2.3v 3A/Phase steppers running at 35v, what amperage?

[guru_florida]

I read the FAQ but not totally clear to me. I even have electronics experience but I want to make sure I have my calc correct?

2.3v 3A/Phase 6-pin unipolar motors that I am connecting as bipolar 4pin motors.
3A/Phase = 6.9Watts , in bipolar mode with two phases in series, wattage halfs as resistance is doubled, but two phases may be active at a time, so total stepper max wattage is 6.9Watts

Amps=Watts/Voltage
My power supply will be 25 or 35v.
@25v, my steppers will draw max .276amps
@35v, max draw is .197amps

For four steppers I will require a power supply that can comfortable serve 28 watts.

Is this correct? It seems low. I assume a unipolar stepper has 4 phases, maybe the center tap is not really included so then my talk of resistance halved would be wrong.

The FAQ talked about the stepper taking more voltage when required, but my electronics says that voltage would be constant and amps would change proportional to mechanical load.

Thanks,
Colin

[pminmo]

Your 2.3V/3A per phase steppers are roughly 14W. Assuming you are using a pwm or chopped bipolar driver, you need 14W x number of motors minimum for your power supply depending on how you wire the motors. If you run them half coil then it's 14W, if you run them series it's more like 10.5W. That gets you in the ballpark. So your talking 56W or 42W, BUT your supply has to be able to deliver much higer currents than simply 56W or 42W at 35V. There will be instantaneous peak current demands at coil current per phase * 2 times the number of motors for durations in microseconds. In practicality though, they won't be simultaneous demands and bulk capacitance can handle most of the peaks. Bottom line is the supply needs to have enough bulk capacitance to handle load fluctuations.

[guru_florida]

Thanks, that clears things up. I anticipated some large caps on the supply, as well as some lower mf caps for quick current demands until the large caps can supply the load. I am looking a t a 100W transformer, so I am well above the limits. I am running the motors in series, as in the center wire is free floating. My driver is a pwm controlled. It has current limiting too, chopped I imagine. Currently, my motors dont generate any heat AT ALL, so I am thinking i am running them too low current. I can probably get faster movements by upping the current, going from the current 18v to 30-35 will do it, but I still have room left on the current pot too.

Thanks for the reply.

Colin

[guru_florida]

Originally Posted by pminmo If you run them half coil then it's 14W, if you run them series it's more like 10.5W.

Should I run them in series or parallel? Operationally, whats the difference. If I run in series do I get more mag flux, thus more strength, in series? I would assume so especially if I up the avg voltage more to increase the wattage.

[pminmo]

If your drives can handle the parallel current, then that is the strongest as you go up the rpm curve. If you have small distances where rapdis speed isn't an issue, or screw ratios that you don't need higher rpms then series is just as acceptable. Half coil when you have higher coil voltages, driver current can't handle parallel and you need higher rpms.

[guru_florida]

Thanks for the help! I will switch to parallel coil, and get faster rapids. My driver can handle the current.

Thanks,
Colin

[pminmo]

Originally Posted by guru_florida Thanks for the help! I will switch to parallel coil, and get faster rapids. My driver can handle the current. Thanks, Colin

Excellent. FYI, the reason is based in the fact that parallel has lower inductance, and the resultant ratio of coil voltage to supply voltage has increased. The result is you have improved the resonse time of coil current changes.