VFD, variable torque application, Efficiency Calculation, 2 motors vs 1 motor

[jeffrey1085]

Anyone have any input or recommendation of a method to determine whether running 2 motors driving fans fed from 1 VFD at half speed is more efficient that running 1 motor at nameplate speed? My gut instinct is to say that running one motor at near nameplate speed is better than running 2 motors at half nameplate speed simple due to the fact that you are wasting energy creating a field in the second motor, basically the extra energy needed for the reactive power, and introducing some loses inside the VFD.

When I started digging for data for this evaluation I found that torque varies as the cube of input power for variable torque applications. So at half the torque I need 1/8th the input power. meaning that unless the motors run ALOT more inefficient at half speed then it would be much more efficient to run 2 motors at half speed from a VFD then 1 motor at nearly full speed. Based on the motor curves I have looked at the efficiency for 460V inverter duty rated motors, efficiency doesn't drop off significantly until roughly 30% or less of nameplate speed.

A few other variable I'm unsure how they effect this evaluation: how does VFD efficiency vary with respect to the motor speed (I'm assuming very little), I'm unsure what powerfactor does when you drop the speed of a motor down when it is fed from a VFD (again i'm assuming very little).

Anyone else looked at something like this? Any articles, formulas, other forum posts, and/or common sense that I can apply to this would be greatly appreciated.

 

[GoldDigger]

What you are not considering, I think, is that a fan running at half speed will not move half the air volume, especially against duct resistance.



Tapatalk!

 

[Jraef]

Rule #1: There is no free lunch.

Rule #2: Whenever someone insists that rule #1 is incorrect, refer to rule #1.

The work that your fan needs to accomplish is to move air. Moving air requires horsepower. How you get the HP is the only possibility of saving energy, but you cannot save any more than you waste. So given that you have two motors, which have losses, and two VFDs, which have losses, instead of just one motor with it's associated losses and no VFD, which of those scenarios is likely to have more waste?

Bottom line, your instinct is correct. And yes, motor efficiency, as well as VFD efficiency, drops as speed drops. There are too many variations in designs of both elements to predict exactly how much, but it is not insignificant as it relates to this application. As a motor LOAD is reduced, it's efficiency is reduced. It is not always dramatic until you get below 50% load (notice I am saying load, not speed), where it gets downright ugly, but I'll get back to that.

Also, the correct issue with the Affinity Law 1b states that the power required by a centrifugal load varies at the cube of the shaft speed speed. But what is often left out of that statement, as pointed out by GoldDigger, is that although the speed of the motor determines the air flow, it is not a 1:1 effect. So half speed does not equate to half flow, in fact Affinity Law 1a first states that Flow varies at the SQUARE of the shaft speed. So at 1/2 speed, you get 1/8th power, but only 1/4 flow.

So taking an overly simplistic view of your scenario, lets say you get Q flow at 100HP running full speed. If you replace them with 2 x 100HP motors, but each running 1/2 speed, you get two motors consuming 12.5HP each (so 25HP), but they are each moving Q x .25 so .5Q. So yes, you are using less power, but you are moving half of the air volume. If you wanted to move the SAME air volume as the one motor, i.e. 100% Q, then each motor must run at approximately 71% speed (.71 squared = .504 Q, x 2 fans). At 71% speed, each motor will consume 36HP, so 72HP, which at first glance makes it appear to work.

BUT, all of this has left out a LOT of losses throughout, both mechanical losses in the fans, turbulence and static pressure losses in the ducts in order to HAVE two fans, and then the extra electrical losses in the 2 motors and 2 VFDs. This is where the rubber hits the road on that motor efficiency drop I mentioned earlier. Remember how I said it gets ugly at below 50% load? Look at the above; EACH of those motors is now below 50% load. So even if you started out with a 90% efficient motor design, at 36% load, it could be as low as 70% efficiency now. So to make up for those losses in your equation and compare to running the 100HP motor at full efficiency of 90%, each motor is now going to draw 45HP, so 90HP total right there. Add in an additional 3% losses in the VFDs and we are up to 93HP, before we even BEGIN to factor in all the mechanical losses we introduced to make it all happen. All in all, you will be lucky if you even get close to breaking even.

In other words; See rule #2.

 

[Ragin Cajun]

Aside from "efficiency", there may be more compelling reasons for having two fans - process reduncancy/realibility.

There are at least two schools of thought on how to operate two redundant pumps; both operating at part load or one at a time.

Lots of options here as well. Operating both at reduced load will likely increase the lifetime of both and allow for faster responce for a fan/motor failure.

RC

 

[GoldDigger]

The redundancy argument makes sense if the air flow is critical and both fans are capable of handling the full load if required.
But if you switch to two smaller fans each loaded to near full power you are actually doubling the chances of a single fan failing.

Tapatalk!

 

[Jraef]

The redundancy argument makes sense if the air flow is critical and both fans are capable of handling the full load if required.
But if you switch to two smaller fans each loaded to near full power you are actually doubling the chances of a single fan failing.

Tapatalk!

Right. But I think the idea was to use 2 fans of the SAME power rating as the single fan, but run them at half speed. So ostensibly if one fan went down, the remaining one WOULD be able to deliver full flow if run at full speed. However, it's at the cost of much more than twice the single fan cost, considering that the single fan has no VFD at all, and as you say, a quantum leap higher in potential failure rate. So at what cost does the redundancy make sense? He doesn't have it now, so has it been an issue so far? These are all good questions worth pondering.