The Right Buying Guide in Choosing Variable Frequency Drive

Variable Frequency Drive - (abbreviated as VFD) is a power electronics-based device for controlling the rotational speed of alternating current (AC) electric motor by controlling the frequency of the electrical power supplied to the motor. The variable frequency drive is also referred to as adjustable frequency drives (AFD), micro-drives, variable speed drives (VSD), AC drives, or inverter drives. As the voltage is varied along with frequency, these are sometimes also called VVVF (variable voltage variable frequency) drives. The right buying guide for variable frequency drive discusses how to choose VFD, factors to consider when buying a VFD, variable frequency buying guide for buyers, and specifiers. They have a wide variety of applications across commercial and industrial spaces extending from small devices and heating, ventilation and, cooling (HVAC) systems, to enormous hardware, for example, compressors, blowers, and water siphons in water treatment plants. They likewise have applications across industries, from mines to schools to office buildings. There are likely open doors for the drives to spare vitality, in any place where there are pumps and fans.

Types of VFDs

The three common types are -

  • Current Source Inversion (CSI) - used successfully in signal processing and industrial power applications.
  • Voltage Source Inversion (VSI) - Have poor power factor, are non-regenerative, and can cause motor cogging below 6 Hz.
  • Pulse-width modulation (PWM) - Most commonly used in industry because of its excellent input power factor.


  • Energy Saving
  • Closed-loop controlling
  • Limits starting current
  • Smooth operation
  • High power factor
  • Easy installation

How to Choose the Best VFD?

The start-up and slow-down speeds of electric motors are controlled by the drives and also to prevent overcurrent in the motor. These devices are extremely useful at increasing the motor's efficiency in a range of AC motor applications where speed control is crucial, and may even be needed in certain applications where energy conservation is the main concern. As there are many variables with each application and system, finding the perfect VFD can be a fairly intimidating task. The variable frequency drive buying guide gives some helpful tips and suggestions to help you select the right drive.

A) How big should the VFD be?

Instead of choosing upon motor horsepower, the size of the variable frequency drive should be chosen based on the maximum motor current at peak demand. The electronics inside the drives are affected by constant starting, stopping, and dynamic loads far more than the effect they have upon a full voltage motor starter and a local power bus. Hence, peak demand current should be used.

B) Which one to choose - voltage/frequency drive and a vector controller?

Picking a voltage/frequency (V/F or V/Hz) drive and a vector controller perhaps would be the first decision to make when choosing a variable frequency drive. Both the control methods may or may not be used with feedback such as a rotary encoder. On the whole, most VFD-controlled motors are operated in an open-loop scenario but take benefit from the drive's soft start and adjustable speed features. Compared to the vector controller, the V/F controller is simpler and often low in cost. Besides, it works well if the load is rather constant and the speed is set high enough to stop overheating of the motor. Whereas a V/F drive is typically designed for specific applications such as a variable torque fan or pump application. Applications such as hoists and conveyors may be programmed to use a constant torque V/F pattern.

More dynamic control of the motor is provided by vector control. The vector-controlled motor can be programmed to limit the torque applied by a motor shaft by independently controlling the output frequency and torque to prevent tool breakage in a drilling application also providing a steady speed for a conveyor independent of load. The drive must be set up with the proper motor characteristics and require tuning it to the system dynamics to properly utilize the vector control benefits and operate at the best efficiency.

C) How the drive slow down at a programmable rate?

The motor becomes a generator as the load increases and the stop time decreases, often causing overvoltage faults on the drive. Turning the regenerated voltage into thermal energy eliminates the problem of adding an external braking resistor.  The external braking resistors that will need to be sized properly are often needed in applications with high inertia, loads fighting gravity, high inertia. The heat will also need to be degenerated, often outside a control enclosure. So, the braking resistors mounting location and guarding against voltage and thermal hazards must be carefully considered.

D) How can Harmonic noise voltage be mitigated?

Harmonic noise voltage is created by all drives, and it gets worse as the drive load increases. The power quality meters can be used if harmonic noise is suspected, or perhaps even as a general installation process, that can measure the quality of the power system prior and after the installation of the variable frequency drive. The line reactors help to protect the wiring and motor from excessive electrical noise on the output side of the controller. An output line reactor is almost a necessity, if the system has a power cable between the controller and the more that is more than 50ft or if the motor is non-inverter-rated.

E) Which cabling should be used?

There's some consideration for cabling used to power a motor with a variable frequency drive. The cables will emit electrical noise and stimulate voltages in surrounding wire and cabling. As opposed to PVC, choosing XLPE insulation offers a better dielectric constant and high resistance, although it's less flexible. Besides, a low capacitance cable is also desired. A VFD cabling should always be routed away from any DC control voltage.

F) How does a VFD work with a Synchronous Motor?

A VFD works with an synchronous motor. But, a variable frequency drive for 3-phase motor is generally employed by most installations. It's a good choice to use inverter duty motors as they operate at a slower speed without overheating. Moreover, the internal wiring insulation of an inverter duty motor is designed to withstand greater voltage spikes. The inverter duty motors are the proper choice for use although they are more expensive.

Some of the expert tips to consider specifying VFDs

  • Determine if a variable frequency drive is right for your application.
  • Consider your reasons for choosing the drive.
  • Select the proper size for the load.
  • Be aware of braking requirements.
  • Determine I/O needs.
  • Select the proper control mode.
  • Understand your control profile requirements.
  • Know your options for communication.
  • Don't overlook installation and operating requirements.
  • Beware of harmonics.

Things to consider before selecting your VFD

  • As VFDs have restricted over-current capacity, so hard-to-start loads may necessitate an over-sized unit to meet higher current demands.
  • To know how many motors the drive control, you must estimate the total peak currents of all motor loads under the worst operating conditions your planned system will see. Size your drive according to this maximum current requirement.
  • If your applications require a quick start or an emergency stop of the load, high currents will be demanded of the drive. Over-sizing of the drive may be necessary.
  • A motor overheating could be a potential concern for any of your planned drive applications like reduced speed, constant torque.
  • As smaller motors are not as efficient as larger ones, so improvements due to the variable frequency drive will likely be readily apparent. However, since large motors consume much more power, even small increases in efficiency can lead to appreciable savings over the life of the motor.
  • In the case of volatiles, be sure all materials in use are resistant to chemical attack and are properly grounded; whereas for an atmosphere full of particulates, ensure proper sealing; and for high temperatures, allow for cooling.
  • Be mindful that you can have a drive that trips immediately in an over-current condition or one that keeps up steady engine force and decreases engine speed to keep up current required
  • How basic is it to understand load downtimes and have the option to do a detailed fault analysis.
  • When power factor correction capacitors are switched, they typically generate power disturbances, and drives can be negatively impacted. Isolation transformers or line reactors may be necessary.
  • While the variable frequency drive is operating, it's possible for the power source of your planned system will occasionally be switched. This might happen, for example, when loads are switched to standby generators during a power outage. A few drives can deal with a brief power outage, others can't.

Buyers and specifiers of VFDs should ask five questions before selecting a drive for an application from the Variable frequency drive buying guide

1) What is the application type? variable torque, constant torque, or constant horsepower

2) What are the power source characteristics? Low-voltage input options typically will be 115 V ac, 208 to 230 V ac, 380 to 480 V ac, or 575 to 600 V ac, and can run as high as 690 V ac.

3) What enclosure is needed? Most of the standard protection ratings that may be needed are represented by UL or NEMA types 1, 12, and 4.

4) Is the drive compatible with the process? Machine builders, users, and system integrators need to ask themselves two specific questions about the process: Is feedback required? What kind of communication is required?

5) What kind of support and service should be expected from the manufacturer?

A decent phone support network encouraging group of people and regional/local professionals is crucial in keeping up a production line's tasks and keeping workers occupied and comfortable. Easy serviceability and support can mean fewer efficiency losses and diminished downtime.


Users should select a variable frequency drive that matches their application. Applications can be categorized as variable torque, constant torque, or constant horsepower and users need to consider communication protocols and maintenance strategies when selecting a variable frequency drive.