Bi-polar/Uni-polar motor P.S amp selection

[sahilkit]

Hello everyone,

I'm building a cnc machine and have STK672-080 based drives ( PEAK 3.5amp and 3amp with heat-sink @ 45volts). I have gone through the forum and found various tips, documents, links.My favorite sites are

CNC Router Source: The Ultimate Information Resource

Stepper Motors

Support

Power Supply Tips

Now, from the above sites i was able to get most of the info, but i'm still stuck at the power supply amp rating and i need some clarification regarding the amperage values used in the formulas ? is it per phase current or 2-phase current value when one talks about motor current and does Bi-polar/Uni-polar motors have an effect on amp selection ?

example: suppose one is using single unipolar stepper motor rated at 3 amps per phase current and needs to find the power supply amp rating

will it be 2/3 or 1/3 or 0.7 time the motor current and will it be per phase current or 2 phase current = 3*2/3 or 3*2*2/3 or 3*2*.7 or 3*0.7 ?

sorry if this has been asked already, i could not under stand which one is correct ?

Thanks

Sahill

[H500]

example: suppose one is using single unipolar stepper motor rated at 3 amps per phase current and needs to find the power supply amp rating

Using Geckodrive's recommendations, your motor current would be 2/3 *3amp = 2 amps.

Keep in mind these are guidelines, not requirements. The actual power will depend on your system. If it's lightly loaded, you might be able to get away with far less. I've been using a single 2 amp supply to drive two 3 amp motors for the past month with no issues on my development system.

[sahilkit]

thanks H500, one more interesting link

AMCI : Tech Tutorials : Stepper Motor Drivers - RMS vs Peak Current

[James Newton]

Not sure if you've seen this page:
techref.massmind.org/techref/io/stepper/power.htm

In addition to the motor type and specs, the type of driver, the way the motor is wired to the driver, and the mode in which the driver is being operated, all affect the current requirement.

In general, double is a safe bet.

[sahilkit]

thanks for the link James, today i hocked up the motor and tested with Mach3, drive was set to 2.867amps/phase, drive input voltage 27.5volts and in 1/4th or 800 steps mode.(P.S is rated for 27.5V@3A max.). The motor is of SOYO make and SY85STH80-5004A original a Bi-polar/parallel winding stepper weights 2.3kg was modified to unipolar 2 phase/6wire by a local stepper motor manufacturer.Spec. 4amps, 1.2ohms, 3.4mH, 4Nm torque.

From Mach3 help/PDF file i calculated that i need 4000steps per inch and 180IPM rapid, in input these values, adjust the acc./dec. value for instant direction change and measured motor rpm, it was above 900rpm with a 5TPI screw = 180IPM ! although this was no-load figures.

Am i going in the right direction ?

Thanks

Sahil

[sahilkit]

pictures

[H500]

You should set it to 16 microsteps. It will run much smoother.

[sahilkit]

well, i can use 1/16th mode but the drive gets there by moving the phases on both the rising and falling edges of the CLK signal for a total of two steps per cycle (which is not true micro stepping) and when in this mode the direction input must not be changed for about 6.25uS before or after a rising or falling edge on the CLK input. But, the mach3 can put out a max of 5uS width pulses?.

Anyway i will be using EMC2, so i a way this is a good thing (because i need to worry less about jitter,latency etc. to limit my system/machine speed!).

sahil

[sahilkit]

update, i have observed that motor gets little hot after 35min of testing/running so used a LM335 temp.-sensor/voltmeter i had and measured the temp. after running the motor for 1hour20min in 1/4 mode, 2.8amps, 28volts continuous and got 50Deg C temperature, is this normal ? i thought the motor would heat up less because i'm only providing 2.8amps to the rated 4amps ! even voltage is only 48% the rated maximum (59volts for 3.4mH) !

sahil

[H500]

I'm using 16 microsteps with mach on my sx3 mill. Mach has no trouble moving it at 200 ipm with a 5 turn/in ballscrew.

Resonance will probably be what limit your speed. It will be much worse without microstepping.

50C is not particularly hot. The motors are usually rated for 85C or so. Mine get almost too hot to touch at 4 amps. However, they don't get nearly that hot when mounted on my mill, which acts as a heat sink.

[RomanLini]

Yes the motor should be bolted to a large metal mass, especially if it is running at high speeds.

If your motor is specced at 4A per phase you need a microstepping driver capable of at least 4A per phase.

I think your STK chip is a little underpowered to get full performance out of that motor. If you don't mind the motor running at only 70% of its torque then the 3A STK chip is probably fine.

[sahilkit]

Actually i calculated that i need 2.315Nm torque from the motor (1.5x safety) at 180IPM (again 1.5x safety) rapids with a linear force of 515.6lb (1.5x safety) these values are the max with 1.5x safety factor my x/y/z tables can take.

But it looks like the motor will not produce 2.315Nm at shaft at 180IPM (rapids) due to losses and under-power or do i need that kind of linear force at 180ipm rapids . Right now i'm re-doing the calculations based on spindle speed, tooling, chip removal rate, feed rate etc and seeing that the spindle power of 0.75kw is not exceeded

sahil