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nuckolls.bob(at)aeroelect
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 Posted: Wed Oct 31, 2012 12:12 pm Post subject: single vs. 3-phase alternators (forgot the picture!) |
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At 01:02 PM 10/31/2012, you wrote:
--> AeroElectric-List message posted by: "user9253" <fran4sew(at)banyanol.com>
> do you know why these brushless alternator designs didn't use three
> phases like a modern BLDC motor?
This is pure speculation on my part; so correct me if wrong.
If the coils of a single phase alternator were rewired to make a 3 phase alternator, then voltage output would be only 1/3 of the single phase voltage. In order for a 3 phase alternator to have the same voltage output as a single phase alternator, it would have to have two more windings of the same size. Then it would weigh more and cost more.
Joe
--------
Joe Gores
The power output from an AC device is roughly proportional
to the weight of the active magnetic material (core), magnetic
strength and speed (rpm). I'm recalling from memory now but
back when I was winding my own transformers, a 1 pound core
was good for about 60 watts at 60 Hz. The same weight of core
in a 400 Hz system was good for 6.5 times that power . . .
400 watts. This assumes similar levels of magnetic flux
that can be carried in the core without saturation or
increases losses at higher frequencies.
An automotive alternator is already crafted as a 3-phase
device. The configuration requires that pole-pairs on the
rotating magnet come in some multiple of 3 (usually
6 pairs but can be more on larger machines).
[img]cid:7.1.0.9.0.20121031151100.01e2d220(at)aeroelectric.com.0[/img]
The output voltage of any given winding is proportional
to speed x flux value x number of turns. When you 'rewind'
to get more voltage, it generally has to be done with
smaller wire . . . the winding window in the core is fixed.
The practical power output is not a linear function because
of heating effects of I-squared x R in the wires and the
windings ability to reject heat into the core and end turns.
There's not a great deal to be gained in power by going from
single to three-phase . . . it's that weight,flux,speed thing.
The BIG toe-stubber is pole piece configuration. The magnets
glued to the inside of the popular PM alternator shells (or
wound-field machines) are already optimized for their as-
manufactured condition. It takes more than a 'rewinding'
to morph from single to 3-phase configuration. But assuming
all other things are held the same, the watts-per-pound-per-
velocity will be the same for both configurations.
The BIG advantage of 3-phase is the smoother output with
simple full-wave rectification . . . about 5% ripple as
compared to 100% ripple for single-phase. But if the designer
sets out to do a clean piece of paper, 3-phase design it
will be only moderately more expensive. The power can be
a little better because the windings share duties . . . i.e.
there is never a time when current goes to zero in more than
one winding.
Bob . . .
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jbr79r(at)yahoo.com
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| Posted: Wed Oct 31, 2012 1:40 pm Post subject: single vs. 3-phase alternators (forgot the picture!) |
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Hi Bob
Is the small Walmart battery tender a good deal. It is quite a bit less expensive than the battery tender brand, Your opinion?
Jim Robinson
Sent from my iPad
On Oct 31, 2012, at 3:11 PM, "Robert L. Nuckolls, III" <nuckolls.bob(at)aeroelectric.com (nuckolls.bob(at)aeroelectric.com)> wrote:
[quote] At 01:02 PM 10/31/2012, you wrote:
--> AeroElectric-List message posted by: "user9253" <fran4sew(at)banyanol.com (fran4sew(at)banyanol.com)>
> do you know why these brushless alternator designs didn't use three
> phases like a modern BLDC motor?
This is pure speculation on my part; so correct me if wrong.
If the coils of a single phase alternator were rewired to make a 3 phase alternator, then voltage output would be only 1/3 of the single phase voltage. In order for a 3 phase alternator to have the same voltage output as a single phase alternator, it would have to have two more windings of the same size. Then it would weigh more and cost more.
Joe
--------
Joe Gores
The power output from an AC device is roughly proportional
to the weight of the active magnetic material (core), magnetic
strength and speed (rpm). I'm recalling from memory now but
back when I was winding my own transformers, a 1 pound core
was good for about 60 watts at 60 Hz. The same weight of core
in a 400 Hz system was good for 6.5 times that power . . .
400 watts. This assumes similar levels of magnetic flux
that can be carried in the core without saturation or
increases losses at higher frequencies.
An automotive alternator is already crafted as a 3-phase
device. The configuration requires that pole-pairs on the
rotating magnet come in some multiple of 3 (usually
6 pairs but can be more on larger machines).
<1ddd5eb8.jpg>
The output voltage of any given winding is proportional
to speed x flux value x number of turns. When you 'rewind'
to get more voltage, it generally has to be done with
smaller wire . . . the winding window in the core is fixed.
The practical power output is not a linear function because
of heating effects of I-squared x R in the wires and the
windings ability to reject heat into the core and end turns.
There's not a great deal to be gained in power by going from
single to three-phase . . . it's that weight,flux,speed thing.
The BIG toe-stubber is pole piece configuration. The magnets
glued to the inside of the popular PM alternator shells (or
wound-field machines) are already optimized for their as-
manufactured condition. It takes more than a 'rewinding'
to morph from single to 3-phase configuration. But assuming
all other things are held the same, the watts-per-pound-per-
velocity will be the same for both configurations.
The BIG advantage of 3-phase is the smoother output with
simple full-wave rectification . . . about 5% ripple as
compared to 100% ripple for single-phase. But if the designer
sets out to do a clean piece of paper, 3-phase design it
will be only moderately more expensive. The power can be
a little better because the windings share duties . . . i.e.
there is never a time when current goes to zero in more than
one winding.
Bob . . .
[b]
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