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smokies
02-07-2010, 09:20 AM
Anyone have an accurate and reliable VFD payback calculator they would like to share? I see a ton on the Internet, but everyone gives a different value. I know the rules of thumb, but would like better data.

freddy-b
02-07-2010, 09:42 AM
Anyone have an accurate and reliable VFD payback calculator they would like to share? I see a ton on the Internet, but everyone gives a different value. I know the rules of thumb, but would like better data.

I don't think there is one definiative calc. It depends on motor and application.

tshort
02-07-2010, 12:03 PM
You can make your own with these formulas and steps if you so choose.

1. First use speed to calculate the VFD Hz using Calc Hz = speed/1.6667
2. Calucate the Number of Poles for the VFD using # Poles = (Design Hz * 120)/Design RPM.
3. Next Calculate the RPM's at the speed using Calc RPM = (Calc Hz * 120)/# Poles.
4. Finally calculate the VFD Power at the speed using this approximation PWR = (Design RPM/Calc RPM)^3 * EFF

Note EFF = .55 @speed of 25%, .73 @speed of 50%, .8 @speed of 75%, & .85 @speed of 100%. You can use interpolation to calculate the EFF at various speeds.

Hopefully these calculations help, they will at least give you an idea of the different power.

You can also approximate the HP at a given speed using the following HP = PWR/.746.

viceman
02-07-2010, 06:13 PM
i have a honeywell vfd pay back whiz wheel.

i have never used it. :rolleyes:

tunaguy
02-07-2010, 07:48 PM
http://customer.honeywell.com/NR/rdonlyres/0841F8C0-4783-4E39-ABCB-C08FCECCEB51/1677/VFDQuickSavingsEstimator.xls

OpenSysTech
02-09-2010, 11:20 PM
[QUOTE=Anyone have an accurate and reliable VFD payback calculator they would like to share? I see a ton on the Internet, but everyone gives a different value. I know the rules of thumb, but would like better data.[/QUOTE]

Accurate... Figures don't lie, but liars can figure...

D1G
02-10-2010, 12:07 AM
FWIW we do a "lot" of Energy Savings Performance Contracts (ESPC). ESPCs have a validation and verification of savings requirement (in case you don't want to waste time reading up on them) that will quantify actual savings.

If you want very precise numbers you'll need to know a lot more than just motor HP, rpm, and efficiency. That said, tshort's calc's are pretty spot on.

Example:
If you're talking about a fan motor you'll need to know building load vs capacity. For instance: If you have an electronics lab and the internal load is predominately equal to capacity than a VFD isn't going to save as much since all of the VAVs would predominately be calling for full cooling placing the greatest demand on the system over the majority of operating time especially in a neutral climate and/or facilities with lots of internal zones. If it's a standard office type space where external ambient loads are playing a larger role you'd see a greater SIR (or shorter ROI) since the load on the motor is more dynamic allowing the VFD to reduce demand on a larger scale.

Any calculation without either load/capacity data (new construction) or historical data is going to make some potentially significant assumptions.

Perhaps a better example:
You have a pump that pumps water out of your tank and you are filling that tank at the same rate the pump will pump it out. You will see zero savings by installing a VFD on that pump. In other words the more dynamic the system (or the more it operates below design) the greater the savings.

R/ D1G

jslimjeff
02-17-2010, 10:17 PM
www.abb.us/drives

Goto Software and search for FanSave Pumpsave 4.1.

It is all as accurate as the data you input. Runtime percentages. Fan design and efficiency, motor efficiency, CFM, Static operating points. The Fan calc you can download has the ability to enter in all of the data. If you need help entering the data you can email me.