Alternator current vs rpm

[Top cat]

The alternator was charging the batteries at 9.5 Amps with the engine at idle, when I increased the revs a bit the current dropped to 9 A. Not what I was expecting.

It's a pretty standard set up with a lucas 70A alternator controlled by a Adverc feeding into a pretty worn 440A battery bank.

Any ideas whats going on ? its not a problem as the batteries do charge OK but I'm puzzled

 

Thanks

 

TC

[trackman]

Slipping drive belt, getting worse with engine speed? Worth checking maybe.

[DHutch]

Batteries becoming charged and the alternator backing off? or did it drop back to 9.5 when returned to idle.

[OldGoat]

The alternator was charging the batteries at 9.5 Amps with the engine at idle, when I increased the revs a bit the current dropped to 9 A. Not what I was expecting.

It's a pretty standard set up with a lucas 70A alternator controlled by a Adverc feeding into a pretty worn 440A battery bank.

Any ideas whats going on ? its not a problem as the batteries do charge OK but I'm puzzled

 

Thanks

 

TC

 

 

at half an amp difference that's hardly significant any anyway if all that Adverc can put it the battery set must be fully charged anyway.

 

Dinna fret, laddie.

[Keeping Up]

I've seen exactly the same effect on my boat quite frequently and I don't know why either. Don't know if it's an Adverc thing (I've got one too) or a feature of the alternator.

[Top cat]

the drive belt isnt slipping and the current went back to 9.5V when I put the engine back to idle. I repeated it several time 9.5A at idle 9A at slightly higher rpm .

I normally consider the batteries are charged when they draw less than 8 A so there was a bit to go.

 

Top Cat

[Justme]

Did the voltage alter?

[nicknorman]

Did the voltage alter?

This is normal behaviour for a dynamo with mechanical regulator, but not for an alternator I would have thought. However:

1) Does it matter?

2) Maybe it's an indication issue? The alternator doesn't output smooth current, there is ripple and the frequency of this ripple is possibly affecting the measurement system if its an electronic meter.

3) if its a mechanical ammeter, maybe vibration at a different frequency is affecting it?

4) based on the concepts in 2) maybe the ripple, which is also sensed by the regulator, causes slightly different voltages to be detected by the regulator at different frequencies and thus a slightly different actual voltage is regulated to.

 

But my money is on 1)!

Edited March 17, 2013 by nicknorman

[Top cat]

Thanks for the suggestions

 

The voltage stayed at 14.40V

The meter is digital , a Victron BVM 600 which measures Volts, amps and Ah put in / taken out. At the time it was alleging we had 6Ah to go before the batteries were recharged but I know that figure is based on gueswork.

 

No it doesn't really matter but I'm curious because the result goes against how I expected things to work and I do like to know what is going on and why.

 

Top Cat

[Paul C]

Probably worth looking at the voltage and amps on an (old) alanogue multimeter, if you can still find one. Then you can see if there's a 'flutter' of the needle.

[OldGoat]

hmmm-

Another angle to consider is-

 

At the top of the charge the batteries start to gas.

The gasses are an insulator

So the charge current drops - 'cos the energy can't flow through the extra "resistance"

 

That's one of the many reasons why an Adverc / whatever is better than a straight alternator, as when it gets to floating I think there's some sort ripple effect to help disperse the gas?

 

 

 

 

[pistnbroke]

batteries dont gas till over 14.4v.........and he said 14.4v

[Sir Nibble]

My theory. As revs increse the regulator backs off the field current, this means there is less voltage drop on the field supply from the diode "trio" therefore the regulator sees a ever so slightly higher voltage. It only need be a millivolt or two to make half an amp of difference.

 

Alernatively it could be to do with the magnetic qualities of the rotor core and the difference in the mark space ratio of the reg switching.

[Top cat]

That's an interesting theory Sir Nibble. I can put my hands on an AVO 8 I'll see if there is a ripple.

I'm sure the batteries aren't gassing as I hardly ever need to top them up.

 

TC

[Sir Nibble]

I doubt you would see much of value on that piece of history, a 'scope would be the thing. Are you really that determined? It's an interesting conversation piece certainly but entirely normal.

[Top cat]

I was just thinking that it might be a good excuse to drag out my old AVO but I'm not worried about the phenomenon just curious. As you say a conversation piece.

 

TC

[Sir Nibble]

Nice instrument if it hasn't been picked up or otherwise disturbed since calibration.

[nicknorman]

Nice instrument if it hasn't been picked up or otherwise disturbed since calibration.

Wot no sarcasm smiley? Good for showing trends etc, not for absolute accuracy. But sometimes there is a need for the former.

[dmr]

An Avo might do better than a digital meter, but will still not tell the full story, but its worth doing anyway as any excuse to use an AVO 8 is a good one!!!

 

The alternator regulator has a tough job to do. The alternator itself does not produce a smooth output but a set of pulses...a rectified AC. The regulator has to try to produce a steady output from these, it needs to respond quickly to changes in load, but not to be so fast as to be fighting these individual rectifier pulses. In practice the regulator turns on for a short time then when the voltage is correct it sort of over-reacts and turns off too much for a while, so the resulting current is a pretty random series of pulses. The exact speed of the alternator in relation to the speed of the regulator might explain your observation, or maybe its the digital meter struggling to cope with the rough current waveform.

Its pretty amazing that regulators manage to work as well as they do!

If anyone is interested I will try to post a charge current waveform that I measured on my boat. Will have to wait till Gillie gets back as I guess this will have need to go into her FlickR account first...I really do struggle to get pictures onto the forum!

 

..........Dave

[Sir Nibble]

Wot no sarcasm smiley? Good for showing trends etc, not for absolute accuracy. But sometimes there is a need for the former.

I am being a bit unfair aren't I. Main problem I found with them was excellent bench instrument but any movement or vibration and the reading is all over the place. Nostalgia is definately a thing of the past.

[by'eck]

An Avo might do better than a digital meter, but will still not tell the full story, but its worth doing anyway as any excuse to use an AVO 8 is a good one!!!

 

The alternator regulator has a tough job to do. The alternator itself does not produce a smooth output but a set of pulses...a rectified AC. The regulator has to try to produce a steady output from these, it needs to respond quickly to changes in load, but not to be so fast as to be fighting these individual rectifier pulses. In practice the regulator turns on for a short time then when the voltage is correct it sort of over-reacts and turns off too much for a while, so the resulting current is a pretty random series of pulses. The exact speed of the alternator in relation to the speed of the regulator might explain your observation, or maybe its the digital meter struggling to cope with the rough current waveform.

Its pretty amazing that regulators manage to work as well as they do!

If anyone is interested I will try to post a charge current waveform that I measured on my boat. Will have to wait till Gillie gets back as I guess this will have need to go into her FlickR account first...I really do struggle to get pictures onto the forum!

 

..........Dave

 

I doubt the alternator field regulator has any effect on smoothing the rectified output from the stator.

 

The nominal 400Hz pulse width modulation that controls the field current will surely have no instantaneous effect on the output waveform due to magnetic hysteresis.

[Keeping Up]

I doubt the alternator field regulator has any effect on smoothing the rectified output from the stator.

 

The nominal 400Hz pulse width modulation that controls the field current will surely have no instantaneous effect on the output waveform due to magnetic hysteresis.

 

Remember that he OP has an Adverc so the internal regulator is probably having no effect anyway

[by'eck]

Remember that he OP has an Adverc so the internal regulator is probably having no effect anyway

 

But I doubt it works any differently with regard to smoothing the output.

 

BTW I think Sir Nibble nailed the answer to the OP's query in post #13

Edited March 19, 2013 by by'eck

[WotEver]

batteries dont gas till over 14.4v.........

:::sigh:::

 

Wrong again, Mr Broke.

 

A flat battery will not gas below around 14.4V but a semi-charged battery will start to gas from around 13.5V - 13.8V depending on that SOC.

 

Tony

[dmr]

I doubt the alternator field regulator has any effect on smoothing the rectified output from the stator.

 

The nominal 400Hz pulse width modulation that controls the field current will surely have no instantaneous effect on the output waveform due to magnetic hysteresis.

 

I am no expert on regulators, and had no idea that they switched at 400Hz.

When I measured the current from my alternator (or strictly the current into the batteries), there appeared to be a lot of "switching" at a much lower frequency. This was the current waveform:.....

 

daves by lovestruck., on Flickr

 

This is with the Adverc connected, though it looked pretty similar when I disconnected the Adverc.

I think most digital ammeters just sample the current a couple of times each second so a waveform like this can give some uncertain readings.

 

...........Dave