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VFD Installation considerations

rabler

Cast Iron
Joined
May 25, 2020
Location
Rural S.W. Indiana
I set up a VFD for use on one of my machines and thought I would go over some things as there are often questions of the "What should I do to power this?" variety. This will be a multi-part post going from "why a VFD" to details of circuits and programming the particular VFD I used.

I purchased an old lathe that had a 3HP, 3 phase motor. No 3 phase in my shop, nor is it available on the power line near my farm. Didn't even want to ask the local power co-op what it would cost at their end, on my end would have meant burying new power lines, new outdoor service panel, etc. That left powering the machine either with a phase converter, a VFD, or install a single phase 3HP motor.

Single phase motors need starting capacitors and associated internal hardware to switch those in/out. Also, single phase motors are somewhat like piston engines, they don't turn with continuous torque, as the rotor goes around they push at some angles and coast through other angles. A 3 phase motor runs with continuous torque through the entire rotor rotation (as long as it's running on real, balanced 3phase). For many machine applications this may help with smooth finishes. It also means less wear on gear trains. 3 phase motors also have the option of using a VFD for speed control. Single phase motors basically need the momentum of the rotor to coast through the dead spots, so using a VFD to slow down a single phase motor doesn't work well. The start capacitors get past this on start-up, but they'll burn out quickly if used for any length of time. So generally 3-phase motors are preferable on both counts. (Yes, this is gross generalization of a more technical issue).

There are some trade offs between VFD's and phase converters. In phase converter's favor, they can power multiple motors, and are fairly simple to set up and use. They are also larger, and noisier. VFD's are small, high-tech, need to be configured, and offer the ability to get infinitely variable speeds out of a motor (within some range). Phase converters and VFD's use some power of their own, more so with phase converters. VFD's can actually save energy by running a motor at reduced power, but this is a consideration for running equipment long periods, like a heating/cooling/ventilation system on a large building. Most machine shops are not going to see any power saving benefit from a VFD.

If you end up with one 3-phase machine there is a good chance you'll get another one some where down the road. So phase converters, typically a rotary phase converter, is the suggested solution. There are some potential gotcha's with rotary phase converters, notably the wild leg isn't as well regulated. This means that anything on your 3-phase equipment that is using a single phase to power some control electronics, DRO, etc, should not use the wild leg. Alternately, better, more expensive (rotary) phase converters have the better regulation on the generated (wild) leg.

An aside, but commonly confusing issue. If you have 240VAC single phase, it is indeed 1 phase not two phases, even though there are two "hot" wires L1 and L2. Three phase has three hot wires L1, L2, L3. Basically a "phase" is a connection between two wires. So L1 to L2 is 1 phase, L1 to L3 is a second phase, and L2 to L3 is a third phase. Obviously you only have one of those options without L3. True, you can split 240VAC into 2 phases of 120VAC. For a motor, 2 phases (180 out of phase) still has the pulsing torque, so there is no benefit to making a two phase motor. Residential power uses those two separate phases to get more total current available at 120VAC on the same size feed wire to the house, and offer 240VAC for larger appliances. While higher voltages increase the shock hazard, you get twice the total power for the same wire size. This is especially advantageous in motor windings, finer windings mean a smaller, cheaper motor (less copper).

So, using a rotary phase converter, you need to make sure any equipment plugged into it uses L1-L2 (assuming L3 is the generated/wild leg) for any non-motor electronics. You can probably get away with running an incandescent light bulb on one of the other phases, after all, a blown out light bulb isn't that expensive to replace.
 
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Cool, a VFD can make 3-phase power for me?
Yes. For motors up to about 3HP, VFD's are available that are designed to take single phase in, and generate 3 phase out. For larger than 3 HP, you generally won't find a VFD that is built for single phase input. But many 3 phase VFDs will run with 1 phase input. To make up for the missing phases, you need a VFD that is rated for more than the motor size, usually by a factor of 2x. So running a 5HP motor requires a 10HP VFD if the input is only single phase. You will also need to check to see if the VFD has input phase loss detection. If so, that needs to be disabled, which is not an option in all VFDs.

So, should I use a VFD for speed control on my gearhead lathe?

The short answer is no.
For machining we like to specify cutting speed in surface feet per min (or metric units). Most cutting is rotary, either a lathe spinning the object being cut or a mill spinning a tool of some sort. Bigger objects or tool bits are turned slower (less RPM) to get the same FPM. But bigger means we need more torque from the spindle. So to make the most of the available motor power, we want torque to go up as RPM goes down.

If you look at the following picture, available torque is on the left axis, RPM is on the bottom. HP is equal to RPM x Torque (times a fudge factor for whatever units you are using. I didn't put exact numbers on the picture so with ignore the fudge factor). For an electric motor running at a certain RPM, the torque will vary to just enough torque to overcome the motors internal friction at a minimum, up to a maximum torque under load before the motor overheats or a breaker trips.
For a 1 HP motor, we can have 1 RPM motor with a maximum of 1 unit, or a maximum of 0.1 units of torque at 100 RPM, or any other combination that multiplies out to 1. But since electric motors are designed to run at a specific RPM corresponding to 60Hz (or 50Hz), without gears, pulleys, or a VFD, we are stuck with that RPM.

Gears (or pulleys) allow different RPM/torque combinations to be selected. I've show 5 red lines illustrating a 5 speed selection of torque/RPM combinations. At higher RPM we get less torque. At low RPM we get lots of torque. This is basically what we want, because as our tool size or turning radius increases, we want more torque and less RPM. So gearing looks pretty good. Of course we're stuck with only those 5 possibilities for our fixed RPM electric motor, unless we add more gears. And gears do add more friction, and loss, so I showed the red lines a bit below the constant HP.

Note Oct 21, 2020.jpg

If there are not changeable gears, the VFD has torque/RPM curve shown approximately like the blue line. A 1 HP electric motor (at 60Hz) actually only can produce 1/2 horsepower if run with a VFD at 30Hz. So if we shift the gears down we get more torque, but if we use a VFD we don't get any more torque, we just get slower speeds. We also get some power savings if we're running a fan, because our fan doesn't need as much power to run slower. This is why VFD's became popular, saving on the electrical bill for fans and pumps. But for a gearhead lathe (or any other geared equipment), the loss of torque is more of a concern. Remember, we wanted more torque as we went reduced RPM.
 
Also, as a motor is turned at lower RPM, it may have problems with cooling. The motor's fan is turning slower, heat builds up more. So turning the motor too slow is not ideal. Inverter-duty motors account for this with a better cooling. My comfort level is probably around 2/3's rated RPM for a non-inverter duty motor. Most inverter duty motors can handle considerably slower. But again, torque is lost.

As a VFD is used to overspeed a motor, the torque drops off as the speed goes up. But that matches up fairly closely with torque needs for machining.

Overspeeding a motor is not without problems though. More friction, more wear, and danger of the motor flying apart if run faster than it is designed to run. Rule of thumb might suggest running up to 1.5x speed is reasonable, 2x is probably pushing it, but you'd need to consult the specific motor manufacturer's documentation. Of course if you are putting a VFD on a machine that wasn't designed as a VFD machine, you may not even be able to find such documentation. And if you do it likely pre-dates VFDs, so it'll assume you're running at 60/50Hz.

So our VFD running our old motor has a "good" speed range between 1x and 1.5x at full HP, and maybe from 1/2x to 1x if we're willing to tolerate 50% torque loss at the low end. That's a lot of lost torque. Much better to use the gears and shift down to gain torque. You could use a VFD to get speeds in between gears.

So a VFD retrofit on an older machine is not as useful as it sounds. Of course we can buy a motor with twice the horsepower and use that with the VFD so we are back to getting the original torque at 50% RPM. But that means buying a new motor. And buying bigger VFD. And if we go over that roughly 3HP limit, we have to double the size of the VFD if we're using single phase input. So if we started with a 2 HP motor, and wanted that torque down to 50% RPM, we'd buy a 4HP motor, but since 4 HP motors aren't common we'd have to up to 5HP, and a 10HP VFD. Quite a bit of expense to save shifting down a gear. (Unless maybe that gear is broken??)

So a VFD might be a quick single-tool 3phase power conversion solution, especially for a single machine 3HP or less. Over that gets questionable in terms of cost. And it isn't really helpful for adjustable speed unless you add a new motor. And if you have a machine with more than one motor, forget it. While technically not impossible to run multiple motors off of a single VFD, it is really beyond the range of anyone trying to refit an existing machine.
 
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Moving beyond theory, I decided to run my first machine off a VFD rather than an RPC since I had limited space, and for a single machine the VFD route is cheaper than an initial RPC. (Again, that may not be ideal long term, more on that later).

One advantage of a VFD is it can simplify the control electronics. So if there a bunch of old contactors, etc, that are bad, a VFD looks better. I wanted to gut and redo the flaky control cabinet on my lathe. So I came up with the following simple schematic:
CK VFD.jpg
This doesn't show the low voltage VFD wiring/switches, or the ground wiring.
While a VFD provides quite a bit of motor overload protection, you could argue that fuses should be added. Also, my 3HP VFD in theory could use more than 20 amps. So I should be running it off a 30 amp circuit, but 20 amps hasn't been a problem.

Here is a picture of assembled control panel:
IMG_3323.jpg

I missed one other technical consideration for VFDs, carrier frequency. I’ll post on that soon.
 
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On your diagram, you illustrate the control contactor being operated by a switch.

Proper way is to use a start and stop button, with the start button pulling in the contactor, an auxiliary contact of the contactor HOLDING it in the run position, and the STOP button knocking the contactor out...

The result, is that if you experience a power interruption while running, or the switch is left on, the machine will not self-start on power resumption.

Also, remove the mechanical cooling fan from your drive motor, and fit a constant-speed fan to the shroud. it'll be quieter (especially when using overspeeding), and also include a 120v receptacle to the box to connect your worklight and coolant pump.

Also, many of the predispositions of performance and application you suggest are not particularly well founded... as an example, a gearhead lathe has no difficulty running on a VFD. I had a Cinci Traytop that worked wonderfully running an old style 2hp motor on an Allen-Bradley 1336S. The only genuine concern one would have with a lathe's drive system, is where the constant input speed is running a lubricating pump that is sensitive to that input speed. In any other case, the VFD provides variable speed, with the advantage of selectable range. Overspeeding is incredibly effective, and 'flying apart' a motor's hard parts simply doesn't happen unless someone has neglected to clean it up and install some appropriate type and quality bearings. When overspeeding, removing the cooling fan as noted above, is an extremely good idea, as overspeed puts a substantially higher load on the motor's integral cooling fan, which if it fails WILL spit out fragments, but the really noticable concern, is that the fan becomes an air-raid siren, An 'inverter rated' motor has some advantages, but there's absolutely nothing wrong with most average 3phase motors that precludes the use of a VFD.

Switching frequency is a subject that is most often made a big deal, when it's not. Once a system is up and running, experimenting with the available choices may prove one frequency somewhat better in a particular speed segment, while another may have a less offensive sound to the operator, but probably a much more offensive timbre to the family dog. Set it, and leave it alone.

While the theory may suggest that a motor will exhibit less horsepower at speeds above or below the synchronous rated speed, a VFD-driven 3 phase motor will demonstrate considerably more tractive power than your notes suggest. My radial drill and lathe are both driven by 3:1 reduction tooth-belt drives, programmed for overspeed of about 210hz, and at just a few RPM of the spindle, would rip my arm off if I were to snag my clothing. The drilling head is a Bridgeport J, and without the backgear engaged, will twist a 3/4" tap in half at a speed slower than I would turn a tap wrench by hand... hence, I have that machine set up with a footpedal 'dead-man', that places the VFD in STOP if I step off. Also note- I've programmed my drives to utilize dynamic AND DC-injection braking to assure that if I DO step off that pedal, it 'throws out an anchor' to affect a most immediate halt... this is one of the greatest safety advantages of the VFD.
 








 
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