This is also regarded as a useful and 'bookmarkable' place by many people in the parallel universe of AMC machinery . . .
http://matchlessclueless.com/electrical/lucas/testing-lucas-dynamo/#readingand this is my simplified take on it . . . with apologies to those who know this stuff better than I do anyway. An important point being that D gives everything (rather than part of itself) to F at pre-cut-in rpm.
There are five stages in a system comprising a (two brush) dynamo and voltage regulator: Static (ie dynamo at rest); excitation; avalanche; unregulated charging; and regulated charging.
With the engine stopped (static stage)
1. The cut-out is open.
The battery can not permanently be connected to the dynamo. If it were, it would try to make it turn like a motor. So, in ‘FADE’ terminology, wire A (to ammeter, switch and battery) is only connected to D when the cut-out is shut. A and D are never connected at rest or at low dynamo rpm.
2. The contacts on the voltage regulator are closed. So D is connected to F through the voltage regulator contacts. Their default position is ‘Shut’. When the revs are high enough for voltage regulation to be required the connection switches off and on rapidly - but at rest and low- to mid-rpm the contacts are closed firmly.
The ammeter shows nothing because Nothing’s Happening unless you've left the lights on.
From Start-up to Cut-in (excitation and avalanche stages)
The dynamo starts by turning slowly.
The dynamo’s body contains a small amount of residual or remanent magnetism from the last time it ran (Richard's point). The armature is therefore turning in a very weak magnetic field.
The armature uses that weak field in the initial ‘excitation’ phase to produce a very small amount (like ½ a volt) between D and E.
This very small amount is fed from the D terminal straight back to F the field through the closed points on the voltage regulator.
With the extra electro-magnetism now available in the field, power builds up fast at D. This is the ‘avalanche’ phase.
The dynamo is working away now, but nothing’s yet showing on the ammeter. That happens when the voltage being supplied from D becomes greater than the battery voltage already present at A. At that moment, the cut-out closes. D and A are now connected. Power from D is now allowed out to play.
So D, F and A are now connected – F to D through the still-closed regulator contacts, D to A through the now-closed cut-out. Because the ammeter is in the A line to the battery, we can now start to see what’s happening for real. This is the end of the ‘avalanche’ and the start of a phase of unregulated charging.
As Dynamo rpm gradually increase in this unregulated stage, so does the power generated by the armature.
D is connected to A as we’ve seen ( via the cut-out) and always has been to F (voltage regulator contacts).
The system voltage rises a little but not much – the battery, a chunky resistor now in play, prevents that (even when it’s state of charge is low) - so the voltage into the field doesn’t increase much and nor does the power it is consuming.
The system’s working fine, but at this intermediate stage the voltage is still not being regulated (D is connected to F solidly by those points on the regulator).
But it gets closer to needing to be regulated with every extra engine/dynamo rpm, as the voltage rises.
As Dynamo rpm rise further (regulated stage) . . .
With D, A and F all still firmly connected, voltage has risen to the point where the voltage regulator says ‘Enough, I’m at my pre-set limit, and I’m going to start regulating’. We’ve arrived (probably around 1800-2000 engine rpm ) at the regulated phase.
(In reality it all happens darn fast – a mere blip of the throttle gets you through most of this in the blink of an eye.)
The voltage regulator is pre-set to prevent D from feeding the field with more current than is necessary for it to enable D to support it (the field) and the electrical needs of the machine at that moment (A). F’s feeding programme and other calls on the dynamo all depend on what’s switched on – and the state of charge of the battery.
If F isn’t overfed, then D can’t overproduce and fry itself or anything else; if F isn’t underfed, then D will produce enough to balance the load and maintain the battery.
The trick is in making sure that F is adequately fed when the electrical load requires lots of support and/or the battery needs a charge, and is not overfed when the load is light and the battery only needs a trickle to keep it up.
How does the regulator regulate?
When it cries ‘Enough’, those voltage regulator contact-breaker points open for the first time so far and F is disconnected from D (and therefore from A).
Immediately, the current to the field drops, so the armature’s output drops and so the current from A to the battery also obviously drops. So does the voltage. A split second later the control box decides the voltage is no longer excessive after all, countermands the order, and the regulator closes and reconnects F to D and A. Next instance the cycle repeats – too high a voltage - disconnection. Then, the voltage drops – reconnection. And so on and so forth, depending on revs and load. The contact-breaker is now in ‘trembling’ mode and we’re regulating.
It’s the relative amount of time the regulator points stay shut versus open that determines the average voltage at A. The higher the proportion of time the points are open the lower the voltage, and vice-versa. Thus the voltage at A is maintained at about what it should be to avoid overcharging the battery, to trickle-charge it once it’s fully-charged and to protect any electrical components that are in use from blowing.
As dynamo revs drop off (or as the electrical load increases to the point that the dynamo can’t sustain the prescribed regulation voltage) the regulator switch stops trembling and closes to let the full output from D go to F once again. D and A remain connected through the cut-out. The dynamo is running unregulated once more.
As the revs drop off even further, the point will be reached where the cut-out senses that the battery needs to be disconnected – and opens to prevent the battery trying to drive the dynamo as a motor.
A is disconnected from D, while D and F remain fully connected at the regulator.
This is precisely where we started.
Cheers, Bill
