Generator specs aren't making sense

[Stainless]

Hi,

My first post here.

I have a treadmill motor that I intend to use for a wind or hydro project. It's an Icon B4CPM-0891 PM 130 VDC 2 HP motor that's rated at 5950 RPMs.

Using the calculator from the FAQ, here are the characteristics this motor should have -- 5950/130 = 45.8 RPM/V. So, it should offer 14 V for charging at 640 RPMs.

However, when I spin it in my drill press to 640 RPMs, I see 9.29 V. It doesn't approach 14 V until approximately 950 RPMs.

I had a look at the motor's brushes and armature, which appear fine. I also calibrated the drill press LCD, so I know that the display is accurate. I have another 96V Icon treadmill motor that doesn't have RPM information on its sticker, but also requires approximately 950 RPMs to reach 12 V charging voltage.

I am fairly new to DIY generation, so please forgive me if I've missed something obvious here... but this doesn't make sense to me.

[Flux]

This may well be the case. The nameplate quoted speed is probably the on load speed, it's not likely to be very efficient, the 2hp rating will be very short term. The no load speed could be significantly higher. The formula is not foolproof but gives a good guide for such motors as the Amtek.

If you are sure your drill speed is accurate then your measured figures will be correct, even at 640 rpm it is very fast for wind power. At 950 rpm it would be too fast for direct drive with wind. Could work for some high head hydro turbines.

Flux

[Ungrounded Lightning Rod]

Another way to look at it is that the nameplate VOLTAGE at the nameplate speed is the sum of the voltage from generation at the nameplate RPM and the voltage drop from the nameplate current.

This is at full load, so the current (and thus its voltage drop) is at its maximum, and thus the fraction of the voltage that's from the generation is minimum.  As flux points out, a treadmill motor doesn't have to be very efficient, so the resistive voltage drop might be substantial and thus the generated voltage quite low.

When you're running as a motor, the resistive losses are ADDED to the generated voltage.  When you're running as a generator they're SUBTRACTED from it.  This is a double-whammy when playing "guess my under-load output voltage" on a motor-to-generator conversion.