Re: Gecko and Anaheim

Hi, The question is what is too hot regarding a step motor. Usually
the maximum rated case temperature will be 85C or 185F. At this
temperature skin blistering (2nd degree burn)will occur if contact is
maintained for more than 5 sec. To my mind this is too hot.

This heating is caused by eddy current and hysterisis losses in the
motor. For those who are interested try this:

Chuck up a step motor in a drill press. Adjust the belts for highest
RPM. Leave the motor leads unconnected. Clamp the motor in a vise and
turn on the drill press. Come back in 5 minutes and see if you can
touch the motor. My prediction is it will be too hot to touch.

The heat is caused by the iron magnetic domains flipping back and
forth under the influence of the rotor magnet. This motion causes
friction and results in heat. This is a gross oversimplification but
it helps visualize what happens.

A 4 volt motor operated at 80VDC will be below this level. The
overdrive ratio is 20:1 (80VDC/4VDC). Above that ratio the temp will
reach or exceed 85C. The ultimate result will be molten nylon winding
bobbins, degradation and embrittlement of winding insulation
resulting in inter-winding shorting. I.E. a destroyed motor.

Don't confuse what happens when a 4VDC motor is being run from an
80VDC supply. The drive is a current source. It will only deliver
what voltage is necessary to maintain the motor's rated current. At
low speeds it will be 4VDC, at higher speeds the voltage will be much
higher. The rated current however will never be exceeded.

Mariss

--- In CAD_CAM_EDM_DRO@egroups.com, john@m... wrote:
> Sorry to butt in on this thread but Mariss's original reply is
copied
> below.
> Mariss states
> >" To get the maximum power from a 7A, 80V drive, pick a 7 Amp, 4
> >volt motor. "
>
> Surely a 4V motor running on 80v supply will burn out.
> Am I missing something here ?
>
> > ----- Original Message -----
> > From: Mariss Freimanis <geckohall@h...>
> > To: <CAD_CAM_EDM_DRO@egroups.com>
> > Sent: Monday, June 12, 2000 6:14 PM
> > Subject: [CAD_CAM_EDM_DRO] Re: Gecko and Anaheim
> > > Now to bring this all together. Size 42 motors I'm sure you have
> > > noticed have much higher detent torque than size 34 or 23
motors.
> > > This is the reason why size 42 motors run out of steam before
the
> > > smaller motors do. Second, for the same amperage, a size 42
motor
> > has
> > > much higher inductance and rated voltage than a 34 motor. That
> means
> > > they come into their own at much higher supply voltages; 160 to
> 200
> > > VDC. To get the maximum power from a 7A, 80V drive, pick a 7
Amp,
> 4
> > > volt motor. This will be either a 4 stack 34 or a single stack
42.
> > > For peak power, gear the motor to run just past the speed where
> the
> > > drive ceases chopping (i.e. current limiting).
> > >