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murph64
05-22-2009, 06:08 AM
Is there any way to stop the iron core of a motor from rusting?

http://pics.montypics.com/mcduff/2009-05-21/1242986194_img_6263b.jpg

It's the I dont know how old pool pump motor. and if I don't run it for four days or more, it's tough to start. Left it outside all winter and this is what I found...I carefully sanded the rust off and got it running. Would LPS3 or Boeshield T9 work to stop this?


Andy

paulx
05-22-2009, 08:23 AM
You can sand off the rust with a drum sander and put a thin coat of paint.

digger_doug
05-22-2009, 08:50 AM
You can sand off the rust with a drum sander and put a thin coat of paint.

Well, you could CAREFULLY sand off the rust, even going so far as to
make a temp guard or shield to keep the sandpaper off the end winding's.

Then purchase the red insulating varnish (it's available in spray cans)
and coat it well.

murph64
05-22-2009, 09:13 AM
I was carefull with the Dremel :) Where would I pick up the varnish -a good hardware store? I can't see Homeys having something like that.


Andy

digger_doug
05-22-2009, 09:46 AM
Andy,
there was a post about it a little while ago.

I believe CRC products, or one of the major paint co.'s

"I was carefull with the Dremel "

We soon shall see how carefull you were with that thing....

murph64
05-22-2009, 11:03 AM
It's been running since yesterday, so couldn't have hacked it up *too* bad :D. I'll search for that post, thanks..


Andy

Langanobob
05-22-2009, 12:32 PM
Grainger's has the insulating varnish. I think it's called Glyptal. Also, I didn't check McMaster-Carr but since they have everything else it's a safe bet they have it too.

bigbill04
05-22-2009, 12:38 PM
Grainger's has the insulating varnish. I think it's called Glyptal. Also, I didn't check McMaster-Carr but since they have everything else it's a safe bet they have it too.

Eastwood also sells it, but boy has the price gone up $25 since two years ago. The link and prices are:

http://eastwood.resultspage.com/search?p=Q&ts=custom&w=glyptal&submit.x=43&submit.y=12

Bill

lazlo
05-22-2009, 12:44 PM
You can sand off the rust with a drum sander and put a thin coat of paint.

The power factor (and magnetic flux) of an induction motor is inversely proportional to the air gap width. In other words, you weaken the motor by increasing the gap width.

I would soak the whole assembly in Evap-O-Rust.

murph64
05-22-2009, 12:45 PM
Eastwood also sells it, but boy has the price gone up $25 since two years ago. The link and prices are:

http://eastwood.resultspage.com/search?p=Q&ts=custom&w=glyptal&submit.x=43&submit.y=12

Bill

I knew I heard that name somewhere before...I might be able to get less than retail at Grainger, ;) let me check there first.


Andy

lakeside53
05-22-2009, 08:36 PM
I would soak the whole assembly in Evap-O-Rust.


I agree 500%.... or more..

Evan
05-23-2009, 06:29 AM
The power factor (and magnetic flux) of an induction motor is inversely proportional to the air gap width

Not quite. It's proportional to the ratio of the air gap width to the diameter of the rotor. With a rotor the size of the one in that motor a few thou isn't going to make a measurable difference.

Paul Alciatore
05-23-2009, 01:12 PM
I wouldn't use sandpaper. Dremel has small wire wheels and they won't remove any metal, just rust. But like Evan says, a few thousanths won't make any noticable difference.

Don't they actually dip motor windings in that varnish to prevent the windings from vibrating as well as to stop rust/

Evan
05-23-2009, 03:46 PM
You want to be careful not to let the wire wheel spin off the laminations and onto the windings. I would scrub it with a plastic pot scrubber and some vinegar to remove the rust and then rinse well in water. When dry then spray with electrical varnish. Not just a light spray but soak it well, let it run off and dry for a day or two.

lakeside53
05-23-2009, 05:17 PM
If you use evaporust, you won't need to use any wire brush... or acid (vinegar). Really is good stuff.

Before any varnish, I'd bake it in a convection oven set to less than 200F for a few hours.

lazlo
05-23-2009, 06:17 PM
The power factor (and magnetic flux) of an induction motor is inversely proportional to the air gap width Not quite. It's proportional to the ratio of the air gap width to the diameter of the rotor.

The magneto motive force F in an induction motor is:


^ 4 (KwN1)
F = --- ------- I
Pi p

KwN1 is the number of turn per phase of the windings, p is the number of pole pairs, I is motor current.

The magnetic Flux Density in the motor's air gap is therefore:


^ ^
B = uo F
-
2l
uo is the permeability of air, l is the air gap length. Notice that rotor diameter is not part of the flux density equation.

I'll run a couple of calculations on Tuesday, but a typical induction motor has around a .5 mm ( 20 thou) air gap, so sanding off the rust is probably doubling the air gap. Worse, you're not uniformly sanding off the rust, so the magnetic flux is no longer uniform either, so the motor is magnetically wobbling.

Spin Doctor
05-23-2009, 06:18 PM
Motor heater circuits

digger_doug
05-26-2009, 08:00 AM
It's been running since yesterday, so couldn't have hacked it up *too* bad :D. I'll search for that post, thanks..


Andy

Andy,
Good to hear.

Yes, I was concerned of a wire wheel slipping down into
the slots, or nicking the wire at the ends (outside of
the slot, where it loops back around).

As far as the air gap change.....The punchings are just stacked
up (after being sheared out, see the shear cut edge), and the
whole stack "wiggles" around a little, so the tolerance isn't
extremely close. I have seen some better motors where a clean
up cut (or grind) is taken in the bore, but that's not we have here.

Evan
05-26-2009, 08:41 AM
Air gap:
---------


... For a given rotor diameter, the slower an a-c machine speed (meaning the larger the number of magnetic poles in the winding), the smaller the gap. Horsepower output will be lower, and the power factor lower as well, so that the electrical effect of a large gap is relatively less acceptable than for a high-speed, higher-horsepower machine having that same rotor diameter. As polarity decreases and both speed and horsepower go up, the trend reverses.

In published material on basic design, the most common guidelines relate power and speed to two basic dimensions: core diameter squared, and core stack length, or DL in shorthand notation. There are a couple of variations. Theoretically, the diameter should be the stator inside diameter, what's often called the bore or gap diameter. Some designers, however, have used the core outside diameter instead. Also, the exponent (the power to which the diameter is raised) has been quoted as either 2.0 or 2.5.

http://findarticles.com/p/articles/mi_qa3726/is_200509/ai_n15349943/

J Tiers
05-26-2009, 08:53 AM
Lazlo and Evan.....

1) the effect of gap is as lazlo says, because you are not able to vary the rotor......... it is irrelevant what the rotor diameter effect is, because you are only varying the gap... the effect of sanding on the rotor diameter is infinitesimal, on a percentage basis.

Evan is correct as to the typical association of gap and rotor diameter, which is as much a practical/mechanical as an electrical decision.

2) Rotor gaps on motors may be 0.25 mm (10 thou). Emerson's small motors were made that way, at least during the time my boss worked there.

3) Your ability to actually remove material is nil, if you sand by hand...... you can sand for days and not affect the gap. All you will remove is rust, and that rust represents material THAT WAS ALREADY REMOVED, IT IS RUST NOW, NOT STEEL. All you are doing is knocking the rust off.

4) The effect of whatever gap increase comes out of the deal will be largely to:
a)slightly increase the no load current.
b)Slightly increase the locked rotor current
c)slightly increase the 'slip", i.e. reduce the full load speed a small amount

5) Who cares? Get some SANDPAPER, preferably a closed coat type like "wetordry". SAND OFF the rust BY HAND to form a smooth surface that the rotor won't drag on. Clean up, re-assemble, and USE THE MOTOR.

Don't soak the stator in anything, don't use oils or other sanding compounds. Sand dry, and vacuum out.

You can varnish the surface with a thinned varnish to prevent further rust. Using the motor will also tend to prevent rust, as moisture won't condense if the motor is warm.

Edit:

The stator punchings are probably 14 or 20 thou thick. If the rust on the end face isn't very far through one of them, it isn't likely to be very far into the bore either, and you know it is considerably less than the thickness of one punching. Unlikely to be a huge deal.