Re: My stepper getting hotter

--- 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