Please explain stepper motor specs

Hi All

Having done a fairly extensive search on the forums and read a number of wiki pages I am still rather unclear as to the relevance of the voltage and current specs...

I seem to have found something somewhere that states a higher voltage rating is better than a lower one? It is all confusing.

I saw some posts stating that the Gen 3 stepper driver controls the voltage so a stepper with low voltage rating is no problem. In fact it allows the motor to be run really fast??

The ones I am currently looking at are SY42STH47-1684B which has a voltage rating of 2.8v and current rating of 1.68A. Only problem is they charge VAT on the postage as well, which kinda sticks in the craw.

Can anyone explain the relevance in resistance and inductance per coil? Should I be concerned about this?

I need to understand these numbers and their relevance so I can decide if other suppliers motors are adequate. for example www.motioncontrolproducts.co.uk

I do know I should have about 0.14Nm (or 1400g cm torque) according to the wiki and that 4 wire seems best although 6 or 8 wires work as well but to stay away from 5 wire for default stepper driver.

I guess I will also go for double shaft (shaft sticking out both ends) so I can clamp the shaft one side when placing the gears on the other side

My fledgling studies have shown P(w) = E(v)I(A) so is the power more relevant than the individual voltage and current specifications?

Thanks for any clarification anyone could offer, specifically the relevance of rated voltage and current.

I'm not terribly well versed in stepper type motors, but I can try to fill in some information here.

Most of the voltage/current/inductance/resistance will only matter to your stepper driver. Unless you are designing the driver yourself, this should be pretty much transparent to you. Just make sure the datasheet for the driver says it can drive a motor with the values you're picking.

Basically, the inductance is inherent because you have a coil of wire in the stepper motor that exerts a force on the rotor when current is passed through it. A resistor is usually put in series with that coil to limit current to the appropriate drive current. This resistor will be based on a particular drive voltage. (Please note here this is around where my knowledge of stepper motors is starting to fade, and I'm explaining the mechanism in terms of how I think it works based on other electromechanical devices like relays).

So remember V=IR. To direct current (DC), an inductor is just a wire, so the steady state current (after transient effects settle out, more on this later) in the coil will be equal to the drive voltage by the internal limiting resistor. The current is what drives the rotor, and the voltage and resistance ratings are just incidental to get that current.

As to your question about higher power meaning better, no I don't think it necessarily does. A motor at 2V@2A and a motor at 12V@2A may (this is a big may) have similar torque ratings, and the 12V motor burns more power in the resistor (P=I^2*R). On the other hand, you're much less likely to have a 2V power supply on hand that can source 2A, so it's somewhat a matter of practicality.

When you apply a pulse of voltage to the inductor (as stepper drivers are wont to do), the current in the coil rises as an exponential of V/R*(1-e^(R/L(t))). You can look up RL circuits on wikipedia for more details on this. You probably shouldn't worry too much about this, because the R/L time contstant will likely be low enough for the speeds you will step a motor for Mendel or any of the other printers.

A stepper driver designer might use some of that knowledge to overdrive the coil for a small duration during the first time constant or so to make the coil current hit the target level faster. Again, probably uneccesary here.

Not sure about the torque, number of wires or the other stuff though. Hope this helps.

I copied this from another post I made:

4 wire motors are bipolar (which work)
5 wire motors are only unipolar (which won't work)
6 wire motors can be wired in bipolar series (which works just fine) or unipolar (which wont work)
8 wire motors can be wired in bipolar series (which works just fine) bipolar parallel (which will work but the amp draw requirement is high, make sure it would be less then 2 amps, otherwise wire in bipolar series) and unipolar (which won't work)

in addition, The amp draw of the stepper motor is the true 'rating' of the motor. Everything else is more like performance specifications. The voltage of the stepper motor is simply a derivative of the resistance of the coil and the amperage rating. for instance on my cnc machine the motor is rated at 2.55v but it is powered with a 48v power supply. This is because the driver is a chopper type.


Inductance is a rating that affects rpm, kind of. the higher the inductance the more voltage you need to run through the motor to get the highest rpm. too low of inductance has a other issues. reprap doesn't move that fast to matter. my cnc machine however can move at 45000mm a minute

dose any any one think these would work?

[cgi.ebay.com]

Hey Rolling RZ

I am not to well versed with these motors as you can see by my query, but one thing I do know is that the wiki stepper motors page specifies that you need at least 1400g.cm torque (which converts to roughly 0.13Nm).

These motors, although temptingly cheap, are cheap for a reason. They specify 2 torque ratings, holding torque and detent torque. Perhaps someone can explain what the difference is, but they are both way too weak. The stronger is 864g.cm = 0.087Nm and the weaker rating is even worse at 36g.cm = 0.004Nm!!!

So I would say no - they clearly are under spec according to the wiki build docs.

Further, my research indicates you want a stepper with a fairly high torque for your extruder. If my memory serves, I think people were saying even 0.25Nm is not enough there.

Hopefully someone will correct me if I am wrong.

Cheers

Holding torque is the maximum torque with the power on and stationary. Detent torque is with the power off and is only relevant if you need something to be held in place with no power. It is the torque generated by the permanent magnets and is always very low compared to the holding torque.

Yes 1400g.cm would not be enough for any of the extruders I have made. I am using a 0.43Nm motor on Wade's extruder, under driving it a little at 1.5A and it is the limiting factor when driving PLA. When driving ABS the grip is the limiting factor.

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[www.hydraraptor.blogspot.com]

I would recommend getting steppers that are as close to the 2-amp rating of the drivers. It is 2-amps right? lol.

You will be lucky to get 2A out of the drivers. That is the rating of the chip, but to get it you need very good heat sinking.

In the UK I don't think you can do much better than the recommended motor: [reprap.org]. It sits right in the sweet spot of the Allegro drivers and is high torque and pretty cheap from Zapp.

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[www.hydraraptor.blogspot.com]

nophead Wrote:
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> You will be lucky to get 2A out of the drivers.
> That is the rating of the chip, but to get it you
> need very good heat sinking.

I have been trying to figure out why the RRRF and Pololu drivers out there seem to have been designed without any thought for thermal considerations. The Pololu drivers in particular provide virtually nil in the way of PCB area for heatsinking. I really hope that the Stepper Driver 3.0 boards have been designed with this in mind, but I fear they have not.

It's entirely feasible to design a board for these drivers which will allow them to reach their rated specs without an external heatsink or fan. In fact, the board I'm working on and will probably release next weekend should be able to do this no problem.

The Allegro specs are very optimistic and it isn't good practice to run electronic components at their max ratings. I think you will find it very difficult to get the full rating, particularly for the QFN package used on the Pololu board. The data sheet calls for 4 layers and 2 oz copper. The Pololu boards have no chance as they simply aren't big enough to have the required area of copper even if they had four layers and 2oz, which they obviously don't.

The RRRF boards have sufficient heatsinking for a Mendel or a Makerbot as neither needs 2A with the recommended motors. The V3 prototype boards I tested go up to about 1.5A, but as they use PLCC packages you could fit heatsinks.

My extruder controller gives 1.5A. To make it do 2A I would have to make the board considerably bigger as the amount of heatsinking goes up with the square of the current. Since I don't need 2A I prefer to keep the board smaller and cheaper.

Edited 1 time(s). Last edit at 05/30/2010 04:38AM by nophead.

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[www.hydraraptor.blogspot.com]

nophead,

Do you have any idea what max current might be usable with the Pololu boards in Adrian's proposed "wind tunnel" configuration? See http://reprap.org/wiki/Pololu_Electronics and the recently checked in related parts:

• electronics/Pololu-electronics/cooling.aoi
• electronics/Pololu-electronics/motherboard-with-cooling.dxf

I'm wondering if that kind of setup is likely to be beefy enough to drive a Wade-style extruder well. Is there any way to know, other than to try it and see?

Thanks,

Jonathan

Jonathon,
I don't know any way of calculating it other than trying it. As far as I know Adrian is the only one to try forced air cooling without any other heatsinking so best ask him. It may be enough for an extruder because moving air makes a massive difference compared to still air.

I use 1.5A on my Wade's extruder. The max extrusion rate rate is limited by the motor when using PLA and by the grip when extruding ABS. 40mm/s second is no problem with either and I have run PLA at 100mm/s (with a 0.5mm nozzle).

Chris

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[www.hydraraptor.blogspot.com]

If someone gives me the continuous PCB area surrounding the driver chip, I can calculate the power that you can deliver.