What is Variable Frequency Drive?
By K. Durgasharan, Director, Versa Drives
The Variable Frequency Drive for AC motors, also known as AC drive, is an electronic device
which receives energy from a fixed frequency fixed voltage AC source (either single phase or three
phase) and converts it into a variable voltage (or current), variable frequency three phase supply
for a three phase motor. In so doing it gives the ability to run the motor at different speeds. Hence
it comes under the category of variable speed drives.
Variable frequency drives come in a variety of configurations. At a basic level there are the single-
stage frequency converter drives such as the cycloconverters and multi-stage frequency converter
drives. Cycloconverters are frequency changers that convert ac input power at one frequency to
output power at a different frequency with a one-stage conversion process. Cycloconvertes are
usually thyristor based and used in multi-megawatt applications. Another single-stage frequency
converter is the matrix converter which uses high frequency bidrectional semiconductor switches
and pulse width modulation (PWM) techniques to accomplish the frequency conversion. The
switches are controlled by PWM to vary to magnitude both the magnitude and frequency of the
However the most commonly used VFDs are multi-stage converters. In these drives the input
energy from a fixed frequency AC source (the AC mains power supply) is converted to a DC form
by means of a rectifier and stored in a energy storage element in the drive. The variable voltage
(or current), variable frequency output is generated from this energy reservoir. Thus there is a
multi-stage AC-DC-AC conversion process. The following diagram illustrates the multi-stage
energy conversion process.
The energy source element could be an inductor or a capacitor. When an inductor is used the
inverter in the output is a current source and the drive is called a Current Source Inverter (CSI)
drive. When the energy storage is a capacitor the output inverter is a voltage source inverter and
hence the drive is called a Voltage Source Inverter (VSI) drive. The term ‘DC link’ is used to denote
the energy storage element as a DC voltage across the capacitor or a DC current in an inductor is
involved. VSI drives are more common than CSI drives. CSI drives are used in higher power
The output inverter is usually controlled by PWM techniques to produce an output voltage or an
output current of variable magnitude and variable frequency. These are termed as the PWM VSI
drives and PWM CSI drives respectively.
The most commonly used VFDs are the PWM VSI drives. These are used in applications which
range in power levels from a few watts to several hundreds of kilowatts. PWM VSI drives are
further classified as V/F drives or vector drives based on the control technique used. V/F drives
maintain a V/F ratio i.e. when the output frequency is changed the output voltage is also changed
in a fixed manner. This simple technique is adequate for many applications. Certain applications
such as variable torque loads (fans, centrifugal pumps) can generally be driven only by V/F drives.
V/F drives are also used to drive AC motors connected in parallel. V/F drives are extensively used
in energy saving applications.
Vector control drives are more sophisticated in terms of the control software and hardware they
employ. They control the AC motor magnetic field’s direction and magnitude to enable
instantaneous torque control. This enables vector control AC drives to be used in applications
which require full torque over a wide speed range and fast torque response; an area earlier
restricted to DC motors. Vector control drives coupled with PMSM and BLDC servo motors serve
the precision motor control applications market.
The PWM VSI drive is a versatile motor control equipment which can be used in any application
which requires variable speed. It is used in applications ranging from simple fan control to complex
high performance servo drives in machine tools. Combined with an AC Induction motor or BLDC
motor it forms a rugged and cost effective solution for most variable speed applications.