RE: [CAD_CAM_EDM_DRO] RE: Stepper questions

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

>
> Hi all,
>
> Dug through my files and found this scope photo of a stepper motor with
> step
> pulses at 460Hz. I guess since I have the ELS on the bench (piled in a
> corner at the moment), when I pull it out again, I can do some more
> detailed tests to show some of the differences in phases and voltages. I
> believe this was watching the current on the sensing resistor of the
> LMD18245. With a 40 Volt supply and a high inductance size 23 motor rated
> at 700mA and 10 Ohms resistance.
>
> http://www.autoartisans.com/cnc/stepcurrent.jpg
>
>
> What you can see on the above picture is that with the armature turning,
> the
> winding current (yellow trace) driven by the applied voltage of about 40V
> (blue trace) doesn't flow until about 1/3 of the step time has passed.
> That's because of the back emf from the collapsing field from the previous
> step and the induced emf from the motor turning.
>
> Then, as time passes the current starts to build until it reaches maximum
> based on the sensing resistor value relative to the reference voltage. At
> that point the hardware starts chopping the supply voltage off and on to
> keep the current at a steady value.
>
> Finally, the step is over, and it's time to reverse the applied voltage to
> the motor again, the current falls to zero.
>
> Notice that full current only flows through the windings for about 30% of
> the time. So for 30% of the time the winding will apply full torque to
> the
> armature.
>
> Now imagine the next step pulse happens about halfway through that 460uS
> pulse time. Above the 't' in the word Current. Only about a third of the
> maximum required motor current is flowing at that point and it's been
> ramping up so overall torque is quite low.
>
> Realistically, because the current was lower, the back EMF time is also
> lower so the forward current will start sooner. But I think the scope
> capture demonstrates why the stepper motor loses torque the faster you
> step
> it.
>
> BTW, using Ohms law I^2R = P where R is the DC winding resistance you can
> see that the area under the yellow trace represents the current consumed
> over time flowing through the winding and that's what generates the heat.
>
> When the motor isn't moving, the current stays at that maximum current
> point (say 1 amp in a 1 Ohm winding is 1Watt). But once you start
> stepping,
> that scope capture shows 1A for 1/3rd the time, and roughly half that for
> another 1/3rd. ((I/2)^2R) + (I^2R)/3 = 0.58W. In other words, power has
> dropped by almost 1/2.
>
> Cool eh?
>
> John Dammeyer
>
>