This is the same general circuit from the previous installment. I have repeated it here for reference purposes.
With several eBay contactors on hand, I decided to continue the shop modernization begun before with the cheap drill press, now modified with the 1 HP VFD. After several weeks of use, the drill press modification is a total success, and a joy to use. RPM requirements for drilling operations change dramatically as the bit diameter decreases, and while swapping a 3/8" bit to a 1/16" bit takes just moments, the traditional swapping of the step-pulley belts took far longer, and was a greasy pain to boot. Now, a quick twist of the speed knob drives the RPM into the appropriate range instantly, and best of all, the RPM can be quickly varied on the fly. as the cut is taking place. The drill press VFD makes use of the internal switch native to the machine, as the current is not high. But, for the mill, the circuit here is required to handle some pretty stout amperages on the input side especially, where single-phase input currents are significantly higher than the output, on the order of 100% greater. Thus, a 7 AMP 3-phase motor output from the VFD will have input currents of approx. 15 or more. While a very heavy mechanical switch would work, the fused contactor circuit is more elegant.
As I mentioned, the drill press mod did not use a contactor. I had ordered a very nice electrical enclosure for the mill, and wanted to "warm up" somewhat with contactor circuitry before actually executing the mill circuit, so I decided to update the original 3-phase rotary converter enclosure pictured below.
|Here once again is the original switch for the rotary
phase converter. It was functional, but one thing I
learned after a couple years of use is that these
switches are not designed to be thrown dozens of times a
day, year after year... there is a lot of force in the
spring-loaded mechanism, and within two years, I had
repaired it more than once, upgrading some of the parts
inside the mechanism to better resist the slamming
The switch was stiff and annoying to activate as well when all I needed was 30 seconds of three phase. Out it went!
|The circuit conduit (PVC) is external to my shop
wall. The switch and outlets were mounted on a handy
piece of particle board, lag-screwed into the wall of the
Upper right is the 240V mains, a dedicated branch from my home's circuit-breaker box. I made a big mistake when I installed this line... I should have installed a 3+1 sheathed cable consisting of 4 wires - 120V, 120V, neutral, + bare ground. That would have allowed me to tap the circuit for 120V power needs. As it exists, without a transformer, only 240V is available here. So if you run your own cable, I strongly suggest a 3+1 cable, allowing both 120 and 240V power, similar to lines run for heavy appliances, where the 120V is used for control circuitry, and 240V is used for high-current portions of the appliance, such as heating coils on a range.
The conduit upper left routes to the rotary phase converter in the attic. The outlets below, still mounted to the backing board, are a pair of 3-phase output sockets, as previously described.
|The new box, along with the contactor, was a lucky
eBay buy, and fit the old location perfectly! I think
that I paid $12 for the box, contactor, and transformer.
I mounted the components to the box and wired it to the maximum extent possible on my bench, before mounting it to the particle board. Much easier that way. Here, the box is attached to the particle board with a number of #10 sheet-metal screws. The 240V mains supply has been connected to the terminal block on the right, and the wiring is essentially complete except for the 3-phase rotary converter feed.
The fuses are not yet installed into the fuse block. Before installing fuses in a block like this, it is handy to throw on a heavy cable-tie around the fuse, leaving enough of the cable-tie tail hanging so that the cable-tie can be gripped with pliers, and a potentially blown fuse removed without trouble. These blocks grip the fuse firmly.
|At the top of the box are mounted the nice
pushbittons from MSC, Cutler-Hammer 22.5mm dia., MSC
stock number 07828031. The green switch has a NO
(Normally Open) contact, while the red has a NC contact,
per the circuit above.
With fingers crossed, I powered the mains and tested the circuit. It worked perfectly! Now, I can engage/disengage the rotary converter with ease. Please note that with a rotary converter, hitting the STOP button will NOT normally power down the tool... in fact the 3-phase motor on your tool will continue to run, once it has achieved a certain RPM. Hence, this is definitely not an Emergency Stop switch, simply something to power-down the rotary converter whenever desired.
|With confidence, then, I began the layout for the
verical mill control
enclosure. (Stock # 54077037) This box measures
8" wide by 10" tall by 8" deep, and is a
nice fit on the right side of my mill column. It came
with welded mounting brackets with holes for 1/4"
screws, and has a moisture-resistant lid. The steel is
heavy-guage, and very nicely painted. When shopping for
enclosures, don't neglect the need for a deep box.
8" worked fine.
Here is the initial layout. Note the two holes upper left in the top of the box for another pair of Cutler-Hammer switches, identical to those used by the rotary-converter box shown directly above. This pushbutton location will allow easy, sighted access when standing in front of the mill.
The fuse-block, lower right, is for the expensive J-class fuses recommended by Hitachi for the VFD. These fuses are much bigger than I expected. Note only two are needed, one each for the two 120V hot lines which combine to form 1-phase, 240VAC.
The contactor is at the top. The contactor outputs, which will feed the VFD, will be routed through the hole in the box just to the right of the contactor.
Warning! Do NOT fail to use wiring of adequate ampacity! #10 copper is fine for this circut. #12 would probably be OK, but why save a few pennies and risk a fire?
|The box could be mounted on a handy wall, with the
240V output fed to the VFD being routed like any other
cable. But by mounting it directly on the mill column,
when the mill is moved or <gasp> sold, all that is
needed to power the mill is to plug the mill into a
30-amp, 240V outlet. Self-contained!
To help isolate the components from vibrations, I cut a gasket from 1/8" rubber, which was then mounted between the mill column and the mounting flanges on the box itself. Of course, the mill was drilled and tapped for 4 ea. 1/4" X 20 SHCS. When tapping cast iron, I prefer coarser threads, as cast iron is somewhat brittle, and fine threads in CI are prone to stripping. Mill columns are hollow, and there is no problem drilling through.
Secret tip: When drilling holes with a hand drill for tapping, select a drill significantly smaller than recommended, as hand-drilled holes are always bigger than one properly drilled in a drill-press. Even with practice, it is pretty typical to drill a hole of diameter 0.220" or so when chucking, say, a 0.208" bit. Tap immediately, or open up the hole as needed. The resulting threads will be of correct depth, rather than being too shallow.
The 240V 1Phase mains enter the box at the bottom with an SJT junction-box connector, and are routed to the fuse block at the bottom. Appropriate wiring is made with #10 stranded copper and crimped connectors. Note the correct mounting of the ground wire. The entire machine must be grounded. Ensure that any paint on either the box or the mill does not interfere with a solid ground.
| The use of stranded copper for
point to point wiring in a tight enclosure greatly eases
the installation vs. solid copper, especially with
May I also respectfully suggest you print a copy of your circuit and fold it up, leaving the copy inside the box? This will be a joy to the next owner of the machine, or maybe for the current owner in 20 years, when other documentation is absent!
This picture, of course, is of the START/STOP switches on the top of the box. I tested the circuit, and once again, the contactor opens and closes in a satisfying manner. Again, all this box does is fuse and switch the input power for the 3HP Hitachi VFD.
In the next installment, the mill upgrade continues, with box-VFD and VFD-motor wiring, as well as the mechanical aspects. So far, so good!