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.

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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