Single phase, 3-phase, VFDs, RPCs, and DPCs... time for some schoolin'

[ArmyDoc]

All of the systems listed above work fine, subject to the limitations noted. It's really a matter of choosing the most economical system for the application. In my shop for instance, I have VFDs, a RPC, and one static phase converter.

So let's look at my applications:

13x40 manual lathe, 3HP, static phase converter. This was installed almost 30 years ago, before the availability of inexpensive VFDs. In this application I don't need variable speed or a soft start so I have not replaced it with a VFD. If the SPC ever fails, I will replace it with a VFD.

3HP milling machine, single phase input rated VFD. This machine requires variable speed, dynamic braking, and full torque at any speed so a VFD is the logical choice. Because it's only 3HP, the VFD is relatively inexpensive and a lot of options are available. This also gives me the option of running the spindle from the computer if needed, but I normally run it manually. The rest of the controls on the machine are 120V and are literally plugged into a wall outlet.

CNC lathe, 10HP spindle, 15HP RPC. While it would be possible to run this machine on a single phase input, a VFD to run the spindle on single phase would be crazy expensive. Also, the hydraulic pump motor and the coolant pump motors are all 3 phase, so add in the cost of dealing with those. The 4 servo drives will accept single phase input, but will not perform at their rated capacity on single phase. So in this case it made sense to build a RPC to run the machine. As an added bonus, the RPC also runs the 5HP press brake motor just fine. Had I not had the RPC already installed, I would have used a VFD to run the press brake. The cost of a VFD is normally less than a RPC.

Air compressor, 5hp, single phase input rated VFD. In this case I got tired of burning up single phase motors every 2 years, so I replaced it with a 3 phase motor. I could have run the compressor from the RPC, but it made no sense to do so. No point in running the RPC to power the air compressor when other economical options are available, in addition the cost of a 3 phase, 5HP motor starter is almost the cost of a VFD, then add in the additional wiring. The air compressor is needed all the time, where the lathe is only used part time. I purchased a cheap 10HP, HY VFD that is single phase input rated. This has the advantage of a soft start so the starting current never exceeds the normal running current, I have it set to ramp up to speed over 10 seconds.

So as you can see, the choice of single phase to 3 phase conversion really depends on the application.

This was very helpful... Not the answer I expected, but very helpful.

I was going to ask at what point you go to RPC or DPC, thinking the decision point was based on number of and cost for VFDs. In other words, once you had enough machines that the number of VFDs cost the same as a RPC or DPC, you would just get one of those. Obviously, this would eliminate the benefits of the VFD's speed control features, but the machines were designed without that, so that's a bonus, not a requirement.

Based on your post and some other reading, the main indications for an RPC/DPC is if one machine has multiple 3 phase motors in it (like your lathe), or if it a machine sensitive to the high frequency noise VFDs create, like a CNC machine.

One thing that I don't understand is why you have to wire a VFD into each motor. I get that the benefit is the variable frequency, but could you use one 10hp VFD to run multiple machines like you can with an RPC?

 

[Dabbler]

but could you use one 10hp VFD to run multiple machines like you can with an RPC?

@ArmyDoc it can't be done using the current technology of VFDs. It is specifically forbidden in my manual to put a switch after the VFD to the motor(s). That's because the VFD interacts with the motor, monitoring its response to the provided energy (a gross simplification). When it is disconnected, even for a tiny interval, the effect on the VFD can be damaging. If your machine has 2 3 phase motors that are always connected together in parallel, I don;t see the problem, but it would require programming the VFD for the combined current.

 

[graham-xrf]

One thing that I don't understand is why you have to wire a VFD into each motor. I get that the benefit is the variable frequency, but could you use one 10hp VFD to run multiple machines like you can with an RPC?

Yes - you can use a single large electronic device to make 3-phase, but it is not a "VFD", nor is "variable frequency" what is done.
It would be a purposed device for generation of 3-phase, at a fixed 60Hz frequency, without any attempt to control the power and torque delivery, because it is not dedicated to a single motor. That would be what you would, and still could, use rotary converters for. I think the price break point, and overload control features may have swung in favour of all-electronic converters by now.

In post #9, I tried to point out the essential thing different about what we are calling "VFDs". They do not simply vary the frequency, and even those asked to deliver very large powers (like in ships, and power stations) do not change low (60Hz) frequency anymore. Wind turbine's megawatts electronics have to deliver at fixed frequency from a very variable input. We called the smaller motor controllers "VFDs", and the name stuck, and that's OK, so long as we understand exactly what they actually do, and what we are using them for.

We have always known that with squirrel-cage type induction motors, one could get a certain amount of "speed control" by changing the frequency. There are very definite limits to how far one can go with this. Go slow enough under load, and the torque becomes feeble. Getting it to start up from scratch (locked torque) is a bit of a trick!

You use a power controller, usually working to obey a speed demand input, but not necessarily. Sometimes we want them to deliver a constant demanded torque, or some other recipe. HM folk usually just want a desired cutting speed, and enough grunt to obey, even if the desire is for something unreasonably slow. A modern pulse-width modulated switch-mode power controller, still called "VFD", does an excellent job. The added value is the power, torque, and speed control features, and the flexibility of being able to conjure up 3-phase power from single phase sources. That is the primary reason we have "one smaller VFD per machine", instead of one large one for the whole shop.

For a shop running with more than one 10HP machine's worth simultaneously, it is a serious amount of power in a domestic context. I can only guess that the property has enough power available. Certainly, you would be taking it from the 230V phases around the 115V neutral. Whether all-electronic, or distributed 3-phase, a central unit would likely get expensive.

[Edit]
Another technical issue
As @Dabbler has pointed out, you cannot suddenly disconnect a power controller from a motor. That is inviting it to try for infinity while deprived of the feedback information about what the motor is doing. There is also what to do with the spinning energy when you ask it to stop. Most controllers will dump it into a braking resistor in a controlled way.

A industrial 3PH power converter operates differently, It tries to maintain an output voltage and phase, within overload limits, and can have the load be switched out without harm. That is not to be found, even in a "big" VFD.

 

[JimDawson]

This was very helpful... Not the answer I expected, but very helpful.

I was going to ask at what point you go to RPC or DPC, thinking the decision point was based on number of and cost for VFDs. In other words, once you had enough machines that the number of VFDs cost the same as a RPC or DPC, you would just get one of those. Obviously, this would eliminate the benefits of the VFD's speed control features, but the machines were designed without that, so that's a bonus, not a requirement.

If I had a number of machines that required 3 phase and did not require variable speed and/or a soft start, then a RCP or DPC might be appropriate. This would be a rather unusual situation, especially in the home shop or even a small commercial shop where it would be unlikely that you would have duplicate or several 3 phase machines that would not benefit by having a VFD on the individual machines.

Based on your post and some other reading, the main indications for an RPC/DPC is if one machine has multiple 3 phase motors in it (like your lathe), or if it a machine sensitive to the high frequency noise VFDs create, like a CNC machine.

In the case of my lathe or any other machine that has a mix of variable and constant speed motors, it is best to supply the machine with 3 phase power. So a RPC or DCP is the best solution. For the reasons stated below, a VFD is not a substitute for a RPC or DPC. While it would be possible to install a VFD for each of the constant speed motors, the cost would exceed the cost of a RCP that would run the whole system.

One thing that I don't understand is why you have to wire a VFD into each motor. I get that the benefit is the variable frequency, but could you use one 10hp VFD to run multiple machines like you can with an RPC?

As @Dabbler said above, a VFD needs to always be connected to the motor(s). They are not designed to be powered on when connecting/disconnecting to the load. A DPC is sort of a VFD that is designed to be connected/disconnected from a load under power, but does not have variable frequency output and does not try to adjust the load output. The electronics inside the box are similar, but the system is designed to function differently. A RPC is really just a big dumb generator and doesn't care if the load is connected/disconnected under power.

 

[mksj]

To add to what Jim outlined, VFD's can drive multiple motors in a V/Hz setting but each motor would need some level of overload protection and there are some limitations if different motors were to be switched in/out of the circuit, the VFD would also need to be oversized. A single VFD may be used on something like a conveyor belt with multiple drive motors all moving at the same speed. When running sensorelss vector this is not possible because the VFDS uses the individual motor feedback to adjust the speed, the VFD is tuned to a particular motor. VFD's can be programmed for different motors (typically 2), so it is possible to switch between two motors, but not while they are running. You cannot use a VFD to run control systems, transformers, capacitors, etc. so you cannot use it as a general power source vs. a DPC which is designed to do so because the generated 3rd leg is filtered to closely mimic the other two legs which are passed through.

 

[ArmyDoc]

So, as Mr Einstein says "no free lunch for you..."









(yes, I know that was John Ruskin, but they didn't have a picture of him sticking his tongue out at me...)

 

[Mitch Alsup]

Single phase is what we have from the socket in the US. 110v, 60 hz. If you want 220v single phase, you are really just running two 110v lines in opposite phase, the difference between the two is 220 volts. (correct?)

It is much more accurate to state that 220V(240V) is the single phase power and the 120V is one leg of 220V and the neutral line.

3 wire [+,N,-]: [+,-] gives 220V, [+,N] or [N,-] gives 120V.

Three phase runs three lines, each with a sinusoidal (AC) current, where each is out of phase by the other by 60 degrees.

120 degrees = 360 degrees divided by 3 phases

The voltage of each can vary, (some industrial machines can be wired 220 or 440) but for most of our machines it seems 220 is the most common.

A lot of motors have 6 independent coils. If you wire corresponding poles in series it is a 2× voltage motor (440V) if you wire corresponding poles in parallel it is a 1× motor (220V) {God help you if you wire non-corresponding poles together in either series of parallel.}

The benefit of 3-phase motors is that they have 3 windings, and each is being powered, which allows smoother power delivery, and about 1.5x higher power

it is closer to 20% more power (83% compared to 100%)

for the same same size motor. (which is why a single phase machine might be rated at 2hp, but the 3-phase version would be 3hp). Three phase motors are self starting, where as single phase aren't (and require a starting capacitor) and because of this are more reliable. Three phase motors provide more uniform torque than single phase. Wiring can be cheaper because 3 phase wires only have to be 75% of the weight required for equivalent power in single phase.

Ok, so 3 phase is the clear winner. And obviously, this why we don't have it run to our houses here in the US. It was recognized early on that you could make a killing getting the guys like us to buy extra stuff to convert single phase to 3-phase. And thus the market for converting 3-phase to single phase was born.

It is cheaper to wire a city for 1 phase. Blame the taxpayers for no 3-phase to most of us.

The next option is a Variable Frequency Driver. VFDs are solid state devices. They basically use rectifiers to convert to DC, and then use inverters and some other fancy electronics to generate 3-phase in a sinusoidal shape. This is not a true sinusoidal curve though.

it is a step from -220V to 0V and another step to 220V. This is NOT good for the bearings supporting the motor as the square wave generate high frequency which causes micro-arcs from race-to-ball-to-race.

 

[Grey]

@ArmyDoc - OK one tiny modification you your post: there are capacitors on the output legs of the VFD. They LOOK like they are there to filter the waveform, but at the current draws they aren't really big enough to do that. they are part of the HF filter, which nearly always includes large inductors in the output circuit as well. My 2HP TECO uses 100MF 600V output capacitors, not enough power storage for more than a few microseconds at full draw.

Manufacturers also recommend and external filter on the output leads to reduce EMF further.

Clough42 did a great youtube video series on installing a VFD - first rate work by him! He has 4 or 5 videos on the subject.

sorry if I was redundant, the other posts above happened while I was thinking!

Thanks for the link. Was looking for the choke info and his video was informative.

 

[Dabbler]

It is cheaper to wire a city for 1 phase.

@Mitch Alsup -- one correction. In all Canadian cities and all the US cities I've visited, the distribution network is all 3 phase. It is far cheaper to use 3 phase all over the city.

Our city distributes at 14,400V, and then steps down every few blocks to 4000V 3 phase. Every block has a single phase transformer to step down to 220V centre tapped, which is distributed to the block.

If you are very lucky, like my friend John, He had a 14,400V feeder line in his back alley. His 3phase 240v (with centre taps) installation came to 4000$ with a 45$ per month minimum on his power bill. At 200A, no RPC could do the work of his electrical panel...

 

[strantor]

@ArmyDoc it's obvious you did your homework, thanks. It makes whatever the follow-up questions easier. So what are the follow-up questions?

This thread is so well covered that I barely have anything left to add other than my opinion that the price of VFDs now is so low that they ought to be the shotgun answer for just about everything in the hobbyist's shop. The typical hobbyist's shop has no motors >5hp and most of the applications are simple re-powers of simple 3ph machines that never had a control system, just a motor with start/stop/reverse. There is no concern about using one VFD for multiple machines* so long as you stick to V/Hz mode** and you don't plug/unplug/reverse (via mechanical reversing switch) motors under power.

* machine implies a simple single motor machine like a lathe or mill. In the case of a more advanced machine with multiple 3ph motors (lube pump, coolant pump, etc), RPC is a better bet.

**V/Hz mode is the simplest mode and does not use feedback (amps, back-EMF, etc.) to control the output voltage. Theoretically you should be able to run a multiple-motor machine from this, and probably even disconnect under power. But I don't recall seeing any VFD manufacturer explicitly condone this, so I won't either.