Determine if a VFD is right for your application.
The primary function of a variable frequency drive is to vary the speed of a three-phase ac induction motor. VFDs also provide nonemergency start and stop control, acceleration and deceleration, and overload protection. In addition, VFDs can reduce the amount of motor startup inrush current by accelerating the motor gradually. For these reasons, VFDs are suitable for conveyors, fans, and pumps that benefit from reduced and controlled motor operating speed.

A VFD converts incoming ac power to dc, which is inverted back into three-phase output power. Based on speed setpoints, the VFD directly varies the voltage and frequency of the inverted output power to control motor speed. There is one caveat: Converting ac power to a dc bus — and then back to a simulated ac sine wave — can use up to 4% of the power that would be directly supplied to a motor if a VFD were not used. For this reason, VFDs may not be cost-effective for motors run at full speed in normal operation. That said, if a motor must output variable speed part of the time, and full speed only sometimes, a bypass contactor used with a VFD can maximize efficiency.

Consider your reasons for choosing a VFD.
Typical reasons for considering VFDs include energy savings, controlled starting current, adjustable operating speed and torque, controlled stopping, and reverse operation. VFDs cut energy consumption, especially with centrifugal fan and pump loads. Halving fan speed with a VFD lowers the required horsepower by a factor of eight, as fan power is proportional to the cube of fan speed. Depending on motor size, the energy savings could pay for the cost of the VFD in less than two years.

Starting an ac motor across the line requires starting current that can be more than eight times the full load amps (FLA) of the motor. Depending on motor size, this could place a significant drain on the power distribution system, and the resulting voltage dip could affect sensitive equipment. Using a VFD can eliminate the voltage sag associated with motor starting, and cut motor starting current to reduce utility demand charges.

In addition to varying speeds, conveyor applications typically require frequent starting and stopping. Here, VFDs substantially reduce starting current to extend motor life.

Controlling starting current can also extend motor life because across-the-line inrush current shortens life expectancy of ac motors. Shortened life cycles are particularly prominent in applications that require frequent starting and stopping. VFDs substantially reduce starting current, which extends motor life, and minimizes the necessity of motor rewinds.

The ability to vary operating speed allows optimization of controlled processes. Many VFDs allow remote speed adjustment using a potentiometer, keypad, programmable logic controller (PLC), or a process loop controller. VFDs can also limit applied torque to protect machinery and the final product from damage.

Controlled stopping minimizes product breakage or loss, as well as equipment wear and tear. Because the output phases can be switched electronically, VFDs also eliminate the need for a reversing starter.

Using VFDs to operate fans and pumps can significantly reduce energy consupmtion, because they can tailor fan speed to the application. Fan horsepower is proportional to the cube of fan speed, so depending on motor size, energy savings can compensate for the initial VFD purchase price in less than two years.

Select the proper size for the load.
When specifying VFD size and power ratings, consider the operating profile of the load it will drive. Will the loading be constant or variable? Will there be frequent starts and stops, or will operation be continuous?

Consider both torque and peak current. Obtain the highest peak current under the worst operating conditions. Check the motor FLA, which is located on the motor's nameplate. Note that if a motor has been rewound, its FLA may be higher than what's indicated on the nameplate.

Don't size the VFD according to horsepower ratings. Instead, size the VFD to the motor at its maximum current requirements at peak torque demand. The VFD must satisfy the maximum demands placed on the motor.

Consider the possibility that VFD oversizing may be necessary. Some applications experience temporary overload conditions because of impact loading or starting requirements. Motor performance is based on the amount of current the VFD can produce. For example, a fully-loaded conveyor may require extra breakaway torque, and consequently increased power from the VFD.