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question for alternator guru's


By bob g, Section Mechanical
Posted on Tue May 6th, 2008 at 02:45:35 AM MST
can't quite get my head around this one

not knowing where to go to ask this question i figure there are some
folks here that might have a clue about this question.

first the preface

an alternator with an iron core stator, three phase
high leakage inductance, typical of automotive alternators.

the leakage inductance goes up with the square of the current in the stator.

(now the question)

for simplicity i will refer to a single pole on the stator.

if one was to wind two coils on the stator pole
one clockwise, and another counterclockwise
both feeding rectifiers to charge a battery.
basically a bifilar winding
wherein one coils back inductance is cancelled by the other coils back inductance

would both coils still produce power?
if they were both assumed to be equal in turn count, wire size, and loading?

what i am trying to get my head around is
the leakage inductance is 6.28*frequency*inductance
(there is also a related formula that accounts for the current)
if the inductance of both coils cancel each other then the leakage inductance
would therefore be zero ( in theory in an ideal example)

what i am trying to do is alter the parameters of a high leakage inductance alternator to either eliminate or dramatically reduce this leakage inductance.

sort of like how a compensating winding is used in a generator, but
by using two equal but opposite coils instead of the pole face slotted winding
that are used on generators.

i am hoping someone here can shed some light on this, and help me to expand my understanding a bit.

thanks
bob g

question for alternator guru's | 4 comments (4 topical, 0 editorial)

Re: question for alternator guru's (3.00 / 0) (#1)
by Flux on Tue May 6th, 2008 at 01:45:18 AM MST
(User Info)

This is a difficult concept and probably beyond my complete understanding, but your idea isn't going to work.

You are confusing leakage inductance with true winding inductance.

The leakage inductance is a consequence of flux not linking the coil completely. If flux linkage was complete there would be no leakage inductance. The actual windings would have inductance when measured on their own with a bridge or inductance meter, but there would be no inductive drop component in the load current.

I think it may be easier to consider a transformer. A perfect transformer has a winding inductance and the normal to consider is the primary inductance and this determines the magnetising current.

The primary and secondary should be completely coupled and any flux linking one winding will link the other. Any current in the secondary is reflected as a current in the primary in the ratio of the turns. The equivalent circuit would just be an inductor across the lines, simulating the magnetising current.

Now if all the flux doesn't link the windings then you have the effect of a leakage inductance being placed in series with the load ( you still have the shunt primary inductance but we can forget about it for now as it doesn't affect the load current.)

For a 1:1 transformer you can regard this leakage inductance as being in the primary or secondary side. If the ratio is not 1:1 then this is still valid but you need to multiply the value by the square of the turns ratio if you refer it to the primary side. In addition you have the winding resistance causing an internal drop.

The alternator is less easy to follow but basically if all the rotor flux links the stator winding the output is determined by winding resistance as in the axial air gap machines. When part of the flux fails to link the stator winding you have the effect of an added series inductance which we know as the leakage inductance. The final load is determined by the vector impedance comprising the winding resistance and the leakage inductance.

As the inductive bit rises with frequency there usually comes a time with Xl greater than R where the thing goes into constant current mode.

This is very simplified and there are effects of the magnetic field in the stator due to load current reacting upon the flux in the rotor and distorting, reducing or increasing it due to something known as armature reaction.

A capacitive component in the load will increase the effective rotor flux and an inductive load will lower it. Capacitive loads will to some extent compensate the drop due to leakage inductance and adding capacitors to an alternator terminals will reduce the effective volt drop. If you can use large enough series capacitors you can actually neutralise the leakage inductance at a fixed speed but it requires rather large capacitors and is not generally practical.

Hope this helps .

Flux



Re: question for alternator guru's (3.00 / 0) (#2)
by bob g on Tue May 6th, 2008 at 06:58:33 AM MST
(User Info)

Thanks Flux :)

yes it does help, and i had hoped you would respond

at this point i am more concerned whether or not the pole would even produce power
with two opposing coils.

late last night i came up with the answer
apparently denso makes  bifilar wound stator
for what reason i am not sure, but at least its existance answers part of my
question.

http://www.poiesisresearch.com/altID.php

(no i am not going to build a pm version,, just found his reference to the denso
bifilar wound stator)
thanks again
bob g



Re: question for alternator guru's (3.00 / 0) (#3)
by Flux on Tue May 6th, 2008 at 07:27:38 AM MST
(User Info)

Yes it will work if each coil is treated as an individual. Naturally it will not work if the coils are connected series opposing  as they will cancel, this is the basis of a non inductive winding for precision resistors etc.

No way similar to leakage inductance.

Flux

[ Parent ]



Re: question for alternator guru's (3.00 / 0) (#4)
by finnsawyer on Tue May 6th, 2008 at 08:46:28 AM MST
(User Info)

I suggest you Google "Mutual Inductance".  I just did and got 158,000 hits.  It might answer some of your questions or put you on the right track to find the answers.
GeoM


question for alternator guru's | 4 comments (4 topical, 0 editorial)
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