Use of a car Alternator

[tytower]

I am sure the learned will have had this question over and over but I did a search for an answer and found a list a mile long ,most irrelevent

What I would like to find is a site which discusses how to use car alternators and what electrical controls need to be installed to control the field coil etc?

Does anyone have any sites that deal with this please?

 

[MikeMl]

Have you read the Wiki?

Basically, the output CURRENT of an automotive alternator is proportional to the Field CURRENT (the alternator is a CURRENT SOURCE). The Field current is PWMed to control the average battery voltage; the output ripple is smoothed by the field inductance, and filtered by the battery.

 

[rmn_tech]

This makes interesting reading

Hope this helps you.

View attachment Alternator Secrets.pdf

 

[tytower]

I have had a look at both of these reference sites and they are both good reading .

They do not seem to address what I want to know which is how do you control the field winding in a wind generator use.

If you use the normal regulator it will put a current through the field coil to produce charging. This will hold back the blades . Though at low revs I suppose this load is minimal and the coil does not really load up until the revs get over 1000.

Hmm , dont know - any other thoughts?

 

[MikeMl]

So if you dont want voltage regulation, what do you want? Sounds to me that you asking the automotive alternator to do some it wasn't designed to do. Of course the rotor will be come almost to a stop if you apply field current and do not apply sufficient torque to the rotor to keep it spinning. In a car, an alternator that is producing ~50A (near full field excitation) is being spun at > 2000 rpm, and it takes almost 1Hp from the engine. How many Hp do you think you can get from a wind-driven fan? How fast will your fan spin it?

 

[tytower]

OK I must not be making myself clear somewhere.
Mike I don ´t say anything about voltage Don´t know where that come from

Assume the battery is low . I turn on the wind generator and it is spinning at 1000 rpm ,say the blades are directly connected to the alternator shaft.
A regulator will see the low battery voltage and bump the field current to produce lots of output. This will slow the fan or stop it depending on how hard the wind blows thereafter. Basicly it will come to a dead halt

If I do the same with a fully charged battery it will not load up the field coil and it will spin away happily in the same conditions

How can the regulation be otherwise controlled to get rid of this?
What am I not seeing that is so plain to you others?

 

[MikeMl]

If the battery is discharged, you have to do work to charge it. In a car, the crankshaft pulley is capable of transferring up to 1HP to the alternator pulley, without the alternator load appreciably slowing down the engine. You try the same with a wind-driven turbine, which cannot deliver 1HP at a constant speed, the alternator load stalls the wind-driven turbine.

The basic problem is that you have an "impedance mismatch" between the turbine and the alternator. You need either a different type of alternator (one with different pole configurations), or a mechanical transmission between the turbine and the alternator.

You could also build an electronic speed regulator which reduces the alternator field current as the turbine slows down; a sort of MPPT algorithm for a wind machine.

 

[ronv]

Seems like with something like a micro you could servo on battery voltage. Adjust for maximum but not to exceed say 14.6 volts on a 12 volt battery. Seems like this would find the best operating point. Might want to limit maximum current as well. Now in a big wind and fully charged batteries you might also want to add load to prevent the blades from spinnig to fast?

 

[tytower]

You could also build an electronic speed regulator which reduces the alternator field current as the turbine slows down; a sort of MPPT algorithm for a wind machine.

Mike this is what I am thinking about . So can you explain what you mean by this please

ronv -- The car regulator built into the back of an alternator puts out a continuous voltage of 13.8V normally. This doesn´t change does it ? Only the amount of current through the field coil is varied which varies the current out . Am I right in this ? Once you start a car its voltage is 13.8V without the battery attached .

I assumed this all would be a common problem faced by anybody trying to use a car alternator - is this not the case ?

 

[ronv]

The difference is that the car always has enough power to spin the alternator at full current: although sometimes you can hear is bog down a little at idle like when you "jump" another car with a dead battery. What you are trying to do is match the available power (wind) to what the battery (load) needs. The variable in this case is voltage. As long as the alternator voltage is below the battery voltage no current.

 

[tytower]

Nah ronv . You do not understand the operation of an alternator regulator and battery on a car .
Once the alternator begins to spin a small exciting current is passed through the field coil by the lamp and diode setup (charging lamp) . Immediately the alternator produces an output and it almost insantaneously goes to 13.8V due to the regulators action. The regulator now has control of the current that will pass through the field coil and it holds it at 13.8V . The amount of current is determined by the regulator sensing the voltage on the outside system including the state of the battery. If the battery is low a large current flows in the field coil and a large output at 13.8V is produced to charge it.
Hope that helps

My interest is in how this effect is bypassed, as a wind generator may hardly have any wind when connected to a flat battery and the sudden load up would stop it

 

[crutschow]

You might want to look at this article on wind driven generators. A car alternator is not a good candidate for wind driven generator.

 

[MikeMl]

If the battery is discharged, you have to do work to charge it. In a car, the crankshaft pulley is capable of transferring up to 1HP to the alternator pulley, without the alternator load appreciably slowing down the engine. You try the same with a wind-driven turbine, which cannot deliver 1HP at a constant speed, the alternator load stalls the wind-driven turbine.

Mike this is what I am thinking about . So can you explain what you mean by this please

The dynamic braking effect on the alternator shaft at any given rpm is proportional to the field current. You can try this on the bench. Remove the built-in VR, and connect a Lab power supply to the field. You can feel the dynamic braking as you spin the rotor with your hand by varying the field voltage/current by adjusting the power supply over the range from ~1V to 12V.

To build a wind charging controller, you would have to know the available wind speed (independent of the actual turbine speed), and then control the field current to achieve a turbine speed which is some fraction of what it would spin at if there was no dynamic load on the alternator rotor.

Obviously, the build-in VR is not suitable for that task. All it knows how to do is to increase field current if the battery voltage is below a preset voltage. Since a car battery can accept a charge rate in excess of what the alternator can deliver, this is what 'stalls' the wind turbine (it cant stall a car engine ).

btw- this thread should really be in the Renewable Energy forum.

 

[ronv]

Your right of course that is how it works in a car with no modifications, but we already know that doesn't work for your application. you need to control the field current to match the output to the available power (from the wind) to your battery.