Amperage to expect from Alternator

[nicknorman]

6 minutes ago, BEngo said:

Yes there will be some voltage ftom residual magnetism, but it cannot exceed the regulated voltage because  it must be there all the time since  it arises solely from the unvarying iron magnetism not the (regulator varied)  field. It would be there regardless of whether the  machine was or was not open circuit.

How would one control an (nominal 12V) alternator with a minimum o/p voltage of, say, even 20 V due to residual magnetism?

 

N

 

No. You are confusing voltage with current. The residual magnetism and rotational speed create current, not voltage. Although of course pushing current into a resistance creates voltage. Therefore it is feasible that an open circuit alternator with residual magnetism creates significantly more voltage than the regulator's preset voltage. Of course as soon as a tiny load is presented to the circuit, the voltage created by that small current will drop right down.

Perhaps this is a clue? Even if the generated voltage rises to the diodes' breakdown voltage, the ensuing breakdown current is very small and thus the diodes are not actually damaged? Just guessing!

[Sir Nibble]

Firstly, these paralleled alternators. How, if at all is seperation between engine and domestic batteries acheived?

On the subject of self excitation. Broadly speaking the residual magnetism is insufficient to raise voltage beyond the forward biase voltage of the diodes. On a good day, with the right alternator, if you rev the balls off it it may just manage to cut in. That holds good for the familiar nine diode machines but more and more six diode units are showing up, especially among the American types. With these the field is directly supplied with either supply from B+ or an earth, with the regulator supplying the other in response to detecting an ac frequency at the stator. This cut in being independant of diode voltages is at a much slower speed and no warning light at all is required, in fact on the test bench the current draw of the field can be seen as a discharge if running below cut in speed. With a six diode alternator I would be inclined to disconnect the B+ to disable the output and jump lead the B+ terminal to earth as a precaution to protect the machine. With a nine diode just jump lead D+ to earth and it will not generate at all.

Back to parallel alternators. Cris W late of these parts built a system where he pressed a button to engage a contactor between alternators which remained engaged until switched out by the otherwise defunct regulator of the sterling pdar controlled domestic alternator at 14.2V. My own idea is to connect the engine alternator to both banks via split charge diode. This gives the combined output of the alternators for bulk charging and the engine alternator can prop up the domestic alternator if output drops off like dropping to tickover. If domestic alternator is set at a slightly higher voltage (and it should be, engine batteries only want a float charge) there is no current flow between systems until load on the domestic needs help from the engine.

[nicknorman]

But wouldn't it be better for the alternators to share the load more evenly, ie by paralleling the alternators as per the OP and using a split charge relay to charge the engine battery? Then much less time with the domestic alternator operating at max field current / min rpm for the output / max temperature.

[WotEver]

Also Snib, you didn't comment on 'disconnecting whilst charging'. What is the mechanism by which it damages an alternator? Or doesn't it?

[Tony Brooks]

3 hours ago, WotEver said:

Also Snib, you didn't comment on 'disconnecting whilst charging'. What is the mechanism by which it damages an alternator? Or doesn't it?

My understanding, although some on here get involved in rotational energy and goodness knows what. is that its the result of an almost instantaneous collapse of the  magnet field in the stator inducing a voltage spike into the stator (generating coils). Having received a belt from both horns and small 12V relay coils as they were turned off I can well believe that to be the case.

That voltage spike (on a 9 diode machine) will also be applied to the rotor. Now how much damping the inductance of the rotor will achieve I do not know if it forces any more current through the rotor that in turn will give an increase in stator voltage.

No idea how they all interact though.

 

 

Edited March 15, 2017 by Tony Brooks
Numpty wording - changes rotor to stator (genrating coils)

[ditchcrawler]

17 hours ago, nicknorman said:

I disagree. You are presuming that all the magnetism comes from the field current. But in reality the iron rotor has some residual magnetism, even when the field current is zero. Regulators don't take the field current into negative territory to eliminate the residual magnetism. So depending on the magnitude of the residual magnetism and the rotor speed, there will be an output voltage into an open circuit of some sort. The only question is how much.

My 24 alternator would happily charge with just a positive and negative cable connected to it, no excitation connection at all.

[WotEver]

3 hours ago, Tony Brooks said:

My understanding, although some on here get involved in rotational energy and goodness knows what. is that its the result of an almost instantaneous collapse of the  magnet field in the stator inducing a voltage spike into the stator. Having received a belt from both horns and small 12V relay coils as they were turned off I can well believe that to be the case.

That voltage spike (on a 9 diode machine) will also be applied to the rotor. Now how much damping the inductance of the rotor will achieve I do not know if it forces any more current through the rotor that in turn will give an increase in stator voltage.

No idea how they all interact though.

Thanks for that, Tony

Edited March 15, 2017 by WotEver
Cloned Tony B's edit

[Tony Brooks]

Tony (WotEver) I have had to edit my reply to your question because of saying something I did not mean - changed rotor to stator in one case.

[WotEver]

23 minutes ago, Tony Brooks said:

Tony (WotEver) I have had to edit my reply to your question because of saying something I did not mean - changed rotor to stator in one case.

I edited the quote for consistency

[mross]

Respect to anyone who can be bothered to edit quotes!  Good on you, sir.