Answer / navin

VFD uses the principle of varying the frequency.
as in motors slip speed ,Ns=120F/P, the only thing we can
change is frequency as poles are built up, even though
Scharage motors are available which can change the poles
also, but it is convenient to change the freq. rather then
poles.
By changing the frequency i.e. reducing it we reduce the
speed, hence torque & hence the current drawn.
Thus reducing the power consumption.

Answer / anubhav sinha

VFD's save power by allowing the motor in consideration to
run at it's optimum speed for the particular load
condition. If we continue to run the motor at it's rated
speed ( rated at supply frequency) even when the load
demand has decreased we would be having unnecessarily high
losses which could be reduced if we use VFD. It is most
advisable in pump or fan applications where power consumed
is a function of cube of the speed.
For example consider a pump running at rated value, now if
we have to control the flow to 50% (as per requirement) one
option (traditional) is to go for dampers, valves to
mechanically control the flow, but the motor running the
pump is still running at full load if it's a constant speed
motor. Whereas if we use VFD the power consumption is only
(0.5)^3 times .. hence we save about 85% of energy compared
to constant speed motor.

Answer / devansh

The above answers only point to theory of VFD. The energy
saving part is derived while the starting of an induction
motor. For a system that requires a high starting torque,
we need a DOL(direct online starter) which may lead to a
high loss while making a provision to limit the initial
current too. A better method is to lower the frequency.
Since torque is inversly proportional to freqency, the
initial high torque is achived, and when the machine has
gained speed, the frequency can be regulated as per
requirement. I hope this one is more satisfactory...

Answer / eswara rao

There are some applications where the the discharge
of a pump/fan are not constant. for the required output, it
is required to control the output valve or damper. For this
type of applications, the motor takes almost same power
whether the output is 0% or 100%
For this type of applications, VFD's are more
advantagious. Instead of controling the discharge by
controlling the output valve, if it is controlled by motor
frequency it saves the power according to the speed of the
motor.
VFD draw the power according to the speed of the
motor. So, for variable discharge applications, VFD's are
suggestible for smoother operation along with power saving.

Answer / kapil mishra

in vfd we change the speed of motor by doing change the
frequency.the power consumpson is praporsanal to speed3 .
thah is why when you run the motor as your required speed
means less than the full speed of the motor you save the
power.

Answer / sivaraman

ofcourse by vfd v can control speed. but if v use t 4 enrgy
consumption y dont v go for low speed(i.e low rated)motors
instead using a high rated one n to reduce its speed by
vfd. so its used onli to speed control not in the point of
view of energy saver. for that we can use capacitor banks

Answer / gopalraju

The speed is purely dependent on v/f constant,until this
state the stator back emf generates z the impedence due
core magnetisation(in order to keep the unsaturated core
with in impedance window)other wise there cannot be back
emf ,hence dc component will start ie ohms law ,hence for
every one volt of increase one ampere current due to this
stator winding gets burnt .
Gopalraju

Answer / pillay

It depends on what type of load your motor is driving.
I agree with eswara rao and Anubhav Sinha.
If the same job can be done at a lower speed
and also you can program the vsd to act
in a manner when to run faster or slow down
automatically so when well programmed you
definetly will enjoy savings.

Answer / bhagavatsinh

power propotional to cube of speed so if you changed speed propotionaly power you can save. but drive is major equipment for generation of harmonics in system and it decrease your powerfactor and create a resonance in system so you have to consider this drawback during design of drive motor unit for energy saving and system.protection

Answer / girish kumar samal, hindalco,h

in vfd we change both frequency & voltage. as we control
frequency so speed will control * hence torque &hence
current. as both current , voltage controlled so energy
saved.

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