Testing alternator rotor (2011)

powerdoc · Nov 9, 2011

I'm about to get a higher output alternator stator and regulator and wonder if the rotor needs replacement too. The bike is 36 years old but only about 6k on the clock. Is there any way to test the magnetic strength of the alternator rotor without assembling the bike with the new components and testing the output?

kraakevik · Nov 9, 2011

The rotor's magnetism should be pretty strong if it's good--enough to suspend a 12-inch monkey wrench or a pound or two of cast iron--I've had them last for twenty years

Tim Kraakevik
kraakevik@voyager.net

powerdoc · Nov 9, 2011

Should I assume at 36 yr old it's bad?

crusadersports · Nov 9, 2011

kraakevik said:
The rotor's magnetism should be pretty strong if it's good--enough to suspend a 12-inch monkey wrench or a pound or two of cast iron--I've had them last for twenty years

Tim Kraakevik
kraakevik@voyager.net

Yes, to test a rotor's magnetism, you can lay an adjustable spanner ('monkey wrench' over the pond) on it and it should pick it up and stay attached as you lift the spanner up.

Or so it is said. The rotor on my mk3 passed this test, yet still gave dismal charging results. I replaced the 40 year old rotor and hey presto, my charging system worked much better.

Draw you won conclusions! I believe it's possible to get them remagnetised...

DogT · Nov 9, 2011

crusadersports said:
I believe it's possible to get them remagnetised...

Hmmm, I'd like to know about that, but it sounds feasible. I may look into this given the price of rotors.

Dave
69S

boz · Nov 9, 2011

From from what I have learned this is very subjective. I was having a problem keeping my battery charged and it was suggested that the magnets may be weak. We took my rotor and a known good one and placed a flat blade screwdriver on the surface. The amount of resistance to pull the screwdriver off the new one compared to my old one was easy to tell when you have them side by side. It comes down to a matter of feel. Not very scientific but it worked. The old one is now a paperweight on my desk.

Webby03 · Nov 9, 2011

You can buy cheap Wassel replacements and in my (limited) experience they seem to work well.
As for remagnatising an old rotor, I think it may be possible, I'll have to put one in a magnetic particle (magnaflux) bench and see what happens.

Webby

powerdoc · Nov 9, 2011

I've seen a lot about the Wassell products; where can they be bought? Interested in the 3 phase. thanks

rpatton · Nov 9, 2011

In addition to lost magnetism, rotors of a certain vintage were known to come apart. I don't know the part number of the suspect ones or if it was fixed during the production run of Commandos at some point, but Lucas#54202298 is the new welded version. Check to see if there is any movement of the steel center on your rotor. Could be a myth, anyone ever see an exploded rotor?

DogT · Nov 9, 2011

I have one of the explodable ones, but so far have been lucky.

How to measure magnetic flux of a rotor? Probably the only way for certain is to put it on a stator, run it up to a certain rpm, and measure the output ac voltage and compare it to another on the same stator at the same rpm. Holding power on a screwdriver or other magnetic item would probably be sort of subjective, although comparing sticking weights may be possible, don't know.

However, I'm certainly interested in trying to increase the magnetic flux of my rotor, it does seem to charge at about the correct rpm's, but after years, would not start with the capacitor and no battery, and I can only assume the flux density from the rotor was the culprit.

Then there's Captain Janeway's always adjusting the plasma flux generator to get out of problems. ugh, how bogus.

Dave
69S

calbigbird · Nov 9, 2011

FWIW, Joe Hunt Magnetos in Sacramento can "recondition" a rotor for under $50. I have heard good reports about their work.

boz · Nov 10, 2011

I have been told that the two wire units are more of a maintainer than a charger. If you have a good charged battery then it will maintain the charge as you run but it was never intended to charge a dead or weak battery.

pete.v · Nov 10, 2011

boz said:
I have been told that the two wire units are more of a maintainer than a charger. If you have a good charged battery then it will maintain the charge as you run but it was never intended to charge a dead or weak battery.

I think this is true.

However, I think some people put in a big battery (14ah) with the 130watt stator and it tends to struggle to keep the higher load battery topped off, whereas a 12ah battery would be a better fit for the 130watt output. Less juice, but a maintainable load.

The 180 or 200 watt stators work quite well the the 14ah batteries. More expensive but cheap insurance. The trick is to have the proper Podtronic reg/rec unit to match. A 130watt pods unit will pop for sure with a 200 watt stator. I know this.

kraakevik · Nov 10, 2011

Powerdoc

I can send you a couple of rotors if you want to play mix and match to determine which of the three gives you the highest output with your new stator

Tim Kraakevik
kraakevik@voyager.net

Torontonian · Nov 10, 2011

I have been told by a scientist geeky friend that magnetism is slowly lost in a rotor situation as you use it ,hence a 6000 mile rotor should have plenty left in it. Also that how the rotor was stored for years is important as new ones come with shells or "keepers" to retain magnetism and prevent accidental bleedoff like say it got thrown into a box of other parts and the steel ones all decided to stick to it like glue. Early rotors are a potential time bomb and I have seen one explode in a primary, thankfully not mine. An improved retaining washer can be used to hold the center and body tight together. Remagnetizing machine supposed to be one big dangerous power sucking monster ,anyone have pictures of the beast ?

Hobot · Nov 10, 2011

http://www.oldengine.org/members/diesel ... Data14.htm

roqueweiler · Nov 11, 2011

I have been told by the local indy that the rotor should be able to easily lift 16-18lbs of steel. He told me to try a plate from a weight bench.

Can anyone verify this?

powerdoc · Nov 11, 2011

Would that be lifting from the edge perpendicular to the weight or parallel to the weight?

kraakevik · Nov 11, 2011

My rotors will lift about four pounds apiece of cast iron when held perpendicular to the smooth side of a ten-pound plate--two won't quite loft the plate entirely clear

Tim Kraakevik
kraakevik@voyager.net

dynodave · Nov 12, 2011

Old alternator rotors I have measured at 1600 gauss and may have been damaged by heat/rubbing on the stator. 1800-1900 is more typical of older used ones. New are as high as 2400 gauss. The field I measure is highly focused at the corners of the magnets where it jumps to the adjacent magnet.
FWIW you are not actually looking at the magnet at the surface but the field shaping armature. The magnet is not visible unless you eat away the aluminum with lye/caustic/sodium hydroxide.

Remagnetising the rotors requires a fixture that looks like a stator on steroids. I built a magneto remagnetiser but terminated building a rotor fixture as economically unfeasible considering all the work though technically I could do it.

Testing alternator rotor (2011)

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