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

Hi Bob, Jon,
Yes, of course, one ohm - not ten...did I say ten? Sorry...

I understand this flywheel/generator thing and that it can charge the
powersupply capacitor to a higher voltage. I have a shunt regulator
circuit that will load the the DC bus to prevent overvoltage. But that
is not the problem or the reason for my original message.

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. (?)

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....

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.

Thanks!
/Henrik.

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>I think you have the right idea, except for the resistance you calculated. In your example you came up with a total resistance of 1.5 Ohm needed, and your motor plus wiring totaled 0.5 Ohm. It seems to me you then need a 1.0 Ohm added resistor instead of the 10 Ohm you stated. Perhaps you meant 1.0 Ohm but made a typo? The continuous power rating would indeed be fairly high. At 19 Amps the power in the resistor would be I^2*R or 19^2*1.0=361 Watts. The resistor would reduce the voltage on the motor by 19 Volts at full current, and would indeed have an effect on acceleration.
>
>The reason something is needed is that a motor acts like a generator when decelerating, and can pump quite a lot of current back into the power supply during this time. If you have a lot of mass (weight) in addition to the motor armature that must decelerate, the time the current flows will be longer, in effect acting like a flywheel. This current will charge the power supply filter capacitor to a higher voltage than normal, and may damage something in the power supply or motor controller if this voltage exceeds the rated voltage of a transistor for example. Sometimes the power supply will have over-voltage protection to protect it but it depends on the design and rating of the protection circuit.
>
>Bob

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