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KiwiJohn Guru Joined: 01/12/2005 Location: New ZealandPosts: 691
Posted: 04:38am 01 Jan 2009
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For clarity I am looking at just one phase and this is how the voltage would look if we had a purely resistive load and there was no inductance or capacitance in the picture.
This is how I think the current would look in our ideal alternator connected to charge a battery. In this example current only flows for about 30 degrees each half cycle, or 33% of the time. If our charge meter shows 10 amps (for example) the actual average current flow during that 30 degrees is 30 amps! Current flow through a conductor (in this case the stator coils) always incurs a loss due to resistance which is according to a square law. Furthermore, in the case of stator coils more current means more magnetic field being created and hence iron losses.
I have always been a bit dubious about capacitor correction of power factor bringing the advantages we have seen quoted so there has to be another explanation, maybe the answer lies that capacitors reduce the average stator current and hence greatly reduce stator losses?
Just my thoughts and I welcome comment.
Gill Senior Member Joined: 11/11/2006 Location: AustraliaPosts: 669
Posted: 06:06am 01 Jan 2009
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Sorry John,
I'm kind of at a loss as to what you are saying. You wonder if reducing stator current reduces stator losses but point to no manner by which there is a battery current increase for having done so?
Perhaps if you could elaborate on your thoughts in more detail or in a different way?was working fine... til the smoke got out.
Cheers Gill _Cairns, FNQ
herbnz Senior Member Joined: 18/02/2007 Location: New ZealandPosts: 258
Posted: 06:15am 01 Jan 2009
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Hi John
Nice to see an active mind, you certainly give mine a active kick to think this over.
I am begining to think members are thinking caps in terms of slow changing dc conditions. We are ac were the cycle of change is determined by the cycle time.
Adding caps will not directly affect the periodic time or even the shape of the sine wave ( Of course I am talking ideal generator here in practice a little change but actually peaks it up from looking at scope )
Look at Gordons scope the charging pulse is few near identical with caps without.
Your theroy would have merit if caps were changing the sine wave to a square wave but this is not the case. More likely to filter out the harmonics that would create distortion towards a square wave.
My of the cuff thoughts
Herb
Bryan1 Guru Joined: 22/02/2006 Location: AustraliaPosts: 1460
Posted: 06:49am 01 Jan 2009
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If I can add my 2 cents worth in this thread,
The way I see the caps working, Ok they accept wild AC but when the voltage/current runs through them the negative cycle is cut out by the caps discharging. This is my explanation from a fitters point of view and I'm a very learning guy in electronics. I did ask the guys in the electro-tech forum about this and they did agree with me. It is a form of active PFC but further refinement will be need to sway the naysayers.
Cheers Bryan
KiwiJohn Guru Joined: 01/12/2005 Location: New ZealandPosts: 691
Posted: 06:58am 01 Jan 2009
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OK Gill, it is rather difficult to describe something I dont (yet?) fully understand, but here goes..
In my single phase example which is a perfect system with no inductance and no capacitance the alternator is turning at some speed where the peak voltage is only just a little above battery voltage, so it is only the peak voltage period that charges the battery. Lets say this peak volts period is only 30 degrees of each half cycle, or in other words only 33.3% of the time does current flow in the stator.
Now assuming that our DC charging meter shows 10 amps, this is the average of the charging current which is in really only flowing for a third of the time and so the actual current, during the short time that it does flow is actually 30 amps.
Supposing the stator resistance is 5 Ohms, our actual, short time, current is 30 amps, power lost in the stator resistance is 30x30x5 =4500 Watts! But only for a third of the time so over time the power lost in stator resistance is 4500/3=1500 watts.
However, if by some magic we could get 10 amps of current flow over the full cycle the loss would be quite different.. 10x10x5=500Watts, a saving of 1500-500=1000Watts!
Of course we cant get current flow over the entire 360 degrees but we might be able to do better than wasting 1000Watts.
Does this make sense?
KiwiJohn Guru Joined: 01/12/2005 Location: New ZealandPosts: 691
Posted: 06:59am 01 Jan 2009
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OK, a couple of postings while I was writing that, and watching Coro St!
I am not specifically thinking about capacitors as I cant see exactly how they could help, not saying they dont, I am just saying I dont understand how they could, but then there is a lot I dont understand. Edited by KiwiJohn 2009-01-02
andrewf Newbie Joined: 24/12/2007 Location: New ZealandPosts: 15
Posted: 07:08am 01 Jan 2009
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Herb, i thinking your thinking about the thinking of some of the memebers is not too far off. :-)
EVERYTHING SHOULD BE AS SIMPLE AS IT CAN, NOT SIMPLER - EINSTEIN
KiwiJohn Guru Joined: 01/12/2005 Location: New ZealandPosts: 691
Posted: 07:19am 01 Jan 2009
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Herb, Andrew, well I suppose F&P frequencies compared to RF (which is the only field where I have even had even the merest training in AC) is not far off 'slow changing DC'.
KiwiJohn Guru Joined: 01/12/2005 Location: New ZealandPosts: 691
Posted: 07:39am 01 Jan 2009
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Bryan, I dont understand your explanation, but I am not scoffing at it either. Are you thinking that the capacitors charge during half the cycle then discharge during the other half so cancelling the second half cycle?
herbnz Senior Member Joined: 18/02/2007 Location: New ZealandPosts: 258
Posted: 07:42am 01 Jan 2009
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Sorry Byran I cannot follow the theroy here at all can you elaborate. But I dare say you are in the same boat as me regards my theroy.
Also a small point could you show me where there has been ppl that are classed as naysayers ?
Most of us know the effects of caps only differ on an ultimate understanding.Edited by herbnz 2009-01-02
Gill Senior Member Joined: 11/11/2006 Location: AustraliaPosts: 669
Posted: 07:52am 01 Jan 2009
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Thanks for some clarification.
This effect is most dramatic at low revs but at higher speeds that difference is reduced dramatically. So much so I doubt it's viability for general use.
Still it doesn't hurt to consider all influences.was working fine... til the smoke got out.
Cheers Gill _Cairns, FNQ
KiwiJohn Guru Joined: 01/12/2005 Location: New ZealandPosts: 691
Posted: 07:53am 01 Jan 2009
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Bryan, is there a circuit diagram anywhere here that shows where the capacitors are connected?
KiwiJohn Guru Joined: 01/12/2005 Location: New ZealandPosts: 691
Posted: 07:56am 01 Jan 2009
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Is this in reference to my explanation Gill? If so I would remark that if I recall correctly the greatest claimed advantages from the use of capacitors has been at the low RPM end. (I still dont understand how capacitors could do that but I am open to explanations)Edited by KiwiJohn 2009-01-02
Notice. New forum software under development. It's going to miss a few functions and look a bit ugly for a while, but I'm working on it full time now as the old forum was too unstable. Couple days, all good. If you notice any issues, please contact me.
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