LOL Ian
May I suggest if you have the time to write a summary of the conclusions you have come to so far.
Ah time, there's the rub - however maybe it is time to conclude this thread and continue elsewhere, I think between us we've long ago answered the original question and I think we're moving into areas well beyond its intended scope.
From my understanding of steppers it is the voltage that determines the speed and the current that determines the torque and I suppose most importantly the way the motors are driven has no relation to V=IR, is this correct?
Kind of, but not quite so simple - as Ian says below.
What I was really trying to say is this: In a real world motor, torque stays roughly constant from 0 RPM up to several hundred RPM and from there it drops inversly with speed. A caveat to this is that below about 500 rpm, torque is affected by patches of resonance, but let's ignore that for the moment. Suppose you have a system that is running with the rated current of 5 Amps, at 12 Volts. Let's also suppose that your system has the torque for the job but has limited top speed in that it stalls at 1000 rpm. If you now double the voltage to 24 Volts - you'll find that you've upped your top speed to around 2000 rpm. This is of course in an ideal system and in reality you may not quite get that increase but you get the idea.
The biggy to notice however is that you have not INCREASED the TORQUE of your motor because you havn't increased the current - it will still not go above 5 Amps. You've just increased the speed at which the torque is available. I suppose in effect you've stretched the curve.
With regard to Ohms Law - we have to be carefull - it's not that it has no relation - we just have to be aware of when it's appropriate and when it isn't. One thing I think that leads to confusion is that I've seen stepper motors variously described as being DC motors and as AC motors. I
think I'm right in saying - well it depends on how they're driven. One thing is for sure though - they are not constant DC motors and Ohms law (as far as I'm aware) only "works" for constant DC. (Constant DC is not to be confused with limited DC)
I am dealing with AC Servos at the moment and from my understanding they have a constant for Torque/Amps and Voltage/RPM and the drive is able to supply the current and voltage independant of each other, this means that I can get max torque no matter what RPM the motor is turning. Obviously it is different from steppers but the principle of voltage and current being independat of each other seems to be similar, and the relation of Voltage to RPM and Current to Torque seems to be similar.
Maybe my understanding of both steppers and servos is totally wrong so I have already donned my hard hat and have taken cover 
Sorry Hood - I have no knowlege or experience with servos but it certainly sounds from what you've said that there are or may be similarities.
Ian and Hood,
just to be clear we're on the same page, we're talking 2 phase motors - yes?
Yes they all seem to be two phase but can be multipole ie more than 4 windings.
I found this website :-
http://www.allaboutcircuits.com/vol_2/chpt_13/5.html
Scroll down to see the full story.
Thanks Ian - interesting read - one of the best I've seen particularly from the construction point of view. But oh boy, where do I start? - Oh well, here goes nothing as they say...
I think it's important to start by saying I'm not disagreeing with anything it says. Rather, I'll say it's allways difficult to be absolutely in synch when we use words to describe things. We use a word and assume the other person thinks the same as we intend them to think - but this isn't allways the case sadly.
I'll put it like this: Is "poles" the best word to use or would "notches" be better or maybe worse? Is "coils" the best word or would "windings" be better or maybe worse?
The "notches" as I'd call them here are what gives a motor its resolution. The "poles" (as in magnetic poles) as I'd call them are the bits (as I know you know) that "intensify" the magnetism when you energise the "coils" or "windings" and so ultimately give it it's number of phases - in a 200 step motor 100 of these would be energised at a time - or is it 50? I lose track. Is one piece of wire wound (or coiled) round a stick a coil or a winding? Is that same piece of wire coiled (or wound) round one stick and then off to another stick, one or two coils (or windings)? In short I would still describe this as a 4 pole motor and only introduce the word "notches" if we were discussing resolution. Maybe I'm wrong, maybe I'm using the wrong words, maybe I'm just really bad at explaining what I mean. ( I know one thing - I'm going to give it a rest after this because this is beginning to do my head in).
The reason we're discussing all this b*ll*cks is because of where we started - remember "rated values" and how I said the interwound coils (or inter-coiled windings) have a heating effect on each other? Well doesn't this article confirm that? In fact doesn't it confirm it with a vengence when it says (near the bottom)...
"The center tap is achieved by a bifilar winding,
a pair of wires wound physically in parallel, but wired in series. The north-south poles of a phase swap polarity when the phase drive current is reversed."
Granted, this is for a six wire (unipolar driven) motor, but an eight wire motor is just where the centre tap is brought out to the outside in two wires instead of one.
Yes, more voltage gives more top speed but if the motor does not develop enough torque to overcome detent torque and friction, you will never get there. So more amps to get enough torque.
Yes, heating effects, from eddy current losses and copper losses, limit how many amps you can feed the motor or it will get too hot and burn out the insulation.
We both agree that heating is the issue, but we
were talking about our different conclusions regards the rated values.
If you could treat a parallel wired pair of coils as totally separate entities then you would be correct with your current calculations but the data sheets suggest otherwise and I'm sorry that seems to have been forgotten. Anyway, I've done my best to explain why I think this is - again maybe I'm wrong. At the very least the fact that the root2 rule and the I^2.R calculation works every time in complying with the datasheets suggests to me at least that it's no coincidence or momentary lapse of reason that the rated values are as they are.
Something for you to think about. A stepper turning at 1000 rpm is being fed by an 8 microstep driver at the rate of 200 x 8 x 1000 = 1,600,000 square wave pulses per minute i.e. about 27kHz.
LOL - like we havn't given ourselves enough to think about already. But for the fun of it I'll play.
You could improve your motor by having 2 rather than one phase - 13.5KHz.
You could half your frequency by swinging positive/negative (bi-polar driver) 7.75KHz.
You're said you're using microstepping so your signal is nearer a sine wave than a square wave, but more to the point, half of that sine wave spans a whole step and is what actually manifests the microsteps - 3.87Khz.
And now nurse is coming with the jacket with no sleeves, so I have to hide. But before I go - group hug anyone?
EDIT: Yes I've just reread this and "notches" is probably no good either because then we'll become confused as to whether we're talking about the notches on the rotor or the stator - poles it is - I give up!
Ian
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