Energy Savings w/ AC VFD

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Thread Starter

stephen

My question is, if by installing an AC VFD on a centrifugal fan load, I reduce my motor speed,by reducing the frequency. The voltage, amperage, torque, HP & KW all will be reduced. saving energy but sacrificing speed.

If I alter my pulley sizes to increase the speed of the fan. Then install my VFD reducing my frequency & motor speed, my voltage amperage, torque HP & KW all will be reduced. the load speed will also be reduced bringing it back to the desired speed.

Motor with out VFD
25 HP,460 volts, 28.5 amps, 3535 rpm, 18.5 KW, 60 HZ, available torque 37.14 lbft, resistance 16ohms,drive pulley6.5", driven pulley6.5"

Motor with VFD
8.1 hp 270 volts,16 amps,2109 rpm,7.25 KW, 35.15HZ
available torque 20lbft, drive pulley 6.5"driven pulley 3.82"

I thought my resistance would stay the same at 16 ohms.

is their something I'm missing?
Thank you
Steve
 
S
Your approach tries to make energy from thin air. The only way a CDS in. a centrifugal blowing/pumping application will save you energy us if you have more volute and motor than your load needs. Then in a starter scenario, you're building more pressure than you need and throttling it away somewhere. By installing a VFD, you simply avoid paying to generate more pressure than you need. Simply adding a VFD to a fan that's already sheaved to the correct speed buys you nothing.
 
Power (whether measured in HP or kW) depends on the load (fan in this case). If the fan itself is rotating at the same speed, it will require the same power input.

What you are proposing will actually consume MORE energy than simply running the motor across the line since no power conversion is perfect. There are losses in the VFD that you have to factor in.
 
C

Curt Wuollet

The VFD idea save you energy if your fan is running harder than need be. Changing things to get the same output as before will almost certainly give you pretty much the same input as before. Pin is directly proportional to Pout adding losses.

Regards
 
Thank you all for your replies.

I'm aware I can't get something for nothing, but I thought I was giving up available torque.

looking at this question I think I see my mistake, a motor will only deliver the REQUIRED torque needed & the required torque hasn't changed. Am I correct?

also could some one show me my mistake on my motor calculations w/ the VFD.

thanks
Steve
 
B

Bill Schwarz

Motor current (power required) is proportional to the characteristics of the driven load. Varying motor speed with pulleys and VFD while keeping fan speed constant does not change the power required to spin the fan.
 
> If I alter my pulley sizes to increase the speed of the fan. Then install my VFD reducing my
> frequency & motor speed, my voltage amperage, torque HP & KW all will be reduced.

This statement is wrong. Torque is the rotary work done by the motor. Same amount of air moved = Same Work done by the fan = same torque required from the motor. You may even reduce the efficiency of the motor, requiring you to provide more energy to overcome losses.
 
sorry for the delayed responce

I read you post & it makes sence. my thought is that my motor is over sized for the job, my required torque is about 20 lbft the motors available torque is 37.14 ftlb. a lot of unuse torque, I'm thinking theirs some waisted energy? motors that are not fully loaded are also not as efficient. what i'm trying to do is keep the torque to speed ratio the same, hopping to maintain my EFF.Am I wrong in my thought?

Thanks
Stephen
 
If the fan requires 20lb/ft to move the volume of air you want, then the motor is providing 20lb/ft. If you put enough electrical power into the motor to generate 37.14 lb/ft, then that power has to go somewhere - So the fan will accelerate and push more air. Pushing more air uses the extra energy supplied by the motor. Forget about Fan Speed - Its about the amount of "Work" i.e. Energy, required to move a certain volume of air per second. (and overcome bearing friction, generate noise and heat etc etc)

This is basic high school physics.
 
I agreed what stephen said.

motor at a particular speed is running at some particular load.

By increasing the speed first and then reducing the speed is just like going a step ahead and come back to same.

Other Ideas is to control the speed according to temperature or pressure or by reducing the speed according to the ambient temp change.

NK
 
I realize this is an old thread but you guys are not correct.
Reducing the size of the drive pulley is like switching to a lower front gear on a bike and pedaling faster. It takes less work.

Reducing the drive pulley size and using the VFD to spin the motor faster (most are good up to 90 htz) WILL reduce amperage.
 
You're confusing "work" and torque. If I reduce the size of the front gear on my bike, it's easier to turn (less torque) but I have to make more turns (distance). Halving the torque will double the no of turns for the same distance. The product of torque and rotary distance is power. So the total power required (neglecting losses) doesn't change.
 
The biggest reason to consider changing to a VFD is if your existing drive, regardless of how its powered, is significantly oversized or if your load demands are constantly or consistently changing. For example, the load may drop off at night or over a weekend. Significant energy savings can be realized if this is the case. Anything else is merely an exercise in mathematics and inefficient expenses.

>My question is, if by installing an AC VFD on a centrifugal fan load, I reduce my motor speed
> by reducing the frequency. The voltage, amperage, torque, HP & KW all will be reduced.
> saving energy but sacrificing speed.
 
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