Author Topic: Do modern car/boat alternators use PFC circuits?  (Read 3965 times)

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Offline Circlotron

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Re: Do modern car/boat alternators use PFC circuits?
« Reply #25 on: July 26, 2019, 07:34:36 am »
PFC circuits exist in devices connected to the mains because the energy (current) has to be transported hundreds or thousands of Km from generation to consumption and PFC provides a reduction in power wasted in transmission.

In a car it makes no sense.
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. Might be the same situation with a sine wave output alternator. PFC= more DC watts for the same AC VAs.
« Last Edit: July 26, 2019, 07:44:58 am by Circlotron »
 

Offline Circlotron

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Re: Do modern car/boat alternators use PFC circuits?
« Reply #26 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.

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?
 

Offline soldar

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Re: Do modern car/boat alternators use PFC circuits?
« Reply #27 on: July 26, 2019, 08:59:41 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. Might be the same situation with a sine wave output alternator. PFC= more DC watts for the same AC VAs.
That's my point. With transmission lines being hundreds of Km long and including transformers along the way you want to optimize the power factor of the load.

With a car alternator it seems more simple and reliable to oversize it than to include electronics which are more prone to problems. If my car has a 100 amp alternator and I need more it is simpler to install a bigger alternator than to add a bunch of electronics which will still waste some energy in the form of heat.
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Offline mikerj

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Re: Do modern car/boat alternators use PFC circuits?
« Reply #28 on: July 26, 2019, 11:20:43 am »
Which vehicles have belt start engines?  Do they have a conventional starter too?

The second generation Smart ForTwo (451) had a belt drive starter/alternator and no conventional starter motor.  It is very problematic on this model, the belt is quite narrow and under a lot of tension and the high forces on the alternator/starter mountings cause the holes to elongate and the resulting misalignment causes the belt to wear and snap.  The same belt drives the coolant pump, so plenty of these cars have died with badly overheated engines.
 

Offline Circlotron

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Re: Do modern car/boat alternators use PFC circuits?
« Reply #29 on: July 26, 2019, 12:37:38 pm »
Which vehicles have belt start engines?  Do they have a conventional starter too?
Haflingers had a combined generator/starter/cooling fan that started the engine through the belt. Called a dynastarter. My uncle had one back in '69-'70.
 

Offline David Hess

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Re: Do modern car/boat alternators use PFC circuits?
« Reply #30 on: July 26, 2019, 02:03:36 pm »
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.

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.

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.

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?

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.
 

Offline Miyuki

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Re: Do modern car/boat alternators use PFC circuits?
« Reply #31 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
« Last Edit: July 26, 2019, 03:42:23 pm by Miyuki »
 

Offline calexanian

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Re: Do modern car/boat alternators use PFC circuits?
« Reply #32 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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Offline David Hess

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Re: Do modern car/boat alternators use PFC circuits?
« Reply #33 on: July 26, 2019, 08:45:35 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

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.
 
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Offline Circlotron

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Re: Do modern car/boat alternators use PFC circuits?
« Reply #34 on: July 26, 2019, 11:14:31 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.
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.
« Last Edit: July 26, 2019, 11:20:51 pm by Circlotron »
 

Offline calexanian

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Re: Do modern car/boat alternators use PFC circuits?
« Reply #35 on: November 18, 2019, 09:15:22 pm »
Just pulled a alternator from a 97 ford apart. Still just three rectifiers.
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Offline Circlotron

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Re: Do modern car/boat alternators use PFC circuits?
« Reply #36 on: November 18, 2019, 10:47:02 pm »
Just pulled a alternator from a 97 ford apart. Still just three rectifiers.
Are you looking at the three small diodes that feed the rotor with excitation current? There should also be six big diodes on two separate heatsinks.

Something like this:

 


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