VFD question

[Greg Urwiller]

I haven't paid much attention to 3 phase tools/machinery over the years because of needing a phase converter and price of such. I see a lot of 3 phase equipment out there, some of which are pretty decent buys. Now I'm reading that everyone is using "VFD's" to run them in their single phase shops. I've googled VFD but the explanations I find are much too technical without confusing the issue (for me anyway!). So now you have me wondering about these. Can someone give me a simpler explanation on them and their usability? Thanks. Greg

[Henry Ambrose]

They just work.

In general you should buy one that is rated about 1.5 times the HP of the motor you are running. Some of them give you many different controls over the motor than you might ever need for wood working. Speed, start speed ramp up, stop ramp down, torque curves, braking and more. You can also just let them do the 3 phase to single phase conversion and leave all that other stuff alone.

I've done business with these guys and would again -

http://www.factorymation.com/

[Joe Jensen]

VFD or Variable Speed Drive. The phase power has three hot lines and the voltage on each line goes from the positive voltage to the negative of that voltage, and back positive. A single phase has two lines that work the same way. The voltage on each wire changes 60 times per second, or 60Hz. If you stopped time and measured the voltage on each wire they would be each be different.

A VFD takes 2 phase alternating current (regular 220V power) converts it to DC, and then the VFD uses a computer chip to create 3 power hot lines. Because a computer is creating the 3 lines, and creating the changing voltage on each line, it can create that changing cycle at 60 times per second, but it can do it faster or slower. Changing faster would make the motor run faster, and changing them slower would make it run slower.

As to how much over size to buy the VFD it depends on the machine and the kind of load. I have a 3.6HP 3 phase motor on my jointer and the manufacturer of the VFD said a 3HP VFD rated at a max of 3HP would be fine in a jointer application because the jointer starts very easily, and it is rarely heavily loaded. Also the VFD I bought can run 50% overloaded for up to 1 minute and then it will automatically shut down, so it's well protected. I bought a TECO because they are well respected.

[David G Baker]

Greg,
Once you go through the learning curve and start using a VFD you will never turn back. TECO has all the information you need to help you through the process. The cost for a VFD unit is not that much until you need to power a high horse power motor. There is a lot of easily understood information here on SMC, all you have to do is search and ask members when you find something that you don't understand.
Good luck.

[Peter Quinn]

I looked into VFD's last year for a shaper, and my conclusion is they are not a forgone conclusion for every application. The basics are all noted above very well. For 5HP and above single to three phase conversion (VFD's in the 7.5-10HP range) a VFD becomes as or more expensive than a rotary phase converter. I don't understand why but the price jumps rapidly above 3HP. One factor not noted yet is that a VFD is generally installed on and used with a single machine, and not swapped between machines. So for each three phase machine you wish to convert you need to buy a VFD. I can use my RFC to power ANY three phase machine it is capable of starting, or multiple motors within its range simultaneously, say a shaper AND feeder, both three phase. So if you plan to add multiple 3PH machines that will either run simultaneously or individually consider the total cost of running each with either VFD's or RFC. Three 3HP VFD's cost more than a single 7.5HP RFC generally.

One great feature of the VFD is the speed conversion and braking. In my case I had a braking motor on the shaper already, and I didn't need precise speed control, the pulley system was simple and sufficient. So consider the particular machine you are trying to run, how it is used, and how many machines you may add. Say you wanted a drill press or a large lathe. Variable speed would be great there. But a compressor? Who cares, speed control is pretty much useless there. There may be cases where speed control would be worth more than any additional cost incurred. Food for thought anyway.

[Hugh Jardon]

Some background:

In a single-phase alternating current (AC) system, the supply varies above and below zero Volts (ground) in a sinusoidal waveform (ie a sine wave shape). In North America, the peaks are 170V above and below, at a rate of 60Hz. (BTW, that is not the same elsewhere in the world). The usefulness of this is somewhat limited, since the "gaps" between the peaks (whether above or below) the ground reference line are potential "holes" in performance, where the power developed is not sufficiently uniform.

To combat this, three-phase power provides for three separate windings in the generator, which are 120 degrees out of phase from each other. This helps to "fill the gaps" between the peaks of the waveform, thus enabling greater loads to be supported. Further, because they are out of phase, if the voltage is measured between any two of the live conductors, it will be perceived as a higher value than if referenced against ground. Thus, some single phase motors can be rewired to use the two phases commonly supplied to homes, and see the benefit of increased voltage. This isn't entirely fair though, since current is 90 degrees out of phase with voltage, and so while there may be a voltage peak, the current will be diminished, so the power developed in the load remains the same as for single phase devices.

As indicated above, variable frequency "drives" synthesise a three phase supply, and can vary the frequency in order to vary the speed of the load.

This is very much the "abc's" of this subject, but hopefully explains the basic theory.

[Josiah Bartlett]

Some background:

In a single-phase alternating current (AC) system, the supply varies above and below zero Volts (ground) in a sinusoidal waveform (ie a sine wave shape). In North America, the peaks are 170V above and below, at a rate of 60Hz. (BTW, that is not the same elsewhere in the world). The usefulness of this is somewhat limited, since the "gaps" between the peaks (whether above or below) the ground reference line are potential "holes" in performance, where the power developed is not sufficiently uniform.

To combat this, three-phase power provides for three separate windings in the generator, which are 120 degrees out of phase from each other. This helps to "fill the gaps" between the peaks of the waveform, thus enabling greater loads to be supported. Further, because they are out of phase, if the voltage is measured between any two of the live conductors, it will be perceived as a higher value than if referenced against ground. Thus, some single phase motors can be rewired to use the two phases commonly supplied to homes, and see the benefit of increased voltage. This isn't entirely fair though, since current is 90 degrees out of phase with voltage, and so while there may be a voltage peak, the current will be diminished, so the power developed in the load remains the same as for single phase devices.

As indicated above, variable frequency "drives" synthesise a three phase supply, and can vary the frequency in order to vary the speed of the load.

This is very much the "abc's" of this subject, but hopefully explains the basic theory.

That's mostly true, but the main original reason for polyphase systems was to provide large motors with an inherently rotating magnetic field, so they don't require brushes or slip rings to the rotor. 3 phase induction motors don't require any fancy starting windings or controls, and are reversible by swapping any two phases. Early polyphase AC power was 2 phase (Niagara Falls was an early 2 phase plant), but 3 phase won out because it is simpler to transmit over long distances due to being able to transmit 33% more power than 2 phase over the same number of conductors. 2 phase power either requires 4 equal size wires or 3 wires with the neutral being twice as large as the hot conductors.

Some people call North American 220V power two phase, but it really isn't. Its split phase, with the neutral being center tapped off a single phase of the 3 phase distribution network. Real two phase power has the two phases 90 degrees apart. 208V power is a little bit different as it is bridged across 2 phases of a 3 phase system but it still produces a single waveform at 60 hz.

Single Phase capacitor start induction motors use the 90 degree phase shift from a capacitor to temporarily create a second phase for starting.

[Matt Van Heiden]

If you acquire more than 1 3PH machine and do not plan on running them simultaneously, you only need 1 VFD for the largest motor you run. In my garage shop I have a '54 unisaw, '44 Delta 17" drill press, and a '70 something delta 8" jointer that have a maximum of 1.5 HP 3 PH motors so running my machines individually works for me. To run these I have one Teco FM50 2 HP VFD. I mounted it on a wall with 220 V wired to it directly. I then ran wires from the VFD into the wall to a 3 PH turn lock outlet. As a precautionary measure, I shut down the VFD prior to unplugging the machines so I don't ruin the VFD. This is a minor extra step, but I paid a total of $475 for three high quality US made machines that were basically plug and play. 3 PH machines can be a steal and as a bonus once you try 3 PH power you will not want to switch back to single phase.

The 2 HP FM50 Teco VFD will run about $150 and will run 2 HP machines at 100% power. Teco also makes a FM50 3 HP that will run 3 HP motors @ 100% for $200. You can get these from Fractorymation. As mentioned earlier, above 3 HP and the 150% rule starts to apply. As the OP was wondering about operation of the VFD, if a guy can run a new circuit to a new outlet from the breaker box- he can definitely wire the VFD in about 20 minutes or sooner. After that it is a simple on/off button. Sorry if I rambled on and restated any information.

[Chip Lindley]

VFDs offer variable speed control and other bells and whistles which may be quite useful, but IMHO, an RPC is the way to go if multiple 3-phase machines with larger motors are acquired.

My 5hp ARCO rotary phase converter could run my Powermatic 5hp shaper, 1hp stock feeder, and 3hp dust collector at one time. (as long as motors are started one-at-a-time)

Buy once, Cry once! But, as a matter of fact, prices of RPCs (especially used) seem to be more affordable than ever, as digital electronic solutions take the spotlight.