Re: [CAD_CAM_EDM_DRO] Re: Mariss' power supply circuit

Yes that sounds about right. I figure peak current is determined
by either armature saturation or magnet coercivity and has a safety
factor. I don't think it's thermal because the time is so short.

A high line voltage perhaps causing an amp failure and a motor over rev
would seem to be one condition where a braking resistor would be
preferable to shorting the windings though... then the regeneration
current could exceed max locked rotor. My floating neutral can
(and quite possibly will) do that. I need a diesel generator or something.
I may just end up running the computer and servo amps off of 240vac.

Les


Leslie Watts
L M Watts Furniture
Tiger, Georgia USA
http://www.rabun.net/~leswatts/wattsfurniturewp.html
engineering page:
http://www.rabun.net/~leswatts/shop.html

----- Original Message -----
From: "mariss92705" <mariss92705@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Wednesday, June 19, 2002 3:23 PM
Subject: [CAD_CAM_EDM_DRO] 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
>
>
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