Author Topic: Car alternator. Ripple question.  (Read 2285 times)

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Offline cs.dkTopic starter

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Car alternator. Ripple question.
« on: December 22, 2019, 10:29:01 am »
Hi,
I just replaced an alternator on a Peugeot 308 (if that matters). It came with a test report, and it just catch my eye, that it can make 42A in ripple current. I think it sounds terrible wrong.
They do however not mention the ripple voltage, which i find strange.

I also find it strange, that the ripple is negative sometimes. How can that happen?
Attached is the test report.

Thanks.
 

Offline Psi

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Re: Car alternator. Ripple question.
« Reply #1 on: December 22, 2019, 11:22:56 am »
Probably just because it's rectified but has no filter capacitor, so the output is full of ripple.
Greek letter 'Psi' (not Pounds per Square Inch)
 

Offline PKTKS

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Re: Car alternator. Ripple question.
« Reply #2 on: December 22, 2019, 11:41:55 am »
Hmm what I find strange is a simple alternator with
specs. like "voltage regulation" and actually a full
table of regulation parameters.

Who can actually provide regulation is the proper
regulator module and that should be provided with
the rectifier.

Such class of regulators (for motor alternators) actually
use a "short coil" technique to achieve cheap and affordable
semiconductor currents and temperatures.

That is the most likely cause of "negative" currents being
absorbed (sunken) by the damped coils.. which can only be confirmed
by looking the proper coil schema. Y or delta with center ground
or not.

That thing should be a whole regulator with very high
brute force DIODE PACK and stupid (big) MOSFETS built in

Paul
« Last Edit: December 22, 2019, 11:43:41 am by PKTKS »
 

Offline Siwastaja

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Re: Car alternator. Ripple question.
« Reply #3 on: December 22, 2019, 01:10:28 pm »
I'm sure the I(t) curve is just AC coupled, i.e., offset to be drawn around zero. The "zero" is likely, according to the first graph, around 150A, so the Y axis of the second curve ranges from 120A to 180A, not from -30A to +30A.

This amount of current ripple seems understandable and normal, it's a rectified AC generator. The battery won't care about this ripple.

Voltage ripple cannot be given because it's a property of the complete system and depends on the ESR of the battery. Current ripple is the property of the alternator, and the voltage ripple is then easy to calculate by U = R * I, given the R of the battery is known.
 

Online coromonadalix

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Re: Car alternator. Ripple question.
« Reply #4 on: December 22, 2019, 02:00:35 pm »
In some circuits  the battery is the "filtering capacitor"

Check every ground wires from the frame to the motor, i had an flat flexible copper (10 inch) wire who got corroded and "cutted" itself.  i had more noise from the radio.

Found a new wire from a scrap yard,  resolved all the noise issues ..
 

Offline mikerj

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Re: Car alternator. Ripple question.
« Reply #5 on: December 22, 2019, 03:56:11 pm »

That thing should be a whole regulator with very high
brute force DIODE PACK and stupid (big) MOSFETS built in

No need for any big MOSFETs, regulation in an automotive alternator is achieved by varying the field current which is only a few amps.  The rectifier diodes are heavy duty devices integrated into a heatsink since they see the output current from each phase.
 

Online Benta

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Re: Car alternator. Ripple question.
« Reply #6 on: December 22, 2019, 07:38:29 pm »
41 A is absolutely realistic as ripple current. The alternator has a 6-diode, 3-phase rectifier feeding current into the battery. This only happens when the phase voltage from the alternator is at a peak.
Same situation as with a transformer -> bridge rectifier -> storage capacitor.
 

Online langwadt

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Re: Car alternator. Ripple question.
« Reply #7 on: December 22, 2019, 08:32:37 pm »
41 A is absolutely realistic as ripple current. The alternator has a 6-diode, 3-phase rectifier feeding current into the battery. This only happens when the phase voltage from the alternator is at a peak.
Same situation as with a transformer -> bridge rectifier -> storage capacitor.

but the ripple out of a  6-diode, 3-phase rectifier should only be ~15%
 

Offline cs.dkTopic starter

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Re: Car alternator. Ripple question.
« Reply #8 on: December 23, 2019, 10:20:43 am »
I'm sure the I(t) curve is just AC coupled, i.e., offset to be drawn around zero. The "zero" is likely, according to the first graph, around 150A, so the Y axis of the second curve ranges from 120A to 180A, not from -30A to +30A.

This amount of current ripple seems understandable and normal, it's a rectified AC generator. The battery won't care about this ripple.

Voltage ripple cannot be given because it's a property of the complete system and depends on the ESR of the battery. Current ripple is the property of the alternator, and the voltage ripple is then easy to calculate by U = R * I, given the R of the battery is known.

If the curve is AC coupled, it makes more sense.

What I could find online says a new car battery is around the 0,02-0,03 Ohms mark - That would give around 1V ripple. Doesn't sound to bad, when looking at it that way.
 

Offline SmokeyTheElectrician

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Re: Car alternator. Ripple question.
« Reply #9 on: December 23, 2019, 01:48:17 pm »
Just guessing, but a half volt of ripple would imply 0.5 / 42 ohms
(10 or 12 miliohms) total resistance in the charging circuit
including the battery's internal resistance.
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Offline DBecker

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Re: Car alternator. Ripple question.
« Reply #10 on: December 23, 2019, 05:43:54 pm »
41 A is absolutely realistic as ripple current. The alternator has a 6-diode, 3-phase rectifier feeding current into the battery. This only happens when the phase voltage from the alternator is at a peak.
Same situation as with a transformer -> bridge rectifier -> storage capacitor.

but the ripple out of a  6-diode, 3-phase rectifier should only be ~15%

You might be thinking of an ideal 3-phase source.

This is the spec from a real-life device, with lots of imperfections.  It's useful to document the current variation into an ideal sink, in this case apparently a 13.00V load.
Presumably the ripple current is measured with a inductive clamp, thus a zero-centered graph.
The rest of the specs are normal for a modern alternator, with the regulator permanently connected to the 12V supply and monitoring one of the phase outputs to activate rotor current when the rotor RPM hits 1533 RPM.  With a pulley ratio of 2.38 (which is a parameter of the engine pulley configuration, not the alternator itself), the engine must spin at 645 RPM before the alternator wakes up.  It then waits briefly (perhaps the 2 seconds listed) before ramping the output current.  A typical ramp is 20 amps/second, so a 140 amp alternator takes about 10 seconds before it is putting out its maximum power.
 


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