Electronics > Projects, Designs, and Technical Stuff
Do modern car/boat alternators use PFC circuits?
David Hess:
--- Quote from: Circlotron on July 26, 2019, 07:34:36 am ---With a transformer driving a bridge rectifier and capacitive input filter you can only get about 60% of the transformer VA rating as DC Watts.
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In the past under those circumstances, they added a series inductor for passive power factor correction.
I have seen some old off-line switching power supplies which used a 60 Hz transformer for isolation followed by a low voltage non-isolated switching regulator but none of them included power factor correction. It seems like it would be easy to add though.
--- Quote from: Circlotron on July 26, 2019, 07:44:17 am ---Some time ago I put a scope on a car alternator winding. Was completely unloaded and on a test bench. The winding voltage was a sine wave of sorts but with fairly flattened peaks. This would broaden the conduction angle and reduce the current peaks, presumably with a better power factor than if it were a normal peaky sine wave.
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The alternator is designed to operate into a constant voltage load and roughly current excitation so an output current measurement would be more meaningful.
--- Quote ---The claws on the rotor are triangular and the air gaps between the claws are as a consequence skewed. Maybe these skewed air gaps introduce the flux into the stator in a more gradual and constant manner and that way produce the flattened top sine wave?
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Car manufacturers are nothing if not economical to the point of being cheap so rotor construction which increases the power factor so that less copper and cheaper diodes can be used makes sense.
Miyuki:
Also is here a frequency problem
Car alternator turn at 4-6 000 rpm at cruise speed with 12 or so pole rotor it makes more than 1000 Hz and even distorted by slow diodes in alternator
I dont think there is any control circuit that can track PFC at this speed and non sinusoidal shape
calexanian:
Baring new fancy, electric, or hybrids......
No. The typical automotive alternator is Typically a 3 phase, half wave rectified output and the output power is controlled by having a couple of different level field strengths controlled by either taps, or a large external resistor. Some are called one wire alternators, which are fairly common today, have this switching internal where only a single lead comes out to the positive terminal of the battery and everything else is controlled internally, but the mode of action is the same. When the RPM is low or the battery is nearly fully charged the field strength is low therefore not either loading the motor at idle down too much as to stall it, or not overcharge a battery at high RPM. All alternators use the metal housing as ground.
David Hess:
--- Quote from: Miyuki on July 26, 2019, 03:36:19 pm ---Also is here a frequency problem
Car alternator turn at 4-6 000 rpm at cruise speed with 12 or so pole rotor it makes more than 1000 Hz and even distorted by slow diodes in alternator
I dont think there is any control circuit that can track PFC at this speed and non sinusoidal shape
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If there was a real need for it, adapting an active PFC circuit to operate with a higher input frequency would not be a problem. I checked a couple PFC controllers and did not see any issue operating them into the kHz range.
Circlotron:
--- Quote from: calexanian on July 26, 2019, 08:33:14 pm ---Baring new fancy, electric, or hybrids......
No. The typical automotive alternator is Typically a 3 phase, half wave rectified output and the output power is controlled by having a couple of different level field strengths controlled by either taps, or a large external resistor. Some are called one wire alternators, which are fairly common today, have this switching internal where only a single lead comes out to the positive terminal of the battery and everything else is controlled internally, but the mode of action is the same. When the RPM is low or the battery is nearly fully charged the field strength is low therefore not either loading the motor at idle down too much as to stall it, or not overcharge a battery at high RPM. All alternators use the metal housing as ground.
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Nope. Automotive alternators have a full wave rectified output with six diodes. At low rpm sometimes as much output as possible is required from the alternator, e.g. on a hot night in stop start traffic, as well as all the usual electrical stuff that is always demanding power, there is the headlights, radiator fan, aircon compressor clutch, aircon blower, and brake lights when stationary. To have the alternator field strength low at low rpm is not a good thing, especially if the battery is not in perfect shape. In any case, all cars with EFI for the last 30 years or so have idle speed control to keep engine idle speed within an acceptable range regardless of the parasitic load of accessories. With my car idling in neutral I can pull the steering wheel a bit and the idle speed comes up slightly to counter the extra load the power steering pump puts on the engine. Same thing happens when the aircon compressor cuts in. A pressure sensor in the ps pump tells the computer there is more load, or a temp sensor tells the computer to engage the aircon compressor. In both cases the computer also tells the IAC stepper motor to open up a few clicks to maintain or slightly increase the idle speed.
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