Re:Servo braking resistor in series with motor armature

Hi Jon,
AFAIK the UHU is a "50% PWM at 0 speed" drive and it does not rely on
software for current sensing/limiting. BUT it does not sense the braking or
recirculating current....bummer....

I know about EMC2, I have the LiveCD and I've glanced thru the manual. I've
even booted it up once... I know you don't like "steps" but I'd much prefer
to use Mach3 since that is what I've come to know.

Thanks!
/Henrik.

>Henrik Olsson wrote:
> That Rutex document I referenced claims that if the motor is deccelerated
> faster than it was accelerated (running into a hard stop for example)
> the returned current (sensed or not) will go thru the diodes inside the
> transistors and possibly destroy them since there's no way to actually
> turn that current OFF. (?)
>

I don't think that's the scenario. I think what the problem
really was was that the Rutex uses sign-magnitude modulation,
like I do, but software sensing of the overcurrent, and it was
too slow to respond, and only sensed current sourced by the
drive, not noticing when the braking current was going high.

A common feature of sign-magnitude drives is they recirculate
motor current internally, not through the power supply, so that
the motor is effectively shorted during off-time. You need to
sense braking current and shut off the transistors if braking
current becomes excessive.

> The new and improved UHU is said to use IRFP264N, rated 250V and 44A. The
> drive, I believe, will be rated at 180V, 25A which seems like a pretty
> good safety margin (as far as the transistors go). Yet it can't seem to
> drive anything over 100V, 10A without crippling the motors performance -
> that is what I don't get....
>

If the logic doesn't handle overcurrent properly, then no amount
of derating will fix it.

> Jon,
> Yes I know about your system and it does seem like a good one. I'd need
> around 120V DC-bus depending a bit on the max dutycyle of the PWM. But I
> want/need Step & Direction.

Have you looked at EMC2? It fully supports my drive & control
system without the use of steps. The only pulses are encoder
counts and the drive PWM to the servo amps.

Jon