[najnielkp]

hello all,
i have a question about braking resistors. the fanuc drives i am retrofitting have a braking resistor of 10 ohms.this is in series with a adjustable overload relay. the motors are gettys type 10. i havent found the specs on the armature resistance. the fanuc drives are 30 amp,170volt dc.
i am using the 2020 rutex. should i stay with the current braking resistor, and leave the overload relay in the circuit? will the overload relay provide protection of the back emf if set below the 40 amp max of the drive?

[NC Cams]

Fanuc used a SCR system to directly convert 3 phase AC into DC - there was generally NOT a capacitor used to buffer the output.

There were essentially a dozen SCR's, (4 sets of 3) that were polarized so that, depending on which 6 were turned on, they'd get the properly rectified DC polarity ("directly off the mains" so to speak )to run the motor CW/CCW as needed.

In light of the fact that they were essentially driving the motor DIRECTLY off an isolated 3 phase AC bus as opposed to a rectified and filtered DC as you'll be doing, they used a complex relay/resistor circuitry to buffer the drive against any EMF feedback so that it didn't screw up the SCR's. I suspect Rutex is doing the same thing via the use of Schottky diodes to protect the drive mosfets. They're simply using semiconductors instead of relays and resistors.

The overcurrent relay Fanuc used would drop out on over current as well as if an SCR shorted close and tried to drive the motor continuously. It would also only come on when an appropriate "enable" check of system functionality was completed. The resistor (as far as I was able to determine) was ONLY switched into the circuit whenever the drive was disabled.

How you trigger the braking resistor with the use of a Rutex drive is beyond me - especially if you are using a capacitive filtered DC power supply. It shouldn't be that hard, however to use an op amp and/or a comparator to sense if the drive is or is not trying to supply power so that you could switch in the braking resistor so as to more completely simulate the Fanuc drive circuitry.

Then again, depending on the capability of the Rutex amp, you may not even need the braking resistor.... I don't have enough expertise to address that aspect of what surely will be your next question. Why not give Rutex a call???

[najnielkp]

thanks nc,