% loads for short periods of time for up to 60 seconds.  If the application requires longer overloads, the drive must be sized to match.
  3. Braking Requirements – For the same reason that drive consideration should include overload capacity, consideration should be given to braking requirements.  If a load carries only a moderate amount of inertia, then the risk of over-voltage in deceleration is not a factor.  When high-inertia loads are incurred, the VFD will extend deceleration over a longer period automatically.  However, if the load is especially large and heavy and if it requires rapid deceleration, a dynamic braking resistor should be used as well. 
  4. Load Type – Depending on application, the VFD chosen will need to control either constant or variable torque.  A VFD can be chosen based on the load type to help control energy use in variable torque situations such as a fan that only requires half the torque when running full speed as it does when starting.  Likewise, conveyors and certain pumps and other devices may require constant torque drives.  While selecting a drive for the specific load type is important, most drive manufacturers offer flexibility in selection.  For example, Allen-Bradley’s PowerFlex 6000 series is configurable for both constant and variable torque applications.
  5. Temperature – Many people have been tripped up by failing to consider temperature in drive selection.  Because AC drives generate a lot of heat, the maximum ambient temperature should be included in planning considerations.  This also impacts enclosure choice, enclosure ventilation and other issues as well.
  6. Control Mode – There are three control modes in VFDs, and selection will depend on application.  Volts per Hertz (V/Hz) use voltage and frequency ratios to supply motor torque based on operating flux.  Sensorless-vector VFDs apply and control torque over a large speed range with no encoder feedback.  And closed loop VFDs use encoder feedback to determine motor speed.
  7. Control Method – Because automation is all about application, how you control the VFD can make troubleshooting, programming and other functions faster, easier and more efficient.  Today’s VFDs work in conjunction with PLCs and other I/Os in a connected factory environment so control method also takes on a critical function in data capture.  This may take the form of EtherNet communication which will allow VFDs to communicate on networks such as TCP/IP, ModBus EtherNet/IP and many others. 

Control method may also be in the form of a programming or display unit to allow changes and monitoring of the drive to be done at the point of use by an operator or technician.  Other control methods include serial communications, Speed Potentiometers, and simple 2 or 3 wire control for simple functionality of stop/start.  And with growing requirements for modularity that allows integrations into large control networks and IoT initiatives, VFDs such as Allen-Bradley’s PowerFlex 400 series and others offer modularity to expand capabilities and enhance performance of a network.

  • Communication – Linked to the discussion of control methods is the communication method.  As more companies explore IoT connectivity, it is important to communicate with all devices in the data stream.  Modern VFDs offer built in communication ports that range widely from simple serial communication to EtherNet and fieldbus communication protocols as well.  By linking communication, the VFD can be controlled with the network interface without having to rely on dedicated I/O ports.
  • I/O Requirements – As today’s VFDs are usually part of a larger control system, most come with a few I/Os.  These range from lower end for small applications through higher-end drives that may have many I/Os as well as communication ports.  In selecting a VFD, one needs to consider not only the application, but also its place within the automation system.  This may require sizing a VFD for a complex automation system to accommodate more I/Os whereas a standalone VFD for the same sized motor would have required a smaller number of I/Os. 

VFDs may have both discrete I/Os to allow interface with devices such as control buttons, selectors, and other machine functions.  as well as analog I/Os which can be used to provide setpoints for other VFDs in the system to follow, essentially establishing one VFD in a server role. 

Integrating VFDs with PLCs

The integration of PLCs with VFDs is intuitive for today’s automation projects.  Because PLCs run programmed instructions for the automation system, many of those instructions will be for control of the VFD.  Because of this, manufacturers for VFDs have improved their ability to integrate with PLCs for data control, command and other functions.  This means that current VFDs may have capability to execute programs as well as control the motor.

Because current VFDs contain features like the functionality of some PLCs, it has become easier to integrate the two into a dynamic system for automation control.  VFDs can now control acceleration rates and deceleration rates more precisely and the addition of open-looped and closed-loop vector control allows better control over motor speed and torque.  Many can also initiate position control as well.

The same is true with the arrival of added I/O onboard VFDs as it allows for remote or external control over the VFD through a variety of end uses.  Combined with added on-board fieldbus or other communication protocols such as EtherNet, PLCs can allow remote access or access from a variety of locations to the entire set of VFD commands. 

While VFDs have their own function and are not intended as replacements for PLCs, having some PLC functionality mand the capability to integrate with PLCs in the device network enhances system efficiency.  It also means that programming of VFDs within a system could result in a reduction of programming needs because these profiles can be duplicated.  It is common to use VFDs such as Allen-Bradley’s PowerFlex drives with AB PLCs such as ControlLogix and CompactLogix to better integrate control systems in automation projects.

Another promising path is VFDs that act almost as “smart” VFDs allowing further integration with PLCs and other controllers within an automation environment.  VFDs such as Allen-Bradley’s low voltage PowerFlex 753 and 755 as well as their PowerFlex 6000 and 7000 medium voltage drives offer Studio 5000 Logix Designer® software that can be used to program across the entire Logix network within a system.  They act like a micro PLC and offer flexibility in application. Choosing the right VFD is a critical decision for a control system.  By understanding the motor requirements, buyers can narrow down choices to those that fit their required motor.  By working through the above considerations, they can identify an ideal VFD for the application based on the motor’s specifications and what the VFD needs to do based on the equipment it is powering.  And by integrating the VFD into a control system and linking it or using it in conjunction with PLCs and other controllers, VFDs can offer more today than their counterparts from years past.


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