Thanks for your ideas. I also posted this question on sci.electronics.design and got several responses in a good discussion. My specific comments follow: --- In electricalmotordesigners@yahoogroups.com, "George Tyler" wrote: > > Hi Paul, > > I waited for a while before replying to this to see what others > said, but I guess they did the same! Perhaps if I say what I think some > others that have a better understanding will correct me. > > Motor power increases with diameter squared, losses are > proportional to dia: therefore larger dia motors are more efficient. I > actually thought the more poles (= lower speed) the more weight. > Yes, this is true, especially for smaller motors (1-5 HP), where a six or eight pole motor may be twice the weight of a two or four pole. Above 20 HP or so, there is not so much difference. A good explanation was that the slower motors have higher torque, and the frame must be able to handle it. I think there is also better winding efficiency with higher number of stator slots, which is more expensive to build and wind for small motors. > 1) Is 20 out of the 40hp motor done by running at a lower frequency > and speed, or is it by reducing the torque? If it is by reducing the torque > the efficiency will drop, especially with multi-pole motors will higher > magnetising current. With some multi pole motors the current does not change > much with reduced load, though the power does as the power factor changes. > The I2R losses in the stator then don't change much. > I would be running at constant torque with a V/F drive. I would expect less copper resistance losses for a larger motor, but possibly higher mechanical losses. Also, usually larger motors have higher efficiency to begin with. > 2) At 2X torque you could have 3X current. (from Alternating current > machines: M. G. Say). Duty cycle? > So if losses are proportional to the square of current, a 90% efficient motor would have only 10% efficiency. But if only half of that is is related to copper losses, then it would have 9 * 5% or 45% losses or 55% efficiency, which is closer to what I've seen from my initial research. > 3) Increase the frequency. Say says that for a given speed a higher > pole count with higher F is more economical. > That is my idea, to make a high pole motor and use PWM to run it at 4x to 8x (of nominal 60 Hz). Essentially making a 400 Hz motor that will also run at lower speeds with similar torque. I've heard that 400 Hz motors are about 2 lb/HP. I would like to make a 20 HP motor that weighs only 40-50 lb. Thanks for the ideas. I will be doing some more research. BTW, there is a fairly lively discussion of motor concepts on the group: > > _____ > > From: electricalmotordesigners@yahoogroups.com > [mailto:electricalmotordesigners@yahoogroups.com] On Behalf Of eti_paul > Sent: Tuesday, 16 May 2006 8:39 a.m. > To: electricalmotordesigners@yahoogroups.com > Subject: [EMD] Efficiency of 3 phase AC Induction motors > > > > I have noticed that larger motors (40+ HP) are typically more > efficient than smaller motors, and also there is not as much weight > difference for 6 and 8 pole motors compared to 2 and 4 pole. I have > several questions: > > (1) Will a 40 HP motor rated at 90% efficiency have better efficiency > at 20 HP? What about at 10 HP? > > (2) What sort of efficiency can be expected if a motor is run for > short times at 2x or 3x nominal torque rating? What are typical duty > cycles and maximum ON times for such conditions? > > (3) What could be done to make a 12 pole motor (about 10 HP) about the > same size and weight as the same HP 2 or 4 pole motor? > > Thanks for any ideas. > > Paul E. Schoen > (also pstech-paul) > > > > > > >

Electrical Motor designers

 > > > > > > _____ > > YAHOO! GROUPS LINKS > > > > * Visit your group "electricalmotordesigners > " on the web. > > * To unsubscribe from this group, send an email to: > electricalmotordesigners-unsubscribe@yahoogroups.com > ribe> > > * Your use of Yahoo! Groups is subject to the Yahoo! > Terms of Service. > > > > _____ >