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 5-10 HP) about the same size and weight as the same HP 2 or 4 pole motor?
Depends on the cooling, a small motor like this would likely burn out within 30 - 90 secs at 3x load, efficiency is very poor less that 15% typicaly.
The more poles you have the slower it runs, so for the same HP you have proportionaly more torque. This means you need a bigger shaft and bigger frame to handle it.
----- Original Message ----- From: Newsgroups: sci.electronics.design Sent: Monday, May 15, 2006 6:33 PM Subject: Re: Efficiency of 3 phase AC Induction motors
I thought possibly a motor run at lower than rating would have less copper heating loss, but I seem to recall that motor current stays fairly constant over a wide range of HP output, so the VA is about the same but phase angle shifts closer to unity at the rated HP. This effect may not occur in PM BLDC motors and other designs.
That jives with my initial thoughts. I am more familiar with transformer overloading, which might handle a 3x overload for about 3 minutes with a duty cycle of 10%. Of course a motor at 3x would be running at close to its breakdown torque, at which it would become 0% efficient with locked rotor.
My conclusion is that it is probably best to specify a larger motor that would not be pushed any more than about 1.5x to 2x, for no more than about
2 minutes. Hopefully, the efficiency at fractions of maximum HP will not be too bad. This for a vehicular application, where loads change drastically, and efficiency is a major factor. There is already a loss of efficiency because of extra weight of a larger motor.
That is one of the best explanations I have heard. I had thought it was more a function of winding efficiency, with more overlap in higher pole numbers, but less so in larger motors with more stator slots. Is that also a contributing factor?
I noticed that the 3 manufacturers now road testing small fleets of hydrogen fuel cell vehicles (Honda, Mercedes and GM) are all using 60 KW permanent magnet rotor synchronous motors. I have not been able to find any information on the stator design of this motor, but it would appear that the 18-phase Chorus design, with it's 3 times greater low speed torque capability than a standard 3-phase induction motor design in the same frame, is a natural for vehicular use if an induction motor is to be used. I am not sure if the induction motor advantages of high phase order can apply to permanent magnet rotor motors or not, but I suspect not because of the need to change stator field profiles with speed in the high phase order Chorus design, and the difficulty in adjusting the permanent magnets :-).
In permanent magnet motors (commutated or brushless) there is a certain power lost just to spin the motor -- this is mostly friction, windage and core loss, but I couldn't for the life of me tell you which ones are more important. So for any given voltage there is always one load (and hence current) that gives the best efficiency.
You'd have a similar effect in any field-wound motor, except the maximum efficiency point would vary with the field. I would expect that a large industrial induction motor would be most efficient at it's rated output, or possibly slightly less if it's designed for varying loads.
From Merriam-Webster online
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Standard AC nduction motors only operate properly over an EXTTREMELY NARROW speed range for a given input frequency.
The synchronous speed minus a few herts of slip frequency.
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At 3600 RPM, with a whip attached, that would be 60 Hurts per second. In the UK you don't get as many lashes. Of course I've heard there are lots of cute red-haired lashes in Ireland :)
Hertz per second is actually a figure for acceleration. Naturally, I'll be using a V/F controller and I might go as high as 360 Hz if the motor does not become too inefficient.
I have been know to slip with some frequency myself...
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