boom

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?