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Registered Member #14
Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383
How we can measure the saturation magnetization of a given material (for eg nail steel)? I was thinking to use a current ramp (triangle wave) generator, a shunt resistor, an o-scope in x-y mode, but how to measure B?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Well, H is proportional to the current in the coil, and if we neglect IR drop in the coil, B is proportional to the time integral of the voltage.
So to draw the B-H loop of an iron cored inductor on a scope, we drive it with a sinusoidal voltage big enough and of low enough frequency to saturate it, and hook the integral of the voltage up to the Y axis of our scope, and a current sensor to the X axis.
The only difficulty I know of is making the IR drop in the windings negligible or compensating it out. The IR drop will skew the diagram. You may be able to add a second sense winding on the piece of metal to drive your integrator: this winding will have negligible current, so the IR drop won't be an issue.
Also, you want an unbroken magnetic circuit of the material you want to measure. If the circuit contains air, you'll need lots more current to saturate it, and the total change in reluctance when the nail saturates might not even be noticeable, since the reluctance mostly comes from the air part of the circuit. So a large nut or washer would be better than a nail.
Of course, you can use a current ramp instead, but you need some kind of high current amplifier to make it. For the sinusoidal voltage, you can just use the mains through a variac and isolating transformer.
Registered Member #14
Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383
Interesting the determination of B through integral. I would be interested in open-reluctance designs, to determine the value in similar conditions to coilguns, but you are right, huge currents involved. I will try using washers of the same material of the nails i use for the projectiles. THanks for the advice, i will try using a mains transformer calculating the number of turns required to saturate at a given rms voltage without unloading the transformer. I like the idea of the integrator
V = dflux(B)/dt = (for a washer with square section) N* (r2-r1)*thickness*dB/dt
the integrator should work like this V2 = Int(V) * 1/(R*C)
so V2 = N*(r2-r1)*thickness/(R*C) * B so B = V2/ K K = N*(r2-r1)*thickness/(R*C)
H is simply n*I, and will be put to X
Got it!, thanx. I will post the pics as soon as i manage to get it going...
Edit: would a square wave work? applaying a square voltage , the current in the inductor will appear ramp linear(ish)
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