Drives / Motors

How to select a variable frequency drive

Many drive choices are available from a variety of vendors, so some of the basics and best practices are important to follow.

By Tom Stevic, contributing editor

When choosing a variable frequency drive (VFD), several decisions must be made besides the obvious voltage and current selections. Even the name should be decided on, as it is often called a variable speed drive, adjustable speed drive, micro drive and inverter.

In general, a VFD takes an ac power source and converts it into dc power. The speed control portion of the drive uses the dc voltage to create dc pulses in varying frequency to drive the output motor at speeds other than the 3,600 rpm or 1,800 rpm or other speed depending upon the number of poles the motor was designed to operate at using a 60 or 50 Hz ac supply voltage.

How big should the VFD be? The size of the VFD should be chosen based on maximum motor current at peak demand and not chosen based upon motor horsepower. Constant starting, stopping and dynamic loads affects the electronics inside the VFD far more than the effect they have upon the local power bus and a full voltage motor starter. Therefore, peak demand current should be used. Manufacturers may continue to list hp ratings more as an historical rating than as a useful one.

Perhaps the first decision to make when choosing a VFD is to pick between a voltage/frequency (V/F or V/Hz) drive and a vector controller. Both control methods may or may not be used with feedback such as a rotary encoder. In general, most VFD-controlled motors are operated in an open-loop scenario but take advantage of the VFD’s soft start and adjustable speed features.

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The V/F controller is simpler and often lower in cost than the vector controller in VFD applications.  The V/F controller works well if the load is rather constant and the speed is set high enough to prevent overheating of the motor. A V/F drive is typically set up to use one of several preprogrammed V/F patterns that are designed for specific applications such as a variable torque fan or pump application. Applications such as conveyors and hoists may be programmed to use a constant torque V/F pattern.

The size of the VFD should be chosen based on maximum motor current at peak demand and not chosen based upon motor horsepower.

Vector control provides a more dynamic control of the motor. By independently controlling the output frequency and torque, the vector-controlled motor can be programmed to limit the torque applied by a motor shaft to prevent tool breakage in a drilling application and to provide a steady speed for a conveyor independent of load. To properly utilize the vector control advantages and operate at the best efficiency, the drive must be set up with the proper motor characteristics and requires tuning the drive to the system dynamics.

VFDs decelerate motors at a programmable rate. As the load increases and the stop time decreases, the motor becomes a generator, often causing overvoltage faults on the drive. Adding an external braking resistor eliminates this problem by turning the regenerated voltage into thermal energy. Applications with high inertia, high motor loads or loads fighting gravity often require external braking resistors that will need to be sized properly. The heat will also need to be dissipated, often outside a control enclosure, so the braking resistors’ mounting location and guarding from thermal and voltage hazards must be considered carefully.

All VFDs create harmonic noise voltage, and it gets worse as the drive load increases. The noise on the input, line side of the drive can be fed back into the plant power distribution system, so it is a good design practice to use input filtering. The filters are often available from the drive vendor or from third-party suppliers.

VFDs sold in Europe have much more stringent filtering requirements then those sold in the United States and other locations. Both passive and active filters are available to help mitigate this noise. If harmonic noise is suspected, or perhaps even as a general installation process, power quality meters are available that can measure the quality of the power system before and after the installation of a VFD.

On the output side of the controller, line reactors help to protect the wiring and motor from excessive electrical noise. If the system has power cable between the controller and the motor that is more than 50 ft (15 m) or if the motor is non-inverter-rated, an output line reactor is almost a necessity.

The cabling used to power a motor with a VFD deserves some consideration. The cables will radiate electrical noise and induce voltages in surrounding wire and cabling. A low-cost improvement would be to use shielded and grounded cables. Choosing XLPE insulation as opposed to PVC offers a better dielectric constant and high resistance, although it is less flexible. A low capacitance cable is also desired. By all means, VFD motor cabling should always be routed away from any dc control voltage.

A VFD will work with a synchronous motor, but the most common installations will use three-phase induction motors. Although general purpose induction motors may be controlled by VFDs, it's a poor design choice for many reasons. Instead, use inverter duty motors as they can operate at a slower speed without overheating. Plus the internal wiring insulation of an inverter duty motor is designed to withstand greater voltage spikes. Although inverter duty motors are also more expensive, they are the proper choice for use with VFDs.

VFDs have become almost a commodity item in control system design. A quick and dirty speed control can be purchased as a no-name VFD on the Internet for very little money. If the design requires greater reliability and longevity, it would be wise to pick a product that has a history of reliability and good support. One would hope that the more money spent on a VFD, the better quality the product. This is not always the case.

Major, well-known, controls manufacturers quite often private-label the products they build. In many cases, the same controller can be purchased for less money buying it from the manufacturer than buying it from another company that simply puts its brand name on the case. But a designer must also factor in support levels both locally and at the end user’s facility. Having field support costs manufacturers money and does add value to a product.

VFDs have a wide variety of programmable functions. Not all manufacturers call these functions by the same name. Research is required to properly utilize a VFD to its best performance. We are fortunate to live in an age where a great wealth of information is available on the Internet. Utilizing manufacturers’ websites and educational information, plus a little time, can make most any controls person an expert in VFD selection.

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