MakerFarm Prusa i3 and i3v 66 oz. in. Stepper Motor Specifications

[clough42]

I finally tracked down specifications for the 66 oz. in. motors that Colin sells and includes in the MakerFarm i3 and i3v kits. It looks like they're not actually the same motors that Circuit Specialists sells (as I have suggested in other threads) but their characteristics are similar: around 18 ohms DC resistance and 35mH inductance.

Specifications from Colin:
MakerFarm Motors.jpg

And direct measurement:
2014-07-29 08.20.37.jpg

The 35mH inductance is accurate, but you can ignore the series resistance value. This meter is attempting to estimate DC resistance from phase angle measurements, and it's out in the corners where it isn't very accurate. The 18.5 ohms I measured with a DC multimeter is more accurate and matches the spec sheet.

[clough42]

As you can see, the resistance and inductance of these motors is tremendous. I think this is why many of us have had trouble getting the extruder motor to retract properly at high retract rates (25-30mm/s).

If you're struggling with blobbing or stringing on your MakerFarm printer, try reducing your retract rate to 10mm/s. The extruder motor can usually keep up at this rate.

[DrLuigi]

So how much should the amps be on the ramps board realy?

I thought i rememberd that Colin told me it was 0.38.
But i do get some skipping sometimes when doing around 60-80mm/s, so i kinda just wanted to ask if it would have been that.

[clough42]

So how much should the amps be on the ramps board realy?

I thought i rememberd that Colin told me it was 0.38.
But i do get some skipping sometimes when doing around 60-80mm/s, so i kinda just wanted to ask if it would have been that.

Colin specifies .39v in his build document for the 8" i3v. This translates to a 2.5x.39=.98A current limit. In practice, the motor never reaches this current because its DC resistance of 18.2 ohms limits the current to .66A, which isn't too much higher than the actual rated motor limit of .5A. When the motor starts moving, the inductance reduces this current even further, depending on how fast you step.

The extra current doesn't hurt the motors, but it does make them get hot. The heat is only really a problem with the extruder motor, which is mounted to a plastic part. You can either turn down the current on the extruder motor, or you can add a fan with an appropriate shroud. My shroud is here: http://www.thingiverse.com/thing:343026

Or you can ditch the Greg's extruder altogether and use something else. Here's what I'm currently using: http://www.thingiverse.com/thing:389105

Technically speaking, the voltage on the drivers should be set to .5/2.5=.2V. Since the motors are running right near their current/torque limits at 100mm/s, this may or may not cause more skipping. I'm running with the .39v default, and I'm printing at 100mm/s and bridging at 120mm/s.

Edit: fixed broken link

[DanSilov]

The extra current doesn't hurt the motors, but it does make them get hot. The heat is only really a problem with the extruder motor, which is mounted to a plastic part. You can either turn down the current on the extruder motor, or you can add a fan with an appropriate shroud. My shroud is here: http://www.thingiverse.com/thing:34302

Looks like the wrong link, or I don't understand how that thing is used with motors

Or you can ditch the Greg's extruder altogether and use something else. Here's what I'm currently using: http://www.thingiverse.com/thing:389105

How is that extruder really different from Greg's? Except for the belt general construction looks very close, motor is still mounted directly to plastic. I have a great heat problem with extruder, it melted original small gear that came with the kit, and now it melted mount I printed with PLA. I gets so hot I can't really touch it, and it's set to 0.37v.

[clough42]

Looks like the wrong link, or I don't understand how that thing is used with motors


How is that extruder really different from Greg's? Except for the belt general construction looks very close, motor is stinductanceed directly to plastic. I have a great heat problem with extruder, it melted original small gear that came with the kit, and now it melted mount I printed with PLA. I gets so hot I can't really touch it, and it's set to 0.37v.

Sorry. The link was missing a character. I fixed it.

You can, of course, use any extruder you like. The heat problem you're experiencing is because of the motor that Colin chose and the current he recommends. You can turn down the current or use a fan to avoid the heat, but that won't solve the inductance problem, so your retract speed will still be limited. Replacing the motor will fix both.

If you choose to replace the motor, you may wish to buy a NEMA 14 instead and try the Itty Bitty Belted Extruder. It has a number of features I like, but not everyone may agree. It's about half the weight of the one that comes with the kit because of the smaller motor. This reduces the torque needed to move the X axis and may help with skipping or allow higher speeds. The belt drive doesn't make clicking noises and has no backlash. It does require more vitamins, though, so that's something to consider.

The motor being mounted directly to the plastic isn't a problem if it doesn't get hot. The NEMA 14 motor (and current setting) I list with the extruder reaches about 45C when printing, which doesn't threaten the ABS.

[clough42]

Oh...I see you're using PLA. I don't think 45C would hurt PLA, but I don't know for sure.

[DanSilov]

If you choose to replace the motor, you may wish to buy a NEMA 14 instead and try the Itty Bitty Belted Extruder.

Is it possible to use NEMA 14 with Greg's extruder? There's quite a lot of vitamins for the Itty Bitty Belted Extruder which I can't get ahold of easily, so I think I have to stick with Greg's for now. But it would be good to replace the motor, it's really scarily hot during printing.

[clough42]

I have not seen a NEMA 14 variant, but it may be out there.

The belt and pulley can be had on eBay for $5 total, including shipping. The rest of the vitamins, save the motor, are the same as the Greg's.

[printbus]

...it would be good to replace the motor, it's really scarily hot during printing.

Although you could do it, replacing the motor or going with a different extruder should not be a prerequisite for you to get your i3v up and running. Continue the dialogue in your print problems thread so we can help figure out what is going on in your case. Does the extruder motor get hot? Yes, as they all do with the driver setup suggested by MakerFarm. So hot that you can't keep fingers on it or hot enough to melt/soften the small gear? IMO, they shouldn't be getting that hot unless something is wrong. Make sure your power supply is set for 12V and not something else common like 13.8V. Lower the driver some. Unnecessary load on the motor will also cause it to get hotter than it should. Too low of a nozzle temp, motor and small gear pushed into large gear too tight, poor lateral alignment between small gear and large gear, dragging at the guidler bearing, large gear cranked onto hobbed bolt too far, and bolts rubbing on the large gear come to mind as some things that would put undue load on the extruder motor. I've since adopted Clough42's extruder motor shroud, but until I did that I ran long prints with a small table fan set next to the printer on extruder motor side. Fan airflow was angled upwards to not affect the bed and nozzle much, but the extruder motor would pass into the airflow when it was on the right-hand side of the prints. That kept the extruder motor noticeably cooler.