"Eric R Snow" wrote in message
...
On Fri, 17 Jun 2005 13:52:37 +0100, wrote:

On Thu, 16 Jun 2005 14:52:25 -0700, Eric R Snow
wrote:

On 16 Jun 2005 13:33:14 -0700, "
wrote:

I don't know what DC voltage a mag chuck needs. So you might need a
transformer in addition to the lamp dimmer and rectifier.
Dan
Oh. The mag chuck needs 100 vdc. Part of the reason for the dimmer is
to limit the voltage to the chuck. But I suppose a transformer would
be OK. I just want to know if a rectifier connected to something that
expects a resistive load will ruin the device.
Eric


The fullwave rectified voltage delivered to an inductive +
resistive load is the mean value of the RMS input i.e 0.9 x RMS volts.
so rectified 110/120v AC supply will be fine.

Although the rectified voltage applied to chuck is half sine
waves, the inductance is so high that the current flow is close to
pure DC. No capacitor should be fitted as it is not needed and its
presence would upset the 0.9 Vin relation.

To avoid inductive arcing, switching should be on the AC side
of the rectifier. If you want to play with a lamp dimmer - it depends
on detail design. Some will accept an inductive load some will refuse
to dim and just deliver full output. Damage to the dimmer is
unlikely.

Demagnetising by applying 60HZ Ac to the chuck is a
non-starter. Because the chuck inductance is so high more than a
thousand volts would be needed. Using the dimmer and successsive
polarity reversals is a miserable business because it needs lots of
graded reversals.

The following kludges work pretty well:-

Remove the rotor from a fractional horsepower motor and remove the
piece slowly from on or near the stator teeth.

Restack the laminations of a power transformer so that a stack of"E"
laminations remain. Remove workpiece slowly from on or near the
exposed ends of the"E".

Raid the dump for a defunct colour TV and remove the demagnetising
coil that is located on the bulbous flare of the tube. Twist it back
on itself so that it forms a triple loop instead of the original
single loop (this the same twist as the one used for tidily storing
spare bandsaw blades). Pass the workpiece slowly through the triple
loop.

These are all short term rated demagnetisers - if power is applied
for more than 30 sec they'll get pretty hot.


Jim


Jim, I have a demagnetizer. The thing is, when parts are ground on an
electromagnetic they stick when power is removed. All the chuck
controls I've used have a demagnetizing button that helps to get the
part off the chuck. I have seen chuck controls without this feature
but have never used one. Maybe that's why chuck controls are so
expensive? It's because of the demagnetizing electronics?
Thanks,
Eric

I have not worked with one, but I would imagine that all you do
for "demagnetize" is to apply AC instead of DC to the coil.
They may make it a little smarter than that to make sure the AC
is turned off as it goes through 0 current so there is no residual
magnetism left. Very much like the degaussing coil in your TV or
computer monitor (when you first turn it on, there is a "hum" - that
is the degaussing coil which goes for a few seconds then turns off.

mikey