Power Saved by VFD

Lower speed uses less power.

Yes. You can. Look at the relationship between torque and angular velocity of the driven load, graph it across the speed range, and apply the principle of Conservation of Energy.

Alternatively, you could bung a wattmeter on the input wires to the vfd.

The choice is yours, really.

Unplug it.

That saves 100%.

The VFD is driving a 50 HP Blower motor of a Paint Booth wherein we need to maintain the air circulation velocity in the booth. If we reduce the speed we loose the specified air circulation velocity inside the booth,

You should have mentioned that in the first place. A VFD is not particularly well suited to that situation.

Since the motor is only 50HP and is required to operate at 100% speed, the energy cost savings may not warrant the expense of purchasing a VFD.

The only savings most likely to be realized would be on the utility demand charges if any apply.

Also: Because it is a fan application, very little mechanical wear would be eliminated by using a VFD.

Dear sir,

Thank you.

In centrifugal pumps running at constant speed can't we use VFD although sometimes load varies/fluctuates,will there be energy savings?.

The terms "constant speed" and "VFD" don't really belong in the same sentence.

It could be that a VFD was chosen to maintain a constant air flow thru out the life of the filters. It could also provide and alarm for filter change.

Varying the speed of the motor to maintain a set air flow would save energy.

To maintain a constant flow of air,do they employ a flowmeter with contacts for sending signals.

If current is proportional to load/flow in a centrifugal pump/fan can we do speed control by measuring current although current can increase due to mechanical problems too?.

Very well put as always Jeff.

There is no doubt that there are many misconceptions in the real world, and many overzealous sales people who believe that a VFD will reduce the power consumed by any motor on any application. "You can prove it by measuring the current into the VFD".

The reality is that you can only reduce power that is being wasted, and the VFD typically adds around 3% additional power losses, so it increases the losses by 3% of rated load.

If you consider modern motors with efficiencies up around 91% to 95%, then the losses in the motor are in the order of 5% - 9% and you add another 3% which is significant in the arena where "you will save power on every motor".

If, and only if, the driven load is operating with a significant loss for part of its operating cycle which can be reduced by slowing the motor without affecting the work done, you can save energy. For constant speed applications, you are best to mechanically modify the system to reduce the losses if possible. Reduce impeller sizes on pumps etc.

Claims about demand charge reduction by the use of VFDs are often incorrect. You need to understand the basis of the demand charge and it is not usually based on an instantaneous peak demand, rather a thermal demand with a 15 minute or 30 minute integral. The short time overload due to starting the machine is very small and has little impact. You still have to transfer the same amount of energy to the driven load to get it to full speed because you need to transfer the kinetic energy of the driven load at full speed.

Very rarely do you hear about the additional losses and costs resulting from the harmonics produced by the VFD. In this region, we are compelled to meet 8%THDi because of the issues these harmonics produce. This adds significant cost to the VFD and another 1 - 3% losses.

The use of a VFD increases (significantly) the losses in the motor and drive system and can only reduce losses in the driven load when the efficiency is improved by slowing it down for periods of the operating cycle.

I have been using VFDs in air blower applications, where previously an AC motor was running a blower at constant speed and then a damper used to choke the air flow down. We also have replaced eddy current drives with VFDs. Your local utility may also provide incentives for such conversions, if the cost savings are real.

From where do you get the input signal to VFD to adjust speed?.

In the applications I am speaking of the speed is set by the machine operator. If more air is required the VFD speed is increased. In the same way that they would have opened a damper in the past.

In your scenario you could look at an air flow meter with a 0-10 VDC output which could be used to adjust the speed of the VFD.

Nicely explained.I would like to to know, is there not any method other than speed changing for energy saving using VFD.