# Variable Frequency Drive (VFD):-
A variable- frequency (VFD) is a system for controlling the rational speed of an A.C motor by controlling The frequency of electric power supplied to the motor.
It is also known as Adjustable- frequency drives (AFD) variable – speed drives (VSD), AC drives ,inverter drives.
Operating Principle:
The synchronous speed of an AC motor is determined by the frequency of AC supply and number of poles in the stator winding. Thus the Synchronous motor operates at the synchronous speed determined by this equation.
Speed in RPM = 120 X f
P
Where, f be the frequency of AC supply.
P = Number of poles in stator winding.
Hence , speed of motor can be controlled by varying frequency of power supplied to motor.
VFD S ystem Description:
A VFD system consists of an AC motor, a controller and an operator interface as shown in figure:-
VFD Motor: VFD system consists of 3-Φ induction motor. Mostly, Induction motors are preferred than synchronous motor. Because induction motor are suitable for controlling and is cheap.
VFD Controller :- It is a solid state electronic power conversion device which first convert AC power in to DC intermediate by using bridge rectifier. This DC power is then converted in to quasi- sinusoidal AC power using an inverter circuit.
VFD Operator Interface:- An operator interface is used to start and stop the motor and to adjust the operating speed.
This VFD operating interface consists of an keypad, display, indication lights and meters to provide information about the operation of the drive. Motor can be controlled and monitored on computer by using an serial communication part on VFD operator interface .
VFD Operation :- When VFD starts ,the applied frequency and voltage are increased at a controlled rate or ramped up to accelerate the load without drawing excessive current.
This starting method allows a motor to develop 150% of its rated torque while drawing only 50% of its rated current.
When a motor is switched ON at full voltage, initially it have at least 300% of its rated current. While it produce less then 50% of its rated torque.
As load accelerates, the available torque usually decrease very little and then increases to a peak while the current remains very high until the motor get full speed.
By using VFD system, the frequency & voltage applied to the motor are ramped down at a controlled rate.
When frequency becomes zero, the motor is shut-off. A small amount of breaking torque is available to help decelerate the load.
Thus the speed of AC motor can be controlled by varying frequency of power applied to the motor.
