Re: Which stepper to buy: 68oz 0.9degrees or 70oz 1.8degrees

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NoobMan

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uncle_bob
Here's the disconnects:
Even if you could put a several thousand volt driver on the motor and run an amp through it no matter what, it's torque wold still drop off. The hysteresis and loss in the magnetic materials used in the motor are the limit here. As the frequency goes up their losses go up as well. All of your added input simply goes to heat. The easy way to look at that is the angle between the voltage and current. You can directly measure what's going on.

No, i havent said i would put 100v to a stepper driver, i intuitively though that would of been your idea behind the affirmation about you designing a stepper to run at 0.1A and beat "my gizmo" 2.5A one. Changing the driver input voltage has its benefits, but not like that and not in that sense. If you didnt thought about changing voltage, then tbh dunno what else you thought about when saying that. I think this part is over anyway - if not then feel free to expand.

The point is that past the point that you can establish the proper sine wave drive signal, there is no advantage to greater driver voltage. That is a very predictable level.

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In a stepper the torque starts off at the holding torque and pretty much just drops as rpms (or frequency) goes up. Your "dynamic torque" has an upper bound in the static torque. That's what makes static torque a very important parameter on a motor. That's why you can always find it specified.

Imo, the starting point for dynamic behavior is technically the detent torque.

The detent torque is simply the torque that will move the shaft with no current through the windings.

The word i believe means jumping distance, and thats what i would be interested in, jumping from one step to the next.

The jumping involved is just the "jump" that you get as the magnets align

The holding torque is about static behavior e.g. "holding" position, i would be somewhat interested for an application where the motor needs to hold weight or otherwise counter forces exerted on the head. And even then still not so much: since any forces on head when one motor holds, can also manifest while is moving, and the moving one is lowest, i guess it means i would still be only interested in detent one just because its lowest value, and i would efectivelly ignore the holding one just coz its higher and thats covered.

Since the holding torque is the number with current through the coils, that's the basis for all the calculations on an operating motor. No current and yes detent torque rules.

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You *slowly* go from static DC to stepping.

Hmm, when the motor stays still, thats not DC. Its still switched mode even then.

When the motor is still there is a constant DC current through the motor. How that current is generated by the driver does not matter. You could use a battery and a resistor to get the same effect.

In case you hear noise, thats where is coming from. If you dont hear noise, then its above 30kHz, hence too high for human ear.

Again, that's just how the driver does it's thing. It has nothing to do with how the motor operates. All it's doing is generating a DC current when the motor is still.

Now you see why i said i care for rise times because simply put, there is lots of it.

They only count if you believe the driver running as an ideal switcher. Due to the Q of the motor coil they are far from an ideal switcher. Since there is no "C" in the system they have some other issues as well. A normal switch configuration has more parts than one of these drivers does.

Rise times dont count much when motor holds, but switching does occurs at all times, it has to, as its the only method of control.

As long as it's DC current the motor will hold. You do not need fast switching to have a DC current.

Without switching, current obviously would rise above setting and would burn something, probably the driver ic. But thats too plain therefore i think you must have been refering to something else by that.

The only thing you need for DC is a PWM pulse width out of the switch. You could be switching at 1KHz with 10us rise times and it would work fine. No need for speed at DC.

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Low RPM's mean low drive frequencies.

I dunno what you mean by this. It makes me think about vfd as in variable frequency drives which are used for stuff like milling heads, but i dont see the resemblance here, so maybe you mean something else.

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Low drive frequency means that you are completely in the ohms law region.

Again, i am at miss. I think this choice of words may be at least partly my fault. I think i remember saying something like that once, but was in some context of choosing the motor values, and we dont care for ohms law about motor's label values because the driver limits the current anyway, its way we can run it at much higher voltage while not caring for the low resistance coil getting burned. This is why there is switching even when motor stays still, because coils would burn otherwise. I think was wrong thing to say, poor perspective/comparison or at least unfortunate choice of words from my part. We may not care for it and even we may sometimes hide it under the carpet, but we are in fact always under ohms law in some form, even if not about label values. Actually its more than just a law, its the very definition of resistance. So there is no practical region or operating regime that is outside ohms law, if we look down hard enough, ohms law gets its finger in all operating points in one way or another. E.g. in the sense that voltage would eventually rise the current to ohms law for a final value, except the driver senses and interrupts the rise edge at the preset current point. I think you meant something similar along those lines. So again i might of misunderstood you and perhaps you meant something else.