demagnetized rotor?

Information relating to the Matchless G2 or AJS Model 14 250cc Lightweight
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demagnetized rotor?

Postby Ian 14 G2 » Mon Apr 04, 2011 7:28 pm

Is it possible for the rotor on my bike to be demagnetized yet still feel like it is actually strong, is there a simple test I can do to find this out?
I have just fitted a new alternator, wiring harness,switches, lights and checked that all electrics are ok and have every thing working but I am getting very little reading on my ammeter when ticking over with lights off and a fair discharege when with lights on and little improvment when running the engine at fast idle.
The bike is a 1961 ajs 14 and is the first bike I have owned, I have done all the mechanical work and any fabrication myself along with some machining and the bike looks ok but the electrics have me beat at this point in time.
I have replaced the the old wipac stator with a lucas 6 volt 3 wire
RM19/LU47204 stator, all dimesions are identical along with the rotors and it fits perfect but I have kept the wipac rotor ,should I have change it to? I dont want to be defeated by this and get in a auto electrician unless I really have to.

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demagnetized rotor?

Postby aobp11 » Mon Apr 04, 2011 8:22 pm

A crude test is given in Tech.Article "alternator-testing".
Michael, because of the incredibly large ampere-windings required to magnetize permanent magnets I am surprised that battery current could demagnetize the rotor.
Albert

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demagnetized rotor?

Postby wilko » Mon Apr 04, 2011 9:41 pm

Just see if you can hang it's weight off a screwdriver or something, she'll be right. I;ve used secondhand rotors all my life, because i'm cheap!!

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demagnetized rotor?

Postby Groily » Mon Apr 04, 2011 10:32 pm

Think I'd start by looking at the AC voltage being produced by the alternator Ian. To do this, a pair of alternator leads have to be paired. Not sure what colours your new three wires are, so not going to guess and say which two should be spliced and which one left alone for the test, but you can look at Goff's web site for a description of the test. It's the same pairing as would be used in the 12v conversion using a combined regulator/rectifier which he describes in detail on the site.
A meter on the AC scale, across the now-paired wires to the single one, with the engine running should show a decent number of volts, mid teens or more, as revs rise. Or put a 12v bulb across them but don't rev things too high or it'll blow if things are working. If there are no volts or just a dribble, then maybe there's a rotor problem, assuming the new stator is good.

If there are loads of AC volts, you then need to see what's happening at the rectifier. Where there are the DC Plus and DC Minus tags as well as the two AC feeds. If you're positive earth as you probably are, then using the DC volts scale, see what you're getting between the DC Minus tag on the rectifier and a good earth. Should be rapidly rising dc volts with revs. If there isn't much to see, then the rectifier may be dead, may have a bad earth on the other (Plus) side, or the connections to the switch and ammeter/battery are bad.
I am wondering whether your switch is not enabling the additional alternator output when in the' lights on' position. This would explain a modest charge lights off turning into a discharge with lights on, and would be nothing to do with the rotor but all to do with not having enough power coming down from the switch to the rectifier in 'lights on' mode.
So I'd check that the switch was truly wired correctly, and also that it's AC side is working properly to connect the extra output from the alternator to the rectifier when the lights are turned on. If it isn't, you'll be running on only half the alternator's output or less, and that won't support all the loads so the ammeter would show a discharge.
If the switch is dead, your easiest option might be to adopt a 6v combined regulator/rectifier and substitute it for the simple rectifier. This avoids the need for the AC side of the switch completely and is cheaper (and rather better too in terms of managing the charge rate) than buying another switch, if one could be got.

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demagnetized rotor?

Postby Ian 14 G2 » Tue Apr 05, 2011 4:48 am

I just read your post and understand what you have said so will give it a go one night this week and see how I get on, thanks for the help. Ian

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demagnetized rotor?

Postby Eric » Tue Apr 05, 2011 8:03 am

Ian I am not sure you have a problem.

Most alternator systems used around the time your bike was made never had any sophisticated control systems, and only gave enough output to balance demand with a little bit more to spare at normal road speeds, not idle or fast tick over.

I am not familiar with the wiring used on the Wipac setup but the Lucas system worked in such a way that with no lights turned on the alternator only outputs its minimum just for the ignition. Unless you were riding along even this would still show a small discharge.

When the sidelights get turned on some more output is included, but it will still not balance or gain a little until you are riding along.

When the headlight is turned on the full output is used, but you still need to be riding at normal road speeds.

In the Lucas system the light and ignition switches also controlled the alternator output to do this.

In one position only one alternator coil was used with the others being switched out and shunted in some way. Then next the shunt was removed, and finally all three coils were included in the output. Thatís all just from memory itís not very good these days but the Wipac system is probably similar.

So you should only expect to see a charge at normal road speed and always a discharge at idle, otherwise with no other form of control you would boil the battery.

P.S. Just looked it up in one of my books and its as I have remebered it except the alternator coils are in pairs, so three pairs is six coils, where I said one coil read as one pair.

In my workshop manual for the Wipac system it says miniumum charge rate at 3,000 rpm no lights 1amp, sidelights 1.3amp, headlight 1amp. It also says with ignition in emergency position and no lights 6amp so this would be the maximum from the alternator as all coils are connected together for this. So as I said dont expect to see vey much.




Edited by - Eric on 05 Apr 2011 08:41:57 AM

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demagnetized rotor?

Postby rex.webb » Tue Apr 05, 2011 10:19 am

AS A CERTAIN STORE SAYS
"EVERY LITTLE HELPS"
Its a wonder anything worked at all with such a little ouput ?.
I seem to remember you had to keep the Demand down as low as possible.
Ride with Care and Live long.
r w webb

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demagnetized rotor?

Postby 1608 » Tue Apr 05, 2011 2:11 pm

I agree with Eric, don't expect too much from idle.If the battery is in good condition the amp meter won't show much anyway.

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demagnetized rotor?

Postby Eric » Tue Apr 05, 2011 3:05 pm

At least its now possible to take advantage of modern technology quite easily without changing very much.

You can purchase a range of LED bulbs that use very little of your small output and a Halogen headlight bulb that will give a lot more light output. All this without changing to 12 volt and technical control units, just changing bulbs as and when needed.

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demagnetized rotor?

Postby Groily » Tue Apr 05, 2011 5:24 pm

I agree with Eric and 1608 100% too, regarding the idle thing. It will discharge noticeably then with lights on. It's at higher revs there's a prob if the thing discharges still - should balance on the ammeter by around 30-ish mph in top with lights on.
So could well be there's nowt amiss if it charges OK at road-going revs - I somehow got the impression it didn't, possibly for looking through a glass darkly.
Good luck!

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