RE: [CAD_CAM_EDM_DRO] RE: Stepper questions

Hi Alan,

I didn't do much more with that motor or even record the exact parameters of
the experiment because my new size 23's (low inductance) showed up the next
day. That's what I'm running on the Lathe Leadscrew now. I have a second
photo of that at the same step rate but with the windings paralleled. It's
obvious that there was a slight difference. At the time I was trying to get
the optimum speed out of the pick and place system for programming the land
mines.

Full motor current is only flowing 30% of the time but current is also
flowing during the ramp up but just not as much.

Also, during the decay there is also motor current flowing but it's not
through the sensing resistors so harder to spot. In the LMD81245 it's fed
back into the power supply I believe.

For that current you have to think about joules of energy. As the magnetic
field created by the current and rotor inertia collapses it could show up as
200V and 2mA or 2 volts and 200mA depending on what the DC load to the
inductor looks like.

Short the windings and 2V is more likely with increased motor heating. Feed
the back emf into the power supply which tends to be low impedance and it's
more likely that the voltage will be allowed to climb to supply voltage with
less current and less motor heating. After all, it will take time to build
up current in the opposite direction and the power supply is a low
impedance.

Think of the back emf in a car ignition coil. The points close and current
builds up in the primary winding to some steady state resulting in 12V
across this winding and for example 5A. Normally the secondary to primary
turns ratio is about 200:1. When the points open the current stops and the
magnetic field collapses inducing a corresponding voltage rise in the
secondary as you get a back emf rise in the primary.

Now you'd think it was 12V collapsing creates 2400V in the secondary which
is hardly enough to create a spark; especially in a pressurized combustion
chamber. Since the primary circuit is wide open when the points open it's
effectively an infinite impedance across the primary so the voltage does its
best to go as high as possible.

It's that high voltage spike across infinity that is translated by the 200:1
winding ratio that really results in a primary pulse that is maybe 150V that
then creates that secondary pulse of 30,000V.

Back to steppers. When the transistor opens, like in the single ended
version Mariss describes, that other winding doesn't just get the supply
voltage (since it's like a 1:1 ignition coil) but it gets the back emf spike
that could go to hundreds of volts. Yikes.

That's why you have to have that reverse diode across the windings in order
to clamp the voltage. Now it's true some of that current does rercirculate
through the windings around and around but because the top of the winding is
also connected to the supply, some of the energy is also dumped back into
the power supply.

Moral of the story for relays, solenoids and steppers, the reverse
protection diodes are really useful.

John

> Hi John,
>
> So the power loss that causes motor heating IS less when
> stepping! That
> confirms our earlier discussion.
>
> Nice 'scope display!
>
> If the motor current is only flowing 30% of the time, then perhaps the
> inductance is still too high for the stepping rate wanted, or
> a higher ratio
> of motor voltage to motor winding is warranted? This is a 7V
> coil? A 2.1V
> 2A stepper should be a considerable improvement (in drive
> "duty cycle")?
>
> Alan KM6VV