Originally Posted by
Torchhead
True unipolar connection is where the full + volts are wired to the center tap of a winding and the two legs are pulled sequentially low . its sometimes referred to as a Push-Pull circuit . The unipolar driver is easier and cheaper to make and the early ones just used a big series resistor in the + leg to limit that current. With the cost of chopper stepper drivers now almost as cheap as the unipolar drivers and the distinct advantages you get with a bipolar setup there is no logical reason to try and move back to the older technology.
The other factor you are not thinking about is the loss of torque with RPM . Just because you can spin a motor faster does not mean you can use all that extra RPM. Yes, inductance does predict the slope of the torque curve (and where the torque "knee" happens ) but as stated you have a lot of connections options with an 8 wire motor. A true unipolar motor only has 6 wires: 3 on each winding.
The correct question is what level of torque and RPM go you NEED. You can get a ballpark on paper but there are too many variables to get really accurate ones. The max theoretical RPM of a motor is a combination of many things including the applied voltage, torque curve at that voltage , series or parallel connection and even the "purity " of the pulse (step) signals. Then you have "microstepping" and step morphing that changes the dynamics of the RPM / torque curve at higher RPM (some drivers have it, some don't) . We have seen a 25% increase of top RPM just from moving from a PC generated step to a pulse train generated by a separate pulse card.
I use the 1/2 rule. You can usually get about 50% of the max numbers in real live moving a load. Its conservative but when you start to push boundaries you start to see problems. With 1605 ballscrews your linear feedrate will be 1/5 of your motor RPM. That RPM has to be at a point where you still have enough torque to both accelerate and decelerate the motion of the toolpath and handle the inertia of the total mass of the object you are moving and any cutting forces .
So---- at 48VDC you can expect to see from 800 to 1000 RPM unloaded motor RPM before it self stalls. USABLE RPM can be 1/2 that . Lets take 500 as a good number. Put that though the 1605 and you are at 100 IPM . Torque and speed and on opposite sides of the equation so while you can get more RPM that often results in less torque (especially through a transmission) so its faster, but won't pull a sick cat out of a litterbox.
The best advice I can give is to look at designs that are similar to what you want to do and use that as a baseline. As they say "your results will vary". There are no simple formulas to predict the overall performance of a set of electronics and motors on a specific mechanical structure