Fan Curve for ECM Blower Motor

[AxisCat]

Hi All,

Haven't posted in a while but I ran into something I am not understanding. And deep down I probably know the answer is way more involved than simple math but I am curious to know your thoughts anyway. Here is the problem. I have an air delivery table from a manufacturer that gives air volume vs SP. Keep in mind this is a variable speed ECM type motor. In the old days I got real fan curves with PSC motors and I could plot my system curve on it and get the operating points. Here is the their table:

I plotted this table in Excel and got this:

The manufacturer also states the RPM range of this motor is from 300-1300 RPM. So my first assumption is at around .6 SP the motor is running at full RPM and we see the normal drop off as the static pressure increases towards 1". From .6 and below the motor has to be ramping down in RPM to give the flat line we see in the chart.

What is did next was to use Fan Law #2 to estimate the motor RPM at the lower static pressures:

Then I solved fan laws number 1 and 2 in terms of RPM2, set equal to each other and all the RPMs canceled out leaving me with CFM2/CFM1 = (SP2/SP1)^.5 ... Have no idea if that works but I got this:

Obviously this is wrong. Like I say I am probably over simplifying what is probably a complex problem to solve. Anyone have any thoughts on the matter?

Thanks for reading,
Axis

 

[Snickster]

My guess would be that the flows shown are the maximum flows at maximum RPM of 1300. The manufacturer's chart states that they are showing maximum flows and static pressures.

If you plot the static pressure on the y-axis and the flow on the x-axis the curve will more resemble a typical blower curve. The pressure rises as the flow decreases to shut-off. As the static pressure decrease the flow increases to end of curve where a point is reached that no matter how much more you decrease the static pressure the flow does not increase any more.

 

[AxisCat]

Thanks for the reply! You are right about the axis being flipped, i was doing a lot charting will excel and didn't post the one i wanted to. But if you think about it the shape of the curve and relationship doesn't change when you flip the axis. You can take my curve above, rotate it 90 degrees and then mirror it. You get the axis flipped but the shape of the curve remains the same. I guess it is all about your point of reference.

But anyway, this was more for my curiosity than anything else. I might chase it some more and if i come up with anything will post it.

 

[HVAC-Novice]

Plot in standard way with flow on X-axis. That is how all other fan curves plot.

Second, have one curve for each rpm. Each curve is for the SAME rpm. Looks like you are mixing different rpm.

Looks like that motor has some sort of control to keep SP fixed until it runs out of capacity. it either employs a pressures sensor, or some algorithm measuring the current and deriving a flow from that. Those fans also have a mode to just run in fixed speed.

What is it actually you want to accomplish here?

 

[Compositepro]

Is not maximum static pressure measured at zero flow, and maximum flow at zero static pressure?