The saga continues. Today's testing:

_COIL HEATING_

I used 160v AC into the bridge. Same as used for the first tests I did
Wednesday.

I assumed a 30% duty cycle (what MagnaBend uses), with 3 seconds on & 7
off. I set up a 555 timer to do this and let it run a half hour. It
probably didn't reach steady state by then, but it's unlikely that I'd
ever use it longer. The temperature rise was 95C (172F). Pretty
modest, given transformer classes are 80, 115, & 150C.

Wondering if the 3 second bend time was realistic, I watched the
MagnaBend video again & measured the time it took to make a bend. It
was about 5 seconds actual bending and 5 seconds or so lining it up. So
I ran another test: 5 seconds on, 10 off (33% duty cycle). I ran this
for 2 minutes (8 cycles). 8 bends being the most I'd ever do at a time.
85C rise.

Then, a balls-to-the-wall test: 250v applied (to the bridge). 5
applications of 4 seconds each & 10 seconds off. 4 or 5 bends is
probably a typical job. I usually only do 1 or 2. A small 39C rise.

_PULL_

My previous pull/grip/force measurements were done with 120v. Looking
for the absolute maximum that I could get, I tried 250v. Knowing that I
couldn't measure the full force, I put 3 equal bars on the magnet and
loaded one of them (the middle one). It lifted 362 lbs, but was heating
up rapidly and as it heated the resistance went up and the current down,
reducing the pull. It let go after a minute or so. I didn't intend to
run a minute's test, it's just that it took me that long to get the load
off the ground. A single bar covering the poles would then exert 3 x
362 = 1086 lbs !!!! Or 290 lbs per inch. The MagnaBend exerts 250 lbs
per inch.

The temperature rise was 120C. The sustained "on" time was the cause.

_SUMMARY_

With 120v applied, the force was 740 lbs (about 200 lbs per in).
Although I didn't measure heating at this voltage, it could probably run
"forever" (100% duty cycle) without overheating.

At 160v, a 30% duty cycle would keep the coil from overheating. Force
was not measured.

At 250v, the force was 1086 lbs (290 lbs per in). A 47% increase from
the 120v force, but with a 108% increase in voltage. Definitely
non-linear. A 30% duty cycle would likely be OK (I didn't test to
steady state), but the "on" period could not be more than 5 - 10
seconds, with a minute's "on" getting the coil very hot.

Were I to actually get around to building a magnetic bender using MOT
electromagnets, I would probably include a variac in the controls. It
would allow the use of 120v when that amount of force would do, without
worrying about heat. But it could be cranked up to 250 when the most
force was needed.

Now, the next step would be to test other MOT electromagnets to see how
much variation there is between them. But that's not going to happen.
I'm going to assume a close-enough between them, but check that the
secondary turns are close. By dividing the cross sectional area by the
wire diameter.

Bob