[GreenKeys] Spice Magnets

[Bob Camp]

Hi

Ok, here's the first pass at a model for the selector magnets in a 
Model 28 KSR:

Each magnet is roughly 4 H, in series that gives 8 H and in parallel 
that gives 2 H.

 From the Teletype documentation they should have a DC resistance of 135 
ohms. The ones I have run 130 ohms each.

The next thing you need for a reasonable model is the parallel 
resistance. Right now that comes out to a range from about 12K to 15K 
per magnet. I'll try to get a better fix on it with some more 
measurements.

The parallel (60 ma hookup) model would look like:

resistor of 65 ohms in series with a 2 H coil. The coil would have a 6K 
ohm (best guess right now) resistor in parallel with it.

That model seems to hold up pretty well from DC up to about 2 KHz.

If you then check the magnets with a simple power supply they pull in 
at 40 ma and drop out at 20 ma. I don't know if that's normal or just 
the machine I have.

A normal 2.5 K ohm 155 VDC loop would pull in the magnets in about 830 
us and release them in about 625 us. That would give a little 
distortion but not much. To pull in at 625 us you would need closer to 
200 volts on the loop. I suspect that the mechanical side of the 
machine has more than 200 us of bias in it so the difference is 
probably not significant. It also does not account for the fact that 
the armature is moving under spring power (constant force) when it 
releases and under increasing magnetic force (force = 1/distance 
squared?) when it pulls in. That also may explain the 200 us 
difference.

Next step is to figure out a good way to measure the voltage on the 
magnets when you break the loop. That should give a pretty good 
confirmation of the parallel resistance.

	Enjoy!

		Bob Camp
		KB8TQ