Calculating the temperature rise in an electromagnet 2

[MAC3382]

Hello, All:

Great Forum!

From time to time my company needs to employ a DC electromagnet for custom pick and place operations. We will typicaly purchase "off the shelf" standard round, rectangular, and bipolar units and assembly them in an array on some sort of backing-plate. This works, but it quite often results in a bulky solution which is not elegant nor optimized.

Recently I have been designing and winding my own DC workholding electromagnets. I have had very good results which are now in a variety of our products. The overall mass and volume of the end effector has been reduced with our custom magnets. I however, always error on the side of lower operating power because I have not been able to accurately estimate the thermal rise in the magnet. By always limiting the current my magnets always run cool during continuous duty operation, but I know that I am sacrificing holding power.

I 2D model the designs (Magneto) and look at saturation and the return path design, but the simulation software will not account for thermal rise. I can purchase a thermal feature for our software, but I am being told by the manuafcturer that we still will not be able to derive the information we need. (Something that the thermal program will not see the coil as a heat source unless I assign surface and volume heat information. It is able to use eddy currents as an electrical heat source, but that makes eveything more convoluted.)

Is anyone familiar with a method which I can use to calculate the thermal rise based on the magnet's power consumtion, mass, etc.? The method would be beneficial if it could even yield first order results.

Thank you

Mac

 

[MJR2]

What is the heat loading you have now? On a watts per surface square inch basis.

For continuous operation the mass doesn't really factor in. At least not for what I do.

Does your power supply run at constant voltage or current?



Mike

 

[MAC3382]

Mile, thank you for the reply:

I do not have a specific case now. All of the magnets are custom so I am looking for a method of calculating the thermal rise and optimizing my coil design for holding power/flux generation.

I modified a spreadsheet I downloaded a few years ago. I can put in the effective coil side-wall (cross-section or plane perpendicular to the turns of the coil) and it resolves the; number of turns, net resistance, coil mass, current at the specicifed voltage, power, etc. I put in a bulge/random wind correction factor (best guess). By changing the applied voltage, cross-section, and the correction factor I can judge the performance (Amp*Turn)vs. power for a spectrum of wire diameters. This is where I judge the heat rise based on previously constructed units, but it is still a guess.

Mass: My background is physics so I was working on a solution with specific heats, mass, and the ambient temperature. I waslooking at heat generation vs. the system's ability to disipate the heat. I may of went way to deep?

The power supply is usually very simple. All passive componenets. Basically just a transformer and bridge rectifier. All of the other componenets are for switching, the current reverse pulse, reverse emf, etc. Some of the electromagnets can be very large inductive loads.

Does this help? I am unfamliar with the watts/ surafce sq inch, is that the first step in a calculation?

Thank you,

Mac

 

[MJR2]

Mac,
Yeah. You may have went way too deep.

DC electromagnets behave a lot like transformers in heat disaption.

I have tried for about 20 years or so to put it together the way you are trying. I'm a bit slow but still come down to a simple surface heat disapation factor. The square inches of surface per cold wattage of the magnet.

I consider everything running continuous.

One thing that I found that within the scope of the magnets I do wire size does not much matter. The voids and insulation combine to drop the average conductivity to near that of the insulation.



Mike

 

[MAC3382]

Mike:

Okay, thanks. I like the way this is going.

Most of our designs are parallel pole or multi-pole which are basically parallel poles put side by side (poles running parallel to each other). The simplest case is a single coil and one SET of mild steel poles. A more complex case is a mult-pole unit with several steel poles with one or more coils between them.

Once I have determined the Heat Disapation Factor (in^2/W), what do I use as a guideline for estimating the operating condition relative to temperature? Are there tables for transformers that I would reference?

Thank you,

Mac

 

[MJR2]

The case(s) as you describe are essentially the same.

That is the direct question that I will not answer for you. Spend a lot of time looking in older books. And look at what you already have.

You can measure the heat flux from the surfaces. That will help you. The internal heat transfer is all conducted. That makes it easy. That is with some of my other suggestions.

Mike

 

[israelkk]

If you will look at the Ledex Lucas catalog you can see that they specify the current/coil resistance/AWG number for continuous, 50% Duty cycle, 25% duty cycle and 10% duty cycle. At those conditions the temperature at the electromagnet will reach 105 Celsius degrees from room temperature (based on practical tests). Just compare your electromagnet size and coil case to the compatible size in Ledex Lucas catalog and you will know which current/duty cycle will give you the 105 Celsius degrees. You probably do not intend to "bake" your electromagnet at higher temperature. This is a practical quick and dirty way to estimate your coil case.

 

[Warpspeed]

As already stated, mass has no real influence, except on how long it will take to reach a state of steady thermal equilibrium.

Also agree that it is watts dissipated versus surface area that is by far the most important parameter. That will get you very close, but it is not quite the whole story.

The heat must escape somehow, either by convective airflow, infrared radiation, or conduction into surrounding structure.

Taking these separately.

Convective airflow only works if the air can circulate freely, the hot air escape, and be replaced by significantly cooler air. Any sort of closely fitting enclosure, or high local temperature is going to effect convective cooling.

Infrared radiation only becomes significant when fairly large temperature differentials exist. It works both ways. A hot object surrounded by cool space can radiate a significant amount of heat. But something can also just as easily be "toasted" by nearby hotter objects.

Conductive heat loss away into a heavy frame or structure can also sometimes form a fairly effective heat sink.

The whole thermal design issue can be simple or complex, and there is no real substitute for practical experience. These things can be notoriously difficult to simulate, but very easy to measure in a practical test.

 

[MJR2]

Mac,
We have been all beating wround the bush somewhat here. You have asked some questions that are often considered proprietary. As well you have told us that you had purchased the magnets from someone but started to build them on your own. Since it is a rather small industry the odds are that 'someone' may have been giving you some help here. Don't be too unhappy with what you have been given. We all continue to question and improve upon what we do. But at a certain point the physics apply equally to all.

Sometimes I give pretty detailed answers to questions. These people do not appear to be a competitive threat. To these people I try to point out as well the dangerous aspects of what they are doing. Magnets as you know can hurt you.

Take what you have been told and what you have learned on your own. Test and analyse the data. Find some older books and reverse engineer the catalogs and magnets you have bought. When you have something to offer back present your findings. We might be able to add to our collective knowledge. And if you don't want to share at that point you'll understand our answers.

Happy new year.

Mike

 

[MAC3382]

Hello, Mike:

I appreciate the explanation. Yes, I see a lot of great information in these forums and I was beginning to wonder why people were being aloof to cagey with the answer to my question. I can very much appreciate the proprietary nature especially when it is based in someone else's historical efforts.

The margin and quantities are too low for an "engineer to order" company who develops electromagnets so we were forced to use common electromagnets kludged together in an array. The diversity of the products the robots are to handle drove us far away from "common"

Development:

In some very old books I have found a few things of interest. The value of the current over and the area of the coil wire in Sq Inches. Depending on the geometry of the coil the value can be compared to tables which show a safe operating zone. (Typically 2,500 ~ 4,500 A/Sq Inch)

I found a method which echoed the suggestions in the forum for calculating the delta between the external coil temperature and the internal coil temperature. this was based on the change in resistance.

I also found Esson's rule: Degrees (Fahrenheit) = 100*Watts/Superficial Area and Carhart's modification of this to account for the radial temperature gradient.

However, as you and other have eluded I need to gather actual data. I am having our plant spin up a variety of coils based on designs from a variety of methods. I have also included a few different core materials to to inspect the saturation induction and the permeability. I hope to apply a variety of voltages and monitor the temperature rise and measure the magnetic moment of the system to judge the relative performance of the core materials. I am using very different core materials; Hyper-co 50 , Si -Fe, and CRS. Non are annealed because I think I can push them hard enough to drive them into saturation.

I do appreciate help, direction, and the recent clarification. I guess the answer to my questions was really no. I have found that there are many ways that people use and most methods were derived by experience and a lot of empirical data with a healthy bit of "gut" feeling.

All the best,

Mac

 

[MJR2]

Mac,
The Amps/sq inch figures you quote would seem to be high to really high. They may be in the force cooled range.

Change in resistance is a standard method. I think it was Carl in another forum that suggested that. It is a situtation of being so common as to be forgotten as a suggestion by me.

Generally speaking you need to be working your steel to saturation. On a practical basis low carbon steels 1010 to 1020 may provide all the magnetic properties you need.

We do many specials. That's all I do. Quantities don't need to be great. But I'd say our margins need to cover all that, so we understand each other. There are many, many things you need to learn about something that seems so simple. Good luck.

Get yourself a 3D FEA package like Magnet from Infolytica. I use it daily.

Mike