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

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