variable voltage inverter
(VVI) An adjustable frequency drive that controls the voltage applied to its inverter section. This variable DC voltage is then converted to the desired frequency in the inverter section. Also called six-step inverter and pulse amplitude modulation (PAM) inverter.

variable torque load
A mechanical device which requires a driving torque which varies directly with speed. At maximum speed, maximum torque is required. Little torque is required at low speed. Starting torque requirements are generally low.

A measure of the energy given to the electrical charges in an electric circuit. Also called electrical potential or emF (electromotive force) It is measured in volts (V).

voltage source drive
An adjustable frequency drive that directly controls the voltage being supplied to the motor. A large amount of capacitance is generally used to help regulate the voltage. This is the most common type of adjustable frequency drive presently being applied to HVAC applications.

A measure of the severity of the voltage notching in an AC power circuit.

volts per hertz
(V/Hz) The ratio of voltage to frequency. This is important in adjustable frequency drives because it determines motor operational characteristics.