On Tue, 16 Sep 2008 11:41:30 -0500, Jon Elson wrote:

Karl Townsend wrote:
My apple water bin dump uses a .5 horse 3 phase 220 VAC motor
attached by pulleys and belt to an outboard motor prop in a tube to
pump water. Maybe 1"
head at 100+ GPM. I just measured the current draw on this motor at
1.5 amp,
2.0 amp, and 2.2 amp with a clamp A-meter.

Na, I'm not totally re doing something that has run 20 years. But I'd
like to try a different prop - it might be better. If not, I'll go
back. My only question is if this motor will have enough power at 1/2
speed. Do you know how to figure that?



I can guarantee you will not get 1/2 Hp out of a 1/2 Hp-rated motor at
30 Hz. You will get 1/4 Hp at rated load. It scales linearly with
frequency (speed) below the rating point. If you try to load the motor
to the rated mechanical output, it will draw twice the rated input
current (roughly) and smoke.

Jon

In this case available torque is more or less proportional to current.
Hence the need to halve the HP when you halve the speed.

--
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com

Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
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