I thought I knew - now I'm not so sure...

[stirling]

Hi - searched the web and the forum - but can't find a specific answer to my question.

8 wire motor, bi-polar driver. Each coil 2 ohms.

So for an example voltage of 10V for the sake of the principal, then wiring...

single-coil wiring = 5A/phase
serial wiring = 2.5A/phase
parallel wiring = 10A/phase

Now - suppose I'm using a driver that can source a max current of 7.5 Amps.

Question: Discounting serial-wiring which is clearly a loser, which of the other two should I use?

The thing is, I first thought - well single coil obviously because parallel is over my driver's limit. Then I thought - hang on - bi-polar drivers are current sensing/limiting drivers arn't they? - if I set the drive to limit current at its max of 7.5 Amps then surely that's the way to go.

Then I thought - do all bi-polar drives do this or is it just chopper drives?

Confused...

[ger21]

Pretty much all drives let you set the current for the motors. You do this yourself, either with a pot on the board, or, in the case of gecko drives, connecting a resistor to the drive. The value of the resistor sets the current, based on a formula they provide.

The serial current for an 8 wire motor is half the parallel current, so that should be 5 amps.

Serial wiring will give you the full current, but high end speed will suffer. Parallel will onle give you 75% of the rated torque at low speed, but may actually have more torque at higher speeds. You really need to see a torque curve for the specific motor to see what you'll get. If you're application requires low rpm's, then serial should work just fine. If you need high rpm's then parallel may work better.

Not sure about single coil.

[jeffs555]

Then I thought - hang on - bi-polar drivers are current sensing/limiting drivers arn't they? - if I set the drive to limit current at its max of 7.5 Amps then surely that's the way to go. Then I thought - do all bi-polar drives do this or is it just chopper drives?

A few of the cheaper bipolar drives have no current setting and need external power resistors to limit current. It is usually best to avoid these drives. If your drive has a current adjustment, it would usually be best to run your motors parallel even if you have to run at less than the rated parallel current.

For an 8 wire motor with a parallel driver, there is no advantage to using single coil. Parallel will have the same inductance as single coil, but lower resistance. For the same current, parallel will run the same speed, have the same torque and run cooler than single coil.

Gerry is right that the serial current limit is normally half of the parallel current limit, so your current numbers just don't look right. For most motors, the serial current is 0.7 times the single coil(ie unipolar) current, and the parallel current is 1.4 times the single coil current.

[pminmo]

I would only add to Jeff and Gerry's summary in that it's all about the complete system, not just a simple if then else....

Stepper motors naturally loose power as you go up in rpm's, they are at their best when running low rpm's. So the application becomes a system question. Assuming you are going to convert rotary power to linear power, by what method. Here is some information to help in the thinking process: http://pminmo.com/PMinMOwiki/index.p...chanical_Power

So the relavance of your question becomes, where do you need your power? If it's in the low rpm region, then the question between series wiring and parallel wiring is unimportant. If the need is to extend the power band of the motor as rpm's increase then it's very important as coil inductance and coil voltage become a factor.

If your system is already set, and your drivers can handle the parallel current of the motor, then parallel is the best power producer through the entire band. The only downside of parallel, is the extra resources required to drive the motor. i.e. more capable driver (current) and power supply size increase.

[stirling]

Originally Posted by jeffs555 A few of the cheaper bipolar drives have no current setting and need external power resistors to limit current. It is usually best to avoid these drives. If your drive has a current adjustment, it would usually be best to run your motors parallel even if you have to run at less than the rated parallel current. For an 8 wire motor with a parallel driver, there is no advantage to using single coil. Parallel will have the same inductance as single coil, but lower resistance. For the same current, parallel will run the same speed, have the same torque and run cooler than single coil.

Thanks Jeff - this is what I was after. I've always used geckos and 4 wire motors so I didn't know whether ALL drives have explicit current limiting adjustment. Also I've mainly used 4 wire motors so with 8 wire, I didn't know whether it was better to run single coil at the rated current or parallel at "under current" if that's all (some other) driver could give. I'll allways use geckos or similar and 4 wire motors if I have the choice but this was a question that "came up in discussion".

That said - I'd like to check my sums which a couple of you have questioned. In my arbitrary example I'm using 10V as the rated voltage and coils with a rated resistance of 2 Ohms. Remember these are RATED value calculations to figure out the MAX RATED current for the coil. I'm not talking about the current flowing in a dynamic system - though to be honest - even if I was, I don't understand some of your figures and statements.

Clearly (or maybe not) the rated current for a single coil is I=V/R which is 5 Amps

For two coils in series we have I=V/(R+R) or I=10/(2+2) = 2.5 Amps

and finally for parallel we have Rp=R1*R2/(R1+R2) which, where R1=R2 (both coils have same resistance - hopefully) reduces to Rp=R^2/2*R which for two 2 Ohm resistances = 1 Ohm.

rated current I=V/R therefore I=10 Amps in parallel.

So you'll understand why I'm completely confused with your statements that...

"serial current is half parallel current"
and
"serial current is 0.7 times the single coil(ie unipolar) current, and the parallel current is 1.4 times the single coil current."

Thanks

Ian

[ger21]

Don't know exactly how to explain it, but the voltage ratings for series is double that of parallel - you're using 10V for both examples.

[pminmo]

Jeff is correct, series current is .707 * the single coil current. Take a look at Vextas site, or Kelinginc site at motors and there ratings. You will find that all are rated this way. It's not as simple as ohms law.

[jeffs555]

The current ratings are usually set so that the motor does not get too hot when static with a constant current applied. The static heating comes from the DC power disipated in the resistance of the coils. Resistance of parallel connection is half of the single coil resistance. Resistance of series connection is twice the single coil resistance. DC power is equal to the current squared times the resistance. If you double the resistance, you need to divide the current by the square root of 2 to keep the power(ie heat) the same. Likewise, if you half the resistance, you can increase the current by the square root of 2.

PS Your only error was assuming the voltage rating is the same for all connections. It also changes and serial is 1.414 times the single coil rating and parallel is .707 times the single coil rating.

[stirling]

OK guys - first off, thanks for your patience. I think I now understood what's going on.

I now see that my original question was at best not very clear and at worst nonsense and apart from anything else I'd failed to give you any rated phase voltages/currents for my theoretical motor.

What I was attempting to do with all my calculations was give the currents through each configuration for an example PS Voltage of 10V where the driver doesn't have current limiting. I picked 10V just as some arbitrary "over voltage" in an attempt to model what's usually done in a real world app.

I realize now (I think) that this was probably doomed from the start because all drivers must have some form of current limiting otherwise how would you avoid exceeding the motor's current ratings, the drive's current rating or both. I guess it would be a nightmare. So working out as I did (and I believe correctly) the potential static phase currents for each of the three configurations at a PS of 10V wasn't a lot of use.

Being used to Geckos I've allways taken their method of setting the current limit for granted but just wondered how you go about configuring a system when you have less sophisticated drivers. And that's probably the question I should have asked in the first place - sorry.

Just a quicky for Phil: Studying the motor ratings you pointed me at made things click - so thanks for that. Have a second look though Phil cos, as far as the ratings figures go, for every motor I looked at, Ohms law holds perfectly. Which for a static system is surely what we'd expect.

PS - Jeff - you just posted whilst I was composing this and clearly you've taken time in figuring out how I got my figures so thanks for that - I appreciate your time. I just noticed I've said "10V as the rated voltage" in at least one post and of course what I meant was "PS Voltage".

PPS I also just noticed I've stated R^2/2R a couple of times - of course this reduces to R/2 - aaaahhh - what a muppet.

PPPS - Believe it or not (discounting motors and drivers it would seem) I'm not bad at electronics theory generally - LOL.

Thanks again to all.

Ian

[pminmo]

Where ohms law isn't applicable and doesn't give you the value you are looking for is bipolar series.

[stirling]

Originally Posted by pminmo Where ohms law isn't applicable and doesn't give you the value you are looking for is bipolar series.

That's just great! - I think I may well go quite insane

Moving on (if I dare)...

Here's a quote from the stepper doc on gecko regarding the current requirements of a PS. (hope they don't mind me quoting)

"The current rating of the supply is based on your motor choice. The drive will always draw less than 2/3 of the motor’s rated current when it is parallel (or half-winding) connected and 1/3 of the motor’s rated current when it is series (or full-winding) connected. That is to say, a 6 Amp / phase motor will require a 4 Amp rated supply when parallel connected and a 2 Amp rated supply when series connected."

Is this just badly worded? or have I missed the point - again?

Seems to me that everything you've told me so far means that a parallel connected motor with a phase current of 6 amps will have a rated phase current of 3 amps when connected in series . So if (according to the above) in parallel I need 4 amps and 2 amps if wired in series - then it's ALLWAYS 2/3 of the rated phase current regardless of how it's wired.

[ger21]

Re-reading it, I believe it is badly worded. Series requirements would be 1/3 of the parallel rating, 2/3 of series rating.