Thermal rise calculation for a DC Electromagnet 1

[MAC3382]

Hello, All:

I am new to this forum and I hope that someone can help me. I would appreciate any and all input.

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. However, I 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.)

The power supplies we use are simple with all passive components. They are basically transformers and a bridge rectifier. The electromagnets are DC work-holding with mild steel return paths/yokes/housings. The switching is slow by electronic standards and we do not suffer from core losses.

It was suggested in another forum to calculate the surface heat disapation factor (in^2/Cold Wattage). I have seen this for many electrical things including electrical enclosures, but I have nothing to relate my calculated factor to. It was also suggested that my system will behave very similiar to a transformer.

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

Thank you

Mac

 

[CarlPugh]

How is the heat getting out of the coil?
Radiating out the sides of the coil?
Conducted through the core the coil is wound on?
Conducting through whatever the coil is mounted on?

The easiest way to design a coil like this is to measure the temperature rise of a coil that you have made.
Merely put an (AVERAGE READING IF COIL HAS LOW INDUCTANCE)ammeter in series with the coil and an AVERAGE READING voltmeter in parallel with the coil. Calculate the cold coil resistance from the volt and amps at turn on.
Calculate the hot coil resistance after the coil temperature has stabilized.
Using the data for copper calculate the average coil temperature rise.
If the coil temperature is not uniform, then use thermcouples or temperature sensitive paint to measure the hot spot temperature.

 

[waross]

Energise at 80% voltage and measure the temperature when it has stabilised. Repeat at 100% and 120% voltage. Extrapolate.
Consider adding some temperature sensors when you wind your test coil to check for hot spots.
As an alternative, the watts consumption is the heat input to the magnet. Include a description of the size and geometry of the magnet and ask the thermodynamics forum.
Thermocouples are good but temperature sensitive paint won't tell you the hot spot or coil temperature.
AS CarlPugh suggests you can use the change in resistance to determine average coil temperature and allow a percentage for hot spots.
respectfully