Energy savings with Inverter drive

[cfordyce]

Hello, I need an approx calculation of energy savings for a fan application. Currently the 44hp motor is run at line voltage and a damper is adjusted to get approx half the rated air flow from the fan (1/2 load). We want to install an frequency inverter to adjust the fan speed to deliver the needed cfm - eliminating the need to adjust the damper.

would moving 1/2 the cfm amount to approx 1/2 the load current to the motor? I know that the motor magnatizing current will remain constant at all loads and that amounts to approx 30% of rated current for a standard AC 3-ph ind mtr.

Mag I = 44A x .3 = 13.2A
therefore load current = 44-13.2 = 30.8A

So really, I'm asking - what does a standard fan curve relating cfm to load (hp) looks like? linear?

Thanks

CAF

 

[Skogsgurra]

Hi,

The magnetizing current should not neter into your energy savings calculation. It is a reactive current and does not consume active energy. There are some losses assiciated with it, but you can forget that for now.

The amount of energy you save with a VFD vs Damper is Head*Flow(Dmpr)-Head*Flow(VFD). So, if your head is 50 mm water and your flow is just right when you have adjusted the damper and flow is just right and head 30 mm water with the VFD, you have saved 40 % energy.

There are so called affinity laws and there are fan curves. Your ventilation supplier should have documentation on this. The curves are not linear. Not at all. Mostly quadratic, but there are deviations. More cfm means more head. And head usually goes up as cfm squared, if you run variable speed fans.

BTW, you cannot add magnetizing current to active current the way you do it. Since they are perpendicular, you shall use SQRT(Active^2 + Magnet^2) for total current.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[ozmosis]

Your VFD supplier should be able to provide free software to show the savings.
However, if you are only reducing the speed by 1/2 and not looking at actually varying the speed, a change of pulley ratio or motor speed (2 speed motor) would bring the same energy savings but a faster return on investment. and this is coming from a guy whose company manufacturers VFDs...prepare to be shot at dawn by the salesman firing squad!

 

[dcasto]

fan law says: 1/2 speed = 1/2 volume, =1/4 head, =1/8 work

 

[OperaHouse]

This is not the kind of work I do, but is there really any energy savings if you produce the same CFM?. Seems like you are trading windage losses for inverter losses. If you block a vacuum cleaner the load drops.

 

[dpc]

OperaHouse - you have to take into account the head loss across the damper. With a VFD, these losses are eliminated. The VFD does have some electrical losses, but these are generally much less than the loss across a partially closed damper or valve. Elimination of the throttling device is the main source of energy savings with VFD-driven pumps and fans.

These savings are often greatly overstated by salesmen, but there are real savings to be had in many situations.

 

[jraef]

Speed vs power is an almost linear function when done mechanically. Speed vs power is a quadratic function when done electronically. If you place the 2 curves on the same graph, the electronic power curve, being more quadratic, has a "bow" that dips below the mechanical (linear) curve, with the greatest delta being right in the middle at around 50% flow. That is where the energy savings lie. At the highest end however, i.e. 100% flow, the VFD losses actually do put its power curve over the mechanical curve. That is why when considering a VFD for energy savings, you must ALWAYS consider the running time at the various speed / flow rates. One of the biggest wastes is to install a VFD and run it at full speed most of the time!

 

[dcasto]

The VFD's I'm familiar with are about 95% eff. When you look at your power factor, you may gain even more on a monetary baisis because your PF goes to 1.0 with a VFD instead of say .9 with an induction motor.

 

[cfordyce]

THANKS ALOT.

CAF

 

[quark]

You can't tell that unless you have performance curve of the fan. If your corrected flowrate, now, with a damper control, is 50% and that is what you need, you have to go for higher flowrate using a VFD if you want to satisfy your final requirement of discharging the air to a place where it is needed. In some cases, I have experienced this to be as high as 80%.

 

[ozmosis]

dcasto
PF on a "standard" VFD will not be 1.0. I think you are looking at the fundamental PF and not the total PF. The total PF will be something between 0.6 and 0.92, depending on the topology of the VFD. Harmonic distortion will predominantly determine the total PF; the >the THD the lower the PF. Not all VFD's are the same.

 

[jraef]

Yes, from a pure sense, but the lower values come from high distortion power factor, not displacement power factor. The simpler displacement PF will usually be .95 or better and that is (usually) all that the utilities meter for because their PF meters are only based upon the fundamental frequency.

They'll probably catch up soon however. The net effect on their equipment is the same.