I'm confused about stepper motor voltage

jens55

Steppers are current based devices. Although general specs usually list a voltage, it is irrelevant for your purposes. When a stepper driver switches on a phase, it measures the current that is drawn by the motor and limits that current.
Since the phases of a stepper motors are coils of wire, they have inductance (which is also listed in the spec's). When you switch on current to an inductor, the current flowing will not go to full current flow but rather the current increases at a given rate based on inductance and voltage. In order to increase speed, you need to be able to bring the current up to the rated current as fast as possible. Higher supply voltage will do that for you. A 12V power supply will supply the same rated current but will not do it as quickly.
Since a stepper motor works by switching on (and off) current in two windings, a higher voltage can substantially increase your speed.
If you ever look at the speed vs torque curve on a stepper, you will see that torque decreases very quickly as the motor speed goes up. This is simply because the drivers can not supply instant current and it takes longer for the current to reach the rated current if the voltage is lower. So, if you happen to trip over a graph plotting torque vs rpm at different supply voltages, you will see that a higher voltage power supply keeps the motor torque higher at a higher motor speed (because rated current is reached quicker).
A side note - the maximum torque a motor can develop is based on it's construction and current handling capability but there is a given maximum. Torque will basically remain at that maximum until the on/of/on switching of phases gets too fast (net current through the coils is reduced because it takes a finite time to go from no torque to full torque). A higher voltage power supply can force the current to rated current faster than a lower voltage power supply.

Not everything in that rambling reply might be correct but the overall gist is correct.

jens55

Forgot to mention - while the construction of the motor causes certain voltage (and current) limits, the supply voltage you choose is limited by the active devices that are used to switch current on and off in the motor coils.
Mot of the Duet boards are limited to around 30V. Only one or two of the newest boards can go up to 48V and then you run into problems sourcing 48V fans or 48V heaters (although you could cheat and supply multiple voltages. The fans could run off the very limited 5V supplied by the Duet board ) etc etc .... 24V is (IMHO) the sweet spot unless you have a really good reason to go higher.

dc42

@jens55 thanks for providing that summary.

The 48V-capable MB6HC 1.02 and EXP3HC 1.02 have on-board 12V regulators which can be used to power 12V fans.

@gnydick you can use the calculator at https://www.reprapfirmware.org/emf.html to see how increasing the driver voltage allows torque to be maintained at higher speeds.

gnydick

@dc42 I wish I had the 1.02. I just (couple months ago) finished building out my machine with the 1.01.

Very cool about the emf tool

gnydick

@dc42 I plugged my info into the form and changing the voltage didn't change anything but low slip angle. I don't know how to interpret that number. I can see changing the torque also brings the number back down after increasing the voltage, but other than that, how am I supposed to use this?

jens55

@gnydick, the first 3 parameters are a function of the motor itself and not related to voltage. Only the slip angle parameters (and I don't know what the low/high slip angle difference is) are supposed change. If you change the supply voltage you can see how the motor maximum speed increases with supply voltage.
As I don't know what the slip angles represent, I can only guess that it is the point when the motor torque starts to dip noticeably.

Phaedrux

https://forum.duet3d.com/topic/5255/meaning-of-low-high-slip-angle-in-emf-calculator

Mandragora

I will add few cents to this discussion:
I supply 37V to my v1.0 6HC Duet:
https://forum.duet3d.com/topic/26758/32-4v-input-instead-of-32v/2?_=1659258404908

If you want to know more about steppers, check this video out:
https://www.youtube.com/watch?v=lftsMrksdTA&list=PLWQs4VcXB7unGrcCVibcisuvt0Hamxb4d&index=21

gnydick

@Mandragora that's cool. I guess I can get a decent bump in performance.

I'm assuming the higher voltage will not be good for my fans that run one VIN?

Also, what about tool boards, heater cartridges, etc.?

deckingman

@gnydick said in I'm confused about stepper motor voltage:

............. Also, what about tool boards, heater cartridges, etc.?

For sure you'd need to either use a second PSU or a buck converter. Doing some rough maths, a 40 watt, 24V heater has a resistance of about 14 Ohms and draws about 1.7 Amps of current. If you supply it with 37 volts, that same 14 Ohms will draw about 2.6 Amps of current. So your 24V, 40 watt heater becomes about 95 Watts at 37 volts.

gnydick

@deckingman, so is it possible to run 1 PSU for the motors and 1 for everything else?

deckingman

@gnydick I don't see why not but someone more knowledgeable than me might say otherwise. I'm fairly sure that the Duet boards switch the 0v side of things like fans and heaters. So you'd connect one side of your 24V heater to the 24V PSU and the other side to the Duet heater -ve terminal while supplying the main board with the higher voltage PSU.

dc42

@deckingman said in I'm confused about stepper motor voltage:

So you'd connect one side of your 24V heater to the 24V PSU and the other side to the Duet heater -ve terminal while supplying the main board with the higher voltage PSU.

Yes that's correct.

gnydick

@dc42 I don't have the confidence to wire things without foolproof instructions/diagrams.

For example, when I hear something like the fact that there's a common ground and I can use any of the negative terminals, makes my eyes water. I have 2-3 years of EE from high-school 30 years ago.

And I'm using tool distro board + 4 toolboards. Only heater directly connected is the bed.

jens55

@gnydick, Danger Will Robinson, Danger, Danger
Most of the outputs are SWITCHED on the ground side. They are NOT common !!!

deckingman

@gnydick In your OP you mention people using 48V to get "crazy speed and torque". If you are uncomfortable about making the necessary changes, then I guess the question you have to ask yourself is how badly you want that "crazy speed and torque". You can't print at "crazy speeds" due to restrictions imposed by the volumetric melt rate of the hot end. If your machine is for cutting metal, then you are restricted by how fast the cutter can remove material. So the only time you can use "crazy speeds" is for travel moves. How badly do you want to use higher travel speeds than those that you can get now? Torque is another matter. It is needed to accelerate the moving mass. So if you have a particularly heavy print head or cutting spindle, and you want to accelerate the gantry on which it sits faster than you are currently able, then you need more torque. But again, can you reasonably make use of that extra acceleration? If so, and given that you are uncomfortable with making the necessary wiring changes involved with using higher voltages, then there are other ways such as simply using bigger motors, or use remote drive shafts and asymmetric pulleys to get a gearing effect, or use two or more motors.

But you really need to do your own cost/benefit analysis (and for cost, I mean time as well as cash). Start with the benefits and look closely at how yould benefit from "crazy speeds and torque". Can you reasonably make use of them? How much would the printing or machining time be reduced? If yours is a commercial enterprise and the machine is running 24/7 then saving a few minutes out of every hour might be worth the cost. But if the machine sits idle for large parts of the time, then probably not.

gnydick

@deckingman that's all correct. My print head is heavy because it's the E3D tool changer setup on a RailCore II. I don't want crazy speed, just a bit more torque. I was merely referring to what happens at higher voltage to see if I could benefit.

I'm not uncomfortable making changes if I know what they are. The lingo is just too casually bandied about sometimes to be sure what people mean. Give me a diagram or explicit instructions, and I can do it.

It's when it comes to terms that aren't identical to what's in the duet docs that I'm not sure. Like, what's Ve? I remember VIN, VOUT, positive, ground, etc.

I just like to be careful. I have a literal pile of duet boards that are fried in one way or another because I carried on when I wasn't sure.

droftarts

@gnydick said in I'm confused about stepper motor voltage:

Like, what's Ve?

I think you are referring to where @deckingman said "... and the other side to the Duet heater -ve terminal ..."
"-ve" is negative. He means the negative terminal.

Ian

gnydick

@droftarts OMG, like i18n, but even slightly nerdier. Well done.

deckingman

@gnydick Yes, sorry. -ve is just shorthand for negative, and +ve is shorthand for positive. Unfortunately such shorthand terms are often used on circuit boards because there simply isn't room to fit the full length term in a legible font size. -ve could also be labelled "gnd" (short for "ground") and you might see vcc which generally means supply voltage (not sure what the "cc" part stands for), or 12v which means 12 volts. But being a nearly 70 year old, retired mechanical engineer, with little in the way of formal qualifications, I'm probably not the best person to advise you on electronics terminology. What I know is just what I've picked up over the years.