Driving bipolar stepper motors in series or parallell  - Page 1
Forum section: electronics / projects

author: HSPalm
author_rank: Regular Contributor
timestamp: July 21, 2016, 09:56:03 am
content: I'm in a situation where running two stepper motors from the same driver would be very beneficial. The stepper motors aretiny floppy drive motors, low voltage type.- Cost. Most drivers FETs handle up to 1A or more, so why pay for so much more than needed if I can run two with the same driver.- Space. I'd like to make the drive circuit PCB as small as possible.- Voltage. Motors seems to want 2V to 5V, most drivers supply a minimum of 8. So with two coils in series I can double the voltage. There are low voltage drivers though, but the selection is not great.The voltage may not be an issue if I use a current chopping driver.The motor pair will not drive the exact same load, they will move the same gantry axis but as you can see in the link they will have their own lead screw.So, parallell or series? Why? Why not?

author: Kleinstein
author_rank: Super Contributor
timestamp: July 21, 2016, 11:27:53 am
content: The rated voltage for stepper motors is valid for really low speed, that is less than about 10 steps per second. To get a higher speed, a higher voltage is needed.So if you need higher speed, switch them in parallel and get a driver that does the current chopping.If you only need low speed, it's ok to have them in series, if the wiring allows this. Motors wired for unipolar drive might not work in series, as some of the wires are already connected.

author: H.O
author_rank: Frequent Contributor
timestamp: July 21, 2016, 01:51:43 pm
content: Yeah, you really need to think of stepper motor as constant current devices. If you drive them with voltage equal to R*I their performance sucks because the inductance of the windings quickly becomes an issue. In the old days massive power resistors was put in series with the windings to limit the current, today active current limit drives using various schemes of regulating the current is used. 5V rating on a stepmotor winding is actually quite a lot, but then I'm used to NEMA23s and up....More advanced drives have active resonance damping and other stuff which probably wouldn't work well with two motors connected to the same drive but with these TINY motors I'd suggest you simply try it. They will (should) "work" either way, it's just a question of if they'll perform as you need them to. You may even want to use something like 12 or 15V with a series resistor to get the current down but I suspect that will violate your requirement for small space...

author: HSPalm
author_rank: Regular Contributor
timestamp: July 22, 2016, 09:03:25 am
content: I have changed my mind a bit and was hoping to drive them via USB 5V. So then there's no driving them in series at least.But so it seems that 5V, even though the motors are rated 2-5V, may not be sufficient if I want to drive them fast.The motor driver I found in the end is a low voltage type:http://www.st.com/content/st_com/en/products/motor-drivers/stepper-motor-drivers/stspin220.html?icmp=tt3848_gl_pron_jul2016

author: H.O
author_rank: Frequent Contributor
timestamp: July 23, 2016, 08:45:41 am
content: Bloody h..l that's a tiny little driver chip.Genereally speaking you won't be able to drive them "fast" if you drive them at 5V - no - then you want a much hight voltage.But how fast is "fast"? It depends on the motor and your requirments of course. And with the non existent data and speed/torque curve on the motor it's impossible to say.Don't know how well high resolution microstepping will work with two motors in parallell though.

author: LeonV
author_rank: Contributor
timestamp: July 23, 2016, 09:25:26 am
content: I would have expected the change in inductance while one stepper is doing a single steep would cause the second motor to not steep reliably if two step motors were in series.Especially of the load is different on reach motor.And that this effect would be minimised in parallel.

author: Rerouter
author_rank: Super Contributor
timestamp: July 23, 2016, 10:07:53 am
content: yeah, with a higher voltage you can run steppers WAY faster than at there rated voltage, to an extreme i played around with linear current limiting with some TO220 transistors i had on hand, and a normal may speed of ~300 RPM at its 12V rating was 1400RPM at 76V, (yes, one transistor per coil leg and those transistors where glowing hot),I would say wire in parallel, and use your driver to do current limiting, or you will burn it out,

author: HSPalm
author_rank: Regular Contributor
timestamp: August 04, 2016, 12:31:34 pm
content: Too bad though as I was hoping to drive them from USB 5v.The application really doesn't say anything about how fast I need to go. But faster is more fun. The project is really just for fun, the XY gantry will move a rounded pogo pin around it's own driver board on exposed pads and do simple tricks... Like pushing it's own buttons etc.