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+--- Thread: Dynamo field coil resistance (/showthread.php?tid=6252)
Dynamo field coil resistance - tomcotez - 13-06-2021
I have ordered an electronic regulator for my Lucas DEL but understand that these need 2.5-3.5ohms resistance in the field coil to work correctly. Mine only has 2.1ohms which I believe is pretty normal. Can anyone explain what the reason for this requirement, and whether the problem could be resolved easily by say adding an extra resistor into the circuit or is there a fundamental issue which would require a more drastic approach- rewinding I assume? . The dynamo has been rebuilt at least once so the windings are good and I don’t want to have to do it all over again if I don’t have to. I have earthed the third brush to allow it to operate as a two brush unit.
RE: Dynamo field coil resistance - JonE - 13-06-2021
That is exactly the reason I gave up on the idea of electronic regulation. The low resistance is a plus - efficient three brush regulation - and we are ditching it or adding extra inefficiency into the circuit to add the regulation. I'm sure others have added resistors - can't remember which thread/who it was. Will be interesting to see who replies!
RE: Dynamo field coil resistance - Robert Leigh - 13-06-2021
Surely earthing the third brush, assuming it is still connected to the field will make the field coils useless because they are already earthed at the other end. All electronic regulator conversions I have done involve removing the third brush and connecting the field to the regulator, or have I misunderstood you?
RE: Dynamo field coil resistance - Bob Culver - 13-06-2021
When adjusted for max output the 3rd brush is near earth potential (on the 2 pole dynamos it can touch the earth brush.)
The field windings seem reliable so presumably can handle a full 6-7v continuously, although normally the 3rd brush setting provides less.
I do not know how the electronic regulators work, or whether the suggested resistance is to protect the field or the regulator or to limit the dynamo max current output. If the regulator does not sense or directly control dynamo output current then the latter likely is the case.
If the regulator simply senses voltage and adjusts the field current , then when the battery is low or lights on it should not allow the dynamo to work any harder than what was considered the prudent max; about 8amps net charge no lamps etc.
A DEL dynamo I use seems to be 1.8 ohm field. 1.8/2.5 is .72 , typically the proportion of 6v across the field when the 3rd brush set for reasonable output.
Whatever, it is important about 8 amps net charge is not recorded on the ammeter with a low battery or calculated with the lights on. 2.5 total ohms will likely acheive this. If dyn max output too high may have to increase slightly. Add exterenal resistance in the field lead
A complication is that the DEL originally had an internal resistor betwen the field and output terminals. This would confuse the regulator and prevent a very low output as approprite for a fully charged battery.With the 3rd brush lifted there shoud be no connection between field and anything else.Most are aopparently defunct or gone.
When measuring low ohms have to allow for meter lead resistance.
Regulators should not produce more continuous peak output then original; purpose is primarily to avoid overchrging, now esp important with Ca batteries.
I assume by earth third brush you mean remove the brush and earth the wire to the brusholder.
RE: Dynamo field coil resistance - JonE - 09-11-2021
When I specifically said about low resistance in Austin 7 dynamos, Al from AO Services suggested that this was one reason the 150W car unit rather than the original 100W VReg regulator (which is happily used by many bike owners with C35 dynamos) was recommended; i.e. I'm inferring from that, that with less resistance in the field coils, presumably more current is taken through that route. i.e. The regulator wiring diagram for the new earthed route still uses the entire field as part of a current flow?
Re. Bob's point about the DEL specifically, presumably there must be some indicator of a wire emerging from somewhere doing that "internal resistance" thing? Has someone got an original-ish unit that can advise. i.e. I'm presuming its just the same function as later dynamos with the resistance in the box, but in this case right inside the casing?