Introduction

The term AC drives covers a wide range of drive types. When talking mostly industrial terms, an AC drive could also be considered a variable frequency drive (VFD), adjustable-speed drive (ASD), variable-speed drive (VSD), and "inverter." If a technician was discussing ASDs and the factory contained mostly DC equipment, then ASD or VSD would refer to a DC drive. The term inverter is actually the final or power section of the drive and is considered an acceptable term for the entire unit.

There are actually many different types of AC drives, but all of them have one concept in common they convert fixed voltage and frequency into a variable voltage and frequency output.

Though they do not meet the strict definition of an AC drive, reduced voltage starters (e.g., soft starts) and wound rotor slip recovery units fall into the variable-speed category. Soft starts immediately deliver line frequency to the motor but at a reduced voltage value for a specified period of time. The result is reduced motor torque, as the motor is accelerating.

Load commutated inverters (LCIs) and cycloconverters are also part of the variable-frequency drive category. Cycloconverters actually use SCRs in very large horsepower amounts that require regeneration. Though the output of a cycloconverter may not be considered variable, the unit does alter the frequency output, thereby reducing the speed. This unit would actually step down the frequency to 1/2 or 1/3 of the line frequency. This frequency would then be applied to...