Field Coil query - E3N Dynamo

Information relating to the Matchless G3 or AJS Model 16 350cc Heavyweight
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AFSMatchie61
Posts: 60
Joined: Sat Oct 01, 2016 7:34 am
Location: Scotland

Field Coil query - E3N Dynamo

Post by AFSMatchie61 »

Hi All,

I've recently bought a cracking 1955 AJS 16MS and am in the process of 'fine fettling' whilst enjoying the ride. Yesterday the lights went out so there is a dynamo issue. I tested output and it puts out only 0.3 volts so I took it off the bike for more tests and to be fixed. Once off the bike I found one of the brushes was stranded in its holder and not making good/full contact. This could be the only fault but I'd like to be sure! I have tested the commutators against each other and they seem OK but....

....the field coil is showing a resistance of 5.5 Ohms compared to the Lucas listing of 3.2 for the E3N dynamo. My electrical skills are primitive so I am hoping someone can advise if this indicative of a fault or within tolerance?

Many thanks for any expert advise!

Dave
Don Madden
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Re: Field Coil query - E3N Dynamo

Post by Don Madden »

With such low resistance the leads & connections are adding to your readings. Always short the test prod tips together & if you have a "zero ohms" control set the meter to read 0. If not, subtract the short reading from the dial for a more accurate value. Cheers! Don.
SPRIDDLER
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Re: Field Coil query - E3N Dynamo

Post by SPRIDDLER »

Welcome, Dave.
If you're testing it on the bench (e.g. driving it with an electric drill) and measuring the voltage you also need to put a load on the output by connecting say, a 20 or 35 watt bulb.
I use a meter but also this Hi-Tec test rig to put a load on the output
Image

Just a tip as I don't know how familiar you are with these old nails: When you get around to refitting the dynamo make absolutely sure that it's positioned laterally so that its chain is running true and clear of the primary chain.
(Damage caused by the chains clashing is a not an uncommon tale of woe on here ;) )
'There is a tide in the affairs of men
Which taken at the flood............'
AFSMatchie61
Posts: 60
Joined: Sat Oct 01, 2016 7:34 am
Location: Scotland

Re: Field Coil query - E3N Dynamo

Post by AFSMatchie61 »

Don Madden wrote:With such low resistance the leads & connections are adding to your readings. Always short the test prod tips together & if you have a "zero ohms" control set the meter to read 0. If not, subtract the short reading from the dial for a more accurate value. Cheers! Don.
Hi Don,
Thanks for the reply! I did short the leads and so deducted 0.3 so the 'true' reading does appear to be 5.5. It might just be a pattern part that's a wee bit off original spec I suppose?
Cheers
Dave
AFSMatchie61
Posts: 60
Joined: Sat Oct 01, 2016 7:34 am
Location: Scotland

Re: Field Coil query - E3N Dynamo

Post by AFSMatchie61 »

SPRIDDLER wrote:Welcome, Dave.
If you're testing it on the bench (e.g. driving it with an electric drill) and measuring the voltage you also need to put a load on the output by connecting say, a 20 or 35 watt bulb.
I use a meter but also this Hi-Tec test rig to put a load on the output
Image

Just a tip as I don't know how familiar you are with these old nails: When you get around to refitting the dynamo make absolutely sure that it's positioned laterally so that its chain is running true and clear of the primary chain.
(Damage caused by the chains clashing is a not an uncommon tale of woe on here ;) )
Thanks Spriddler, even my rudimentary skills might be able to cobble a test rig together. Then again...

Good tip on the chain alignment, thank you. I will be wary.

Cheers
Dave
AFSMatchie61
Posts: 60
Joined: Sat Oct 01, 2016 7:34 am
Location: Scotland

Re: Field Coil query - E3N Dynamo

Post by AFSMatchie61 »

Oh, and here is the bike :-)
AJS.jpg
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aobp11
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Re: Field Coil query - E3N Dynamo

Post by aobp11 »

When the field coil is really 5.5 Ohm then I don't think it's a bad replica. The maximal magnetism would have been too low and you would have noticed this as a (slight) discharge when lights are on. Even when more turns of finer wire is used in the same space (or volume) the maximum magnetism will be reduced.
But then the question is what caused this high reading. Whatever the cause, it won't be stable if the original value was 3 Ohm. Maybe your multimeter has a current range for 2 A or more. Then you could feed the coil straight from the battery with the meter in series. The reading should be about 6 V/ 5.5 Ohm = 1.1 A. If you keep things connected then the coil will get warm and the current will decrease (the resistance increases about 4 % per 10 degrees temperature rise). Maybe you see a sudden change or even a break after some time.
In theory the higher temperature won't damage the coil; when the dynamo is heavily loaded the coil also receives the full battery voltage during most of the time. But better stop when the current dropped to 80% or so (temperature rise some 50 degrees or so).

Albert
AFSMatchie61
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Joined: Sat Oct 01, 2016 7:34 am
Location: Scotland

Re: Field Coil query - E3N Dynamo

Post by AFSMatchie61 »

Thanks Albert, much appreciated. I've ordered a new one so will see how I get on.
Cheers
Dave
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