Re: My stepper getting hotter

John,

Your request prompted me to do "science" since yesterday. I have been
gathering data and will post the results. I had suspected the
new "square" steppers are different than the round ones, I just
didn't know how much different they are (and better).

Mariss

--- In CAD_CAM_EDM_DRO@y..., "jmkasunich" <jmkasunich@y...> wrote:
> --- In CAD_CAM_EDM_DRO@y..., "mariss92705" <mariss92705@y...> wrote:
>
> > Step motors get hot for two reasons, copper losses and iron
> > losses. Copper losses are due to current passing thru a
> > resistance, (I^2times R) and are independent of power supply
> > voltage in switch-mode drive.
> >
> > Iron losses are primarily eddy current heating of the motor
> > laminations. These losses go up with the square of the power
> > supply voltage. Inductive reactance currents are proportional
> > to the supply voltage and eddy current losses are proportional
> > to the square of this AC current.
> >
> > Iron losses range from inconsequential at low voltages to being
> > the majority cause of heat at high supply voltages.
> >
> > Mariss
> >
>
> Mariss:
>
> Can you elaborate a little about copper vs. iron losses and how
> they are affected by voltage, switching freq., etc.? I know
> quite a bit about how those losses behave in induction motors
> at high hp, but very little about fractional hp steppers.
>
> For example, step frequency current causes losses when moving.
> The current is large, but frequency is small compared to the
> PWM carrier frequency. Carrier frequency ripple current causes
> eddy losses all the time. Are carrier frequency losses usually
> greater or less than step frequency losses?
>
> How much carrier frequency ripple is there? On our VFDs, the
> carrier is 4KHz, and the fundamental (corresponds to step
> frequency for steppers) is 60Hz. The carrier frequency
> ripple current is usually less than 10% of the 60Hz current.
> Does a similar statement hold true for steppers on Gecko power?
> Or is the ripple lower, due to much higher carrier frequency?
> BTW, what is the Gecko carrier frequency?
>
> At rest, heating is due to only to IsqR and carrier frequency
> eddy current. Which dominates, or are they somewhat balanced?
> I^R depends on the idle current, eddy current depends on DC supply
> voltage. (And carrier frequency, which I assume is fixed.)
>
> At speed, I would expect carrier frequency eddy losses to
> remain constant. IsqR losses would drop once the current
> begins to drop (voltage limited), and IsqR heating would
> also be distributed over all the windings. (Skin effect
> would increase R - does skin effect show up in steppers?)
> On the other hand, step frequency eddy losses will keep
> rising with speed. Do they eventually become the dominant
> heat source, or does the motor run out of torque before
> you reach that speed?
>
> Thanks for your insights!
>
> John Kasunich