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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 5-10 HP) about
the same size and weight as the same HP 2 or 4 pole motor?

Thanks for any ideas.

Paul E. Schoen
 
J

Jim Backus

Jan 1, 1970
0
(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?

It's too long since I did electrical machine theory, but I think the
motor could stall at that load.
 
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?
No, the motor is most effiecent at rated load.

(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?

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.
(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?

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.
 
P

Paul E. Schoen

Jan 1, 1970
0
----- Original Message -----
From: <[email protected]>
Newsgroups: sci.electronics.design
Sent: Monday, May 15, 2006 6:33 PM
Subject: Re: Efficiency of 3 phase AC Induction motors

No, the motor is most effiecent at rated load.

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.
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.

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.
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.

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?

Thanks for the information.

Paul
 
G

Glen Walpert

Jan 1, 1970
0
<clip> 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.

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 :).

http://www.chorusmotor.com/
 
T

Tim Wescott

Jan 1, 1970
0
Paul said:
----- Original Message -----
From: <[email protected]>
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.
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.
That jives with my initial thoughts.

From Merriam-Webster online
(http://www.meriam-webster.com/cgi-bin/dictionary):

Jive: 2 a : glib, deceptive, or foolish talk b : the jargon of hipsters
c : a special jargon of difficult or slang terms

Jibe: : to be in accord : AGREE

(sorry for the compulsive proofreading)


--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html
 
D

Don Lancaster

Jan 1, 1970
0
Paul said:
----- Original Message -----
From: <[email protected]>
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?

Thanks for the information.

Paul

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.

--
Many thanks,

Don Lancaster voice phone: (928)428-4073
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
rss: http://www.tinaja.com/whtnu.xml email: [email protected]

Please visit my GURU's LAIR web site at http://www.tinaja.com
 
R

Rich Grise

Jan 1, 1970
0
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.

Would that be 60 hurts per second in the US, and 50 in the UK?

Thanks,
Rich
 
P

Paul E. Schoen

Jan 1, 1970
0
Rich Grise said:
Would that be 60 hurts per second in the US, and 50 in the UK?

Thanks,
Rich

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...

Paul
 
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