Naïve stepper motor question about voltages

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eplatypus
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Joined: Mon Apr 16, 2012 7:05 am
Location: Briar Hill, Victoria, Australia

Naïve stepper motor question about voltages

Post by eplatypus » Mon Apr 16, 2012 7:13 am

Okay, for starters, I know that living south of the equator my motors are going to spin backwards ;) , so I have a bit of an issue with sourcing a stepper motor. I know I'll be running it at 24V, but the motors I can find all seem to be rated at 3V (such as this one: http://littlebirdelectronics.com/produc ... -steps-rev).

My question is: is the 3V part just an indicator of at 3V the motor will do this and it'll run at 24V fine, just with a bot more oomph; or is it a matter of putting 24V through the 3V motor will cause many flames and smoke and a $75 hole in my wallet.

Many thanks in advance.

nogthree
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Joined: Mon Apr 09, 2012 8:48 pm
Location: Brisbane, Australia
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Re: Naïve stepper motor question about voltages

Post by nogthree » Mon Apr 16, 2012 10:54 am

I'd say they're all 3v motors for smaller projects. I know that littlebird resell some sparkfun parts, your best bed is likely to find the sparkfun motors that Edward likes being resold for littlebird.

Also hola there fellow downunder dweller (Yes, some of my motors do spin backwards. :oops: )
- Brendan Halliday.
ShapeOko Wiki Userpage

eplatypus
Posts: 4
Joined: Mon Apr 16, 2012 7:05 am
Location: Briar Hill, Victoria, Australia

Re: Naïve stepper motor question about voltages

Post by eplatypus » Mon Apr 16, 2012 12:01 pm

Cheers for that. I think I've found what I'm after: http://ausxmods.com.au/index.php?main_p ... cts_id=174. Now all I need is for the Shapeoko to arrive.

aldenhart
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Re: Naïve stepper motor question about voltages

Post by aldenhart » Mon Apr 16, 2012 7:48 pm

The voltages on the motors don't matter and you can safely ignore them - unless you are going to run the motor above the insulation breakdown voltage (600v? 1200v?). It's the current that makes a difference. Don't exceed 2 amps on Polulu drivers or 2.5 amps for the TI drivers used on the grblshield or TinyG.

eplatypus
Posts: 4
Joined: Mon Apr 16, 2012 7:05 am
Location: Briar Hill, Victoria, Australia

Re: Naïve stepper motor question about voltages

Post by eplatypus » Mon Apr 16, 2012 11:32 pm

Okay, correct me if I'm wrong (and I'm fully expecting to be), but as voltage and amperage is linked, a 3V motor at 1.7A taken to 24V would be at (about) 0.25A? Or am i veering off into the blissful world of naïvety again?

kargofab
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Joined: Tue Apr 10, 2012 12:30 am

Re: Naïve stepper motor question about voltages

Post by kargofab » Tue Apr 17, 2012 12:50 am

Most stepper motor drivers are the "chopper" type and provide constant current rather than constant voltage. Don't worry about the voltage applied or the theory of operation, but rest assured you won't blow up a "3VDC" stepper motor at 24VDC.

-marc

levlandau2
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Location: Minneapolis, MN, USA

Re: Naïve stepper motor question about voltages

Post by levlandau2 » Tue Apr 17, 2012 3:19 am

Kargofab,

What you said in the post above is wrong. Using ohm's law V=IR, as the voltage increases, the current will increase as well (assuming the device is ohmic). So for 1.7A and 3V the motor has R=1.76 ohms, and at 24V the current through the motor would be 24/1.76 = 13.63 Amps. However, this is not what happens because the motor current and voltage are controlled non-linearly by the stepper drivers.

The wikipedia link below gives some explanation of why the high voltages are needed. Using high voltages increases the speed of the motor by allowing the poles to be switched more quickly. This is done by first applying a high voltage e.g. 24V, monitoring the current applied to the windings, and then limiting that current to 1.7A (or whatever the spec is) using electronics which monitor the current.

Basically, high voltage is needed to enable an electronics trick which allows faster control of the motor.


http://en.wikipedia.org/wiki/Stepper_mo ... e_circuits

aldenhart
Posts: 132
Joined: Tue Apr 10, 2012 2:17 pm

Re: Naïve stepper motor question about voltages

Post by aldenhart » Wed Apr 18, 2012 11:44 am

Levlandau2 - That's my understanding as well. The implications of the correlation between voltage and switching time is interesting. If the switching time is faster, there is less time taken in transition and therefore the stepper driver chip runs cooler (as it's the switching that dominates the power consumption). Another aspect is how this affects microstepping. At very high speeds (pulse rates) and high microstepping values there may not be enough time for the current to reach it's rated value in the motor. In these cases you don't actually get full power out of the motors. This explains the observation that higher microstepping values can reduce power. Again, a higher drive voltage reduces this.

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