Thread: Magnabend
View Single Post
  #70   Report Post  
Posted to rec.crafts.metalworking
[email protected] pentagrid@yahoo.com is offline
external usenet poster
 
Posts: 116
Default Testing MOT as electromagnet - this just in

On Wed, 19 Jan 2011 15:06:08 -0500, "Michael A. Terrell"
wrote:


Bob Engelhardt wrote:

wrote:
If I recall correctly when one designs a transformer, you calculate
what the ET product is. ...

snip
So the flux ( B ) depends on the voltage and frequency. And using the
same line frequency and the voltage at 240 volts RMS ( much less than
the usual 2000 volts on the secondary, you should not have to worry
about saturation. ( this assumes you are using AC voltage.).


Thanks, that helps!

I've been thinking about those 2000 volts. If that's the normal voltage
on this coil, it's probably going to take close to that to saturate the
core. I mean, wouldn't they design for the core to be close to
saturation, to minimize the core size needed?



Only at line frequency.

With DC exitation the same flux density will be reached when the
DC reaches the same level as the normal line freqency magnetising
current (i.e the no load current). With DC, the voltage needed to
reach this current will typically be about 5% of the rated line
frequency voltage provided the magnetic circuit is closed (no air
gap)



My interest in saturation is 2 fold: I don't want to run beyond
saturation 'cause of the extra heat, and at saturation is where the
maximum pull will be.

Now, about ACC. To use as an electromagnet, I have full-wave
rectification, unfiltered. Although the coil's inductance will do some
smoothing (I wish my scope was working). My intuition is that the DC
component of the current will be determined by the coil resistance and
that will produce flux proportional to the number of coil turns. So,
the question is whether the DC current will saturate it before 240v. Or
be too high for the coil's wire guage.


Because the rated line frequency full load current is very much
larger than the no load magnetising current, DC excitation will
saturate the core long before rated full load current is reached.

This applies when the magnetic circuit is as fully closed as in
the original transformer. If it is only partially closed with
perhaps inferior iron or small airgap, proportionally
more current will be needed.

But the bottom line is that I'm going to be using 240v max (full wave)
and as long as it isn't saturated then, I'll be getting the maximum pull
available.


240V should be ample.

Jim