Answer / binay agarwal

VFD stands for 'variable frequency drive'.
Generally its speed is control by the input frequency,
For example in induction motor, VFD used.

Answer / sak

given to the answer above, i would like to add few things,
to start and to run the motor at different speeds we require VFD, coz variable speed can be achieved easily by changing the frequency, as change in voltage is complicated.

Answer / shahulsnfa

In addition to Variable speed operation, VFD can be used as
motor starters which normally makes the load to take only
one time of the rated current at the time of starting. in
large motors you may find VFD as starters also it reduces
the harmonic level in the AC supply to which the load is
connected.

Answer / p.subash chandrabose

VFD- VARIABLE FREQUENCY DRIVE
VFD can be used as Variable speed operation
VARIABLE SPEED ACHIVED BY AC MOTOR IN 2 STEPS
1. VARY THE FREQUENCY
2. VARY THE POLE
THE FORMULA WAS N=120F/P
Eg:N=120*50 /4
N=1500 RPM

WE CANOT CHANGE THE POLE EASSY
BUT WE CAN CHANGE THE FREQUENCY ACHIVE BY VFD.

Answer / mehul g. c.

vfd id variable frequency drives by that you can change
input frequency to motor or machine .

it is generally used in motor smooth speed control.
like conveyer, lift, electric overhead cranes, special
operational motors.

in that you can get different speed with changing output
frequency.

Answer / shivakumar

variable frequency drives, its used for control frequency &
aswell as motor speed.

Answer / sbb

VFD is defined as "Variable Frequency Drive". The main
purpose of VFD is to control the speed of the drive for
different applications.
The principle behind that VFD as we know that N=(120F)/P:

Where N=Speed
F=Frequency (Indian Freq. 50 Hz.)
P=Poles of motor.(Constant)

So speed is controlled by varying the poles of frequency,but
poles is constant and it will be designed when motor is
manufactured.but we can change the frequency though
PWM,Cyloconverters and by thyristor controlled circuits.
So for AC, speed control through VFD is by V/F controll.By
controlling Voltage or by Frequency or by both we can
control the speed.

In constant power region of 3-phase IM torque-speed characteristics, if frequency increases voltage A does not alter B decreases C increases D none of above

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