re: boom

Keeping in mind that you can express the back emf caused by a collapsing (or
building) magnetic field as
EMF = -L*di/dt;

This means that the voltage produced (EMF) will always oppose the change in
current. Whether the current is increasing or decreasing. The only real
time that we have to worry about this however is when energy is stored in
the magnetic field of the inductor (aka winding). The energy stored in
magnetic field of the inductor will be
E = 1/2*L*I^2;

Thus when the magnetic field is made to collapse, via disconnecting the
current source (either mechanically via switch or electrically via
transistor) the energy will be expended in trying to keep the current
flowing, this will cause a large voltage to be produced which will be
proportional to the rate at which the current was reduced.
Generally a transistor will cause the current to reduce more gradually than
a simple switch will, although the switch will also bounce more and thus
take a longer time to finally settle in the off condition.

In either case the driving circuitry should be protected from the emf (to a
certain extent) by the proper use of a snubbing circuit. Disconnecting the
windings whilst current is flowing through them (in either the hold state,
or whilst moving) is not a good idea, although I'm sure we're already
decided on this. The electrical switching (via transistor) should be done
in such a way that the the reverse voltage produced by the winding is
lessened, and wont cause damage to the driving or switching circuitry.

>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?

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