Thursday, July 5, 2012

Levitation Frequency Characterization: Train of Lab Work Notes

Great news!  The bridged output mode on the Peavey amplifier I'm using here works!  That means I can apply roughly twice as much power to the levitation force providing electromagnet.  The bridge mode appeared not to be working two days ago.  A little reading of the manual provided the reason.  When in bridge mode, the speaker outputs don't like to be applied to any kind of ground.  I was trying to measure the output of the amplifier using a grounded oscilloscope.  Apparently this is what was causing the amplifier to shut itself off in bridged mode.  After removing the oscilloscope from the speaker outputs today, everything is working great.  Instead of hooking the scope directly to the electromagnet inputs, I attached it to  the pick-up coil.  As I mentioned a few days ago, this a better measure of the power available for levitation anyway.

I don't have time to write up everything in a classy fashion at this point, but I would still like to post on what's going on around the lab, so today I'm going to try something new.  This post is more or less what was going on around the lab as it was happening, (stream of lab work).  Hopefully in addition to showing what's going on in the lab as it happens, this type of post will give an idea of how results have to be reviewed, questions asked, and new tests performed.  I put in some notes after the fact, and they're marked.

A quick disclaimer.  I'm characterizing our equipment right now, so if it seems like there's not a great experimental procedure yet, that's because there's not.  All the data right now is just quick and dirty to get an idea of what our minimum and maximum experimental parameters are.

There are two questions for today:

What is the highest frequency that levitation can be attained at with the new bridged amplifier mode?  The old record is 152.62 Hz.

What is the minimum voltage at which levitation is achieved for each frequency?  [It looks like this question will have to wait for tomorrow].

Lev was immediately attained at 200 Hz!  Lev available at 400 Hz!  No lev at 600 Hz with power ramped up until the amplifier protection circuits kicked in.

400 Hz seems to be associated with changes in the level.  300 Hz looks like it might be level change related as well.

Comments after the fact
There are two types of levitation that we can get with the apparatus.  Each of them is shown in the following video.  The first one is what I'm calling steady-state levitaiton for the moment.  The superconductor floats up when the current is ramped up and stays up with no further changes in the current level.

The second one has to do with quickly changing magnetic fields.  If the electromagnet is quickly switched off, (as seen at the end of the video), there is a rather large, rather fast change in the magnetic field.  The amount of electromotive force induced in the superconductor is proportional to the rate of change of the magnetic field.  Consequently, you see a larger though very short jump in the levitation force.  (Look up Faraday's law of induction and Lenz's law for more information).

Back to train of lab work
200 Hz actually does provide steady levitation after a slow ramp up of power.

Trying again at 250 Hz.  Got steady levitation from 250 up to 300 Hz.  See the data on movie 108...:

Starting at 300 Hz, cannot get steady levitation.  The protection circuits kick off immediately after the superconductor moves the first time.

Trying again at 250 Hz after giving the SC time to cycle.  Got steady lev.  Did a successful run at 275 Hz as well.

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