Unlike the CNC mill project, gathering components for this mini-project was significantly easier. Because the cost was so much lower, I purchased many of the parts from MSC, the "big one" being the motor. The only major component which I decided to pursue on eBay was the contactor. For what is essentially a big relay, commercial contactors are very expensive. Plan on $100+ for a four-circuit contactor capable of handling 20 amps.
Executing an eBay search for "contactor" within the business and industrial category will reveal dozens for sale at firehouse prices. You should be able to pick up a NIB contactor for $10 to $20 with ease. You will want a four-circuit contactor; three contacts are dedicated to lines T1, T2, and T3 (three phase) and the fourth is for the contactor coil itself. By integrating this fourth contactor switch with the contactor coil energizing circuit, you can make use of a momentary, normally open switch to energize the coil, and keep it energized.
Here is a diagram of the basic circuit which makes use of these switches to power the VFD and supply it with fused, 240V single-phased power. The 240V supply, starting at lower left, consists of two hot lines of 120V each. This is very typical of a household 240V circuit such as a hot-water heater. The lines are fused through some rather expensive type-J fuses, per the VFD instruction manual. These fuses are from MSC, and cost approx. $10 apiece. For this application, I have sized them at 15 Amps. After the fuses, the two lines are tied to two of the three contactor inputs. Also, the 240V power is connected to a control transformer, center-top in the diagram, and pictured below. The electrical system now has 240VAC, and 120VAC available, the latter being produced by the control transformer.
This contactor coil is powered with 120V. To close the contactor, and thus power the VFD, I need to get 120V through the contactor coil, and it must be maintained. There are two switches on the diagram, switch "R" (Run, Normally Open) and switch "S" (Stop, Normally Closed). Let's trace the power... note that pushing the Run switch energizes the contactor coil. <CHUNK>. All three "switches" within the contactor are now closed. The 120V power goes through both the Run switch and contact X. Releasing the Run switch does nothing... contact X picks up the entire 120V load. The contactor coil remains energized/ON, and 240V is delivered to the VFD through contacts Y and Z.
To turn off the VFD, the Stop button is pressed. This switch, being normally closed, is now opened, and the 120VAC feeding the contactor coil is interrupted. All three contacts open, and the 240VC to the VFD is now shut off. Simple!
|On to the components!
This is not a contactor... it is actually a 100 VA control transformer, also shown in the diagram above. The primary side of the transformer accepts 208, 220, 240, or 480, and the secondary side will output 115VAC, and 24VAC. These lower voltages will be used for the contactor coil, and to power the 115VAC muffin fan to augment the airflow through the motor.
|Don't underestimate the need for a pulley (more
correctly termed a sheave) of good quality.
You can buy a cheap zinc sheave with a sloppy bore for $5
at a hardware store, or you can get this nice cast-iron,
precision bored sheave, fully balanced, for $16, from
MSC. Minimizing vibrations is important in a machine
Be sure that the bore is correct for the motor that you will use. In this case, the bore is 5/8", keyed for 3/16".
|Of course the motor combines with the VFD to form the heart of the retrofit. In Part 1, I explained why it was desireable to have an inverter rated motor. Roughly $200 new. You could probably buy a very similar motor on eBay for ~ $75, but of unknown, possibly not inverter-rated insulation, and perhaps used and sloppy bearings.|
|Also in Part 1, I mentioned that Grizzly Tools
carries the correct adapter plate to mate this NEMA 56C
face to my vertical mill. Even
though I knew that this plate would fit the motor, based
upon my conversation with the nice Grizzly salesguy,
there is always a bit of tension when two parts like this
are first mated. In this case, the fit was perfect!
Four ea. 3/8" X 16 bolts secure the motor to the plate through the counterbored holes seen in this photo.
|Along with the major components, you'll need a pretty
good supply of industry standard electrical components.
You simply can't find switches like these at Radio Shack,
or a home improvement store like Lowe's. They are
Cutler-Hammer 22.5mm pushbuttons, MSC
stock number 07828031. The Green switch has a NO
(Normally Open) contact, while the red has a NC contact.
A quick word on MSC's customer service - In 12 years of being an MSC customer, ordering hundreds, if not thousands, of individual components, I have had perhaps 2 items not in stock when the order was placed. I ordered these switches Wednesday morning, April 7th, at 10:00 A.M. The switches were on my doorstep the next day, and I had specified UPS ground, and they were shipped UPS ground!! How do they do that? You will pay a very little more than the dirt-cheap suppliers, but trust me, it is well worth it.
|Two eBay kills... a pair of NIB Moeller contactors.
Both of these have 120V coils.
Contactors are nothing more than big, heavy relays. On the contactor shown, which is a 3-circuit, you can see the tie-points for the wiring. Two additional tie points will energize the relay. Application of 120V through the coil draws the contacts sharply downward with a loud <clunk>, and as long as the coil remains energized, the three circuits will remain closed, held there by electromagnetism.
The smaller of the two contactors here is rated at 4kW, which is more than adequate for a 2HP motor.
|Since I had another VFD rated at 1HP, left over from
the CNC mill project, I decided to put it to work on my
crappy drill press, which is now, thankfully, a nice
drill press! The VFD is worth ~3X the drill press, but it
can be easily removed and used on another project later.
This project was a matter of a couple of hours, most of which was consumed with creating the VFD mounting bracket.
The bracket backing is evident - it is a short section of extruded aluminum T-slot table. The cable leading from the VFD to the motor is 4-lines of #10 stranded copper, encased in a long section of green heat-shrink tubing.
The motor is 3/4HP, three-phase, an eBay surplus Rockwell, which I bought over a year ago as a possible motive source for the CNC mill. The motor was way too big and underpowered for the CNC project, so it recieves a second-chance here.
|The motor pulley here is a cheap-o from True Value
hardware and was manufactured for a lawnmower. This
drill-press didn't deserve one of higher quality, and
this steel sheave is adequate.
Of course the original belt no longer fit (do they ever?) and a section of Fenner link-belt does the honors. The link-belt is a fantastic product, guys, I highly recommend it, even more than standard V-belts, as it produces a smoother running drive-train.