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rotor saturation
By electronbaby, Section Homebrewed Electricity Posted on Mon Sep 29th, 2003 at 08:30:12 AM MST
has anyone experienced this?

During the rework of one of my dual rotor alternators, I had noticed something strange. It appears to me that the late model 11" volvo rotors do not have enough density or steel to accomodate 2" x .5" neo disc mags. I should not say it this way because they work fine but I have noticed flux leaking from behind the rotor (on the back side of the rotor). This can only mean that flux is being wasted and not returned in the "circuit" to the front of the rotor for use across the stator. I thought this problem would disappear when I place both of the rotors facing each other on the alternator, but with both of them in position, it is still detectable. The magnets I used are the same ones DanB used for his last 4 alternators, and they are very powerful. Dan, you are right when you said that the 2" x 1" discs would be overkill, when I am seeing leakage from the .5" mags. Possibly some 11" rotors are made with more iron than others? The pair that I had got a hold of seemed quite lightweight. If they were not vented, this would not be a problem. I have thought of a way to correct this, and I have come up with a 1/4" steel disc that would fit over the back of the rotor and just add a little more steel to try and neutralize the field and return it back into the rotor. BUT, this may not be an issue at all. Does anybody have any input? Ill stop rambling now. :-)
RoyR
KB2UHF

rotor saturation | 8 comments (8 topical, 0 editorial)

Re: rotor saturation (none / 0) (#1)
by kww on Mon Sep 29th, 2003 at 08:35:52 AM MST
(User Info)

I bet your rotors have a lot of wear on them(not very thick). All like rotors are going to be a different thickness, unless new, because of wear from braking.

Re: rotor saturation (none / 0) (#2)
by DanB (danb@*no spam*otherpower.com) on Mon Sep 29th, 2003 at 08:41:08 AM MST
(User Info) http://www.otherpower.com/

Hi Roy - yes, I have noticed this too. It's not much - but I can definitley detect a weak magnetic field on the backside of these rotors. Ideally - that should not be the case, but I think it rather minor really - it is a very weak field.

I suspect you'd probably notice this too on 1/4" mild steel with such large magnets.

In large computer hard drive magnet assemblies, where the magnets are about 7/16" thick, I notice they are using soft steel behind them about just as thick! I think our brake rotors are actually cast steel - not the best material, but not bad.. and it is quite thick and I think this saturation is a minor issue which is possibly outweighed by the convenience of "ready made parts"... but I could be wrong!

Re: rotor saturation (none / 0) (#3)
by electronbaby (roy<at>windsine.org) on Mon Sep 29th, 2003 at 08:51:55 AM MST
(User Info) http://www.windsine.org

I was thinking the same thing Dan. I was just curious, thats all. The field is strong enough to hold a 5/8" galvanized nut pretty securely to the back side of the rotor. The field is strongest over each magnet, but almost falls to nothing in between. The rotor was not worn too bad. I took pics but they are still on my camera. I will post them soon. I really have to make a new stator using the 14AWG wire. My current one is using 16AWG and I still have it wired in star. It does cut in at 60-70 rpm HAHAHA I really should change it over to delta and use 14AWG. In its current state, I can spin it to 62 volts by hand. Interesting but not practical. I must make a few adjustments. Thanks for the quick response. Oh by the way, in a previous post I was talking about bushings and 1/2 rod, I ended up drilling out the rotors for 5/8" rod and everything is great. :-)

RoyR
Have Fun!! RoyR KB2UHF
[ Parent ]

Re: rotor saturation (none / 0) (#4)
by DanB (danb@*no spam*otherpower.com) on Mon Sep 29th, 2003 at 09:17:21 AM MST
(User Info) http://www.otherpower.com/

Hi Roy - yes, I've not noticed the saturation nearly that much... barely enough to grip a paperclip on mine. Is this the case with both rotors on? ARe they good thick rotors? Are they vented rotors? (mine are... not sure if that affects things much or not - its probably not ideal though)

If they are vented rotors, perhaps you could fillup the vents with resin and iron filings or something - that'd probably help some.

Regarding your stator.. yes, cutin at 65 rpm is a bit slow :-) - why are you in star on that? Delta would put your cutin speed at a more reasonable rpm and lower resistance a bunch.

I think if I do this again, I'd probably try AWG 13 wire, as I think there is room for it, and maybe even add just a couple more windings per coil to get cutin speed down a touch. I believe then a larger 12' diameter prop might be reasonable.

Good thoughts about the 5/8" allthread, I think perhaps next time Ill use larger stuff...

[ Parent ]

Re: rotor saturation (none / 0) (#5)
by electronbaby (roy<at>windsine.org) on Mon Sep 29th, 2003 at 09:53:44 AM MST
(User Info) http://www.windsine.org

The only reason Im in star is because its the stator from a previous machine where the mags werent as strong and I was gearing it from the start for low wind speed. I definitely will not keep it this way. I am giving a lecture on friday of this week and I wanted to bring this machine in for a demo (a little show and tell) and I just want to make sure there is plenty of voltage when I turn it for the sake of the audience. :-) When it actually goes up on the tower, it will be changed for delta and some time before winter, a new stator will be made with 14AWG wire wired in delta. Probably around 65 windings per coil. In this 16AWG stator Im using, there are 80 turns of 16AWG per coil. There are 9 coils to make it a 3 phase unit which certainly works well also. There are 12 mags per rotor. Like I said, this is too much resistance in the coils for the 2"x.5" neos, but worked fine for the smaller mags I was using previously. I have to get used to these monster magnets. They certainly make all the difference in alternator output. Im happy because, Im starting to free up all these rotors with mags glued to them that I have played around with in previous but inferior designs. I will use these to start playing around with more experimental alternator designs. I am very happy with the output from the 2" x .5" neos in my new machine. Forcefield rocks. :-)

RoyR
Have Fun!! RoyR KB2UHF
[ Parent ]

Re: rotor saturation (none / 0) (#6)
by wind pirate (nap35dh-at-nospam-yahoodotcom) on Mon Sep 29th, 2003 at 10:17:17 AM MST
(User Info)

I just finished putting together my dual rotor using (24) 1.25" x .5" neos (12 per rotor) on (2) 14" x 1/4" steel plate rotors. I'm not detecting any leakage on the backside of the plates at all. I've got to think that the density of a brake rotor would at least be equal to that of the 1/4" plate wouldn't it? or is it that the 2" magnets are just that much stronger than the 1.25"?

Re: rotor saturation (none / 0) (#7)
by Electric Ed on Tue Sep 30th, 2003 at 06:29:08 AM MST
(User Info) http://www.electric-ed.com

Magnetic flux lines like to travel in straight lines. I think the "fins" in the vented rotors are guiding the flux to the back side of the rotor.

In addition, cast iron has more reluctance than mild steel.

Ed.

Re: rotor saturation (none / 0) (#8)
by electronbaby (roy<at>windsine.org) on Tue Sep 30th, 2003 at 08:18:27 AM MST
(User Info) http://www.windsine.org

Thanks Ed. I dont think its a big deal. I agree with Dan in that for the price and ease of using the 11" rotors, it probably isnt a big deal. I just have not experienced such strong magnets before and it just occured to me that there might be flux being 'wasted'. Ill keep you guys posted on the output and performance of my latest toy. :-)

RoyR
Have Fun!! RoyR KB2UHF
[ Parent ]

rotor saturation | 8 comments (8 topical, 0 editorial)

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