Heat Shock test For Magnet Wire

Again with too much free time I have been revisiting some previous discussions. Regarding the problem of heat shock failures, regardless of the enamel used or the oven set up:

a) You might want to check your testing procedure and the standards that you are using. As previously mentioned, typically a sample of wire is elongated a specific amount, shaped into a coil, inserted into an oven that is at least 20 degrees centigrade higher than the factory thermal rating of the enamel. It stays in the oven a specific number of minutes, removed and quickly cooled. it is then looked at with 3x magnification for cracking, etc.
You can get more info at the following:

ASIEC60851 Methods of Test for Winding Wire;
TM D1676 - 03 Standard Test Methods for Film-Insulated Magnet Wire;
NEMA MW 1000 - Magnet Wire

b) It is inconceivable that every enamel that you are using is failing heat shock. Suggestion - get curing temperature recommendations from your enamel suppliers and then try to duplicate in your oven. Generally there is no single oven temperature profile that will work with every enamel!

c) I would have my lab oven tested to see that it meets the requirements of the test. BlueM and I am sure others make ovens that meet this standard. Most of these are recirculating ovens. If your placement of the sample in the lab oven exposes it to uneven or intense heat as opposed to average chamber heat, you are probably creating the failure. We used to leave our lab ovens on all of the time and would adjust the temperature as needed per test.

Most companies want heat shock values that are considerably higher then the minimum required. See example from Ess*x.

HEAT SHOCK RESISTANCE
Ess*x/GP/MR-EXTRA® magnet wire passes all UL heat shock resistance testing at 20°C above rated temperature.
TYPICAL PERFORMANCE: 300°C
REQUIRED PERFORMANCE: 240°C

This means that their wire while required to have a heat shock value of 240 will typically pass 300 degrees C.

d) Lastly as said many times, go back to ground zero, clean oven, etc. and start over at the oven and enamel mfg. suggested temperatures.

Richard

Hello,

For a start, we suggest you read this two page WAI Forums thread from 2006 very carefully:

www.wireassociation.com/forum/viewmessag...fm?Forum=8&Topic=277

Likewise please note the comments regarding the Dansk system for tangent delta equipment www.dse.dk and the pull mandrel test.

Since you have tried many enamel manufacturers, it would seem to us that it is indeed your process. We would guess that you are slightly over curing the enamels.

Thank you.

Regards
Peter J. Stewart-Hay
Principal
Stewart-Hay Associates
www.Stewart-Hay.com
519 641-3212

What ever came of this heat processing idea? I am anxious to hear a little bit more about what happened and why this process was not going according to plan?


C

www.bwire.com/

Peter,
I'll comment tomorrow. I've tried twice and don't know where my repy went. Will do in word and then paste.
richard

Hello,

Heat shock is generally thought of as a test of cure (or over cure). As you say the enamels you are using are from reputable companies. You take a sample, elongate it some, wrap it around a mandrel, and then place in a lab oven for a period of time at a temperature that is 20 degrees hotter than the thermal rating of the wire.

Over the years I have learned a few things:

1) Get input from the enamel supplier as to suggested temperature profile in the oven.
2) Get input from the oven manufacturer regarding what they think are good temperature speeds.

The best performance occurs when you set the temperature profile, air flow, fan speed, and wire speed for each type enamel and wire size. This means you are operating at optimal conditions.

If you try to run all products and enamels and only vary the speed, you compromise production and quality.

There is no normal reason that I can think of that all of your enamels would fail heat shock. My first thought is that I would check the accuracy of my test oven. Normally heat shock tests are 20 degrees over the thermal rating of the wire for a specific time. If your oven is off even a little it would make a difference.

If the lab oven temperatures are good, then you need to look at your oven. Maybe you have a hot spot or the wire is dragging on the oven or touching a heating element (assuming they are in the wire chamber).

- Is the wire centered in the wire chamber?
- Is it too close to one wall?
- Are you running the oven too hot?
- What kind of oven are you using? Electric? Radiant? Recirculating? Single direction air flow?
- Does the air flow with the direction of the wire or against the movement of the wire?

There are a lot of questions that perhaps you can answer.

Richard

For a definition of Heat shock go to www.spsx.com/uploadedFiles/News/White_Pa...al-class-ratings.pdf

The idea behind heat shock is to see if rapidly changing the temperature of the wire will make it fail. Typically you elongate the wire, make a coil, hang in an oven heat opeating at 20 degees C above it thermal rating of the wire. After a period of time you remove the sample, let it air cool and check for cracks in the insulation. You mention BSS specs - is that a Boeing spec. Exactly what is that standard. If it exceeds the basic NEMA heat shock requirements your samples may fail. In NEMA once you reach 200 degree rated wire the oven temp is the same as the rating of the insulation. If it placed in an oven that is too hot or stays in too long it will fail!