VFD Conversion Project Start page | Home

I thought that I could wrap this up in the last installment, but I dislike having more than about 7 pictures max. per page, and I had a few more photos that I wanted to share.

One thing I immediately noticed when I attached the bracket to the mill head was this: the mill head housing is of cast iron. Being cast, it has draft... the side of the casting slopes inward by 3 or 5 degrees, and when the bracket was firmly mounted, the VFD head itself sat at an annoying angle relative to the head, with the outer left edge sloping down relative to the right edge.

This was unnacceptable. To correct this, I installed a pair of #10 set screws on the lower edge of the sideplate. These protrude slightly to the right when facing the mill, and by adjusting them, I was able to level the VFD control head.

You can see the slight, triangular gap that this creates between the sideplate and the mill head casting. I'd rather have this gap than have the VFD canted badly. The mount still locks up very firmly, no wobble when working with the VFD itself.

From below, looking up at the power inputs and outputs from the VFD. I took care to neaten (and protect) the connections, as the entry of metallic swarf into this area would be pretty dramatic.
Behind, you can see the armored cables, both in and out from the VFD.

As I mentioned, the mill is now a strange combination of colors. Maybe some day I'll paint the various parts to match, but for now, it'll remain a mongrel.

There are all sorts of 120VAC goodies mounted on or near the mill. The list includes two halogen spots, a Sargon DRO, and an X-axis power feed among others. Previously, all of these 120V lines were haphazardly routed to various 120VAC outlets. In keeping with the "modular mill" philosophy, I mounted a modern power strip on the rear of the mill, using a NEW type of velcro I found at a local store. It is called "heavy duty" and rightly so, as it consists not of hooks and piles, but rather mating plastic hooks of a special shape and depth.

When mated, the hooks + hooks create a terrific grip that absolutely does not shift, will never wear out (unlike hooks and piles), and makes maintenance a snap.

Cable ties keep the cables neat.

Poorly lighted picture, but you get the idea. The VFD control is very accessible. Aesthetically, I like it!

Initial Tests: The VFD was programmed for a maximum speed of 60 Hz. Given the roughly 1/1 pulley ratio, this would give a top speed for the moment of 1750. The acceleration ramp was set for 4 seconds, while the deceleration was set for 2 seconds. The VFD display can be toggled between frequency output and current output, among other parameters. I set it to amperage, and powered up the motor. The spindle accelerated nicely, and the RPM followed the VFD potentiometer as expected - perfectly!

Current: The motor was drawing 6.2 Amps according to the VFD display. The big test, of course, is under load. I mounted a scrap of aluminum, and began to hog with a 3/4" roughing end mill. The additional 1/2 horsepower (the original was 1.5, this one is 2 HP) paid dividends at the slower speeds, with the cutter happily chopping huge swaths even at very low RPM.

RPM: The top-speed was too low. Knowing that the motor and VFD combination was delivering excellent torque, I replaced the motor sheave with my largest, which will reduce available torque, but increase the top speed to about 3,000 RPM. Even with the new sheave, there was plenty of torque to do everything I'd need.

As a test, I set the VFD upper frequency to 80 Hz rather than 60Hz. This increases the top speed. It worked fine for a while, but eventually something internally caused an "E7" error, which was revealed to be a "DC bus overvoltage". Since the new sheave gave me all the RPM I wanted, I reduced the top frequency back to 60 Hz.

Noise: Even with all the metallic shielding, this thing is electrically noisy. Any AM radio in the vicinity is blown out of the water when the VFD is operating. By increasing the PWM carrier frequency (via software) to it's maximum of 16 Hz, both electrical and mechanical noise is greatly reduced. Note that when the VFD is at or near its peak output current, increasing the PWM carrier may force a current derate. At 6 amps, I am nowhere near this unit's limit of 10 amps. Also, the noise could create issues with CNC controls. If this ever turns out to be the case, Hitachi makes a series of filters which should cure that problem.

Heat: This is written a couple of weeks post-completion of this job. I have had the chance to do some serious milling, and am happy to report that the motor gets no hotter (in fact a lot less hot) than the original motor. This is probably due to the fact that 80% of my milling is at higher RPM's, where adequate airflow is generated in the TEFC motor. I feel no need at this time to augment the motor fan with an external unit. The VFD heat sink itself warms up not at all. It's so cool that I am tempted to disconnect the VFD cooling fan, which creates an irritating buzz when the VFD is powered.

Overall Impression: It is a real pleasure to use this VFD and the bigger 2HP motor. The electrical braking is great! The spindle smoothly halts in 2 seconds without actuation of the mechanical brake. Quick RPM change is wonderful. Only step-pulley users can appreciate just how nice it is. During end mill cuts, with the X power-feed engaged, I can adjust the RPM for best cut. For drilling operations, often you want to start the hole at a slow RPM, then increase it as the drill penetrates; this too is easy to do with the VFD. Installation was more involved than I thought, mainly due to the contactor. There are mechanical switches available which can easily handle these currents, but you must fuse the circuit as well, and along with the fusing, you may as well make the setup slick with a contactor and a pair of switches.

This VFD and Leeson motor gets a big "Thumb's-up" from me!