Electronics > Projects, Designs, and Technical Stuff
Do modern car/boat alternators use PFC circuits?
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duak:
An AC generator (alternator) equivalent circuit includes a series inductance Ls.  This can be a good thing for limiting peak currents such as those using rectifiers to convert to DC or should it be overloaded.  Automotive DC generators have current limit relays, but alternators don't.  I don't know what the value of Ls is for a typical alternator. I used an old alternator to test a PWM driver I was developing.  I recall Ls was in the low mH range but definitely not more than 10 mH. Ls provides a measure of PFC and I think you won't see the extreme current pulses that occur in a capacitor input offline supply.
David Hess:
Low millihenries is what I remember for car alternators and about 3 or 4 ohms so 3 or 4 amps at 12 volts of excitation.

I once built a PWM switching regulator based control circuit for my car charging system and measured the alternator characteristics with an impedance bridge for modeling purposes which allowed me to get the pole-zero frequency compensation close on the first try.  Load regulation at the battery terminals was better than 10 microvolts for a 30 amp change in load current where the stock regulator was 100s of millivolts under the same conditions.
NiHaoMike:

--- Quote from: MagicSmoker on July 24, 2019, 11:44:14 am ---Really? That's exceptionally stupid, even for an automotive OEM, which can usually be counted on to do things too cheaply, rather than the other way around. PM rotors are much more expensive and can't withstand even moderately high temperatures (some types of rare earth magnets start to suffer demagnetization at less than 100C), to say nothing of then needing an SMPS to provide voltage regulation and current-limiting. It would make far more sense to simply replace the crappy claw-pole rotor with a salient pole wound-field rotor as found on larger synchronous machines since it could use the same field current regulator design and 3-ph. bridge rectifier.

--- End quote ---
The added cost is offset by removing the need for a separate starter motor. Then add auto start/stop, to boost MPG and help otherwise conventional cars stay competitive with hybrids and EVs.
TheUnnamedNewbie:

--- Quote from: NiHaoMike on July 24, 2019, 04:49:43 am ---
--- Quote from: MagicSmoker on July 23, 2019, 10:30:33 am ---Given that the typical claw-pole rotor alternator is around 55%-60% efficient, it makes absolutely no sense to spend lots of money (relatively speaking) adding PFC to it when it would cost far less to simply change to a more efficient rotor design (the claw pole rotor does a very poor job of concentrating the flux from its field winding).

--- End quote ---
The move is towards permanent magnet rotors, then stepping down the voltage with a switching power supply.

--- End quote ---

I thought the general push was to a much higher voltage. I know this is already electric cars, but I think there is a big push to 48 volts for internal stuff. Now with better on-chip DC-DC capabilities, the reduced cost of wires (esp the weight gains) easily outweighs the cost of fancier DC/DC.

12 V used to just be nice with lead-acid (I think, don't know why you wouldn't want to put more lead-acid cells in series?). I think some bigger vehicles, like trucks use 24 V? .

SMPS technologies are really becoming cheaper, and the increase in EV stuff is only helping their price go down further (lots of research going into that). Same with GaN and SiC - their cost is coming down to the point where their increased cost can be justified for the benefits they offer in system design. 
MagicSmoker:

--- Quote from: NiHaoMike on July 25, 2019, 01:06:39 am ---The added cost is offset by removing the need for a separate starter motor. Then add auto start/stop, to boost MPG and help otherwise conventional cars stay competitive with hybrids and EVs.

--- End quote ---

Ahh, yes. The mild hybrid/belted-alternator-starter thing. I was so fixated on conventional ICE vehicles changing their alternator design over to PM that I totally forgot about mild hybrid systems. I'm still not sure this makes any economical sense, but it is definitely true that mating a PM rotor alternator - that is, a PM synchronous motor/generator - to a 6-switch inverter with field-oriented ("vector") control allows the alternator to act as a motor or generator and with ideal power factor at all times. Whether the higher efficiency of the mild hybrid system offsets the vastly higher cost over the lifetime of the vehicle is another question altogether, though.


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