Car alternator. Ripple question. - Page 1
Forum section: electronics / projects

author: cs.dk
author_rank: Unknown
timestamp: December 22, 2019, 10:29:01 am
content: 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.

author: Psi
author_rank: Super Contributor
timestamp: December 22, 2019, 11:22:56 am
content: Probably just because it's rectified but has no filter capacitor, so the output is full of ripple.

author: PKTKS
author_rank: Super Contributor
timestamp: December 22, 2019, 11:41:55 am
content: Hmm what I find strange is a simple alternator withspecs. like "voltage regulation" and actually a fulltable of regulation parameters.Who can actually provide regulation is the properregulator module and that should be provided withthe rectifier.Such class of regulators (for motor alternators) actuallyuse a "short coil" technique to achieve cheap and affordablesemiconductor currents and temperatures.That is the most likely cause of "negative" currents beingabsorbed (sunken) by the damped coils.. which can only be confirmedby looking the proper coil schema. Y or delta with center groundor not.That thing should be a whole regulator with very highbrute force DIODE PACK and stupid (big) MOSFETS built inPaul

author: Siwastaja
author_rank: Super Contributor
timestamp: December 22, 2019, 01:10:28 pm
content: 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.

author: coromonadalix
author_rank: Super Contributor
timestamp: December 22, 2019, 02:00:35 pm
content: 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 ..

author: mikerj
author_rank: Super Contributor
timestamp: December 22, 2019, 03:56:11 pm
content: Quote from: PKTKS on December 22, 2019, 11:41:55 amNo 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.

author: Benta
author_rank: Super Contributor
timestamp: December 22, 2019, 07:38:29 pm
content: 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.

author: langwadt
author_rank: Super Contributor
timestamp: December 22, 2019, 08:32:37 pm
content: Quote from: Benta on December 22, 2019, 07:38:29 pmbut the ripple out of a 6-diode, 3-phase rectifier should only be ~15%

author: cs.dk
author_rank: Unknown
timestamp: December 23, 2019, 10:20:43 am
content: Quote from: Siwastaja on December 22, 2019, 01:10:28 pmIf the curve is AC coupled, it makes more sense.What I could findonlinesays 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.

author: SmokeyTheElectrician
author_rank: Contributor
timestamp: December 23, 2019, 01:48:17 pm
content: Just guessing, but a half volt of ripple would imply 0.5 / 42 ohms(10 or 12 miliohms) total resistance in the charging circuitincluding the battery's internal resistance.

author: DBecker
author_rank: Frequent Contributor
timestamp: December 23, 2019, 05:43:54 pm
content: Quote from: langwadt on December 22, 2019, 08:32:37 pmYou 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.