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pole inductance | 29 comments (29 topical, editorial)

Re: pole inductance (3.00 / 0) (#18)
by joestue on Sat May 17th, 2008 at 12:46:50 PM MST
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AFAIK the leakage inductance is mostly the result of the energy stored in the magnetic path, 95% of which is in the air gap.

The air gap is often a compromise between magnetizing force, tooth tip losses, windage losses and cooling requirements.
Older texts talk about this extensively, though often in confusing terms like armature reaction. (none of this is complete in any means).

Basically your desire to reduce losses is in a sense like asking how do you design a car. Per unit losses as a function of cost is actually a sh*t ton of variables.

Lagging power factor serves to increase copper loss, so use more copper. really nothing you can do about it. Leakage inductance caused be the end connections are probably negligible, but not at 12,000 rpm, a factor of 50X what you are dealing with. so essentially 2 opposing coils are not going to have much effect.

Re: pole inductance (3.00 / 0) (#19)
by bob g on Sat May 17th, 2008 at 04:16:45 PM MST
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joestue:

respectfully :)

unless i am mistaken, which i have been on occasion :)

i have asked or at least tried to ask a specific question
and many have taken that question and made some rather broad assumptions.

for instance the 12k rpm statement, no where have i mentioned wanting to run anywhere near that,, for the record between 2500 and 3800rpm
my target efficiency peak is at 3800rpm specifically.

in the oem design of the alternator i am working with, the leakage inductance is primarily in the slots and it is by design that this is a fact. the high leakage inductance serves in this specific unit to protect the machine from burn out under extreme high current loads by dropping the voltage. but again that is the oem design
which is something i am working away from,, i don't need this max current level output so therefore i don't need the high leakage inductance protection of the oem design.

that is one factor, not necessarily the primary factor.

correct me if i am wrong, but
in an alternator with an open stator, no load connected
there is no displacement (or very little) of the flux across the airgap
as current rises in the stator the pole is magnetized by this current in the stator winding which fights and displaces the flux crossing the airgap.

now if you wind two coils around an iron core (opposing coils) and you place a
current in these two windings the net effect is no magnetization of the pole. no magnetization and you have no displacement of the flux across the airgap.
at least in theory, the result should in any event be less displacement or armature reaction.

here is the thing
a typical automotive claw pole alternator is ~50% efficient, give or take
it is built with a ton of variables and an equal amount of compromises.
because it has to be all things to all people, it has to provide power at idle
and at cruising speeds, which is a very wide bandwidth
if you pick a specific rpm that the alternator will run at, one can then redesign
to optimise the alternator for that specific rpm.
the downside of course is losses at lower and higher rpm,, but i could care the less
because i am running at a specific design rpm.

my target is to attain 75% efficiency from a particular alternator running at a specific speed doing a very specific job.
the researchers have attained 71% efficiency with the same alternator design
(lundell/clawpole) for use in automobiles, which again require a very broad rpm range and dramatically wider loading, high temperature environments etc.

they have attained 71% while still having to comprimise all sorts of stuff, things i don't have to comprimise on. (cost being one, an extra 10 bucks at the manufacture level can kill a design, where an extra 50 bucks might otherwise be acceptable to me)

so for the record, i am not trying to get something for nothing, overunity or any other bullcrap.

btw,, i have now 5 or 6 textbooks on alternator design and a couple of very good engineering texts as well.

the only issue i have not been able to resolve was the bifilar or opposing wound
stator design. i can only surmise that the reason bifilar is not mentioned is basically most alternators run at a specific speed and other things can be done to improve efficiency, or on the the otherhand alternators that are built for broadranging rpm would not benefit from such a winding.

thank you very much for you input, it is all appreciated even if it doesn't answer my specific question. sometimes an answer i get will come in handy somewhere down the road to a different question.

thanks
bob g

[ Parent ]

Re: pole inductance (3.00 / 0) (#20)
by herbnz on Sun May 18th, 2008 at 01:28:12 AM MST
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Ok my first post here lets see if I can throw more confusion into this.

Lets actually push reactance inductance etc to background for moment.
As the as the stator pole flux builts up due to the passing of the rotor magnet
an emf would be induced into both windings
as a load is connected to both a currents will try to flow.
these currents will in turn pruduce another magnetic force mmf that will oppose the mmf of the rotor (Lenz's law)
both coils will therefore combine together to oppose and reduce the rotor flux. Immportantly no flux will be produced by the currents in the coils as mmf is always going to be less than rotor mmf.
Hence no back emf or transformer effect. ( no self inductance no mutual inductance )
The output emf will be reduced however as if there was inductance by the fact both coils oppose the rotor mmf. Even to the extend that phase shift will occur due to greatest opposition as current passing though zero.
In conclusion IMHO the effect of winding coils in opposite directions and having separate but equal loads would bethe same as wound in same direction. Dueto there only being one flux from the rotor. The mmf from the coils cannot overcome this as they cease to exist if even equal.
A very good query though and one that makes one go back to basics.

Will post now but bound to think of better way to explain as soon as i do

Herb

[ Parent ]

Re: pole inductance (3.00 / 0) (#21)
by Flux on Sun May 18th, 2008 at 01:37:33 AM MST
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Bob

If you want higher efficiency at a specific high speed then rewind with fewer turns of thicker wire. The less turns you have the lower the effects of armature reaction and leakage inductance.

Work the field hard and use the minimum turns to get what you want and that is the best you can do. Forget the bifilar windings but you may have to wind several wires in hand to get the wire cross section so you could connect each strand to its own rectifier just for fun, it will not be any different from paralleling the wires.

Run it at your speed ( 3800?) and try it into higher and higher voltage batteries until you find the peak power point, then divide the original turns by the ratio of optimum volts above 12. If 36v works best use 1/3 turns etc.

Good luck.
Flux

[ Parent ]

Re: pole inductance (3.00 / 0) (#23)
by bob g on Sun May 18th, 2008 at 08:41:03 AM MST
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thanks Flux:

my target voltage is 120 line to line Y connected 3 phase at ~380 hz
with the final product being AC and not DC

the slot fill with the original winding is about 75% or so, and i can go with
much smaller wire. my hope is to replace the 7 turn/pole with 21 turns per pole
which i think is doable.

with the increase in turn count i would expect higher slot leakage inductance
but am thinking that current also plays a significant role
perhaps i can triple the windings if i reduce the current by a factor of 4 or 5
with the net effect being reduced?

that i don't know for sure

this is why i was interested in bifilar
i knew all along that i was faced with a rewind of the stator.
and while most of the machine is technically locked in by design (i am not going to recut a stator core, or fabricate a rotor, etc.) winding however is something i can make a change in.

i am now thinking that perhaps it would be interesting to wind one phase
bifilar as a test, then remove it and replace it with a standard double wound
(2 in hand) convensional winding and compare the results.

in theory the bifilar method should reduce of eliminate the back emf in the stator (under load) and provide much less armature reaction (if you wanna call it that)
whereas the standard winding under the same load would have a back emf established in the pole that bucks the flux across the gap.
(here again i may be using the wrong terminology)

anyway, thank you very much for your input
it is always appreciated

bob g

[ Parent ]

Re: pole inductance (3.00 / 0) (#24)
by bob g on Sun May 18th, 2008 at 08:51:18 AM MST
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Herb:

thanks for the input

i would agree with your assertion in re to the coil once current is flowing creating an opposing magnetic field in the pole to that of the rotor pole, if
you are using a single turn coil or two coils wound in the same direction.
if however you use two coils that are wound opposite to one another
their setting up a magnetic field in the pole would cancel each other out.
with in theory no back field being generated.

what am i missing here,, it feels like i am just on the edge of understanding
what you are stating, but can't quite grasp it yet.

you have given me something to think about :)

bob g

[ Parent ]

Re: pole inductance (3.00 / 0) (#25)
by herbnz on Sun May 18th, 2008 at 10:38:44 AM MST
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Hi Bob

It certainly makes one think and get back to basics that many of us have either never studied or brushed aside. Most of the text books you refer to properly asssume basic knowledge.
The discusion here is off track in the fact its confusing self inductance where the current flowing in a coil by itself can and will produce a flux. (note flux is like current mmf is like emf )this flux cuts coil creating a back emf - opposition
Also Mutual inductance is mentioned this is where th above flux cuts another coil -transformer
Neither of the above are in a generator as the flux flowing can only come from the rotor, if mmf from any coil equals rotor mmf no flux no induced emf.
In your twin coil a mmf is set up by each coil but must both be in a common direction to oppose the one rotor mmf and resulting flux is reduced. Its not relevant that the coilsare wound in different directions currents will flow to create opposing mmf to rotor and as they can not produce a actual flux themselves self inductance is not a issue. bifolar is a means canceling self inductance . Here we have inductance yes but due to rotor flux. Its much easier to follow the theory in transformers where the flux is produced by the primary winding. Read up sometheory on transformers, I just looking though my old books most authors do avoid discussing fundementals of inductance in generators.
another term i have seen here armature reaction was originally given to DC machines its the distortion of the rotor flux due to the opposing mmf it caused the brush position to be incorrect as load increased. only effect in our generators is crowds flux to a smaller pole face in fact with air cores no real effect.
Leakage flux is when the opposing mmf makes it easier for the rotor flux to bypass the coil. All the effects tend to reduce output voltage as load comes on. Inductance of the rotor coil combination is the main cause though in our small units, we often kill the rotor flux creating the flatterning of of out put current.

Enough .......

Herb

[ Parent ]

Re: pole inductance (3.00 / 0) (#26)
by Flux on Sun May 18th, 2008 at 12:40:47 PM MST
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Bob
The only way a bifilar coil can cancel any inductance is if it is wound opposing, it then becomes non inductive and useless as far as generating power is concerned.

If the coils are separate it doesn't matter in the slightest which way they are wound, you have 2 windings each supplying a load, if you reverse one coil you will reverse the phase of the external emf, but each load will still contribute in opposing the rotor mmf.

Think of a transformer with 2 secondaries, if you connect them opposing you get nothing out. If you connect them in parallel and in phase and ran 2 10W bulbs you would have exactly the same thing as each secondary running a 10 watt bulb. If you wound 0ne secondary the other way the thing would still see the same load with separate windings.

Flux

[ Parent ]

Re: pole inductance (3.00 / 0) (#27)
by Flux on Sun May 18th, 2008 at 01:47:16 PM MST
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Bob
Just a few more thoughts that might help you. The power factor of the load influences the effect of armature reaction. Leading power factor increases rotor flux, lagging pf reduces it. In theory unity pf just distorts the flux across the pole but as you will be pushing the iron anyway, part of the pole will saturate so the net effect is a reduction as with lagging pf.

If you are rectifying then the rectifier presents lagging pf. You may see marginal improvement by adding capacitors across the lines to bring pf leading. Unfortunately again saturation will prevent you seeing much improvement and the higher leading current will increase the resistive loss.

Now as you are using high voltage and high frequency I am going to offer you one ray of hope, if you use SERIES capacitors then if you can get the right value you should form a series resonant circuit with the leakage inductance and it should cancel leaving only R. I can't promise that this will work but it does in the case of a phase converter and you can raise the third leg voltage quite a bit. Normally the capacitor values are beyond reality but in your case it may not be so bad.

Flux

[ Parent ]

Re: pole inductance (3.00 / 0) (#28)
by joestue on Sun May 18th, 2008 at 05:03:34 PM MST
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Flux, the capacitors serve to correct the pf, and to an extent the voltage depression, at the expense of copper loss.

How much of the inductive voltage limiting has to do with efficiency is unclear to me.

Certainly for constant power output the effect is not a lot, but if you want to extract the maximum power out at the 50% efficiency point, it will certainly drop the output power away from the KVA = ~rpm^2 curve.

[ Parent ]

pole inductance | 29 comments (29 topical, 0 editorial)

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