Re: [CAD_CAM_EDM_DRO] Re: Question for Kim Lux

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From: Kim Lux <lux@...>
Date sent: 05 Jan 2004 13:28:29 -0700
Subject: Re: [CAD_CAM_EDM_DRO] Re: Question for
Kim Lux
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Kim

I suspect the problem is a freq. prob. Increase the freq. to increase
the rpm. Just my humble opinion.

Bob McKnight
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On Mon, 2004-01-05 at 12:02, Robin Szemeti wrote:
> On Monday 05 January 2004 16:02, Kim Lux wrote:
> > I'd like to "over voltage" the motors so that they can run at a
> > higher speed. These motors have significant armature resistance
> > and with a 40V power supply we run out of supply voltage long
> > before we are anywhere near the peak current of the motors.
>
> wrong solution, infact you still haven't stated exactly what
> 'problem' you are trying to address

You seem to be a pretty knowledgeable individual considering you
don't
know what the "problem" is !

I stated that I would like the motors to run faster than they do
presently.

> if at max rated voltage the motors are not providing the speed you
> need out of your machine and are pulling well below max rated
> current, then the answer is to lower the gearing. Over-volting your
> motors won't 'solve' anyhting.

The motors are already direct driving the screws. The issue is that
the motors have more torque than they really need and also run slower
than I would like.

> If you are already achieving the speeds you need, but feel that the
> max current capabilities of your motors are never being used, then
> you have simply over-specced your motors.

Or not... The Amatek motors that I am running don't have hard specs...
Kv is 1800 RPM for 40V input = 22ishV/1000RPM Max current is
stated
as 10A. I suspect that the 40V was arrived at as being the max
voltage for which it was impossible to exceed an armature current of
10A, ie I suspect that the armature resistance is about 4 ohms. Using
the motor in conjunction with a current controlled voltage source (ie
a Gecko 320) allows the motor to be operated at a higher voltage
without inducing any damage because:

a) the controller (G320) will change the pulse width to achieve the
desired motor speed. They are rated for 1800 RPM and I'd like to see
2500 RPM or so.

b) the controller will limit the current according to the setting on
the current limiting pot.

> When correctly geared and appropriatly loaded, you will find that
> you can pull maximum rated current at or below the rated voltage,
> trying to correct you miscalculation of gearing by running the
> motors out of spec will not benefit anyone, except perhaps the motor
> salesmen. Remember, two wrongs do not make a right.

Where do you get your knowledge from ? Provided the windings don't
fly apart, operating the motor at a higher voltage isn't going to harm
anything.

Motor speed = Varm/Kv
= (Vterm - Iarm*Rarm)/Kv

Lets say that the motor is pulling 5 amps when running at the fastest
speed it will allow: Varm = 40V - 5A*4ohm = 20V. At this current the
motor will run only 900 RPM.

If I increase the power supply voltage to 60V, I can run a lot faster
because Varm will be higher.

I've never seen these motors get hot, or even warm. Increasing the
supply voltage isn't going to make the motors run warmer at the
current speeds because the controller will still be providing the same
current/voltage to them. They *MIGHT* run warm/hot at higher loads
(ie higher speeds), but I'll find that out when I actually run them
fast.


--
Kim Lux <lux@...>


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