Re: boom

--- In CAD_CAM_EDM_DRO@y..., bjammin@i... wrote:
> At 05:22 PM 8/4/02 -0000, mayfieldtm wrote:
> >A step motor's winding with current applied is simply an inductor
> >sucking up power and generating a magnetic field. If the current
flow
> >is abruptly interrupted, such as when being physically
disconnected,
> >the magnetic field will collapse, the speed of collapse is
determined
> >by the size of the inductance of the winding, resistance of the
> >winding and other factors. The collapsing magnetic field induces a
> >reverse voltage into the windings. This Voltage can be several
> >hundred volts under certain conditions, jumping the broken
connection
> >and frying the drivers.
>
> All true. Except frying the drivers. Magnetic field collaspes
exactly in
> step with spontaneously with current slowing and stopping. Neither
is
> instantaneous, and induced voltage is probably low. But the point
is moot,
> see below.
>
> >include the protection circuitry for such events. or... as
mentioned
> >above, the winding resistance and inductance are such that only
low
> >back EMF is generated.
>
> Field collapse isn't back EMF. Steppers holding don't have a back
EMF
> anyway. Back EMF is product of moving fields. Happens in electric
motors
> because they are also at same time generators with reversed
polarity,
> limits current draw when idling near synchronous speed. Neither
fields not
> rotor moving in stepper in hold state. At the point of actually
stepping,
> then only it will have a slight back EMF on one set windings.
Completely
> different effect from field collapse pulses; hence this point
constitutes a
> logical discussion error: red herring. See below.
>
> >In actuality a step motor's current is turned on or off for each
> >step, and can be turned on and off thousands of times per second,
> >(even at rest) if a "chopper" drive is used.
>
> Ah I knew that, it's the basis of my previously ignored
argument ... when a
> stepper motor winding is switched off by the driver, the magnetic
field
> collapses in the excited windings, probably faster than when I
unplug it.
> <=Nothing you can do about it! The stepper is connected to the
driver when
> this happens, and so all the induced voltage generated is imposed
on the
> driver output as you noted thousands of times per second. As
opposed to
> whatever partial pulse may jump the connector once when I break it.
Now ask
> yourself, is therefore the driver immune to pulses of that nature,
no
> matter their cause?
>
> There was another logical error in someone's note: shooting the
messenger.
> Said he was hard to stir up, but that I did it. Neither is true!
First, I
> didn't stir you up unless I reached thru your screen and personally
shook
> your cage. Second, therefore you did it yourself, and ... that's
all it took?