Permanent magnet alternator charger. - Page 1
Forum section: electronics / projects

author: DeanA
author_rank: Regular Contributor
timestamp: October 06, 2020, 09:40:26 am
content: I’ve been asked to investigate the design of a voltage regulator for a single phase permanent magnet alternator charging a 12V lead acid battery for a kit aircraft.The existing design is quite simple, there is a control circuit of a few transistors and a Zener diode which turns off the SCRs of the bridge rectifier once the battery voltage reaches the cut off voltage of about 14.5V. (see attached)I’m advised the existing product works quite well, but there is no current limit (other than from the alternator) and the availability is not good.The wish list for the new design is for 3 stage charging of the lead acid battery, bulk – absorption – float. My initial thoughts were to use a PWM buck regulator in constant current / constant voltage mode to do the battery charging. But I’m beginning to think I’m overcomplicating things, especially considering the simplicity of the existing design. Being for a small aircraft I really wouldn’t want to end up make it less reliable than the existing product.Customer has advised maximum output current of the regulator is 18A (I have not verified but it sounds quite high to me considering it’s only a 21Ah battery). Also, the high output voltage from the alternator of about 70V starts to limit the choice of PWM controllers, I was starting to look at something like LM5146 which is when I started thinking I’m overcomplicating things.Does anyone have any thoughts on what I might be missing or other things I could considered ?

author: voltsandjolts
author_rank: Unknown
timestamp: October 06, 2020, 09:49:51 am
content: I'm curious - what are the potential consequences of this alternator system failing during flight?

author: DeanA
author_rank: Regular Contributor
timestamp: October 06, 2020, 10:00:10 am
content: Plane will still fly if the regulator fails, catching fire might be an issue though

author: mikerj
author_rank: Super Contributor
timestamp: October 06, 2020, 11:32:19 am
content: Quote from: DeanA on October 06, 2020, 09:40:26 amDoesn't seem unreasonable, in cars it's not unusual to have an alternator with an output significantly higher than battery capacity (e.g. my car has a 75A alternator and a 41Ah battery). Don't forget that the alternator is there to power the electrical systems in flight as well as charge the battery.A permanent magnet alternator is inherently current limited, indeed other vehicles using this kind of alternator (e.g. motorcycles) often use shunt regulators that basically short out the outputs to provide regulation.

author: Zero999
author_rank: Super Contributor
timestamp: October 06, 2020, 12:12:30 pm
content: It depends on the alternator. Presumably it will have a high leakage inductance to limit the current. If that's not enough, then add an external inductor. As the voltage from the alternator increases, so will its frequency and the impedance of the inductor, which will limit the current.

author: ConKbot
author_rank: Super Contributor
timestamp: October 06, 2020, 07:50:44 pm
content: Quote from: voltsandjolts on October 06, 2020, 09:49:51 amrequires landing quickly, as time is now ticking before fuel pumps(only needed in some phases of flight), electronic avionics, (vacuum operated ones still functional) , lighting, radios, etc stop functioning. If it's a magneto ignition engine, you may not fall out if the sky on a dead battery, but you are in a very bad place.I'd not touch the project with a 10 foot pole, at least not without a test and validation budget. The engine compartment of anything is already a harsh environment, let alone with aircraft reliability tacked onto it. EMI needs to be tamed, vibration needs to not pull components off, etc. .If replacement part availability is low, buy your spare, store it, and worry about the availability after when you swap the spare in, rather than having down time.

author: floobydust
author_rank: Super Contributor
timestamp: October 06, 2020, 08:46:18 pm
content: The stator winding will cook and burn up if the regulator fails (short). I've seen it happen on motorcycle PM generators. It's a high-torque load on the motor and the windings get hot enough. The energy from the rotating magnets, or backfeed from the battery (which is why a fusible link is always there) causes high current in the stator. Other failure modes are high or zero output.I would keep it simple, there are numerous PM generator regulator circuits out there, 1970's style still using the usual two SCR/bridge rectifier and a few transistors. A TL431 and temperature compensation would be an improvement. The old designs I've seen are cheap and do nothing to tame voltage transients. Current limiting is only used on dynamos but here the magnetic circuit ultimately limits power output. MOSFETS for the rectifier would run cooler than SCR's but their low voltage would be hard to ensure.Overall, reliability is poor due to heat and vibration and voltage spikes. It's hard to do measurements on the running engine, best to do a good Spice model of the generator and build the circuit that way.A bit more here:https://www.eevblog.com/forum/projects/ac-generatoralternator-charger-rs1105-replacement/

author: geggi1
author_rank: Frequent Contributor
timestamp: October 06, 2020, 09:35:09 pm
content: If this is for a kit plane i would seriously consider something off the shelf.There are plenty of small alternators with built in chargers used in small cars.Small cars like the WW golf, Toyota Starlet, WW Polo, Honda Civic and similar cars have small alternators.Most of the well known brands like Boch and Delco have built in voltage regulators.Another thing to also consider is that the it is handy to just swing in to a auto parts store for spares considering price and availability.Auto parts are also well tested by the factory and also by the number of units used in the auto industry.

author: DeanA
author_rank: Regular Contributor
timestamp: October 07, 2020, 01:16:36 am
content: QuoteYes, I think you're probably correct there. Liability could be a bit of an issue with this project

author: DeanA
author_rank: Regular Contributor
timestamp: October 07, 2020, 01:20:20 am
content: Quote from: floobydust on October 06, 2020, 08:46:18 pmHumm, yes, the circuit for the existing product is the same as the Kubota one in that thread you linked.

author: ConKbot
author_rank: Super Contributor
timestamp: October 07, 2020, 06:44:31 am
content: Rather than focusing in narrow, look back at the big picture and see what the goal is. Is it to lower maintenance costs by bit charging the battery less aggressively? Could the same thing be achieved by using a certified lithium battery? It's more expensive up front, if it's going to last longer, and save fuel by dumping 10 lbs of weight, perhaps the TCO of it could be less in the end. No idea how such a small amount of weight effects fuel burn, and I suspect it would be dependent on the number of hours flown if it ever was in the black.The other thing is keeping the factory regulator in adjustment(or monitored if not adjustable), keeping the battery appropriately watered, and making sure it doesn't see huge charge currents if it ever gets deep discharged (3-5 A charger instead of hitting it with a jumper box and the alternator charging with full current) may extent the life and provide TCO reduction vs treating it like a car battery and letting it roast till it doesn't work anymore. Obviously, just guesses here, and the unique situation needs to be considered before choosing a path.

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 07, 2020, 12:43:33 pm
content: What about a small AC generator (or BLDC motor used as a generator) and a switching power supply module?

author: strawberry
author_rank: Super Contributor
timestamp: October 07, 2020, 08:41:18 pm
content: current is limited by generator internal winding resistance, airgap ... battery resistance...https://www.eevblog.com/forum/projects/ac-generatoralternator-charger-rs1105-replacement/...original regulator failed when battery was installed in reverse polarity by accident

author: floobydust
author_rank: Super Contributor
timestamp: October 07, 2020, 09:02:48 pm
content: Current is not limited well enough when the regulator shorts to stop the stator from damage. It depends on engine speed, and cooling but the stator is usually inside the engine case so no airflow.Another way to look at the regulator design is it dissipates about 2W/A of heat, so at 20A output it's making 40W of heat- which is a lot. So you'll see these regulators have big heatsinks or not... they assume full load is for the minute until the battery recovers from cranking.

author: bdunham7
author_rank: Super Contributor
timestamp: October 07, 2020, 09:26:51 pm
content: Why do they see a need for a system like this? Are they having battery life issues? If so, is the existing system accurately temperature-compensated?

author: strawberry
author_rank: Super Contributor
timestamp: October 07, 2020, 11:55:04 pm
content: Quote from: floobydust on October 07, 2020, 09:02:48 pmit is like 3 phase bridge and rotating electromagnet is not dissipating any powerwhat if brushes fail in generator ?

author: DeanA
author_rank: Regular Contributor
timestamp: October 08, 2020, 07:41:37 pm
content: Quote from: bdunham7 on October 07, 2020, 09:26:51 pmI will ask if there are battery life issues (but I don't think so as he said existing product works well). I suspect he has just heard somewhere that 3 stage charging is better for lead acid battery so has put that on the wish list.

author: floobydust
author_rank: Super Contributor
timestamp: October 08, 2020, 08:35:43 pm
content: Quote from: strawberry on October 07, 2020, 11:55:04 pmAn alternator is not a generator nor a dynamo. Without getting into semantics, here for small engines the stator is a single-phase two-wire deal typically used in applications like motorcycles, tractors up to about 32A. There are no brushes, no field coil. With the AC output rectification must be done in the regulator.I find the regulators don't include temperature compensation and if they do it doesn't work because the regulator temperature has little to do with battery temperature.The three-stage charging is ideal in deep-discharge or cyclical use. In vehicles the battery is cranking only so float charging works well if the voltage control is accurate.

author: bdunham7
author_rank: Super Contributor
timestamp: October 08, 2020, 09:26:38 pm
content: Quote from: DeanA on October 08, 2020, 07:41:37 pmI would respectfully suggest that reliability is the paramount concern here. I don't know of any vehicular (airplanes included) that use a 3-stage charging sytem like that. I would think it would be very difficult to manage, as your power availability and needs vary wildly. One of the things an automotive battery does well is manage those issues, sinking and sourcing a lot of current when needed, without too much fuss. Modern calcium-type batteries are just fine for years on a float charge as long as you don't overdo it. Probably the single best improvement you could make would be an accurate (sensor near or on the battery) temperature compensation system if it doesn't have it. Just make sure it is fail-safe, as in having limited authority and mid-level voltage default.

author: bdunham7
author_rank: Super Contributor
timestamp: October 08, 2020, 09:31:52 pm
content: Quote from: floobydust on October 08, 2020, 08:35:43 pmWhich regulators? The cheap/simple/integrated kind that you find in products that eat a battery every year? Proper automotive designs nowadays include a sensor near or at the battery and often very sophisticated controls both for charging control and efficiency. As for the accuracy of the sensor vs the actual plate/electrolyte temperature, you can try to factor that in or just live with it knowing that you are compensating over a range that can approach 200F and just getting to within 20F will do pretty good job.

author: floobydust
author_rank: Super Contributor
timestamp: October 08, 2020, 10:41:07 pm
content: I've never seen a dedicated battery temperature sensor in a car. Only on solar charge controllers.Before I was talking about the PM generator regulators, their circuits are cheap and primitive, any thermistor is potted inside and probably ends up protecting the module.Car alternator regulators have temperature compensation built-in but are out to lunch due to internal self-heating & engine heat always making things seem hotter than they are, so chronic undercharging results. I've also seen automakers vary widely with the voltage setpoints they choose, and battery chemistries. So it's tough to be accurate at all. I think best to err on the side of overcharging a little as Canadian winters can need over 15V. Modern ECU's, knowing air temperature you can model battery temperature and do more precise correction.I'm not sure what airplane battery and air temperatures are like but surely a big swing?The old Delco SI regulators are around 1mA quiescent drain and prev generation (still non-PWM) i.e.CS3341draw 0.15mA. For reference, a car battery leakage current is around 3.5mA just sitting there by my math.

author: bdunham7
author_rank: Super Contributor
timestamp: October 08, 2020, 11:24:21 pm
content: Quote from: floobydust on October 08, 2020, 10:41:07 pmWell, to be diplomatic, your info is out of date. Using an external sensor, whether an explicitly named battery sensor or derived from another sensor like the ambient temp sensor, and controlling the battery charge via the ECM has been common for 30+ years and I'm pretty sure they are universal now. They're often hidden and not always called the battery temp sensor in the data stream, but they exist.Yes, the integrated regulators from the 70's and 80's pose a bit of a problem in determining the proper offset, but many other cars had external regulators that wouldn't have that issue. Even the old Delco 2-stage mechanical regulators were temperature compensated. Very old batteries could tolerate just being boiled as long as you added water regularly and didn't expect a long life. Modern calcium-recombinant types need a temperature compensated voltage ranging from 13.2 to 14.8 volts to survive very long. I can't imagine a modern system not having it.https://usparts.volvocars.com/p/Volvo_2005_S40/Battery-Temperature-Sensor-Battery-Box-and-Mounting-Parts/42816434/30658826.htmlhttps://shop.advanceautoparts.com/find/chrysler-battery-temperature-sensor.c6388

author: floobydust
author_rank: Super Contributor
timestamp: October 09, 2020, 03:19:59 am
content: I have issues when engineers sperg and use maximum technology i.e. FreescaleMM912J637intelligent battery sensor (IBS) 413pg datasheet - it's a shit ton of engineering time, software development, 8 page silicon errata. I think these approaches are overboard in complexity because the bugs end up causing big drama and cost to a car owner.Would you rather fly with several MCU's controlling the charging system... or five transistors?https://www.nxp.com/design/designs/12-v-lead-acid-battery-management-system-with-lin:RD9Z1-638-12Vhttps://www.nxp.com/products/power-management/battery-management/battery-sensors/battery-sensor-with-can-and-lin:MM9Z1_638latest using CAN.

author: Alti
author_rank: Frequent Contributor
timestamp: October 09, 2020, 08:04:44 pm
content: I think to answer this question a failure analysis has to be conducted.I suspect a permanent magnet alternator was chosen because it does not have any excitation, rings, wired rotor etc. So sth tells me it has a higher reliability, when compared to a classical approach which is already quite a reliable design (w.r.t. other mechanical gear of similar part count).So, quantify the reliability and move from there. In the end, you do not want to end up with somehting less reliable than off-the-shelf automotive alternator.

author: BrianHG
author_rank: Super Contributor
timestamp: October 09, 2020, 10:07:17 pm
content: Quote from: DeanA on October 06, 2020, 09:40:26 amHow is it that a single IC buck regulator which only requires 2 external mosfets and an inductor is over-complicating things. It has everything you need. Your other solution still requires 2 SCRs and some form of smart gate drive circuitry. Not to mention any other required circuits to clean the pulses plus even worse, track the frequency of the generator and accommodate so as to not overdrive the battery being charged and a voltage reference as well. Not to mention what to do during mention over-current and under-voltage situations.

author: bdunham7
author_rank: Super Contributor
timestamp: October 09, 2020, 10:51:58 pm
content: Quote from: floobydust on October 09, 2020, 03:19:59 amSimplicity is great in moderation. Or to paraphrase a common saying, the ideal solution is as simple as possible, but not simpler.Modern cars with engine controllers already have most of the hardware needed, so an external regulator is neither simpler nor more reliable. Since the ECM is doing the load control at idle (earlier systems) and at all times (newer), it makes sense to simply have it control the alternator and AC, as well as sensing other loads like power steering, and just work that all in to its control programming.QuoteWhichever one has been shown to be more reliable. I'm sure a previous generation had the same discussion between mechanically regulated generators and newfangled alternators with solid state regulators. On a simple, single-engine plane, I'd want whatever has the best and longest track record, not some new invention. On a complex, multi-engine plane that already has electronic controls for everything, the answer might be different.

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 10, 2020, 02:05:52 am
content: Quote from: floobydust on October 09, 2020, 03:19:59 amFor close to the last 20 years, Toyota has been using custom ASICs to manage 12V battery charging. Why they went that route instead of using one of the many off the shelf switching power supply controllers I'm not sure. The design is very odd with an active clamp forward converter (I normally expect full bridge for that power level, as most large PC power supplies do) and plain Schottky rectification. Therefore, I doubt efficiency is the reason for the ASIC. All the discrete components and the lack of programming test points near the ASIC suggests it's mostly analog with some simple state machines at most for digital logic.https://openinverter.org/forum/viewtopic.php?f=14&t=439&start=10QuotePretty sure every passenger jet built since the 90s used the microprocessor approach. Only time I ever heard of that being a problem was buggy software that potentially had problems after staying running for too long.For small aircraft, I would say that CVCC implemented with a common switching power supply controller would be plenty.

author: james_s
author_rank: Super Contributor
timestamp: October 10, 2020, 04:42:39 am
content: Quote from: NiHaoMike on October 10, 2020, 02:05:52 amI don't know a lot about the electrical systems but most small aircraft still use carburetors for the fuel, magnetos for ignition and lead acid or NiCd batteries, it's all pretty low tech stuff. Not only is there an advantage in simplicity but to get anything approved for use in a certified aircraft is quite an involved process. It's not like cars that people replace regularly and new models come out all the time, huge numbers of light planes built in the 50s-60s are still in regular use. They're designed to last decades and be maintainable. You don't want something depending on a microcontroller with firmware written by some guy who died 20 years earlier.

author: akimpowerscr
author_rank: Regular Contributor
timestamp: October 10, 2020, 10:11:39 am
content: Good morning to all,This is my first intervention on this forum, I thank you in advance for not being too harsh on me.I'm just trying to contribute a little with a modest collaboration to this wonderful forum.I notice a great similarity between the alternator + regulator + battery assembly which equips the majority of vehicles with this single-phase alternator with permanent magnets + its regulator + its battery.The function of the alternator + regulator is not at all to be a battery charger, but to maintain a constant voltage across the battery terminals (from 14 to 14.5V) to keep it fully charged and supply the entire electric circuit powered from the battery.It is therefore not necessary to provide different voltage levels, or even current limitation, since current limitation exists, by the characteristic of the alternator.But the examination of the diagram of the regulator shows that this one works in all or nothing, the thyristors being only able to be fully on or blocked.This principle has the advantage of simplicity but subjects the battery and the equipment supplied with 12V to strong current and voltage variations.This regulator could therefore be improved by using another principle, that of phase control which is a linear and progressive control.

author: akimpowerscr
author_rank: Regular Contributor
timestamp: October 10, 2020, 12:10:49 pm
content: If DeanA wants to develop and design a new regulator with phase control, I can, if he wishes, provide him and this forum with a number of ideas on how to make such a circuit.

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 10, 2020, 04:31:26 pm
content: Thinking about it, the simplest way to add charging current limit would be to add some way to adjust the output voltage between about 12.8V and 14.5V, then put a Hall sensor on the battery cable and an opamp circuit to reduce the voltage if the current is above the setpoint.But aren't even Cessnas running 24V nowadays?

author: akimpowerscr
author_rank: Regular Contributor
timestamp: October 10, 2020, 05:35:26 pm
content: Why add a battery current limitation?It is totally unnecessary, the millions of vehicles that run flawless with an alternator/regulator without battery current limitation prove it.

author: james_s
author_rank: Super Contributor
timestamp: October 10, 2020, 06:05:05 pm
content: Quote from: NiHaoMike on October 10, 2020, 04:31:26 pmAFAIK aircraft have been 24V (28V) at least as far back as WWII, even light planes. Large planes also typically have 115VAC 400Hz.

author: floobydust
author_rank: Super Contributor
timestamp: October 10, 2020, 08:01:34 pm
content: We don't know the requirements well enough, so the thread is going to be a bit turbulent.12V PM generators I see them on ultralights and Cessna 152. At 13-18A output it's a small engine, little current for lighting and accessories.It seems "Owner Built and Maintained (OBAM) aircraft" don't have to meet strict regulatory requirements and formal DO-160 testing. So the requirements are a bit loose, i.e. "short-circuit" protection is commonly for the FIELD winding of an alternator (regulator) as well as the obvious output fusing. It seems designers implement bits and pieces of DO-160 with good intentions, but can miss transient overvoltage or EMC. This pic below(from)the ignition system is surely making a lot of EMI to the generator wiring.I searched 'airplane voltage regulator' and it's kind of a gong show between home-made looking stuff, retrofits and pro stuff that is $1,000+. A lot of Piper stuff is three transistors and a relay.OT - I checked with a engineer in the automotive industry about the advanced battery SOC monitoring. He says it's actually all about efficiency and meeting fuel economy and CO2 standards. Because you can't just lean it out (to get best fuel economy) without causing NOx to surge and scuttle emissions. So during deceleration or braking alternator output is increased with the small bit of regen to the battery.The ECU is controlling alternator output mainly for emissions spikes due to transient loads switching on/off like cooling fans, and idle stability which is best that you can forecast the alternator load change before commanding it.