steppers and current / amps...

[Ksoggs]

I am just curious how some steppers of the same torque can have drastically different power needs?

I have 3 steppers sitting in front of me right now... all three are right around 1200in/oz one is rated for 7amp, another 3amp, and yet another at 5amp.

I was told that it is really voltage that matters to these guys. the more volts you shoot at them in a pulse the more torque you get.

one guy i talked to told me that the amperage requirements are really nothing more than an indication of efficeincy. he said that older motors needed more current to do the same work. due to better stronger magnets, and better design, you can now do the same amount of work with less juice. i asked if i would gain anything going with a higher amp stepper... and he said no, i was just getting something less effiecient and would have to spend more money on larger power suppies and all. I am not sure i believe all that... it appears that manufacturers still make steppers in various currents... so why would they bother making a 9amp when they can do the same thing with a 3amp? there has to be something different between different amp rated motors. is one faster? one cheaper? one last longer?

and then what happens if you under or overpower a stepper current wise? say i put a 9amp motor on a driver that supports only 7amp... or if i accidentally put a 3amp stepper on a driver set to 9 amps? will i burn out one of the components? will i just loose or gain power?

I was just hoping someone could explain the effects of voltage and amperage on a stepper to me... just trying to understand this a little better.

I have one driver that needs external power... it says up to 36v... and outputs up to 3amp per stepper.

i have another driver that is self contained, and you plug it in to 110v, and it outputs up to 7amp.... but never tells you voltage output anywhere.

I dont have anything rated for 9amps, so i am afraid i will be unable to power my one stepper... i could use the 7amp driver... but i dont want to risk anything... like will i fry the driver if it is trying to power a 9amp stepper?

what cases would you want or need a higher amp stepper?



also... is ther a way to improve the speed of a stepper? i bought three 1200in/oz 3.0amp steppers to use on my X Y and Z... and was thrilled with the power they had, but man are they dog slow. the guy who sold them to me told me that is the trade off... smaller ones are faster but less powerful... and the bigger ones are all slow.

[Mariss Freimanis]

Actually motors of the same size have identical power needs. Power is amps times voltage so a 7A motor needs only half the voltage a 3.5A motor needs for the same performance. A 3.5A motor has twice the number of turns of wire in its windings as a 7A motor. Torque is proportional to ampere-turns (amps times number of turns of winding wire) so the more turns, the less current is needed.

Why not just use a whole bunch of wire turns and have nothing but 1A motors? Good idea except it takes twice as much power supply voltage to get the same performance every time you double the number of turns of wire. Low current motors need high power supply voltages for equal performance.

Mariss

[Ksoggs]

Ok. I knew that theory. Just wasnt, sure it applied here.

But now if that is the case.... Then if I had all three motors hooked to the same controller and the same power supply... The higher amp motor should significantly out preform the other 2, assuming the driver could supply the needed amps.

As I mentioned, I have 2 different drivers. One is a toshiba based 3 axis control board. It claims to handle up to 3 amp motors, and up to a 34v power supply. I bought this controller to go with my 3 1200in/oz 3amp steppers. I have a 24v and a 36v supply to use with it (the 36v I adjusted output down to 32v for safety).
the other controllers I got from a guy selling some spare parts. He sold me 2 1200oz steppers and 2 all in one controllers/ps's. 2 phase micro-stepping motor driver MD1105.
No voltage selection, just amperage.

So with both of these controllers I'm pretty stuck on voltage. So I assume despite the 1200in/oz I am going to get more out of the higher amp motors?

[RomanLini]

The higher amp motors will have lower inductance, so they will generally perform better at high RPMs.

As for "getting more" that depends on what the limiting factor is. With big Size34 steppers the limiting factor can often be the problem of breaking into resonance, and the high amps low inductance motor will often be worse for that as they have higher resonant excitation energy at the same step rate compared to a higher inductance motor using the same driver and same PSU voltage.

Beware of tuning everything for "max speed", as it can result in reduced low speed performance. Ideally you should tune everything for good all round performance doing the tasks it needs to do. That's a good philosophy for any design.

[Mariss Freimanis]

Not true.:-) Mid-band resonance is caused by a system phase shift of 180 degrees. The motor is a mass-spring and has 90 degree phase lag. At low speeds the drive appears as a 0 degree phase lag current source. This makes the total phase lag 90 degrees and everything is stable. As speed increases the drive runs out of headroom as a current source due to inductive reactance and reverts to a voltage source. This introduces an additional 90 degree phase lag for a total system phase lag of 180 degrees. This is a setup for unconstrained oscillation of course.

The "onset of mid-band resonance" speed is a function of power supply voltage and motor inductance. Increasing the supply voltage increases the mid-band resonance onset speed while increasing inductance decreases the onset speed.

Mariss

[James Newton]

As far as I can tell, Mariss and Roman said the same thing, in different ways: Low inductance motors will spin faster but resonate sooner.

[Mariss Freimanis]

No, for the same power supply voltage and same motor size, a low inductance motor will spin faster (provide greater power output) and will break out into resonance at a higher speed.

Mid-band resonance (aka parametric resonance) starts at the end of the constant torque section of the motor's speed-torque curve. It is also most intense at that location but can be induced at any speed above that point (anywhere in the the inverse torque region).

Here is a low-tech way of determining how susceptible your motor, drive and power supply system is to mid-band resonance:

1) Set the motor on its side on a hard surface like a desktop. Make sure there are no papers or any other soft materials underneath the motor.

2) Run the motor up in speed until you are in the inverse torque region of the speed-torque curve. Measure the DC current going into the drive from the supply. Inverse torque begins past the point where the power supply current drops rapidly.

3) Pivot the motor up from the desktop about 1" (20 - 30 mm) using one corner of the motor's mounting flange as a fulcrum point.

4) Rapidly slam the motor back to the desktop using the motor corner as a fulcrum point and press the motor hard against the desktop.

5) The motor will immediately go into mid-band resonance (will make a growling or warbling sound) and stall within 2 or 3 seconds if the drive doesn't have mid-band compensation. If it has compensation, nothing notable will happen at all.

Mariss

[James Newton]

I'm sorry, but these two statement appear to me to be contradictory:

Originally Posted by Mariss Freimanis Increasing the supply voltage increases the mid-band resonance onset speed while increasing inductance decreases the onset speed. Mariss

Originally Posted by Mariss Freimanis ...a low inductance motor will spin faster (provide greater power output) and will break out into resonance at a higher speed. Mariss

Which is it? Does a lower inductance motor go into resonance at a higher or lower speed than a high inductance motor?

Or can you explain how it is that I'm misunderstanding what you are saying?



I don't doubt anything else you've said in your posts (frankly it's pretty much over my head) but I've always understood that Inductance in electro-magnetics acts like Mass in mechanics. Higher inductance means bigger magnetic fields that take longer to expand or collapse. So a low inductance motor can change it's field quicker, which means it is more likely to spin faster, but also to be "vibrating" or starting and stopping more sharply and therefore be more likely to resonate. I would have expected the speed of that resonance to be more or less the same for a higher or lower inductance motor, but that the lower inductance motor would be more /likely/ to resonate, because the higher inductance in the other motor would be more likely to "smooth out" the changes in the field and thereby cause less vibration.

That last paragraph is very likely to be wrong; as I freely admit I'm not an analog electronics expert, but that is a separate issue from your statements appearing to contradict each other.

[boblon]

I was reading this thread and thinking how good it was.

I don't know if it is the way the question was posed that prompted it to go this way, but the short discorse that follows is one of the best and most concise explanation of how steppers/voltage/current work and relate to speed/torque and why that I have seen.

I was disappointed that the final question wasn't cleared up and hope a bump will put it there.

If that happens I think it should be 'stuck' somewhere to make it an easy find.

BobL.

[James Newton]

There were so many questions in the original post that I'm frankly not willing to take the time to figure out which were answered and which were not. What is unresolved in your mind?

[boblon]

Originally Posted by James Newton I'm sorry, but these two statement appear to me to be contradictory: Which is it? Does a lower inductance motor go into resonance at a higher or lower speed than a high inductance motor?

This one please

BobL.

[James Newton]

My guess is: That is a question best resolved by careful experimentation. It will require two motors which are identical in every way except for the inductance and, of course, will involve hooking them up to the same exact drive system and supply. I don't have motors like that, but I might be willing to test them if I did.