stepper motor help

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jbenedetto84
 
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stepper motor help

Post by jbenedetto84 »

hello all - posted this in another section but i think the other section was the wrong spot.

i purchased 2 stepper motors from amazon, nema 17 2a per phase, 1.1ohm resistance. i know they say voltage isn't always an issue with steppers, but i think it may be the reason for my problem (explained below). even still, by using ohms law, that should mean that the rated voltage for the stepper is 2.2 volts.

my power supply is a 12v, 5a power supply.

i purchased 2 big easy drivers from sparkfun originally. my wiring was quadruple checked. the steppers worked, but seemed to have practically no holding torque whatsoever. the rated torque for the steppers is 45Ncm(63.7oz.in). i'm using them to roll up thin plastic sheet for a make-shift greenhouse. it weighs less than 3 pounds for the entire side, including the pipe weighted at the bottom, and has no friction during operation.

long story short, i was turning the pot on the big easy drivers to adjust, and running some sample tests to find the best settings, and somehow managed to burn both big easy drivers to a crisp. on one of them it looks like the trace from the chopper driver to the filter cap is what burned, but it is difficult to tell because it is such a small footprint.

i just purchased the adafruit motor shield, and i'm going to give that a whirl (i've heard big easy drivers aren't so big and easy, and them burning to a crisp is very common, even when used properly)

i should also mention i have an arduino motor shield on another project working perfectly for about 2 years now. i'm not ruling out the possibility of me being the issue, i just hope that helps show that i'm not a complete moron when it comes to wiring these things up =)

my one thought - is it possible that by running a stepper motor thats "rated voltage" is 2.2v with a 12v power supply to the driver is what caused the driver to burn? because the motor is rated at 2.2v 2a per phase, does this mean that the chopper driver has to absorb the rest of the voltage, and current that goes along with it?

any thoughts are very much appreciated. i want to try and prevent burning up the adafruit motor shield like i did sparkfuns big cheesy drivers.

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adafruit_support_mike
 
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Re: stepper motor help

Post by adafruit_support_mike »

Those motors will kill the Motor Shield.

The H-bridge chips are rated for 1.2A, with peaks up to 3A. Even at the rated voltage, those steppers will draw 2A while holding, which is more than the chips can handle.

Check over at https://www.pololu.com/category/120/ste ... or-drivers for divers that can handle higher current. They specialize in robotics, so their drivers can handle more current.

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adafruit_support_bill
 
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Re: stepper motor help

Post by adafruit_support_bill »

This guide has some info on how to find a good match between motors and drivers:
https://learn.adafruit.com/all-about-st ... he-stepper

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jbenedetto84
 
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Re: stepper motor help

Post by jbenedetto84 »

Shouldn't a stepper of this size be able to handle 3lbs on a shaft with little to no friction?

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adafruit_support_bill
 
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Re: stepper motor help

Post by adafruit_support_bill »

3 pounds connected how? The motor should have a torque rating. Torque can be expressed as inch-pounds. If the motor has a torque rating of 30 inch-pounds, it will be capable of creating 3 pounds of force at the end of a 10 inch lever.
http://en.wikipedia.org/wiki/Torque

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jbenedetto84
 
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Re: stepper motor help

Post by jbenedetto84 »

63.07 OZ/IN. - the offset is 3/8" off of the shaft. I measured the friction at virtually 0. Again the total weight is around 2.7 pounds.

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adafruit_support_bill
 
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Re: stepper motor help

Post by adafruit_support_bill »

There are 16 oz in a pound, so 63 oz/inch is about 3.9 pounds/inch. If you are only .375" from the center of the shaft, you should have plenty of torque.

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