motor coil dip and bake, why varnish not paint etc?

[Bill D]

Motor main coils are dipped and baked in shellac or varnish to keep the wires in place and prevent vibration rubbing through the factory insulation. I suppose they now use fancy epoxies or glue. I assume the bake was just to dry it faster.
Why use varnish instead of paint. I can see not using red oxide primer since it may be conductive. I though paint was cheaper. Is it too thick to soak in all the way or what, tradition?
Bill D

 

[technocrat]

High temperature resistance was not easily available with old paints. You don't want high solids as it restricts the penetration. The bake is also to remove water. Epoxies are used now. I used to epoxy rally car alternators, now they come that way from the factory.

 

[technocrat]

glyptal - a polyester resin was one of the first synthetics wide used in the electrical world. It's still very expensive.

 

[Bill D]

I had forgotten the name glyptal. Wasn't that used as an interior coating on gearbox castings to sell off sand and keep oil in?
BilLl D

 

[technocrat]

Yes, it was used in engines and gearboxes where the cost was justified. It can seal off porosity and also gives faster oil draining.

 

[Bill D]

I understand for wood ladders only clear varnish is allowed no paint. Varinsh allows the user to see any cracks and splits. Not really an issue with wires all coiled up on top of each other.

 

[GregSY]

It's an issue of voltage as much as anything. Low voltage windings (like 600V and under) are copper wire coated with enamel then dipped in a resin and baked for curing. Higher voltage (like 601-13,800) usually receive a VPI treatment...that's vacuum pressure impregnation....where a higher quality resin in forced into all the voids through the use of vacuum and pressure. The resin can be polyester or epoxy....epoxy being the better of the two. In those cases, the baking is necessary to get the resin to properly 'kick'. It'll dry if you don't bake it but with much poorer insulating and mechanical properties. Epoxy resins are more expensive, not only because the materials cost more but because they have to be kept very cold....constantly chilled.

The entire process is there for mechanical retention but also for insulation value...so naturally the better job you do at applying the varnish/paint/resin the better the result. The topcoat of Glyptal paint really is for uniform appearance and smoothness; a smooth finish holds less dirt.

 

[Robert R]

The Glyptal web page:

Glytpal-Products

The pigment in red Glyptal is iron oxide. It is a 60% solids by weight paint using xylene as the main solvent.

https://sds.chemtel.net/docs/Rexel%20Holdings%20USA-0004804/Glyptal%20Incorporated_Glyptal_1201A_01-01-1989_English.pdf

 

[Scottl]

I had forgotten the name glyptal. Wasn't that used as an interior coating on gearbox castings to sell off sand and keep oil in?
BilLl D

Yes, many years ago I worked for a small company that made reduction gearboxes for use with large valves and on the marine units we sealed all the internal non-machined surfaces with Glyptal. These were mostly for use in oil tankers.

 

[Joe Gwinn]

As others have mentioned, there are multiple reasons to dip and bake the windings of electrical motors and transformers, even at 12 V DC and 240 Vac.

The dip and bake cycle is done twice, to ensure complete fill of all voids in the windings, and saturation of all paper or fabric spacers/insulators. Now days, the dip part is often done with vacuum, to speed full penetration of the varnish. Using pressure above atmospheric is for large units, again to speed things up.

The first reason is mechanical, to prevent wires from vibrating and chafing against one another and the separators et al. The second is to keep moisture and electrolytes out of the windings. The third is to prevent corona in cavities from causing failure.

Glyptal was the origional varnish used for dip and bake, but many others have been invented since. If I recall, the glyptal was thinned a bit for dip and bake.

 

[Bill D]

I know the smell of older overheated motors and coils. Do the modern epoxy dipped ones smell different? Do they still smell close enough you know there is an electrical problem or do you have to train your nose.
Bill D

 

[GregSY]

Generally if you can smell any motor, it ain't good. Modern epoxies and varnishes stink plenty when burned.

 

[tomjelly]

Seems to me that if Glyptal is 60% iron oxide it might be conductive. Is this not a potential problem in the motor windings or laminations? Or because the winding wire has lacquer of its own the glyptal never touches the wire anyway and just holds everything in place?

 

[boslab]

I read dielectric varnish low bake on the tins in the bin of the local motor rewind shop, smelly stuff whatever, I saw them heat a motor then lower it in a tank, raise it to drain them.
They went in the oven next, clever how they wind those coils
Mark

 

[chipss]

Most dipping varnishes are water based these days, And heat set typically needs 275 degrees to cross link and harden. I have some 'Dolphs' brand insulating varnish and it's almost odorless when you bake it out. I vacuum dip the parts to get it into the crannies to displace any air.

 

[technocrat]

Seems to me that if Glyptal is 60% iron oxide it might be conductive. Is this not a potential problem in the motor windings or laminations? Or because the winding wire has lacquer of its own the glyptal never touches the wire anyway and just holds everything in place?

Most metals become insulators when oxidised, so it's not a problem. However often the oxides are soft and easy to scratch through to the base metal (and electron soup) and the oxidised piece of metal may seem to be still conductive.

 

[boslab]

I suppose you could do it differently, the recent submarine did that, I’d stick to the proven method myself ( whatever that is as I only visited motor shops to drop things and collect things, I reckon they do human sacrifice and all sorts of witchcraft in those places)
I am impressed with the young Chinese girl rewinding motors out back of her house on YouTube, bloody clever girl.
Mark

 

[dwebster]

I used to work for a company that developed nanoscale wire coatings for motor windings. Processed miles of wire during testing. The current vacuum process to draw the standard epoxy type coating into the windings was hit and miss as far as depth of penetration and the tolerance of the coating on the surfaces of the wire. Outer edges had thicker coating than the inner edges and the heat distribution areas varied wildly in the different methods of winding. We were making semi flexible ceramic coating to decrease the performance lost from the temperature differential across the entire stator. We were coating the winding cable before it got wound on the stator assembly, eliminating the problem of uneven insulation and providing a coating that could withstand heat that would melt the winding cable before the coating would be damaged.

 

[GregSY]

I used to work for a company that developed nanoscale wire coatings for motor windings.

I assume these are low voltage windings. Medium and high voltage windings, using form wound coils, already has a coating directly on the wire...but that's not enough to provide adequate insulation. There still needs to be wraps of mica tape, etc. and VPI after insertion into the slot.

Low voltages allow for all sorts of games and shortcuts. Higher voltages don't.

 

[dwebster]

This was a project we were doing for Honda.