Re: MircoStepping Drive Which One?

Power supply current actually decreases with increasing supply
voltage on switching type drives.

Switching type drives (choppers, PWM, etc) are current sources. This
means a drive set to 7A will push 7A thru a motor winding regardless
if the supply is 24V or 80V.

Current sources are like a mirror image of a voltage source (battery,
regulated voltage supply, etc). A 24V voltage source will be 24V
whether you draw 1A or 10A. Substitute "current" for "voltage" and
you have a current source. This is what step motor windings need.

Motor windings (inductors) resist rapid changes in current. On the
other hand, the faster a step motor turns, the more rapidly you have
to inject and remove current in the windings. You have only two ways
to do that; decrease the inductance (higher phase current rated
motors) and/or increase the supply voltage. A higher supply
voltage "insists" the current change more rapidly, a lower inductance
decreases the "resistance" (reactance) of the winding to rapid
current changes.

Some other things to consider when you are evaluating drives:

1) If the drive is a microstepper, does it revert to a full-step
waveform at higher speeds where microstepping is no longer a benefit?
If it doesn't then you will get only 75% of the motor's potential; it
will be an under-achiever. Ask.

2) Does the drive have mid-band resonance compensation? Mid-band
resonance is a terrible performance robber that puts a huge notch in
your motor's speed-torque curve. At higher speeds the motor makes a
descending pitch growling sound, then stalls for no good reason at
all. You can test for this:

Bench-run the motor up to 5 or 10 revs per second. Have the motor on
its side and pivot (rotate) it up on one corner of its mounting
flange about 1/2" or so off of the desktop. Bring it sharply back
down onto the desktop. Press down on the motor hard immediately
afterwards. If it doesn't have mid-band compensation it will
immediately breakout into resonance (growl) and stall. You don't want
that.

3) Can you trim out low-speed vibration? A good drive should have
accurate sine-cosine currents (<1% distortion) and have an adjustment
to trim-out any remaining vibration. You can test for this as well:

Bench-run the motor on a hard desktop; have no papers or anything
else under the motor. Have the motor on its side. Slowly run the
motor up from zero speed to about 1 rev per second. Listen to the
motor as you do. You shouldn't hear any ticking or buzzing sounds
over that speed range.

4) Kind of minor but does your motor hiss, squeal, grunt and whistle
at you while stopped or turning slowly? A good drive won't let that
happen; it's very irritating and poor manners on the drive's part.

----------------------------

Cheap Commercial Announcement:

At the end of this month we will be offering a beefed-up G201/G210
called the G202/G212. It will have the following improvements:

1) No heatsinking required at 7A. New MOSFETs (140A peak current)
having only 1/5 the resistance of the current IRF530s will be used.
Very efficient and very low heating at 7A.

2) No external 470uF/100V capacitor required at 7A. Bought them and
put them inside the G202/G210. They are big so the package is 0.625"
wider, (3.125" by 2.5" instead of 2.5" by 2.5"). Same pinout, same
mounting-hole centers.

3) Short-circuit protected. Short your windings together, miss-wire
your 8-lead motors, use motors with shorted windings, short your
motor cables to ground, disconnect your motor under power; do it at
80VDC and 7A per phase while you are at it. The G202/G212 won't care;
it will go into "Hulk" mode and laugh at your feeble attemps to kill
it. Otherwise it's just another mild-mannered G201/G210. Think of it
as a G201/G210 on steroids.

4) Power-on Reset. Waits one second after power comes up and is
internally stabilized before it unleashes its fury on your motor.

5) More features = more parts = more money. In this case, a $20
surcharge above the G201/G210 pricing.

More interesting new stuff coming towards the end of May as well.

Mariss

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Luke1027 <luke1027@p...>
wrote:

> Ignorance here on voltage/amps for RPM.
> What exactly do I need to consider if I am running
> bipolar steppers? If I put more voltage to the motor
> will that increase the amperage? I assume yes...
> but is this linear? 20% more voltage would mean
> 20% more amerage? Which in turn would up my RPM...
> by what? Any rule of thumb here?
>
> Thanks,
>
> Luke
>
>
>
>
> Monte wrote:
>
> >
> >They are both good drives, but the Gecko's will allow you a higher
> >voltage to drive your motors. If you can get by with 28vdc the
> >Xylotex would work. If you need more (up to 80vdc) the Gecko's

would

> >be worth looking at. The Gecko's will cost a bit more. I have a
> >Xylotex and can get 40ipm rapids on a Taig mill with 28vdc power
> >supply and 258 oz-in steppers and Mach2.
> >
> >Monte
> >
> >
> >
> >