12 Volt Field Coils

[Zetomagneto]

Sorry to start another thread relating to charging.
I have converted my dynamo to 2 brush operation and  used a 12v DVR regulator.

As I only had 1.5 ohms field coil resistance I fitted a 1 ohm resistor .
The dynamo is charging well , probably too much. The resistor is 10 watt and gets hot

I am told I need to get the field coils rewound ,and that they should show a total resistance of 6 ohms.
I have seen advertised somewhere new field coils for £95 but can't remember where.

Anyone had their field coils rewound latey?

Before anyone suggest, I don't want to go down the Acuspark route. 

Thanks in advance

[Bob Culver]

The original field coil was intended for somewhat less than 6v so the total resistance now should be somewhat more than double the original.
The amps drawn will be more than double what need be on 12v; a waste of current and power but many tolerate. The coils seem to survive more than the original current.
A good little exercise in Ohms law.
6 ohms is the field resistance of the common 1950s/60s Lucas 12v generator.

[Speedex750]

Hi Z,

This something I dallied with a while back without completing. I ran my Opal with a 12v RB106 regulator and it worked for a while but the regulator was working really hard and the points quickly burnt. 

Bob's summary is pretty close to what I was doing. I thought to double the resistance to about 4 Ohms. From memory, if you go too high on resistance you start limiting the output plus it's hard getting the number of turns you need into the physical space? By dropping the winding wire one size you can get almost double the number of turns into approximately the same sized coil. I had a jig for winding a square coil but found it difficult to then bend it into the curved shape to fit inside the small diameter body. I intended using it with a standard armature rather than rewinding that as well.

It was around about this point that other problems took over my time and I've never got back to it, there's a little project-ette pile of bits on the corner of the bench. The Opal seems to work fine on the 12v battery for an all day 150 mile run without charging...…………..

I can dig out the photos of the homemade winding gear, numbers and rough calcs if that helps?

Dave

[Bob Culver]

Ideally the number of turns times the new current should be no less than the original current times original turns.
And ideally for 12v the armature should be rewound with thinner wire and more turns but that further reduces the total current carrying ability (although at 12v not a great loss.) Some old books on rewinding car dynamos suggest a gauge thicker and I suspect this with some Sevens. Then more resistant to burning out but output reduced even on 6v.

I gather some locals here have rewound their own field coils.

(The weakness with Sevens is the armature. The heating effect is proportional to the amps squared, bearing in mind that includes amps to supply the field and which do not appear on the ammeter. Hence minimising field current is desirable)

If anyone has run 6v with an  electromechanical volt regulator and retained 3rd brush would be interested in outcome

If the 12v regulator was designed for a 6 ohm field would expect it to reasonably cope at 4 ohm. it is the guys who claim to run 12v two wire with an e.m  regulator and original 1.5 ohm field who mystify me!

[Special Bits]

Hi Bob

Regarding your last line, about two brush 12v conversion.
We have had exactly the set up you describe for several years, initially with no problems , then the regulator died! Since then I have had nothing but trouble, with it apparently doing in new regulators with ease. As I do long distance trials with the car, involving running all night, in all weathers, this is becoming a pain! My question is, will fitting a resistor of 4.8 ohm in the field coil wire to the regulator fix this problem, or does it need the dynamo rewinding? My next trial is at Easter, so a rewind isnt an option before then.

All the best

Stuart Palmer

[Bob Culver]

i have had no experience of these mods but.... quite apart from the suffering regulator contacts, when the battery is low or load heavy the field current is continuous. At near 3X th original current and so 9x the heat dissipation it is a mystery the field coils survive.
 I gather there are mods (diode?) to reduce sparking. Perhaps the original had something wired in underneath. A total resistance much more than double the original will reduce output. Some additonal resistance seems desirable; presumably could tolerate say 4 ohm total. Electromechanical reguators, whether one bobbin or two, have windings which respond to output current and moderate, but in Seven use probably not brought into significant operation.

[Zetomagneto]

Hello Stuart, you seem to have the same question as myself.I have now found a company who will wind 4 field coils for £120 plus post, plus VAT.I don't really want to spend that particularly as the dynamo is charging.

I have now bought a length of 24 gauge resistance wire which I will cut to the length which gives me 4.5 ohms, and will wind it into a coil .

https://www.ebay.co.uk/itm/Nichrome-Wire...QDHMMcTZFw

will let you know how it goes/

[Special Bits]

I ordered a 4.8 ohm, 50A capacity, panel mount, resistor from RS Components last night for the princely sum of £5 delivered. Will give that a go and see what happens.

I use an 11A regulator to limit the load on the dynamo, but Im coming of the opinion that the disparity in resistance is causing rapid points wear on the regulator. The current one lasted about 3 hours of running, failing about 3 miles before the start of the MCC Exeter trial! We now carry an auxiliary battery and have had to resort to it on the last 3 events, having started with a working charging system.

[Bob Culver]

Bar heater elements are a source of free resistance wire. The modern enclosed tube type have wire inside.

[Speedex750]

Hi Z,

My calcs below were for the later C35M dynamo but may help you in having a go rewinding. I used varnished copper wire available from the Bay of E under 'copper magnet wire' with tape from the same suppliers and a home made winding jig. If I were to remake the jig I'd make the sides stronger, the pressure from keeping the wire tight tended to bulge out the thin ply. Keeping the wire tight, laying it down evenly while keeping a good score of the number of turns as you're winding is a bit of a challenge!

Dave

Standard Lucas C35M 6v field
 
90 coils, 0.88mm diameter, 20 gauge wire per pole. Approx 310mm per turn = 27.9m per pole.
 
From look up table 20g wire has resistance of 26.3 Ohms per kilometre = 0.0263 Ohms per metre. One pole should therefore be approx 0.733 Ohms
 
Amp turns (an expression of the magnetic field generated);
F = NI
At 6v; V= IR; 6 = I x 1.4, Field current is approximately 4A
F = 90 x 4
F = 360
If used with 12v; 12 = I x 1.4, Field current is approximately 8.5A
F = 765
 
12v fields
 
21g resistance of 33.2 Ohms per kilometre = 0.0332 Ohms per metre.
Density = 0.00463Kg/m, At 0.31m per turn, there’s approximately 350 turns on a 21g 0.5Kg drum.
 
If get +50%, 135 turns, in the space;
Resistance = 1.39 Ohms, field resistance of 2.78 Ohms
Wire length = 41.85m = 0.19Kg per pole, 0.39Kg per field
F = 582
 
If get +75%, 155 turns, in the space;
Resistance = 1.62 Ohms, field resistance of 3.24 Ohms
Wire length = 48.83m = 0.23Kg per pole, 0.45Kg per field (two from a .5Kg reel)
F = 574
 
If get +100%, 180 turns, in the space;
Resistance = 1.85 Ohms, field resistance of 3.7 Ohms
Wire length = 55.8m = 0.2Kg per pole, 0.4Kg per field
F = 583
 
Go for 160 turns 21g, 0.75mm, 0.81mm over varnish
Design
Resistance of 33.2 Ohms per kilometre = 0.0332 Ohms per metre.
Density = 0.00463Kg/m
160 turns of .31m gives wire length = 49.6m, 0.17Kg per pole, 0.34Kg per field
Resistance = 1.65 Ohms per pole, field resistance of 3.3 Ohms
F= 582
 
Actual built
Weight wound into 160 turns = 0.22Kg of wire
62.15m, giving 2.06 Ohms for pole, field resistance 4.1 Ohms
Field current 2.9 Amps
F = 468