Re: [CAD_CAM_EDM_DRO] Servo braking resistor in series with motor armature

Hi Jon and Roger,

It was me who wrote the message on CNCZone that Roger quoted here. The
reason is that the UHU have gone thru a major redesign to be able to cope
with higher voltage motors. The (re)designer is 'Kreutz' and it is him I've
been exchanging PM's with as I mentioned in the original message.

The complete schematic isn't released yet so I don't have much details but
the circuit has apparently been improved on a number of points. A LRC filter
on the output is one of those. What I DO have is the BOM and it states TWO
0.02, 3W resistors as sense resistors so it may actually sense current in
both legs of the bridge.

Obviously the UHU-chip is still the same so what ever switching scheme used
on earlier implementations is used on this new one too.

The reason I'm "into this" is that I'm on the lookout for a suitable drive
to run my motors. Rutex is out of the question for now but this new UHU did
look so promising until this came up. But as you can probably tell from my
original message I'm not "into it" enough to understand the how's and why's
fully.

IIRC the new UHU-drive is going to be rated at 160V, 25A but it seems it
won't be able to drive anything that a Gecko can't already
handle.....putting a resistor in series cripples the performance of my
motors, costs a lot of money and is a waste of power but right now there
aren't that many S/D servo drives available that can be used for these
motors (that doesn't cost $1000+) Aaargh.... :-(

/Henrik.



-----Original message-----
Från: CAD_CAM_EDM_DRO@yahoogroups.com
[mailto:CAD_CAM_EDM_DRO@yahoogroups.com] För Jon Elson
Skickat: den 14 februari 2008 18:36
Till: CAD_CAM_EDM_DRO@yahoogroups.com
Ämne: Re: [CAD_CAM_EDM_DRO] Servo braking resistor in series with motor
armature

Just an additional comment:
The quoted document said:

> What determines the need of a series resistor is the current rating of
> the diode inside each MOSFET.

These diodes are parasitic, ie. they are not even desired to be
there! They are horrible diodes, and have turn-on times in the
microseconds. Therefore, they really can't be used for any good
purpose, and my (and other's) opinion is that ever letting them
become forward-biased is a big mistake.

> As I said at the beginning: I don't fully understand how the
> regenerative current can be so large so that it destroys the diodes in
> the MOSFETS but the expertise says that is a fact. So my only hope now
> is that someone jumps in and tells me my understanding of the issue is
> all wrong.

Using the first example, the motor has .24 Ohm, and can deliver
98 V open circuit. Well, I=98/.24 = 408 A, neglecting the
resistance of the cables and transistors. I'm not sure if it
makes any difference whether the transistors are in the shorted
condition, which the UHU doesn't do, or doing the usual 50% duty
cycle idling pattern of a synch-antiphase drive, the current
would still be the same, except for the PWM modulation on top of
the 408 A. Since this would be a circulating current, the
single current sense resistor wouldn't see the true nature of
it, it would see + then - 408 A at the PWM frequency, and
depending on the time constant, it might average it out to zero.

With separate sense resistors and comparators in both low-side
transistors, this condition would not be missed, and the
transistors could be turned off. An output filter inductor
could give the electronics time to respond before the current
built to a damaging level. Proper high-speed freewheel diodes
would protect the transistors from the crummy characteristics of
the body diodes and divert the braking current back to the power
supply, or an external braking resistor switched into the DC bus.

Now I've revealed almost all my secrets.

Jon