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Old 10-31-2020, 09:40 AM   #11 (permalink)
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I got your post Crashy in my e-mail about alternators. I did not post the argument against your suggestion, and I am not sure what the discussion is about, although I do understand the alternator in cars started in the 1960's with Chrysler. The reason for switching from a generator to an alternator is the ability to charge a battery even at low speeds and idle. I did not read your total post so I am not sure of the argument.

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Old 10-31-2020, 06:42 PM   #12 (permalink)
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Originally Posted by Crashy View Post
Who are you talking to?
Did you even read the thread before commenting?
I said NOT to use active rectification, instead use AC rotor current rectification.
I said use a 24v alternator and convert it to 12v. I never suggested changing everything in the vehicle to be 24v.
Okay fair enough, an AC rotor works, but it costs more to have commutation with brushes...probably a lot more. Diode losses are half as much at 24V though, so it might not even be worth the expense.

Uh, I did say 24V and implied conversion to 12V. That's what a DC-DC converter is for. They're expensive! It's really not a surprise that 24V hasn't caught on, with alternators being as cheap as they are.

Personally, one day when I have some spare time, I'm going to run a sensored BLDC motor with a controller and high voltage battery to make a mild hybrid. Thin laminations from a high quality motor takes care of the efficiency issues with a permanent magnet, but alternator makers are too cheap to use them.

Last edited by serialk11r; 10-31-2020 at 06:53 PM..
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Old 10-31-2020, 07:02 PM   #13 (permalink)
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Commutation is another way I guess, but it could be done electronically using existing slip rings and a hall sensor. There's also the option of only doing half wave rectification at double the current, it should make no difference to the heat, but it will only have 0.3v drop across one Schottky at something like 40v at 1000rpm (0.75% lost). I played around with inductor capacitor combinations in parallel, and was able to get AC converted to DC without any active components, but it would not work once rpm changed.

A buck converter has only 3 components, so should be cheap, even at high current. It does not need fancy filtering, remember that the output from an alternator is very dirty
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Old 10-31-2020, 08:08 PM   #14 (permalink)
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What happens if you run two alternators in series? Half the efficiency?
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Old 10-31-2020, 08:29 PM   #15 (permalink)
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Er, wait you can't turn the 3 phase summed power into DC if the rotor is switching? You would have to ditch at least one phase to get AC which you could then convert to DC by switching the polarity of the rotor.

Half wave rectification is an interesting idea; as you say it's not heat limited but winding resistance still limits power. Using half wave rectification on an oversized alternator for a car with low electrical demands could be a quick way to boost efficiency by a few percent. You could trim the cooling fan to reduce parasitic load further.

24V half wave rectification would keep power around the same, but raise efficiency at least at lower speed (iron losses will double). This actually seems DIYable, though a 12V alternator field coil running at 24V isn't great.

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Old 11-01-2020, 01:52 AM   #16 (permalink)
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Originally Posted by Charlie Cheap View Post
I am still debating if I should install a standard Air-Conditioner in my Sunbeam or hold off for total electric AC. My fear is it will be a 24v system...though I can convert if necessary. Just a thought.
There are both 12-volt and 24-volt systems already available, they're mostly targetted to the RV market and long-distance truckers.
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Old 11-08-2020, 08:21 AM   #17 (permalink)
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The Insight stock engine has a very large permanent magnet 3 phase motor bolted to the output shaft on the engine. For a while I had this connected to a Schottky rectifier and then to a DC-DC converter with the internal rectification bypassed. This was connected to a bank of large supercapacitors and a small lithium battery. It worked well, but there were a few things to consider:

-At idle, the motor produced around 65v, and at redline it was around 400v. Both of these extremes were outside of the range of voltage my DC-DC officially operated at, though I never had any issues with its protection tripping. That said, it wouldn't surprise me if a vehicle that wasn't so electrically frugal at idle might have overloaded it under certain conditions, e.g. defroster, A/C + blower, headlights, maybe heated seats or something running. Not an unrealistic situation.

-No more bump starting. An alternator basically starts producing electricity right away, the DC-DC did not.

-The Mean Well unit I used would likely not have worked in very hot climates. Imagine leaving your car parked in the sun, only to find it wouldn't charge your 12v battery until it cooled down.

-Electrical loads were typically under 20 amps. Reducing electrical loads probably *realistically* goes farther than increasing efficiency of it's production.

-Supercapacitors and lithium batteries can be dangerous under certain circumstances.
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Old 11-08-2020, 11:17 AM   #18 (permalink)
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Old 11-09-2020, 02:57 AM   #19 (permalink)
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Originally Posted by Ecky View Post
-Electrical loads were typically under 20 amps. Reducing electrical loads probably *realistically* goes farther than increasing efficiency of it's production.
This is a good point, which is why I never got the motivation to go tinker with the alternator. Once I had switched out all the lighting to LEDs on the car, it just didn't feel like a good use of time chasing down a few watts.

There's also the fact that the usual 1.5-2 ton car really doesn't recuperate very much energy from just an alternator pushing a few hp into the battery. On a Lotus Elise or Honda Insight, a few hp counts for more.

Last edited by serialk11r; 11-09-2020 at 03:04 AM..
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