Make a car alternator voltage regulator - Page 1
Forum section: electronics / projects

author: Zeyneb
author_rank: Regular Contributor
timestamp: July 25, 2020, 04:07:06 pm
content: Hi there!I’d be interested to see if I can make a voltage regulator for a car alternator. I already bought a replacement alternator at a junkyard and I was able to install new bearings in the alternator.Also I was able to bench test the alternator by connecting it to a car battery. The car battery provides current to the rotor field winding so it will become an electromagnet. Then I spin the rotor around with an electric drill. When a certain speed is reached the voltage increases from 12.6V to 14.5V and the battery gets charging current.I know a voltage regulator can become really complex to deal with various electric loads, temperature compensation and engine RPM variations. For now I just want to see if I can get a stable charging voltage of 14.5V in the bench test.I believe the first thing to focus on is to provide a stable field current to the rotor. Maybe it is best to make a new setup without the car battery but with an lab power supply, so when I end up with an oscillating stator voltage it can do no harm.My first approach is the following coil driver circuit with an opamp and a mosfet. I know the actual rotor field inductance is 6.3 mH (measured). But that doesn’t matter to understand the principle right?Ok about the circuit, I know the current needs some time to develop so Vf cannot keep up with Vi, I also see the gate voltage being maxed out so the opamp is a bit of an hyper control freak in this circuit. This is a stability concern right? My question is how can I expose this problem in a simulation?I also did an ac analysis. But I’m wondering if inputting sinewaves at a range of frequencies (the way I understand ac analysis) will expose all the possible stability issues in this circuit. You know in transient analysis Vi during the pulse is 500 mV and inductor current gets to 1A. But in ac analysis Vi are 1V sinewaves but the inductor current only gets to 6 mA, however there is a peak at 700 kHz of 14 mA. Do you know some application notes or internet search terms to learn about ensuring stability in circuits like this?Thanks for your time!Edit: How can I sort the pictures attached here?

author: duak
author_rank: Super Contributor
timestamp: July 25, 2020, 08:55:19 pm
content: Older regulators used PWM to vary the field current. This was first done with voltage sensitive relays and then with transistors and zener diodes in a hybrid. Motorola/Onsemi came out with a chip that does much the same thing:https://www.onsemi.com/pub/Collateral/CS3341-D.PDFThis data sheet might might give some ideas about controlling an alternator.

author: Zeyneb
author_rank: Regular Contributor
timestamp: July 25, 2020, 10:08:57 pm
content: Thanks for that IC. I also found some of these ICs dedicated for alternator use. They all seem to operate with PWM. I'd be interested to see if I can make an analog one. The reason is that I'm aiming to minimize voltage ripple.I hope someone is willing to give me advice about the shown circuit regarding stability.

author: elektrolitr
author_rank: Contributor
timestamp: July 26, 2020, 07:19:06 pm
content: There is no point to make linear field regulator.First, with PWM you control field voltage, but what actually produces magnetic field is current. Make PWM frequency high enough, and inductance of rotor will filter all the ripple outSecond, the highest ripple in car alternator comes from rectifier. It will easily overpower any ripple from field current variations.Large synchronous machines in power plants use the same technique - with static excitation system the output of 6-pulse thyristor bridge is routed directly to rotor winding. You look at field voltage- it's terrible. Look at the current- it's smooth enough (and they really care about power quality!)

author: amyk
author_rank: Super Contributor
timestamp: July 26, 2020, 11:22:27 pm
content: Being a mass-produced part, they are quite simple:http://www.xs650.com/attachments/delco-10si-jpg.94744/http://www.xs650.com/attachments/delco-12si-06-jpg.94743/

author: David Hess
author_rank: Super Contributor
timestamp: July 27, 2020, 07:25:27 pm
content: I have built a custom voltage regulator for an alternator twice in the past. The first time I used a 74S40 and it worked great but the 20% constant off time limited maximum output power to 80%.The next time I used a more conventional Unitrode voltage mode PWM controller with an external LT1007 error amplifier and reference with a Kelvin connection to the starter battery posts. Load regulation was better than 10 microvolts from a couple amps to 10s of amps load; I am not sure how much better because my best handheld meter only had 10 microvolt resolution.The first design being a constant off-time architecture did not require frequency compensation but the second one did. I used an impedance bridge to measure the inductance of the alternator's field winding and this was enough to estimate the needed frequency compensation, which turned out to be close to ideal. The "gain" of the alternator was estimated from the field resistance and rated output current.

author: bdunham7
author_rank: Super Contributor
timestamp: July 27, 2020, 07:49:20 pm
content: First, keep the car battery. The average car battery provides sink/source stability and withstands abuse several orders of magnitude greater than a typical bench power supply.Second, for most (including automotive) applications that include a lead-acid battery in parallel with the alternator, stability of the field (rotor) current drive is irrelevant. Early designs simply used one or two relays that turned on and off 4-5 times a second. If, for example, your op amp was "hyper-controlling" so that it turned the field current completely off at 14.51 volts and completely on at 14.49 volts, your overall system would work fine. Even if it oscillated at a high rate, the field winding wouldn't care.And, a safety note. A typical alternator can put out several hundred volts if not controlled properly, and at full current too--although your electric drill won't be able to sustain that level of power. That's just another reason to keep the battery.

author: Zeyneb
author_rank: Regular Contributor
timestamp: July 27, 2020, 08:13:45 pm
content: Quote from: David Hess on July 27, 2020, 07:25:27 pmHi David,If I may ask, are you willing to share something of that design? Also regardless of the alternator application I would better understand opamp stability issues. Even if this circuit would be fine. But when looking around on the internet I can hardly find something with a transistor or FET in the opamp feedback loop. Every example that talks about poles, zero's and Laplace just has RLC components apart from the opamp but no transistor. I'm a bit stuck on this. I don't know where to start.

author: Zeyneb
author_rank: Regular Contributor
timestamp: July 27, 2020, 08:16:06 pm
content: Quote from: bdunham7 on July 27, 2020, 07:49:20 pmI didn't plan to connect the lab power supply to the stator winding. My intention was to use it just for the rotor field current.

author: bdunham7
author_rank: Super Contributor
timestamp: July 27, 2020, 08:24:14 pm
content: Quote from: David Hess on July 27, 2020, 07:25:27 pmWas the purpose of the 'frequency compensation' to prevent oscillation or to decrease the rise time and improve transient response? I'm assuming the latter, but I don't know the application this was for--and I'm sort of curious Actual automotive applications have so many additional parameters nowadays that this function is almost universally done within the engine controller system. Precise voltage control is pretty far down the list of priorities.

author: David Hess
author_rank: Super Contributor
timestamp: July 27, 2020, 09:16:31 pm
content: Quote from: Zeyneb on July 27, 2020, 08:13:45 pmIt was a long time ago and my notes and schematics are buried somewhere but I remember it well.The operational amplifier measures the difference between the output voltage, and a reference, and applies that difference, multiplied by massive DC gain, to the modulator for the switching controller to produce a PWM signal to drive the high side power switching transistor which drives the field winding. (1) Internally the switching regulator controller has a ramp generator and comparator to make the modulator, which will be available at the "compensation" pin. I did it this way because I wanted to use an external operational amplifier, which made a 4 wire measurement of the battery voltage easy, but the internal one would have worked and is probably the way to go, but the design is the same.I remember that this was the first time I got Unitrode to send me samples, but they were happy to do so.A feedback network from the operational amplifier output to inverting input tailors the frequency response. The way I calculated it was knowing the field inductance which I measured, alternator "gain", in amps per amp, an estimate of the starter battery's ESR, and the modulator "gain", allows for drawing a bode plot and I worked from there to estimate the feedback network, just a series RC network, for stability. Then I ran load response tests to tune the frequency compensation network but it was very close. The alternator's field winding inductance is massive, like more than a henry, so it dominates.The part of the circuit outside of the switching controller looked like that below, with the linear regulator replaced by the switching controller and high side power transistor, and the frequency compensation network across the operational amplifier. At the time I had recently used a circuit based on that to make some very high performance low noise precision power supplies at work. I would do it differently now with more safety features but with the same general idea.(1) I used a fast recovery rectifier located at the regulator as a flyback diode across the field winding which is not ideal with the long lead length but it worked fine.Quote from: bdunham7 on July 27, 2020, 08:24:14 pmIt prevents over and undershoot with load changes and as I recall, I tuned it for slightly more than critical dampening but honestly it is difficult to get it wrong for the reason you identify; the inductance of the alternator's field winding is so high that it dominates the response. You could discard the PWM modulator and drive the field winding with a comparator output that has a little hysteresis and get fine results.I remember worrying about integrator windup at startup, and I had a plan to fix it, but it was never a factor.

author: amyk
author_rank: Super Contributor
timestamp: July 28, 2020, 12:33:35 am
content: Quote from: David Hess on July 27, 2020, 07:25:27 pmI guess you were trying to do something with the output other than power a car's electrical system...?

author: Renate
author_rank: Super Contributor
timestamp: July 28, 2020, 02:08:49 pm
content: There are a few reasons why I'd build / modify an alternator.The first is that the output voltage is compensated for temperature.Unfortunately that "temperature" is the temperature of the alternator mounted on a hot engine.You really want the temperature of the battery, an external sensor.I see my voltage go down to 13.3V as soon as the engine warms up.If you have two batteries, you might want to use one and a half 3 phase bridges (9 diodes) instead of using external diode isolators.Also, you'd need to supervise both batteries.Finally, real battery management has more brains and goes by stages.

author: David Hess
author_rank: Super Contributor
timestamp: July 28, 2020, 06:16:51 pm
content: Quote from: amyk on July 28, 2020, 12:33:35 amNo, not really. The reason I designed and built a second version was that the constant off-time 78S40 limited maximum output power to 80%. There *was* some alternator whine which I hoped that a better regulator would remove, and it did, but that was not a specific goal. My triband mobile FM transceiver and HF through 6 meter transceiver had nothing to do with it.The reason I used an LT1007 and external precision reference, which was massive overkill, was to see just how good the regulation could get. I had recently designed and built some precision power supplies for work using the topology I showed earlier and just replicated it with the PWM switching controller replacing the linear regulator. It was fun.If I did it again, I would use a TL494 or TL495 or whatever and take advantage of the dual error amplifier to implement fault protection like current limiting. It does not take much to improve on a stock automotive regulator.For the housing, I used the old regulator enclosure and kept the connector so it was a drop in environmentally resistant replacement.

author: Renate
author_rank: Super Contributor
timestamp: July 28, 2020, 07:00:30 pm
content: Quote from: David Hess on July 28, 2020, 06:16:51 pmOMG, I can't stand alternator whine."I'm slaving here over a hot engine cranking out the amps and you're just sitting in the air conditioning."