Burning drivers on Motor Shield

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btmost
 
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Burning drivers on Motor Shield

Post by btmost »

I have been wrestling with motor shield issues for a few months now and really need some help.

My project requires a fair amount of torque at high RPM. I was originally trying to do this with the Adafruit stepper (https://www.adafruit.com/products/324) motor but it didn't have enough torque at the speed I needed. I switched to a higher torque motor (http://www.lulzbot.com/?q=products/nema ... per-motors) and was burning driver chips (clearly since the rated motor current of 1.5A/phase is way higher than the L293D) so I switched to piggybacked SN754410s with a heatsink (like in the FAQ). This worked great for some test runs, but then when I got down to business the chips fried again (I have a pile of about 14 fried SN754410s on my desk), So following advice I have read on here, I switched to 24V steppers (model 4418S-51 http://www.linengineering.com/contents/ ... /4418.aspx) to lower my current to 1A/phase. When I hooked everything up and ran the same code I was using before I almost instantly burnt out the drivers. I shut the power to the shield off, waited a little while, then turned it back on and the green LED of the shield was blinking (causing little puffs of smoke to come out of one of the driver stacks, great.....). This was long after the code had finished running so I am not sure why it was still sending power to the chips unless the shield is now toast too. I should also note that I have a computer fan blowing on the chip stacks/heat sinks for extra cooling and I am using this power supply (http://www.jameco.com/webapp/wcs/stores ... 2101518_-1).

Any thoughts, help, suggestions? Am I missing something completely obvious?

Thanks!!

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adafruit_support_bill
 
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Re: Burning drivers on Motor Shield

Post by adafruit_support_bill »

The coil winding on that motor is pretty low (4.25A). I suspect that the specification assumes driving it with a chopper-drive that limits the current. Using a simple controller like the L293D, the current at 24v will be closer to 5.6A.

btmost
 
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Joined: Mon Mar 04, 2013 12:32 pm

Re: Burning drivers on Motor Shield

Post by btmost »

Ahhh, that would make sense. Is there any way to limit the current? Sorry for my ignorance, I haven't worked much with this setup before.

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adafruit_support_bill
 
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Re: Burning drivers on Motor Shield

Post by adafruit_support_bill »

The only way to limit the current is to lower the voltage. By Ohm's Law (V = IR), 4.25v will give you 1A. A chopper drive will use a much higher voltage at the start of the step when the induction of the coil will limit the current. But once the coil is fully energized, it becomes a resistive load and Ohm's l;aw takes over.

The chopper drives will give you higher performance (starting torque, max step rate etc.) But at 1A, the holding torque will be the same.

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