Re: Mariss' power supply circuit

Hi,

My assumption has been this: A motor has a stall (locked armature)
current and torque rating. The current is equal to the motor's rated
voltage divided by its armature resisistance; the torque is equal to
the motor's torque constant multiplied by that current.

The motor will momentarily draw that rated stall current as it
accelerates from rest when the rated voltage is abruptly applied.
That is exactly identical to shorting a motor running at its rated no-
load speed. Both currents will be identical and within ratings.

Mariss

--- In CAD_CAM_EDM_DRO@y..., Jon Elson <elson@p...> wrote:
> Doug Fortune wrote:
>
> > "John H. Berg" wrote:
> > >
> > > I've been reading these comments on braking techniques for
servos/steppers,
> > > and though I don't think any of the conversations to date have
reached
> > > sufficient current levels, one needs to watch the peak current
level in
> > > permanent magnet motors so as to not exceed the demaginazation
current
> > > level. This is the point at which the magnetic field in the
motor becomes so
> > > high that it can destroy/demagnatize even rare earth magnets.
This problem
> > > is most common in emergency stop situations where a resistor or
short is
> > > used to brake a motor.
> > >
> >
> > Mariss:
> >
> > Speaking of current levels in DC Servo motors, the ones I have
are rated
> > 90VDC at 7.6 amps continuous (giving 300 oz*in at up to 2500
rpm). Of
> > course I only run them at 75 VDC on my power supplies.
> >
> > However, they can surge for brief periods (5% duty cycle) to many
times
> > that amperage. I presume the limit is mostly heat-dissipation
related.
>
> No, if they are permanent magnet, a momentary peak can 'erase' the
field
> magnets, runining the motor. You have to watch that limit
carefully.
>
> > Should I set the G340 at 20 amps (so the full 20 amps is
available for
> > infrequent surges), or should I set the G340 closer to 7.6 amps
(perhaps
> > you have made provisions for brief surges over the maximum set on
the
> > potentiometer).
>
> You probably want to set it above 7.6 A, but you need the
manufacturer's
> specs on the peak current to know what it is. It is often about 4
x the cont.
> rating, but it varies depending on the motor's design, magnet type,
etc.
> (A forced-air cooled motor may have a much higher continuous
rating, so
> the peak would not be as high as for a free-air cooled motor.)
This peak
> would be used mostly for accelerating the table, so if you need fast
> acceleration to high rapid feedrates, then you need more peak torque
> (and current) than if you won't be using such accelerations.
>
> Jon