Checking dynamo

michipons

michipons

Hero member
Location
Palma de Mallorca
Hi all,

I just finished to rebuilt the dynamo, I was wondering if there is a way to check wether it is working properly or not before I assembly it to the engine...?
And I take advantage to ask.. What does DF stand for? I guess is the negative pole, but there is another screw that is just screwed to the case of the alternator that has the D- :-\ (as far as I remember the is no cable attached to this one)

Thanks in advance,
Regards,
Miguel

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There are two ways of checking a dynamo - either (1) operate as a motor or (2) measure its voltage output as a generator.  To do the latter, you'll need to fit the generator back on the bike and have the engine running (unless you can rig up a way of easily spinning the dynamo off the bike, maybe with an electric drill?)

1.  Clamp the dynamo firmly in a vice or similar to stop it from spinning across the workshop. 
Connect DF (Dynamo Field) to earth (the dynamo body)
Connect a 12v battery negative terminal to the dynamo body
Connect 12v positive to the D terminal.  The generator should turn in the same direction as it would when fitted to the bike.
Do not let the generator run for more than 5 seconds to avoid overheating

(Carrying out this check will avoid the need for repolarising - or 'flashing' your rebuilt dynamo)

2.  With the same connections as above - but with the battery completely disconnected - you can measure the generator voltage between the generator body and the D terminal when the generator is turning.  The generator will only needed to be spun slowly.  You can use a 12v lamp to apply some load instead of just measuring the no load voltage.
 
Ok! thanks for the instructions! I will try it tomorrow.

Apparently I did a mistake when trying it: I was connecting negative pole to the battery to D- and the positive to D+, it was not spinning... I will try it the way you said. Thanks!

Regards,
Miguel
 
Hi Rob,

I don't like electricity, what for you is so obvious...  :LOL: For me it was clear that current had to flow, but never mind, I gave up with electricity time ago :LOL:

Of course, I couldn't wait until tomorrow, so I went down and tried it, it worked straight away!

I post the photos for next generations...

Thanks a lot, I will sleep like a baby today! :LOL:

Regards,
Miguel

https://www.youtube.com/embed/3kWzsqAtRcQ

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As is a restoration to standard, I would like to leave the original alternator.
I will drive only in daylight or rarely in the night and the bike battery is allways pluged in when parked... so I guess this alternator should suffice, or is there any other big dissadvantage that I am missing?

Regards,
Miguel
 
No disadvantages other than the battery doesn't charge when the motor is idling. That's one of the reasons they came with a massive battery - you can reduce the size no probs.

They put out plenty of power (BTW, it's a generator, not an alternator) ... Lee and I have done many interstate trips in Oz with std 60W halogen headlight and a 60W Cibie halogen driving light ... loads of light and no charge problems whatsoever. Absolutely NO reason not to ride an SF at night! They are sports touring bikes, built for anything.

Auto elecs charge very little to test your system ... it's simple and reliable.
 
I checked my dynamo output a couple of months ago and it was putting out 14.3v at max. I'm using an electronic regulator.
 
Just for the benefit of anyone picking up on Ron's comment - the Bosch dynamo is rated at 150w.  Anyone replacing the OE regulator should keep in mind that it needs to limit the current output to around that figure to avoid overheating.  An 11 amp rated regulator fits the bill.
 
The dynamo is trouble free and gives plenty of watts. You can ride all night with the flashing blue lights, additional beams and siren, park the bike in the morning and restart it 3 months later.

The only things that need changing every 30 to 50 years are the brushes and the bearings.

Paul
 
Piranha Brother 2 said:
No disadvantages other than the battery doesn't charge when the motor is idling. That's one of the reasons they came with a massive battery - you can reduce the size no probs.
Click to expand...

Probably true of the 750 twins, but the massive battery on the 3C was because they had no regulator, just a rectifier. The larger the battery the more stable the system voltage. It's kinda like the sea level not rising when you piss in the ocean. If you fit a smaller battery to a 3C, you really need to fit a rectifier/regulator. Probably not a lot of 3Cs around with the original ignition and charging system these days.
 
Nat Magee said:
I checked my dynamo output a couple of months ago and it was putting out 14.3v at max. I'm using an electronic regulator.
Click to expand...
No. You checked your charging system, not the dynamo itself. Your charging system is fine on the volt side, to fully test the charging system you need to put in an amp meter as well. To test the maximum voltage output of your dynamo use the procedure I posted, generally only done either after rebuilding a dynamo or to fault find/eliminate a charging problem.
I have been checking out electronic regulators on my bike lately, three of them, and so far the Bosch replacement has been the best both on voltage level and stability and on amps delivery. More expensive than the other two I have but worth it I would say.
 
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