Answer / surinder kumar

Hi Below is small write-up in answer of your query

A Variable Frequency Drive (sometimes abbreviated VFD) is
system for controlling the rotational speed of an
alternating current (AC) electric motor by controlling the
frequency of the electrical power supplied to the motor. A
variable frequency drive is a specific type of adjustable
speed drive. Variable frequency drives are also known as
adjustable frequency drives (AFD), variable speed drives
(VSD), AC drives or inverter drives.

Variable frequency drives operates under the principle that
the synchronous speed of an AC motor is determined by the
frequency of the AC supply and the number of poles in
the stator winding, according to the relation:

RPM = 120*f/P

where
RPM = Revolutions per minute
f = AC power frequency (Hertz)
P = Number of poles (an even number)
Synchronous motors operate at the synchronous speed
determined by the above equation.The speed of an induction
motor is slightly less than the synchronous speed.The motor
used in a VFD system is usually a three-phase induction
motor.Variable frequency drive controllers are solid state
electronic power conversion devices.

The usual design first converts AC input power to DC
intermediate power using a rectifier bridge. The DC
intermediate power is then converted to quasi-sinusoidal AC
power using an inverter switching circuit. The rectifier is
usually a three-phase diode bridge, but controlled
rectifier circuits are also used. Low power models are often
configured for use with single-phase input power.

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