wrote:
On Sep 29, 5:26 pm, Lew Hartswick wrote:
Spehro Pefhany wrote:
On Mon, 29 Sep 2008 13:42:11 -0700 (PDT), the renowned
wrote:
I have an old fleamarket growler for checking electric motor
armatures. It warns not to be turned on without armature in V. My
physics has faded over the years, what will happen with an incomplete
magnetic circuit? Can this be modified to use as a de-magnetiser or
is it a piece of junk just worth the copper?
Thanks, John.
If the armature is not there then the inductance will be very low, and
thus the current will be very high, so the coil will likely burn out
unless it has been designed to handle the power dissipation.
I think you could limit the current through the coil (for example,
with a series electric heater element) and use it as a demagnetizer
(but it might not create a strong enough magnetic field to demagnetize
a strongly magnetized object). You could also try a series degaussing
PTC thermistor out of a CRT monitor or TV.
Best regards,
Spehro Pefhany
Or operate it from a "Variac" (reg TM). Lots of generic versions. :-)
...lew...- Hide quoted text -

- Show quoted text -

What if I try increasing the inductance by placing something of
suitable size in the V, could there be sufficient field in the air to
be useful?

John.

You'll "use up" the field in the thing, instead of the thing you want to
demagnetize. This could result in your thing still attracting things,
which would be a bad thing. Something would have to be done, before
things got out of hand.

When you slap anything ferrous into that 'V', you complete the magnetic
circuit, and provide an easy path for the flux. Take it out, and it
takes a lot of ampere-turns to drive the flux through the gap -- this is
why the current goes up when you don't load the 'V'.

Now if you put _two_ things in the gap, then the flux will get split up
between them -- so your "thing" in the gap will steal the flux from
whatever thing you want to demagnetize. So it's best to just fling one
thing into the gap while demagnetizing.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

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