Re: Stepper current in partial-step positions

Hi Mariss,

Nice "aside" calcs! I plan on trying them. And great half/step
comments!

I tried running MaxNC's 4-phase (normally runs 1/2 step) at FULL step,
and boy did that make a difference! :>(

I've heard of two sequences for half step, how many other sequences are
there, Assuming simple on/off control for the phases?

I'd also like to find data on the 5-phase sequences, probably some good
motors out there! :>)

Thanks again for your comments.

Alan KM6VV


mariss92705@... wrote:

>
> Hi,
>
> Actually both coils carry the motor's rated current in full-step
> operation. You can think of it as a single-bit approximation of a
> sine and cosine. Microstepping is just a progressively finer
> approximation.
>
> Motor heat dissipation is independent of partial step angle (sine
> squared + cosine squared = 1). The exception is a common half-step
> technique where both windings, then one winding carries current.
>
> Based on power dissipation, one would assume the current can be set
> to 1.41 times the unipolar rating when microstepping. Doing this can
> have unwanted effects on motor smoothness however. The motor windings
> do not share a common flux path; you can prove that by trying to use
> the motor as a transformer. This means the iron may saturate at 1.41
> times rated current and cause the motor to become non-linear.
>
> I recommend using the motor's unipolar current rating for the
> microstepping peak winding current for best all around performance.
> High speed torque is not affected by the set current because at high
> speeds inductive reactance limits motor current, not the drive.
>
> Power supply current (and watts delivered) is a combination of motor
> heating and mechanical power generated. This means power supply
> current increases as applied motor load increases.
>
> As an interesting aside, you can calculate the motor's torque at any
> speed using just a multimeter. Here's how:
>
> 1) Measure power supply voltage.
> 2) Set multimeter to DC Amps to measure power supply current.
> 3) Accelerate motor to the speed you want to measure torque at.
> 4) Measure the power supply current at no motor load.
> 5) Gradually load the motor while monitoring current.
> 6) Note the supply current at the moment the motor stalls.
> 7) Subtract (4) from (6), then multiply with (1).
> 8) Multiply (7) by 4506, then divide result by (3) full steps/sec.
> 9) The answer will be in in-oz
>
> in-oz = (4506 * V supply * (I stall - I noload)) / (full-steps/sec)
>
> Mariss