|The vast majority of small to medium-sized machine
tools used in the home and in light industry are powered
by one of the following types of motors: Single phase AC,
Three phase AC, and Permanent Magnet DC.
Typical home wiring in the U.S. is single phase AC, at a frequency of 60 Hz. A 120V circuit uses one HOT line, and one NEUTRAL line, while a 240V circuit uses two lines of 120V each, which combine to form 240V. Industry, on the other hand, uses three-phase power. A 3P line consists of three lines whose waveforms are related to each other. One benefit (among many) of powering a motor with 3P is that the motors are cheap, powerful, instantly reversible, and require no start or run capacitance. They can be found surplus at ridiculous prices in a wide array of styles and voltages. Simple to install, efficient, they are perfect for machine tools. The problem, of course, being a home machinist, is "How the heck do I obtain/create 3-phase power in my shop?"
There are three common methods to convert 1P to 3P in the home. You can use a static system, in which the 1P power is electronically converted to 3P. You can use a rotary phase coverter, which makes use of an idling three phase motor to create the correct waveform. Finally, you can use a VFD like the one shown here.
Each of these has advantages and disadvantages. Static converters are the cheapest, but produce the lowest quality 3P output of the three. Static converters can often be purchased for less than $100. Rotary converters produce a quality 3P waveform, but are expensive ($300 and higher), wasteful of energy, and often noisy. That leaves the VFD. The VFD as we know it today can be though of as a static system with very advanced features, microprocessor controlled. The BIG advantage of the VFD is the ability to vary the speed of the motor; ideal for a machine tool like a mill or a lathe.
|VFD's in a nutshell - what they do, and why
one might want to use them
|In a simplistic way, here is what a VFD does...
Most smaller VFDs (typically 3HP output and less) allow one to "feed" it with either 1P or 3P power. Larger VFD's, above 3HP, normally require a 3P input. So if your goal is to use a VFD to convert 1P power to 3P, be sure the model that you have selected does in fact allow a single phase input. This VFD is a Hitachi L100 with a 3HP, 3P output, and accepts either 1P or 3P 240V input.
You can see the input terminals, to the left, labeled L1, L2, and N/L3. For 1P to 3P, the 240V 1P lines are connected to the L1 and N/L3 terminals. The outputs, to the right, are labeled T1, T2, and T3. These are connected to the 3P motor one desires to power and control.
Inside the VFD, the AC input, at 60Hz, is converted to DC! The DC power is then inverted back to an AC waveform, this time in 3 phase. Further, the voltage, and more importantly, the frequency of the output, is variable. This particular VFD produces output from 0Hz to 360Hz. Remember, the rotational speed of a 3P motor is tied to the number of poles built into the motor, and the frequency of the motor input. Zero Hz = Zero motor RPM. 60Hz = Full rated motor RPM.
|The original plan was to use the VFD to power this
original motor. The stock motor is a 3P, 1.5 HP,
two-speed motor. A 3P motor can have more than one speed
if there is more than one set of poles wound. I
won't get into poles, mainly because I don't understand
it completely myself, but the number of poles relates
directly to the rated RPM, minus a certain percentage
known as slip. A theoretically perfect 2-pole
motor at 60 Hz has a speed of 3600 RPM. 4-pole motors
turn at 1800 RPM. 8-pole motors at 900 RPM. You get the
idea. This motor has both 4 and 8 pole windings, and
depending upon how the windings are energized, will turn
at either 1800 or 900 RPM, again, minus the slip.
It is a TEFC (Totaly Enclosed, Fan-Cooled) motor with a cast-iron case. While very heavy, and of questionable origins, it has bravely powered this vertical mill (an ENCO 1525) for 10 years and shows no signs of quitting. I made the decision to replace it for two reasons - the first is the primary drawback of the VFD. VFDs tend to produce voltage spikes and surges which play hell with standard wiring insulation. In recent years, motor manufacturers have recognized this, and accordingly have beefed up the insulation on motors they now term inverter duty, inverter, of course, referring to the VFD.
The second reason I elected to replace it was to gain a torque boost at lower speeds. The motor selected is a Leeson 2HP, 1725 RPM 4-pole motor.
|The original step-pulley arrangement. Top speed with
this setup is slightly greater than 3,000 RPM. 99% of my
milling with this machine has been at 2,500 and below,
and I have selected a single, fixed pulley for the new
motor, which will mate with the smallest spindle pulley,
at the top of the stack. The ratio should give me a top
speed of close to 3,000.
Note the "odd" belt. This is a Fenner drive systems product, called a link-belt, which allows one to hook belt segments together to form a belt of any length. It looks clunky, but it actually outperforms a standard v-belt, due to the Fenner belt's lack of harmonization.... it produces a better finish and smoother running than its standard counterpart.
Best of all, you can link it together in place and so avoid tearing down the spindle. Nice! Get it from MSC.
|A real problem cropped up when I dismounted the
motor. The first thing I noticed was that the Chinese
motor did not have an industry standard face mount,
meaning the base plate (pencil) would be unuseable for
the 56C Leeson motor face. Argh!
I knew I could machine a new plate (lots of work) but wanted to avoid that if I could. I remembered that Grizzly sells this same milling machine, except that they install an American-made 1P motor. Thinking that the plate for the Grizzly mill would have the correct mounting format, I took a chance and called their spare-parts people. Yup, they had one in stock, and it had a 56C mount! The Grizzly parts folks were very nice and personable. I previously had a bad experience with a Grizzly bench roll, but overall, I've heard lots of good things about "Grizz." I won't hesitate to do business with them again.
Here is my mill under the Grizzly label. Guys, this exact mill under an ENCO nameplate has impressed me and served me faithfully for 10 years now. At $2500 U.S. it is a real bargain for a new machine. Please consider this mill if you can't afford a 9" X 42". It is light-years beyond a mill-drill in quality, accuracy, and performance.
|As part of the overall 3P upgrade, I am going to
install magnetic contactors for my rotary phase
converter. The current setup is shown here. The rotary
converter is up in my attic. I installed some external
conduit, which comes down from the ceiling, and slipped
in some 10/4 sheathed wiring. A 30 Amp dedicated breaker
serves this system.
To get three-phase power, I simply throw the safety switch handle, completing the circuit. I can hear the rotary converter "hum" to life. The true 3P power is routed into the two sockets below the box. One feeds (and will continue to feed) my Hardinge HLV, while the other juiced the mill before I got into the VFD!
No problems running both machines simultaneously. If I have a friend over, he can work the lathe while I work the mill at the same time.
|The routing of the wiring for the rotary phase
converter, inside the safety-switch box, is not
intuitive. I simply followed the directions supplied with
the phase coverter, and had immediate success.
The raw 1P 240V enters the box from the conduit, upper right. The white and black wires are posted to the upper left terminals of the fuse block.
Throwing the switch completes the circuit and starts the converter. The wiring from the converter is attached at the bottom of the fuse block, phases T1, T2, and T3, black, red, white. Of couse, everything is grounded properly where needed.
Finally, the true 3P output is tapped from the top of the block, and routed down to the outlets.
|These big locking bayonet plugs and sockets get the
3-phase to the machines. I considered a permanent wirng
installation from the junction box to the machine tools,
and not using the plugs. In retrospect, I probably should
have, and avoided the cost and effort of the plugs and
sockets. Servicing of the machines can be safely done in
this former case by tripping the circuit breaker.
Oh well, the plugs are cool. I liked them, and still do.
|Mmmm. Nothing like unwrapping a fresh, new motor. I probably could have scrounged eBay and picked one up at 25% of cost, but very few motors are advertised on eBay as inverter-ready. This one is, I know that for a fact; the windings are fresh, the bearings are new. In this case, it was worth the cost.|
I don't know if this plate is readable, but most guys who are into stuff like this (you probably are if you've read this far!) appreciate the data thus provided.
There are nine wires inside the motor junction box which must be correctly tied to configure the motor for either 240V or 480V operation. This will, of course, become a 220V setup.