Stator Rust 1

[actnup]

I have some fractional hp, 120vac motors that we have found quite a bit of rust on the rotors. These are rated at about 35oz.ins. of torque and I need every bit I can get. Will the rust on the rotors drive up the amp draw or reduce the torque output or both? Any help is appreciated.

 

[actnup]

I am sorry I titled this "Stator Rust" when it is rust on the rotor that is the problem. I was looking at one thing and typing another. Thank you for your help.

 

[electricpete]

I would think the biggest concern would be reliability of the motor. Over time are pieces of remaining rust going to get into bearings or winding.

Small cast rotor motor I don't think there are any rotor cooling air ducts like large motor, so doesn't seem likely any concern about reduced cooling.

The only other thing I can think of is that if there is very large removal of metal or change in structure=> magnetic properties of metal, the effective airgap may increase, resulting in increased magnetizing current and decreased breakdown torque.

Just my thoughts.

 

[rhatcher]

I agree with electricpete. Unless you are loosing large amounts of metal, the effect on motor performance should be negligible. He does have valid concerns about contaminating the bearings and windings, I would worry about that more than anything else.

If it is a concern for you, why don't you paint the rotors?

 

[UKpete]

I go along with the previous posts, it would have to be pretty bad to give an increase in airgap.

There isn't usually a rust problem when there are rotor windings which are impregnated, and often they are subsequently spray-coated with an anti-tracking varnish. It is not uncommon to varnish induction motor rotors either.

 

[electricpete]

Interesting comment by ukpete about varnishing or 'impregnating' squirrel cage motors. I think I have heard of that but was never quite sure why.

Is the primary reason corrosion prevention? Or prevent rotor bars from coming loose?

What's the reason for using varnish or other resin instead of other coatings more specific to those functions (corrosion protection or mechanical retention)? Nothing to do with electrical properties, right? Just because it is readily available at a motor shop?

 

[jbartos]

Suggestion: It may be a good idea to check motor bearings since they may become rusty, if detrimental chemicals were present. Visit

http://www.edtcorp.com/c.applications/01poultry.htm

http://www.finishing.com/181/20.html

http://www.skatedork.org/pr/pr9.htm

http://www.grogz.com/bora.htm

http://www.canyonchasers.net/equip/shop/tls_ohlins.htm

http://www.ebearing.com/news2001/news127.htm

http://www.pirate4x4.com/tech/billavista/PR-35Spline/index4.html

http://www.walleyesinc.com/walleyeinc2/boatstorage.htm

etc. for more info on rusty bearings

 

[UKpete]

The AC and DC machines I am aware of with varnished rotors are actually large open-ventilated traction motors, not fractional-hp or industrial I admit.

The treatment was intended for corrosion protection. The insides of the DC traction motor stators (confusingly referred to as "the magnet&quot were also varnished internally for the same reason. This air-drying varnish (red or grey usually) was applied by spraying and went over the impregnation varnish - the whole stator assembly including the frame having already received a VPI.

I don't know what the practice is on industrial induction motor rotors.

 

[edison123]

pete,

Normally cage rotor bars are caulked along the length to prevent looseness. Impregnating the cage rotors is unnecessary. A finishing air drying varnish is applied on the rotor cylindrical surface to prevent surface rusting.

Kumar

 

[electricpete]

Thx Pete and Kumar. I was a little confused but now it makes sense that we just use a spray varnish for corrosion protection (not electrical varnish).

 

[pablo02]

All good posts -- I wouldn't worry too much about the loss of metal increasing the air gap -- due to the properties of rust, it generally takes only a minute amount of iron to create a large amount of rust(or so it appears)... BUT, as pointed out, it can create problems in the bearings and in the windings...

 

[Wasson5395]

Did you ever find a root cause?
Ever heard of protective paint on the rotor out-gassing at temp. The Chlorine gas then ate into the varnish which started the corrosion