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MG Midget and Sprite Technical - Bench testing dynamo

Hi,

After cleaning up the dynamo I decided to bench test it.

Watched some videos and read some threads on it.

Understood that I needed to put a jumper cable between the two connections then put the volt meter positive to one of the connections and the negative to ground, that could be the casing.

It’s the reading I get which I don’t understand. As I turn the pulley the voltage rises, but it is alway 0. something.

It never gets to even 1 volt. But the faster you turn the reading does rise and vice versa.

Wondering if anyone knows if I’m doing something wrong or what might be wrong with it.

Thanks

James







James Paul

Have you already tested the field coil and the armature? I can't remember the values but the videos should tell you. How are you turning the pulley, and at what speed? And in which direction? I have also wasted many hours on these tests, and after decades of unreliability gave up and fitted an alternator. Half the cost of a new dynamo and the lights are brighter! Gets rid of the dodgy control box as well.
Les Rose

Thanks Les,

Just read up this article

https://www.matchlessclueless.com/electrical/lucas/testing-lucas-dynamo/

My field winding levels are way too high.

The dynamo windings are spot on!

I’ll put it all apart and see if I can find where the break is.

Alternator conversion is for stage two of my project.

Thanks

James
James Paul

Easy simple bench test-join both posts and try running it as an electric motor--if it works it should generate

let me find a simple test video for you- I'll be back
William Revit

Well, there you go, this Moss youtube vid does the trick for simple straight forward tests

Also, when you were bench testing how fast did you spin it---Normally in car it's fast idle speed but on the bench you'd need 2000-2500 rpm


https://www.youtube.com/watch?v=RjLW_eb3D-c&t=129s
William Revit

Thanks,

Yes seen that video was very helpful.

I used a drill to spin it up. Think it goes up to 2000 rpm

I think though it has seen better days.

Time to replace I think. Now it makes sense why the PO had a volt meter fitted on the dash.







James Paul

James

A very kind midget & Sprite Club (MASC) member (Dr John E Davies) who tested and refurbished my dynamo, voltage regulator and rev counter reduction gearbox for the cost of parts and whatever I wished to donate to his chosen charity. All now works great.

If you are a MASC member worth a try?

Cheers
Mike
M Wood

Time to bring stage two of your project forward and invest in an alternator; much, much better performance and reliability.
Jeremy MkIII

I decided to give all the bits a big clean up this afternoon in a last hope to see if that would make a difference. Carefully sanded down all the rusty bits. Cleaned up all the contacts points. Ended up looking quite good.

Then I remembered watching one of John Twists videos from University Motors.

https://www.youtube.com/watch?v=sqCYcW0bAYA&t=179s

In this video he connects up a current to the field coil to demonstrate the magnetic force the coil produce. I have a car battery charger so connected that up the same and tested it with a screw driver as John does in the video and to my surprise both coils had quite a force on them.

Made me think that maybe my cleaning had made a difference. So put it all back together and tested it again.

Still the same result. Maximum I got was 0.20 volts. But rising and falling perfectly as I increased the decreased the revolutions.

I'm no electrician, so I wondering if I was reading the volt meter wrong. Popped over to my next door neighbour who is a professional electrician and asked him how he would set up the volt meter.

Turns out I hadn't put the black and red test leads in the right jacks!!

Went back and tested again....

14 volts!!

What a numpty

Think I'll leave it there.
James Paul

James brilliant! 🤣

Don't worry we've all been there, some of us many times.

I put in a thread quite recently that the very basics are often missed for some else to point out the issue immediately.

Some of the very basics are forgotten just because we can't sometimes think so basically where as someone with little or no knowledge might ask a very basic question like are the wires the right way round.

Decades back I was trying to help a friend of a friend over the phone as to why his new CD player wouldn't play as he was new to CDs. I went through all the wiring from 13a plugs onward, then the switches but got nowhere and said he'd have to take the player back to the shop. Found out he'd been putting the CDs in the player upside down, that just never entered my head but it should have as its one of the basic points to start from.

Nigel Atkins

This thread was discussed between 22/07/2022 and 23/07/2022

MG Midget and Sprite Technical index

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