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Dynamator output

Posted: Tue May 19, 2020 12:11 pm
by RadfordJim
I've just replaced the battery on my SP and checked the voltage across the terminals with an accurate multifunction tester, which indicated 12.8V dc.
With the engine running I checked again and my tester shows 12.9V ac with a frequency of 34.5 Hz at 650 rpm tickover.
When I increase engine speed the voltage increases slightly but Hz goes up almost in proportion. I would have thought the Dynamator should give a pure DC output like a standard alternator, as confirmed on my daily driver.
Any thoughts would welcome.

Re: Dynamator output

Posted: Tue May 19, 2020 12:50 pm
by Vortex O'Plinth
Perhaps you have a diode down?

Re: Dynamator output

Posted: Tue May 19, 2020 3:31 pm
by Ian Slade
Nick, there ain't any diodes in a dynamo :lol: :shock:

Re: Dynamator output

Posted: Tue May 19, 2020 3:35 pm
by bobtills
You've got your meter set to AC? The squiggly line, instead of the straight line?

Re: Dynamator output

Posted: Tue May 19, 2020 3:39 pm
by Vortex O'Plinth
Ian Slade wrote: Tue May 19, 2020 3:31 pm Nick, there ain't any diodes in a dynamo :lol: :shock:
That's true Ian, but Jim is running a Dynamator i.e. an alternator disguised as a dynamo, and alternators do have diodes.

Re: Dynamator output

Posted: Tue May 19, 2020 4:01 pm
by Ian Slade
Whoops sorry Nick, read it as a dynamo :lol:
If that is the reading from a multi meter then that is the RMS value the AC component is higher, about 18v and could well cook the battery if left too long or even melt wiring insulation

Re: Dynamator output

Posted: Tue May 19, 2020 5:35 pm
by RadfordJim
Many thanks for the replies gentlemen, a faulty diode does seem most likely, thank you Nick. I posted in case anyone has experienced similar or measured the output of a Dynamator.
I suspect it's been like it for a long time as it's probably the reason for any LED that I fit flickering dimly when switched off. I couldn't say as I've never checked the voltage before. The output can't be a pure sine wave otherwise the battery wouldn't charge at all!

I'm thinking a 50A bridge rectifier (£3 fleabay) across the output might work. My local alternator repairer won't even look at it. He said he can't get spares so suggested I get a new one!

For info, my multifunction meter just has one V setting and differentiates between ac and dc automatically, ranging from 10 to 600 volts. It shows 2 parallel lines for dc and the squigly sine wave for ac, along with the frequency. It's cleverer than me!

Re: Dynamator output

Posted: Tue May 19, 2020 7:03 pm
by classiclife
Hello Jim,

Paul Baker who owns Smart Power may well be worth contacting. I had an original Lucas 11AC built from scratch and he did a superb job. He has extensive knowledge along with a second to none spares access.

He is based in Barby near Rugby, CV23.

Regards.

Richard.

https://www.smartpowerclassicandvintage.co.uk/

Re: Dynamator output

Posted: Tue May 19, 2020 8:48 pm
by Brian-H
It depends on the exact waveform as to what the meter is reading when it displays the "~"

i.e. the waveform could be anything, not just sine, nor offset sine (on top of DC).

But ether way, as said, it won't be doing a battery any good if it's able to push several amps at its peak voltage (whatever that is)

If you have access to an oscilloscope and capture (or photograph) the waveform, then it might be possible to come up with a solution "external" to the "generator".

Re: Dynamator output

Posted: Wed May 20, 2020 3:20 am
by Ozzsp250
I changed to a Dynamator and had one fail the diode assembly and there were no spares diode assemblies available. I purchased a replacement and it constantly supplies (outputs) 14.2 - 14.3 volts from just above idle. The first dynamator failure also destroyed my electronic ignition module. I think quality may be an issue as they appear a cheaply made product.