Karl Townsend wrote:
I want to add tapping ability (need to stop and reverse fast) to my CNC mill
with a VFD on the spindle. Its a 5 hp Hitachi SJ100.
http://www.hitachi.us/supportingdocs...ok_NB585XF.pdf

Pages 5-8 to 5-10 discuss adding a brake resistor. If I understand correctly
I can put 2 resistors in parallel for more braking. This unit calls out a 35
ohm resistor. Two in parallel would be 17.5 ohms. Am I reading these tables
right?
A 230 V VFD has a 340 V DC bus voltage. A 35 Ohm resistor would
draw about 10 A. The 17.5 Ohm would be about 20 A.

I seen a 500 watt 16 ohm power resistor on eBay. Close enough or do you have
to be right on?
340 V/ 16 Ohms = 21.25 A, sounds well within reason.
(340 ^ 2) / 16 = 7225 W, so this resistor needs to have a REAL
good surge rating. Those big, hollow-tube vitreous enamel
resistors generally qualify. Mount it vertically so convection
can help cool it.

With only about 20 A worth of energy disposal, you can't reverse
this spindle like plug reversing on real 230 V 3-phase mains.
You should still be able to stop the spindle in well under a
second, though. You can reduce the decel time until the drive
faults out and then increase it a little.

Jon