Alternator regulator – voltage or current?

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BabaYaga

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A recent thread about battery chargers also drifted into alternator charging and the question how voltage regulation actually works.
Some comments made me realise that there is something very fundamental about the regulator that I don't understand, because I seem to have read conflicting descriptions.

My basic understanding is this: The regulator registers voltage between the alternator output terminal (or the battery positive post) and ground/alternator case.
If the voltage registered is lower than the set point, typically 14,4V, the regulator does nothing. The alternator then runs full-fielded and the current output is only limited by what the alternator can produce (capacity, rpm's) or possibly by what the battery is capable of absorbing.
When the voltage registred reaches the set point, the regulator begins to limit the field current to the rotor, thereby also limiting the output current to a level that will make sure that the voltage does not overshoot the set point.

Is this description right?

Or is it more correct to say that the regulator adjusts the output voltage, thereby limiting the current that flows into the batteries?

So my question is essentially, what is the regulator limiting:
Current in order to reduce voltage or voltage in order to reduce current?

Or perhaps neither of the above is correct?

For those interested, please discuss!
 
I'm certainly not an expert but I have always believed that any lead acid charging regulator regulates the voltage and the current at that voltage is then effectively self-regulated by the battery internal resistance.

I'm sure that's not the whole story though.

Richard
 
As far as I understand it the regulator switches the current flowing through rotor on and off very rapidly to alter the strength of the magnetic field generated. This field then generates the output voltage in the stator. This gets fed back to the regulator to be measured and therefore the output from the alternator is regulated at the desired voltage.

As the output of the alternator is loaded up by various circuits or a discharged battery the voltage will try and fall so the regulator will naturally increase it back to the desired voltage.

As battery voltage rises due to charging the current will reduce through the battery and the load on the alternator will reduce.

Some bedtime reading - http://www.olajedatos.com/documentos/alternatorhandbook.pdf
 
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OP is essentially correct. Quote
Or is it more correct to say that the regulator adjusts the output voltage, thereby limiting the current that flows into the batteries? Is a better statement except the reduction in voltage does not limit the current going into the batteries but rather say that the current into the batteries reduces because the voltage falls. Actually the regulator holds the voltage at the set point (or as you say limits the voltage to no more than set point). So current going in to the batteries falls as batteries charge up. (inherent battery voltage rises)
But as you say current from an alternator is limited by rotation speed and winding resistance (simply max capability) olewill
 
The regulation in a typical alternator is 'upstream' of the main rectifier diodes.
Therefore the resistance of these diodes reduces the volts available at high current and give the alternator its V/I characteristic.
Modern alternators may have more complex current limiting in addition to that imposed by the magnetics inside.
 
Many thanks for your input.
I think we all agree that voltage is what the regulator uses as a reference to adjust its output, so in a sense I suppose it is right to say that the alternator regulates its voltage to the desired level. And that this voltage level determines how much current will flow.
But I have still some problems with the concept of the alternator producing a voltage. I would rather have it that alternator produces a certain current from which a desired voltage results (or perhaps more accurately, a desired voltage level is not overshot).
Thanks a lot for the link to the alternator handbook, I have still to study it.
 
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BabaYaga said:
Many thanks for your input.
I think we all agree that voltage is what the regulator uses as a reference to adjust its output, so in a sense I suppose it is right to say that the alternator regulates its voltage to the desired level. And that this voltage level determines how much current will flow.
But I have still some problems with the concept of the alternator producing a voltage. I would rather have it that alternator produces a certain current from which a desired voltage results (or perhaps more accurately, a desired voltage lever is not overshot).
Thanks a lot for the link to the alternator handbook, I have still to study it.
Click to expand...

The alternator will only deliver a current if there is a load connected to it, but it will still produce nominally the same voltage at the output terminals even if there is no load and therefore no current flow .

To go back to basics if you dig out your old copy of Electricity and Magnetism by Nelkon you will find that there is an expression for the EMF ( in volts ) generated in a coil rotating in a magnetic field of

E = ωnAH/108​

here ω is the angular rotation of the coil in radians per second, n is the number of turns in the coil, A is the mean area of the coil and H is the magnetic field intensity.

No mention of current. An EMF is produced when a coil rotates in a magnetic field.
 
Thank you Vic.

I agree some studying of the fundamentals on my part would be in order.

Until I have found time for that, I have written the following example, in very simple words, to back up my present understanding of how the regulator works.

I would be grateful if you, or anyone else, would care to comment.

Consider this situation: A 70 A alternator running at full speed charging a battery of 200 Ah capacity, voltage is held at set point (14,4V) and the battery is absorbing, say, 5 A. So this is ongoing charging in the absorbtion phase.
What the regulator does, I think everyone agrees, is monitoring the output voltage, 14,4V, and adjusting/limiting the field current in such a way that this output voltage is maintained.
Now a 5 A load is switched on. Suddenly the alternator must put out 10A instead of 5A. What has to change? The regulator must increase the field current in order for the alternator to double its output. However, the regulator knows nothing about current, it reads only voltage. So how come that the field current increases? In my understanding it is because the additional 5 A load causes an instant voltage drop in the circuit, which the regulator very quickly picks up and compensates for, by increasing the field current. Which in turn doubles the alternator output, restoring the voltage to 14,4V, all within a fraction of a second.

Would you say the above description is about right or have I got it all wrong?
 
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BabaYaga said:
Thank you Vic.

I agree some studying of the fundamentals on my part would be in order.

Until I have found time for that, I have written the following example, in very simple words, to back up my present understanding of how the regulator works.

I would be grateful if you, or anyone else, would care to comment.

Consider this situation: A 70 A alternator running at full speed charging a battery of 200 Ah capacity, voltage is held at set point (14,4V) and the battery is absorbing, say, 5 A. So this is ongoing charging in the absorbtion phase.
What the regulator does, I think everyone agrees, is monitoring the output voltage, 14,4V, and adjusting/limiting the field current in such a way that this output voltage is maintained.
Now a 5 A load is switched on. Suddenly the alternator must put out 10A instead of 5A. What has to change? The regulator must increase the field current in order for the alternator to double its output. However, the regulator knows nothing about current, it reads only voltage. So how come that the field current increases? In my understanding it is because the additional 5 A load causes an instant voltage drop in the circuit, which the regulator very quickly picks up and compensates for, by increasing the field current. Which in turn doubles the alternator output, restoring the voltage to 14,4V, all within a fraction of a second.

Would you say the above description is about right or have I got it all wrong?
Click to expand...
That is roughly right.
 
VicS said:
The alternator will only deliver a current if there is a load connected to it, but it will still produce nominally the same voltage at the output terminals even if there is no load and therefore no current flow .

To go back to basics if you dig out your old copy of Electricity and Magnetism by Nelkon you will find that there is an expression for the EMF ( in volts ) generated in a coil rotating in a magnetic field of

E = ωnAH/108​

here ω is the angular rotation of the coil in radians per second, n is the number of turns in the coil, A is the mean area of the coil and H is the magnetic field intensity.

No mention of current. An EMF is produced when a coil rotates in a magnetic field.
Click to expand...

But the current flowing in the stator reduces the H.
 
BabaYaga said:
...
But I have still some problems with the concept of the alternator producing a voltage. I would rather have it that alternator produces a certain current from which a desired voltage results.....
Click to expand...

That is more or less what happens in the flywheel magneto charging circuit of an outboard or small Italian motorbike.
Many mopeds used to have a lighting coil designed for a 6V 30W headlamp bulb. They would run equally happily with a 12V 55W bulb!
The volts or current on these things don't increase linearly with rpm, because the magnetics saturate above a certain current.
It's either subtle or black art depending on your point of view!
 
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