FEM induced by a moving magnet... so easy?

Question

I am trying the following experiment:I would like to estimate the fem produced by falling magnet (athigh=d=70mm) through a tube with a coil at its end.I use cylinder neodium  magnet with remanence of 12T (Bo), radius=6mm,h=10mm.I made a coil (area=A=7.5mm,  N number of coils=250). Could you pleasecheck if what I am doing is correct?t0=sqr(2d/g)=0.12s [s]   time when the magnet reaches the coil.B=Bo t/t0  [T]    Equation of the magnetic field. B is 0 when themagnet is outside the coil and it is equal to Bo when it is inside thecoil (at t0)Phi=NAB=0.053 [Wb]  flux of the magnetic field inside the coilFem=-dPhi/dt= -NABo/to=-0.43VUsing a scope, the max fem voltage results of -1.2V instead of -0.43.Am I doing something wrong?I am not confident about the strength of the magnetic field, since Iam using a small cylinder, I don't know if the B equation is correct.I looked at the table of the neodium and I found the remanence value=1.2T. Can I assume this value equal to the magnetic field B of themagnet? Could you please help me out?Thanks in advanceNick

John Larkin · Accepted Answer

Wrong t0?John

Paul Hovnanian P.E. · Answer

I think that's it. The OPs t0 is the time to fall 70 mm. What is needed is the time it takes the magnet to fall through the coil and some idea of the magnet's flux distribution along its axis of travel.

I get a time of 8.5 ms for the magnet to travel its height (10 mm). If I plug this into the OPs formula, I get 6.2V. The coil should produce a pulse of one polarity as the magnet enters the coil, followed by a pulse of the reverse polarity as the magnet exits out the bottom.

John Larkin · Answer

Yup. Sounds like a case of "there's an equation, here's a value, plug it in!"

Two fun things to try:

Drop a supermagnet down a vertical section of copper water pipe.
Drop a supermagnet through a coil, a couple hundred turns maybe, connected to an LED.

I once dropped a quarter down the vertical bore of a megabuck superconductive magnet, just to see how long it would take to come out the bottom. It didn't come out... it got stuck on a flange inside. There was some consternation.

John

Robert Baer · Answer

??? Nickel in the quarter did that?

John Larkin · Answer

No, it just landed on the flange. My host, after giving me a proper dose of dirty looks, found a stick, wound some tape on the end in some secret pattern, poked it down the bore (standing on a ladder... this was a big magnet) and snagged it. Got my quarter back.

We were exploring eddy currents created by pulsed gradient coils. They make a pulsed, z-axis tapered magnetic field on top of the main 200 KG static field, using a cleverly would self-shielded gradient coil. The gradient coil pulse winds up exciting eddy currents (not to mention impressive acoustic pings) in all sorts of nearby structures - the self-shielding isn't perfect in 3-space - so we built a little probe thing to poke here and there in the sample space to measure the decaying eddies; it turned out be be an interesting problem.

John

Rich Grise · Answer

...Quarters these days have a bunch of nickel in them. You should havedropped a penny or a little chunk of Al. :-)Cheers!Rich

FEM induced by a moving magnet... so easy?

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