Answer / vipul dhruv

basically vfd is a soft starter which is used for induction
machines. there are two methods of control on which a vfd
works... one is the vector control & the other is the scalar
control method. your question does not concern this so no
explanation is required i guess... now the function of the
vfd is to maintain a constant flux in the air gap of the
machine it is used for and that can be achieved only when
the V/HZ ratio of the machine is kept constant.so a ramp is
created in a vfd by the controller which gives a constant
v/hz ratio... and the flux is kept constant... by flux i
mean the magnetic flux.. so as the frequency varies the
voltage has to vary accordingly to keep the ratio constant
which is fed into the controller....the controller is the
heart of the vfd...

Answer / ashok

yes voltage is reduced (out put) when frequency is reduced..

50 hz = 415v
25 hz = 270 v
0 hz = 0 v

if u reduce the frequency voltage reduce vice versa

Answer / rahul waghmare

I think in VFD When frequency increases voltage also
increased & vice versa

Answer / lallan chodhu

simply v proportional to 4.44 phi f n
v n f variable phi n 4.44etc constant.to maintain constant flux whole ratio v/f is changed agar sirf v badhaoge f kam hoga n vice versa.toh vfd ek prakar se v/f contriol hai

Answer / amrinder

arvind please give reason why voltage will increase??

Answer / arvind

Dear Amrinder
we know through v/f ratio, if frequency will reduce that
means the voltage will increase.

Answer / amrinder

yes voltage is reduced when frequency is reduced..normally
for 1 hz, 7.42 volts is reduced.

Answer / arvind

In vfd when frequency is reduce the voltage will increase.
it is difficult to say that upto what level it will
increase or decrease, but speed can be find out by using
the formula.
Ns=120f/p
where f is frequency
p is poles

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