Re: What is Rms?

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "robertokx" <robert.okx@s...>
wrote:

>
> Hello,
>
> I was hoping someone could explain what RMS means in simple terms.
>
> I have a driver with the following Specs:
> Output: 5A Rms Max.
> Motor : 5A Peak Full-Step
> Phase: 7.1A Peak Microstepping.
>
> I have a motor with a documented Phase Current of 6A.
>
> Does it mean that I'll be able to run it OK Microstepping given its
> within the 7.1A peak however won't be able to run it at Full-Step?
>
> Gecko drives and other bipolar drives on the market appear to just
> quote the microstepping peak phase rather than mentioning Rms.
>
> Thanks in advance.
>
> Robert

I'll try to give it a go without any technical verbiage.

RMS usually is on Alternating Current. I am sure you have seen the
ocean wave type pattern that is used to describe AC. google RMS
WAVEFORM and you'll get 483,000 hits. Click on IMAGES above the
search boax and you'll get 85 pictures.

Anyway, AC alternates, goes high and then low.

Since it spends some time at the peak and some time zero crossing and
some time, well, suffice it to say there is no way to measure a
snapshot of the AC at any instant and say that is what it is.

So, there has to be a way to get an average.

OK, so we do need math. but that math allows us to determine how much
voltage is available or very loosly speaking the average of the peaks
and valleys.

the voltage meters are designed so they measure the 'average' as
determined by the RMS.

The reason capacitors charge up to 1.414 times the AC voltage is that
when you change the AC to a DC voltage, you get the same ripple, but
the capacitor acts like a little storage tank. it fills to the
average (RMS) pretty quickly, and then the peaks of the AC keep
nudging the stored voltage higher and higher until that voltage equals
the peak itself.

But, your post says 5A RMS so you are really talking CURRENT and not
voltage.

but, with an alternating voltage, doesn't the current jump around too ?

Well, yest it does. still playing fast and loose with the terms,
there are generally two types of loads. one is continucous like a
resistance, a resistance heater or a light bulb. the other is a
varying load like a motor. As the motor spins, it switches the power
to the coils very quickly. A switching power supply is another type
of load that takes slices of power.

There are two types of power, the actual power and the apparent power.
if they are equal, like a light bulb, they are in unity, or there is
zero difference.

if they are not equal, then the power lines have to give tiny bursts
of power. the problem is that your 5 amps rms may get 7 amp pulses.
That means the wires have to be larger to handle the pulses. If you
had a manufacturing plant with thousands of hp of motors your bursts
may be higher than expected and you might need bigger lines.

So, to deliver the higher amps, everything has to be rated for that
current.

Now, it will take someone much smarter than I to explain why you have
3 amp listings and why you have an RMS on what I assume would be a DC
current.

My total guess would be that all you need look at is the motor amps
for the step rate you want. full or microstepping.

hope that is of any help.

Dave