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Dynamo not charging?

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Dynamo not charging?

Posted by tom_dolby at July 10. 2017

The bike in question is a 1956 Dommie 99. It has a magneto, dynamo and control box.

When I started it a few days ago, the ammeter was showing no sign of a charge. When I put the lights on, the ammeter showed some discharge. I tried again with the battery disconnected and it showed the same discharge. It's the same whether the engine is running or not , so the lights are only working because of the battery; the dynamo isn't charging.

I checked that the brushes are free in the holders and that their springs are OK. I did the test of linking the dynamo F and D terminals and then connecting a bulb to this link and to earth, but not the slightest glimmer from the bulb when running the engine.

I tried 'flashing' the dynamo but no change. Is there anything else I can do before removing the dynamo and having it checked and probably repaired?

Electrics are my weak point and I don't have a multimeter.

Thanks - Tom.

Re: Dynamo not charging?

Posted by les_howard at July 10. 2017

Looks like you have done the basic tests, so you could have a field coil open circuit...If you do the test outlined on the service sheets (Attached) you will soon find the problem.

Another basic check is that the brush pig tails or field coil connections which come together on screw post terminals on inside and rear of the end cover can be loose and thus become disconnected. It is fairly easy to remove the Bakelite end (single central screw) for inspection and to get to the connections screws & nuts on the rear of it.

Les

Attachments

Re: Dynamo not charging?

Posted by tom_dolby at July 12. 2017

Les - thank you - Tom.

Re: Dynamo not charging?

Posted by alan_osborn at Monday 23:32

Just a few basic 'tests/checks' further. First always polarise the dynamo 'flash it'. Obviously if it does 'flash' then there must be a Field coil in place. Next do as you say connect F to D and start engine, at this point you need a lamp to earth, but only earth the lamp after you have started the engine, as sometimes the cold current of the lamp stops the dynamo starting. If that fails then if the dynamo has been known to work on that machine, then it is off with it and off to the repairer. But if the dynamo come from an unknown source and is of unknown parentage and it is not charging then the Field might be out of phase with the armature, so you have to swap the Field wiring wires over. It would be very costly if you had the Dynamo 'rebuilt' only to find the Field and Armature where out of phase. By the way if you have a direction arrow on the dynamo that will tell you what direction it went in 60 years ago!

Further information on this point can be found on my web site A O Services.co.uk under the item, using the V reg with 'Bosch' type dynamos.

Re: Dynamo not charging?

Posted by Barry Carson at Monday 23:55

Re: Dynamo not charging?

Posted by tom_dolby at Tuesday 09:24

A bit more detail - I bought the bike partly dismantled about 10 years ago and the dynamo came with it. Eventually, I got the bike assembled and running, and used it on the road. But as I have too much Old Iron, I don't ride/drive each one regularly. I do start everything every few months and the last time I started the Dommie, the dynamo was charging.

Yes, the dynamo shaft is turning. There are no obvious loose connections under the end cover.

Is it normal/possible for it to stop charging without warning rather than fail gradually?

Re: Dynamo not charging?

Posted by Barry Carson at Tuesday 09:56

Re: Dynamo not charging?

Posted by les_howard at Tuesday 10:49

Hi Tom. Yes it is possible for dynamos to stop charging suddenly which would mean that a wire somewhere inside the dynamo has broken or become unconnected,* hence my first suggestion. To carry on testing it is better ( I find it much more insightful) to use an analog meter simply because by watching the needle gives far better idea of what is going on rather than be hit with spinning numbers going up and down all over the place.

Also it's useful to have a better idea of how a dynamo works. So just as a reminder then: The armature, has lots of sections with their corresponding coils wired out to the commutator. It spins within the steel field magnet which has a magnetic remnance (slightly magnetic).

Spinning the armature generates a small current and is taken off the armature by the brushes and fed to the D output connector socket and the other side to earth. The field coil is earthed on one side only too and its other end connected to the F output connector. When this small current is fed, by linking the D to F connection, it flows into the field coil to energise it and raises the magnetic flux in the steel field magnet. Because the magnetic field is now stronger, the armature can now produce a higher voltage and current which in turn feeds the field coil and a "runaway" increase in current occurs only limited by the wire resistance and magnetic saturation limit of the field magnet.  This connection between D and F is normally controlled by the regulator to reduce the output when conditions require a lower output, but by making a solid short cct for testing maximises the dynamos output.

So back to the testing. One quick test is to connect an analog meter set to MILLIAMPS between the D and F terminals to act as the short (no other wires connected) and turn the dynamo by starting the engine. You should get a reading instantly the engine turns even at tickover….Do this test first. I won’t suggest more tests at this moment. (BTW…you can use a digital meter but as said, you will just see numbers flashing around but you maybe able to get a reading that you can decipher….(Digital meters are best suited to measure fixed voltages and currents but the most expensive sometimes have an analog bar type trend indicator)  Do this test first and let us know…..Les

* PS...one thing that can cause a sudden output stoppage is that one of the carbon brushes gets stuck just clear of contacting the armature. Make sure they are free to slide in and out of their holders...they can be removed easily by pulling back the spring with a small screwdriver but the Pig tail connection wire will still be attached on the connector screws or frame. No connection via the brushes to the armature is THE most likely fault that results in ZERO output (if connections are all OK) ...so don't dismiss this check....Les

Re: Dynamo not charging?

Posted by David Cooper at Tuesday 13:57

Thanks for all the detail, Les.  I need to re-check a dynamo as well.

Your comment about non-contact by the brush reminds me of the dynamo failure I had years ago in an old MGB.  The wire which should be jammed into the back of the brush was loose.  It ran hot, and the spring burnt its way into the carbon.  The brush looked OK on inspection - but eventually I opened the bonnet at night and it was glowing red.

Jamming in a bit of silver fag packet foil liner from the gutter fixed it.

It had been blowing halogen bulbs (cost about 7 pints of beer at the time).  Years later I discovered that, if they don't get hot enough, evaporated tungsten settles back on the glass and not the filament.  So low voltage dramatically shortens the life of halogen bulbs.  I imagine it also does if they are dimmed at home.

Re: Dynamo not charging?

Posted by David Cooper at Tuesday 13:58

sorry - site placed the same text twice

Re: Dynamo not charging?

Posted by tom_dolby at Tuesday 21:55

Thank you for the further replies. I'll check the basics again given the potential costs of a prof rebuild.

Al Oz - I have a 3 page handwritten reply on NOC headed paper you sent me from your West Ealing address in answer to a query about my ES2's charging system. It's dated 17/01/1978 - is it a Collectors' Item yet?!

I also have a typewritten reply from Titch Allen on Motor Cycle News paper dated 31/03/1977.

Yes, I'm a hoarder!

Re: Dynamo not charging?

Posted by Neil Wyatt at Wednesday 01:08

I have this problem with my 16H because of lack of use.

What works for me is to fully charge the battery and then go for a ride. Usually, after a mile nothing happens so while on the go I flick the light switch on, there is a discharge and a sudden right swing of the ammeter and all remains well unless I leave the bike standing for many months as usual. Main thing is to give the bike a good run before you flick the switch.

Hope this works for you too.

Re: Dynamo not charging?

Posted by les_howard at Wednesday 09:47

Re David Cooper: "It had been blowing halogen bulbs (cost about 7 pints of beer at the time).  Years later I discovered that, if they don't get hot enough, evaporated tungsten settles back on the glass and not the filament.  So low voltage dramatically shortens the life of halogen bulbs.  I imagine it also does if they are dimmed at home."

A good point David, but if you think about it, at low voltages and hence low filament temperatures the tungsten filament isn't going to be boiling much of itself off and coating the glass. Its only when you take the filament temperature up to near its boiling point that evaporation is a problem but then the halogen process steps in and starts to react and returns the tungsten molecules back onto the filament. The evaporation of tungsten limited the top temperature of a conventional bulb, but taking the temperature higher* greatly increases the light emission and creates a whiter light colour for the same current consumption and this can be done by using the halogen process.

* This is achieved by, in effect, designing the filament to be overpowered by the available drive voltage...similar to running a 6V bulb on 12V.

Les

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