Hi

I have the bigger drives (10 amp) from Routout CNC and I also looked at the smaller ones they supply, both have current limiting, the larger you set by using a volt meter across some special contact to set the current, a table is supplied. Can't remember how the smaller ones are set.

Just looked and here's the pdf setup file for the smaller 2.5 amp drivers http://www.routoutcnc.com/RoutOutCNCStepper.pdf

Hope this helps.

Steve

Yes- thanks -

I have the instructions, the set up is much the same as for the 10 amp, you turn a pot til the voltage on two pins in 1.4v multiplied by the amperage you need. It is working well - as must be by definition, current limiting. I feel I could go back to the motors in parrallel and run them - as long as I set the outputs right.

Someone is lending me a Gecko drive - rated I think, at 7 amps and voltage way up.
I am going to try this on my leadscrew so I can put the full current through the motor. If I add another battery to my system, I can put the voltage up to 36 volts on the Gecko - that sould make the sparks fly.

I am thinking of adding a forth axis to the system - so that will be a chance to get a more powerful driver card.

As your driver can only source 2.5A it can not supply the rated parallel current of your motor which is 3.5 Amps. The data sheet describes it as a PWM driver which I think is synonymous with it being a chopper driver. If so, then I don't think it will be damaged by your proposed parallel wiring scheme - but don't take my word for it. That said, at 24V in both cases your motor will have the same available power of 60W. Had you been able to source 3.5A then clearly the power would have been 84Watts in parallel as opposed to the 60W in single coil.
Despite the fact that in the first case you're "overdriving" voltage wise by a ratio of 3.2:1 and in the second case by 4.52:1, I can't see it actually makes any difference - power is power is power after all. But from your reaction when you briefly tried it I get the feeling it did make a difference. So if you feel it's better in parallel go for it. The only thing I can think of is that if it is better in parallel then I'm guessing it's because reluctance is coming into the equation - back to the books!

If you do switch to geckos (other great chopper drives are available) and up your V - make sure your rig's well bolted down - that thing's going to go so fast it'll make your head spin.

Stirling,

Just some thoughts on current into a stepper. When you apply a voltage, because of the inductance of the coil, it will generate a current in the coil. This will start at zero amps and rise rapidly to whatever maximum you have set your driver to, then decay to zero with a time constant depending on the inductance. So the driver is not generating a square wave pulse of current and you cannot just multiply amps and volts. In a reply on the Yahoo Gecko group, Mariss said that a 23 frame 2 stack(2.5A) motor would generate about  35 watts  with 30 volts applied and doubling the voltage would double the watts(power) available to overcome inertia, friction, detent torque and drive the cutting tool.

So how does this help Jim? It looks like you were right to point out that he could use parallel connection of his 8 wire drive if he sets the current limit on the drive and gain some useable torque perhaps at the expense of top speed and that if he can try his borrowed Gecko and increase the voltage even more power will be available.
Keep th information coming Jim, we are all learning!

Ian

Hi Ian

I think we're on the same page more or less with this. I understand that the driver does not induce a square wave of current. As I understand it, that's exactly why we use over voltage in order to try to get the leading edge of the current waveform as steep as possible.

I wasn't my intention to suggest that the output mechanical power of the motor is VA, that's why I said "your motor will have the same available power of 60W", meaning from the PS - which is VA. Clearly some of this is not converted into mechanical power otherwise we'd have a 100% efficient motor and we'd all be rich

I agree that the output power of the motor effectively doubles as the PS voltage doubles - Mariss told me that too when he said this (I'm sure he won't mind me including it here.)

"Look at it this way: Doubling the power supply voltage doubles the mechanical power output of a step motor. The efficiency of the motor doesn't change, so twice as much electrical power has to be supplied to the drive. Electrical power is Volts times Amps. Because you doubled the voltage, you doubled the electrical power at the same amperage.

Conclusion: Power supply current is independent of supply voltage for a given motor. It is the same value regardless of power supply voltage.

Mariss"

What I still don't really understand though is if Jim does indeed get more oomph by wiring in parallel as opposed to single coil, where is that extra oomph coming from? Like I said, there is 60W of electrical power available to drive the motor in both cases. As Mariss says - the efficiency of the motor doesn't change at the same amperage, and as you've said - not all of that will be converted into mechanical power, - but whatever proportion it is, is the same.

The only thing I can think of at the moment is prbably absolute rubbish but I'll throw it in anyway. We know that the current is induced in pulses. We know that the quicker we can get the current to rise to its 2.5A the better. Inductance of parallel is less than single coil. This presumably allows the current's leading edge to be steeper. Therefore the 2.5A is "present" in the motor for longer. Therefore the power is available for longer on each pulse than it is for single coil. - just a thought.

Ian

EDIT: As you correctly say - we're all learning. A thought just crossed my mind. A chopper will apparantly only draw about 2/3 of the rated current of the motor. Is this (I wonder) because as we know it's applying that current to the motor in pulses, it's only actually applying it for 2/3 of the time?

I cant nsupply the answer yet, becasue I've been away and only got back tonight, Went straight down to the post office and picked up my Gecko drive - looks good. Will try and fit it tonight.

As far as power in a motor is concerned Watts is the measurement. Watts = Amps x Volts. The only trouble is you need to increase voltage to increase the amperage becasue Voltage = Resistance x Amps - and for the sake of the argument resistance of the winding is constant ( in this case at 3 ohms - given in the rating of 2.5amps at 7.5volts).

There are, we have found, several different ways to connect steppers ( the four coil variety).

1.) In my first effort - with two windings in series - I was effectively putting 6 ohms of resistance across my voltage. Using the 7.5 volt rating as a rough guide, then this would mean that the two coils would draw 1.25 amps. Again using 7.5 volts this would give 9.375 watts x 2 = 18.75watts absolute maximum.

I am not aware what effect the chopping of the applied voltage (24v) has on the final voltage at the motor. It must be similar to the PWM output and vary with the number of pulses per sec opposed to the carrier frequency. Since the carrier frequency is usuall well above the maximum chopping frequency, then there must be quite a cut in apparent voltage at the motor - hence I will use 7.5 volts in all the examples. (It does - however - increase with speed)

2.) In the ONLY WIRING TWO COILS ON THE MOTOR scenario - each coil is 3 ohms resistance - and draws 2.5 amps (the rating for the drive) but bear in mind there is only one coil operating - this give 18.75 watts - BUT THE EFFECTIVE VOLTAGE INCREASES AS THE SPEED INCREASES - THEREFORE THE APPARENT POWER OUT INCREASES. To me it seems that twice the speed (of pulses) would give twice the apparent voltage right up to the point where current limting came into play. It seems to me also, that the current in the coil would rise much more quickly than in No.1 and therefore be higher longer and give more power earlier - perhaps helping it get over the limits of the series wound option.

3.) When I connected the motors with two coils in parrallel, the effective resistance Rt = R1 x R2 / R1 + R2 = 9/6 = 1.5ohms. This has the potential to draw 5 amps - and the current limiting would be working over time to keep it down to 2.5 amps - BUT there are TWO windings working on each pulse therefore twice the power - so although there was only 2.5 amps to share between the coils, I think it was sucking a bit more out and then with twice the number of coils, giving a little over that at No.2. Again because resistance was less, current would rise more quickly giving more power per pulse.

It certainly had a bit more Omph.

I will wire the Gecko up and try it wired to give the full five amps to a motor connected in parrallel and let you know the result.

 

Sorry Jim but there is waaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaay more to it than you've suggested. You're talking pure Ohms Law which is wholly inapropriate other than to work out the static ratings for a motor. Once that motor moves you're into a whole different ball game.

Can I suggest you re-read Ian and my posts because we've moved a lot further on since you were last here.

Thanks

Ian

Jim,

Like Ian Stirling said, you are not quite there with your understanding of what's happening when you apply a voltage to a stepper coil. It is as if the coil had the mechanical equivalent of inertia, you apply big volts to get it to move, but you need the amps quicker to get power. Yes, two coils in parallel have less resistance, but they also have less inductance and therefore the current rise time to maximum is shorter, so within a given pulse, maximum current is on for longer.

Ian (Who lives in hope that one day he might reach a reasonable plateau of understanding!)

I will wire the Gecko up and try it wired to give the full five amps to a motor connected in parrallel and let you know the result.

Jim - if you do this you will be overloading your motor current wise which is a really bad idea. You run the risk of significantly shortening its life. The rated current of your motor in parallel is 3.5Amps - not 5Amps. Putting figures to it, the coils will heat up to just over twice their rated value. (This is not to be confused with acceptable motor heating which is the result of overdriving VOLTAGE wise. I know it's confusing but the two are entirely different.)

the rated parallel current of your motor which is 3.5 Amps.

Ian and Ian - Yes I know I was missing out a great deal about inductance, and current lag and all the other things I've read about. I would understand it if I spent enough time reading and studying it - but I haven't the time.

We are, though, near the end. The Gecko works a treat and I have it up to 30ins per minutes, which from a start of 4" in quite remarkable.
Ian or Ian - sorry are you saying from coils in parrallel I should settle at 3.5amp - not 5amp. I will keep checking the motor - it was not getting warm (although it was only on test - and not working). I can drop the rating on the Gecko.

Certainly where I went wrong was - when the motors said they were rated at 2.5amp at 7.5 volts - per coil - I then opted for 2.5amp drives. There are four coils of course - two pairs - and it follows that you need more current capability.

I have done all this by merely altering the motor wiring and upping the current capability of the system - I have the voltage side to go at yet - although that would mean Gecko's right through, rather than just the one I have at the moment.

I better stay as I am for the moment (otherwise the financial director will complain) - I have the two motors using only 2 coils on the y and z - using their original drives upped to the Max at 2.5amps. The X (or Z on the lathe) with the four motor coils in two pairs, powered by the Gecko.

Either way we have a big improvement in performance !!! Y and Z at 16 ins per min and X at 30. I am settling for 14,12 and 28 for accuracy and see how it will go.