0
\$\begingroup\$

I'm trying to chose correct stepper motors to drive 2 wheeled robot. I started with geared DC motors, but found they are too fast (I have 27:1 geared), and they provide little torque at low speed.

Therefore I'm looking at stepper motors that will give enough torque at low speed. The criteria are:

  1. Motors need to be small. Top are Nema 17, Nema 23 are going to be too large
  2. Need to be driven from low 12V accumulator
  3. Need to output 1Nm torque, to move 5kg dolly on potential 5 degree slope with 10cm wheels
  4. Need to use as less current as possible
  5. Need 30RPM on output shaft

I researched this topic a little and found that low inductance motors will need less volts but need more amps. As the setup doesn't require very precision movement I think I can get more torque if I go for geared stepper motors.

I found 2 identically geared motors on ebay, one having 34mH and another having inductance 2.8mH

http://www.ebay.co.uk/itm/DE-Ship-27-1-Planetary-Gearbox-Nema-17-Stepper-Motor-0-4A-DIY-CNC-Robot-Camera-/231657603766?hash=item35efde82b6

http://www.ebay.co.uk/itm/DE-Ship-27-1-Planetary-Gearbox-High-Torque-Nema-17-Stepper-Motor-DIY-CNC-Robot-/231657805043?hash=item35efe194f3

If anyone had experience with geared stepper motors and can advice on the selection, I would really appreciate it. 27:1 is not set in stone, I can go for 5:1 if it provides enough torque, or higher if not. I can even use bare motor without gearbox, but I'm afraid it will be either too weak (even at low speed), or will have to be too large to give enough torque.

\$\endgroup\$

closed as off-topic by PeterJ, Ricardo, Leon Heller, Null, nidhin Oct 5 '15 at 18:13

This question appears to be off-topic. The users who voted to close gave this specific reason:

If this question can be reworded to fit the rules in the help center, please edit the question.

  • \$\begingroup\$ Their ebay store gives a 'contact us' link. Armed with the information from Marko Buršič's answer, maybe providing extra information (e.g. required speed) ask them. You might discover how well they understand their products. I feel that silent 'close' feels a bit harsh for a mostly specified problem, even if it does ask about specific products. \$\endgroup\$ – gbulmer Oct 5 '15 at 13:59
1
\$\begingroup\$

I took a look on those steppers, but from their datasheets I haven't found the speed. Even from your description there is no mention about the speed of motors, keep in mind that power is a product of torque and speed.
A useful data of stepper motor is back EMF voltage V/krpm, which in both models is absent. 34mH has more turns than 2.8mH and higher BEMF voltage respectively. At 12V they won't reach the same final speed, also the dynamics is different - the 2.8mH is faster in terms of dynamics and it will reach higher rpms at 12V, but less torque than the 34mH model. The 34mH would be high torque , low speed motor.

\$\endgroup\$
  • 1
    \$\begingroup\$ Thank you for your reply, Marko. I found similar dependencies on CNC forums. If I use formula from http://www.daycounter.com/Calculators/Stepper-Motor-Calculator.phtml I'm looking at a bit more than 60RPM for 34mH motor itself, which divided by 27 will give me just 3PRM from gearbox, which is way to slow. 2.8mH motor will be faster with 382RPM (motor) and 14RPM (gearbox), which is still not fast enough. As 2.8mH is probably low enough, I'm either need 5:1 geared motor, or stepper without gearbox but with high inductance. \$\endgroup\$ – zmechanic Oct 5 '15 at 14:56
  • \$\begingroup\$ The only question is, will 34mH or similar motor provide enough torque without gearbox? \$\endgroup\$ – zmechanic Oct 5 '15 at 15:00
  • \$\begingroup\$ There is a rated output torque (after gearbox) on the datasheet, so you could divide that by the gear ratio. \$\endgroup\$ – Brian Drummond Oct 5 '15 at 16:43

Not the answer you're looking for? Browse other questions tagged or ask your own question.