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Date: Sun, 13 Sep 1998 07:30:41 -0400
To: lancair.list@olsusa.com
From: John Cooper <heyduke@digital.net>
Subject: MT Prop electrical noise
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          <<  Lancair Builders' Mail List  >>
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>>
For Dan Schaefer <dfschaefer@usa.net>:

The filter which has now been installed in the power lead to the MT prop
controller box was obtained at a car-audio store. Clarence has no idea of
the part number or type <sigh>. If I have the access, I will try and read
any numbers off of the thing when I am out at the hanger today.

BTW, his flap motor still causes a line of false hits on his stormscope,
but he has not bothered to filter that line. He says he tried a filter in
the power lead to the Stormscope itself, but that didn't work. I wasn't
involved in that particular test, so I am not sure what exactly he tried.

Also for the record, both MT and 3M say they have never heard of a similar
problem. Sure...

Here are some old threads on the subject from AVSIG:

------------------------
#: 1287924 (C) S12/Homebuilt Corner
    15-Jun-97  20:08:52
Sb: Stormscope/Elec Prop
Fm: John H. Cooper[X68] 76226,634
To: Bob Nuckolls 72770,552

Bob--

  My friend has about 400 hours on his Lancair 235. He has "all the goodies" 
in his panel, including a Stormscope. For years he has been fighting noise 
problems with this device which would appear as spurious "hits" on the 
screen -- usually in a N-S line. Most of these were corrected when I 
convinced him to install a B&C LR-2 linear regulator instead of the switching 
type sold by Neico.

  Lately, he determined that whenever his MT electric prop changed pitch, it 
would splatter a line of "hits" on his Stormscope in the N-S axis -- he asked 
me to try and see if I could fix it. Amazingly enough, I was successful <G>.
[ 'bout time my college degree paid off <G> ]

  We had guessed that the slip rings might have been arcing and generating 
some RF, but the noise was produced with the engine not running. Nope, that 
can't be it.

  OK, maybe the motor inrush was causing some radiated noise spikes. I 
bypassed the prop-motor lines with various sizes of capacitors with no luck. 
Besides, the Fluke 87 was telling me that the prop control box had snubbers 
in it, since the voltage across the motor leads never exceeded +/- 14V. Only 
a few millivolts of noise on the motor leads anyway....Hmmmm..... For 
reference, the prop motor draws roughly 0.5A.

  I looked at the 12VDC feeding the control box next, and discovered about 80 
mv of noise at 110HZ constantly on the power lead to the box. Another 
Hmmmmm...  I put a 1000mF capacitor across the power lead to the prop-control 
box, which seemed to help a little, but not much.

  Next, I took the battery out of my plane, and hooked it up to the 
prop-control box on my friends plane, totally isolating the power to the 
electric prop from the rest of the avionics. The noise went away completely 
-- not so much as a zot on the Stormscope when the prop was cycled. So I 
figure the problem is either conducted or radiated noise in the power lead.

  We're getting somewhere now. So I hooked the power back up as before, but 
with an alternator filter in series with the power lead. No luck at all -- 
noise still as bad as ever. So I'm thinking poor lead dressing/radiated 
noise must be the problem. I told my friend that I was out of ideas, and that 
he would have to shield his power lead to the prop-box <sigh>.

  Oops, one more trick in my bag. I wound a 10t toroid around a ferite core I 
had laying around the shop, and put it in the power lead to the prop-box. 
Bingo! Totally fixed the noise in the Stormscope. Doncha' love it?

  Go figure -- bypass cap did nothing but the inductor worked. I wound a few 
more turns on the core for good measure, and my friend should be back in 
business tomorrow. I told him he was going to have to stop saying all those 
horrible things about "smart-ass college boys" <VBG>.

  My only guess is that the MT prop control box has a DC-DC inverter inside 
which is sending "hash" back down the power leads. Doesn't make sense, 
though, since the prop motors are 12V.

  Maybe you could file this away for future reference. Your comments would be 
appreciated.

Sometimes you just get lucky--

   --John

[TAPCIS: Message # 1287924 posted]

#: 1288483 (C) S12/Homebuilt Corner
    17-Jun-97  06:26:03
Sb: #1288317-Stormscope/Elec Prop
Fm: John H. Cooper[X68] 76226,634
To: Bob Nuckolls 72770,552

Bob--

<<Does that model of Fluke have a peak storing mode?>>

Yes. Min/Max/Avg. It also has Hz/Duty Cycle, Capacitance, Conductance.  It's 
a true RMS meter, down to 50Hz. Wish it went lower...

<<What's the narrowest transient it will catch and measure?>>

The manual and meter are at the airport, and I don't recall. I do know that 
it is selectable. I'll get back to you.

<<Sometimes, BIG capacitors are not as good as LITTLE ones depending on the 
nature of the noise>>

I first tried a 100mF tantalum in parallel with a 1000pF silver-mica. When 
that didn't work I tried the 1000uF electrolytic by itself -- It didn't help 
either. Glad you mentioned the high internal resistance of electrolytics 
above 10KHz, though, for those not aware.

<<What kind of filter?>>

Yes, an "alternator" filter like ACS sells. Axial terminal on each end, 
ground strap in the middle.

<<Did you end up with JUST the series inductor or was there some capacitance 
on the bus side too?>>

My friend will install the inductor I wound today to see how it works. The 
one he will install has 16 turns on one of those ferrite cores designed to 
slip over a 3/8" dia power lead. The core is about 1.25" long with an OD of 
3/4" or so. If it works OK, I'm planning to just leave it as is with no added 
capacitance. I also have some Arnold tape-wound toroid cores laying around -- 
maybe they would be better, but we'll see what works...

If I do need to add a capacitor, what ballpark figure would you suggest?

The story about your son made me laugh. I have seen *my* son do exactly the 
same thing! Mine's 23 now, and still hasn't developed an analytical way of 
thinking. Product of the modern school system, I'm afraid <sigh>.

And as you know, at that age they *know everything*.

<<You should send a copy of your letter to MT also. They should be aware of 
customer difficulties and SHOULD be pleased to know what the solutions are >>

Gee, maybe they would send me one of those nice 3-bladed electric props for 
my Lancair in appreciation <G>.

Regards--

   --John

[TAPCIS: Message # 1288483 posted]

#: 1288706 (C) S12/Homebuilt Corner
    17-Jun-97  16:25:24
Sb: #1288522-Stormscope/Elec Prop
Fm: John H. Cooper[X68] 76226,634
To: Bob Nuckolls 72770,552

Bob--

Thanks for the tips.

My friend installed the 16 turn choke and says is works perfectly. He was so 
happy he even bought me lunch! He says I should *sell* my idea to MT. Hmmmm...

Still looking for the Fluke 87 manual.

I'll be passing throught the Delta terminal at LAX Friday morning about 10AM. 
Maybe I'll see you there.

Regards--

   --John

[TAPCIS: Message # 1288706 posted]