Clarification on VVFD

Hi,

firstly VVFD is using for 2 purposes, 1st to save the energy and 2nd to have variable speed, In the case of VVFD both voltag and frequency are inrecesed and decreased simultaneously to maintain the touque produced by the motor constant.

V/F ratio of drive maintains the constant flux in the machine and that will take care of torque,

example

one water pump is capable of dilevering water of 1000 LPM at the pressure of 1.8 bar

now if i want to ruduce the pressre, either i have to reduce the speed of the motor or to close the output valve of the discharge pipe, in 2nd case motor will run at its rated RPM whereas in the 1st case motor itself run at some slower speed so that output can be of required value,

divyesh patel

Bombardier Transportation India Ltd

Vadodara

yes......

same thing also i had studied.............

if we reduce our frequency means, torque remains constant(since we will be definitely varying the Voltage also to make the flux constant), and the speed will definitely decrease since we are reducing frequency.......then how will be the power increased if the speed & torque behaves as like as mentioned by me above??????????

this is my question.........

Hi,

firstly VVFD is using for 2 purposes, 1st to save the energy and 2nd to have variable speed, In the case of VVFD both voltag and frequency are inrecesed and decreased simultaneously to maintain the touque produced by the motor constant.

Dear Friend

VVFD driven motor has a principle i. e.

V1/F1=V2/F2=V3/F3.......Vn/Fn

It means that the ratio of voltage and frequency must be remain same at any voltage or frequency. Then it comes that if you are reducing freq. from 60 to 50 hz then voltage output from VVFD also reduces and vice versa (if freq. increase above 60 hz.) In motor the power is defind as P=VI cosΦ, hence if you change freq. then voltage keep changing (IN VVFD only) and simultaneously the power (as per above formula)

So the motor can be derated by changing Freq. and hence by Voltage in case of VFD application.

Regards

Ramesh Kumar

Sr. Electrcial Engineer

Based In India