Re: What is Rms?

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Jon Elson <elson@p...> wrote:

>
> turbulatordude wrote:
>
> >I was under the impression that RMS only applied to sine wave type
> >power (or half wave) but not to square wave. After all, isn't duty
> >cycle used to calculate current ?
> >
> >
> >
> No, RMS can apply to any waveform. Ordinary voltmeters are corrected
> for sine waves, only, and will give incorrect AC readings on

anything other

> than a sine wave. Duty cycle can be used simply to calculate the

average

> voltage or current. But, if a square wave voltage is applied to a

resistive

> device, the power delivered will not match the duty cycle * voltage.

That

> is because power is volts X amps. When the voltage goes up, the current
> ALSO goes up, by the same factor. Therefore, a change in voltage causes
> a change in power equal to the SQUARE of the change in voltage.

This is the

> whole key to the thing.
>
> So, if you were to apply 120 V DC to a 120 Ohm resistor, it would draw
> one amp, and dissipate 120 Watts. If you applied 240 V 50% of the time,
> it would draw 2 A, and dissipate 480 W during the on time, and of course
> zero during the off time. The average power dissipation would be 240 W
> (480 / 2). So, although the average VOLTAGE is the same 240/2 = 120,
> the average POWER is twice as large, in this example.
>
> Jon

I assume then that this is the basic idea behind using a higher
voltage on steppers.

The higher voltage can deliver power faster, so the motor has more
'snap' OF course, there are other circuits that regulate the power.

AND.... THIS is why one does not take that 5 watt stepper, and
calculate 5 watts at a 20x voltage for the power supply.

Dave