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

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