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

[ArmyDoc]

I've been researching single phase and three phase power and various ways to convert from one to the other. Here's my understanding so far. Please correct me if / when / where I am wrong.

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?)

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

So, about that. As near as I can tell, you can convert single phase to 3 phase in one of 4 ways. Three are good / safe / etc. These are: Rotary Phase Converters, Variable Frequency Drive and Digital Phase Converters. (There is another way, which is kind of ghetto, using capacitors to shift the phase, but the guy demonstrating it had his motor over heat while on camera... so yeah, I think I'll avoid that one).

The first way is Rotary Phase Converters. RPC's are mechanical in nature. They use a 3 phase motor as a generator, where single phase is applied to two of the three windings, and this induces a current in the third. The problem is the current generated doesn't exactly match the other two legs, so this simple set-up isn't balanced, or well stable with power loads etc. (They couldn't make it easy where all you have to do is hook up a motor as a generator and you'd be all set... I mean, really, where's the money in that?) So, they add a bunch of other expensive electronic stuff in a box to clean it up, and voila - 3 phase power. The benefit of RPCs is that with one good appropriately sized RPC, you can use to power all the 3 phase equipment in your shop. It outputs 3 phase that is just like what you would get from the power company. Plug your 3 phase machine in, and it works. The down sides are you just get straight 3-phase. You can't vary the frequency up or down to use it as a speed control. (Although, you could run a 3-phase to 3-phase VFD for to each machine, which would let you do that I guess...) Also These are pretty expensive - (~1k for 10hp)

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 generated digitally so it's more of an approximation of the sine-wave, and they use capacitors etc to smooth it out. The benefit is that by adjusting parameters, you can vary the frequency, which enables you to control your motor speed. There are some downsides, though. You need to have an induction rated motor (most newer motors are induction rated, I understand, so this is less of a problem). And the output can have some high frequency distortion spikes etc that does not play well with some machines. Also, it is my understanding that you need to wire it to the motor. In other words, it doesn't just output to a 3 phase receptacle that you can just plug the machine into. It also voids the warranty on some machines (all?). It's less expensive for one machine (~$330 for 3 hp) but adds up if you have multiple machine (3 x 330 = ~ 1k for 10hp)

The last option I hadn't heard about till today, and that is DPCs. Digital phase converters, as near as I can tell, are the newest category, and are set to replace RPCs. They are similar to a VFD in that they are a solid state device. They take single phase input, and convert it to a very clean (<2-3% distortion) and ballanced 3-phase output without the high frequency issues of VFDs. In fact, they are advertised as producing cleaner output than 3 phase direct from the power company. You can run multiple machines off one device, just like an RPC. The downsides are that you loose the ability to vary the frequency, and they are quite expensive (~3-3.5k for 10hp).

OK, I'm going to stop there. I have some questions, but I want to make sure my baseline understanding / assumptions are correct before I do.

 

[mksj]

More or less correct, In the US we have single phase, 240VAC is split phase, 3 phase is staggered at 120 degree phase angles. Three phase thus gives smoother power delivery, is more efficient, typically last longer and do not have limitations on number of start/stop cycles over a given time period. Single phase motors are not natively designed for variable speed, there are some VFD's designed for single phase motors, but they typically remove the capacitor and run the motor more as a 2 phase motor using the starter coil. The expense of all this makes it a poor option in my opinion.

RPC's there are issues in that wild leg which cannot be used for the control systems and there are also grounding issues at the motor end which effect the wiring configuration of the motor. "The three-phase output is delta configured. This means that voltage measurements between phases will be similar to input voltages. Voltages should only be measured line to line. Do not measure voltages line to ground. Most machinery requires a delta three phase voltage and this third line voltage to ground should not affect operation. If a piece of equipment requires wye voltage and a neutral connection, the output of the phase converter must be run through a delta-wye transformer to create the neutral. The phase converter must be started before the transformer is connected electrically. This is done with a safety disconnect switch.". RPC's need to be sized for the load, usually the idler is sized at about 2X the motor to be run, so requires a higher panel breaker, running costs and noise. There are some plug and play options for RPC's, but for they are expensive unless you build your own. On the plus side of RPC's you can run multiple machines, and there is no rewiring of the machine other than being cognizant of the wild leg and not using it to power the transformer or sensitive electronics.

VFD's for the most part need to be direct wired to the motor, and usually used for single motors when run in sensorless vector mode. They are relatively inexpensive for units in the 1-3 Hp range, beyond 3 Hp running off of single phase they get expensive and there is derating for 3 phase input models as well as additional expenses for a DC choke and noise filters due to increase electrical noise generated. There are also a lot of install considerations when installing VFD's and additional expenses for enclosures and such. Since the VFD output is PWM there are issues with insulation breakdown, and electrical noise, but in general this is not an issue with 240VAC motors and shorter cables. There can also be some issues with bearing currents and damage to bearings, this is more of an issue with higher voltage and industrial settings. Pluses for VFD is small size/weight, inexpensive for smaller motors, have the ability to be programmed for multiple functions (soft start, variable speed, fast braking, etc.), and ability to drive a motor at very high overload for short periods with overload protection. As far as voiding warranty, well that varies and you need to check with the individual distributor. My experience is it does not void a mechanical warranty issue.

As far as SS 3 phase converter's such as the Phase Perfect, they are very expensive and more directed to the industrial setting where small shops do not have access to 3 phase or it is prohibitively expensive to pull in. It is the closest to native 3 phase, and their newer converters have models with voltage doublers which is an attractive options for individuals needing to run 400-480V equipment off of single phase 240VAC. They do have some issues such as noise and heat which some people find annoying, there are also some issues around capacitor failures so they do need to be replaced every couple of years. They provide fixed output 3 phase, so one does not have the additional control with VFD's, but they can run VFD's and/or sensitive electronic equipment. They are rated for full output of the motor size, I believe they are in the 90% efficiency range so one does not need to upsize the connverter like with an RPC. Cost is probably the biggest limitation, and that is not going to come down in the future. I wouldn't be surprised if you start seeing these being sold through AliExpress some time in the future.

 

[markba633csi]

I thought the digital phase converter is essentially like a 1/3 VFD in that it synthesizes the third phase and passes the other two thru. So pulse width modulation like a VFD, with filtering on the third output to get a clean sine wave. I've heard of fancy PWM methods to minimize the unwanted harmonics using math tricks to make the filtering easier which is more than the cheap VFDs do- magic sine waves and such stuff- digital signal processing
At least that's how I understood it- maybe there are other types?
-Mark

 

[Dabbler]

@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!

 

[Dabbler]

@markba633csi SS 3phase converters take a single phase connection and synthesize 2 more phases, each 120 degrees apart.

 

[matthewsx]

You left one out, the static phase converter. I bought one from these guys and I'll leave it to them to explain how it works.

Static Phase Converters | Static 3 Phase Converters for Sale

Get consistent motor power with a static phase converter! These American-made converters are designed to operate 3 phase motors on single phase power.

www.northamericaphaseconverters.com

Basically with this kind of unit you just get one for each three phase machine and wire it in to 220v single phase. They can apparently also be used with a three phase motor to make a rotary converter.

It's working well for me until I do the VFD conversion Mark has so kindly documented for us on here.

John

 

[ArmyDoc]

You left one out, the static phase converter. I bought one from these guys and I'll leave it to them to explain how it works.

Static Phase Converters | Static 3 Phase Converters for Sale

Get consistent motor power with a static phase converter! These American-made converters are designed to operate 3 phase motors on single phase power.

www.northamericaphaseconverters.com

Basically with this kind of unit you just get one for each three phase machine and wire it in to 220v single phase. They can apparently also be used with a three phase motor to make a rotary converter.

It's working well for me until I do the VFD conversion Mark has so kindly documented for us on here.

John

As I understand it, these aren't true 3-phase. They use motor start capacitors to start a 3 phase motor on single phase power. The capacitors are disconnected once the motor reaches full RPM and the motor continues to run using just 2 of the windings. The only benefit is that you can use it to start up and drive a 3-phase motor, but you lose the power benefits and smoothness of real 3-phase.

 

[matthewsx]

As I understand it, these aren't true 3-phase. They use motor start capacitors to start a 3 phase motor on single phase power. The capacitors are disconnected once the motor reaches full RPM and the motor continues to run using just 2 of the windings. The only benefit is that you can use it to start up and drive a 3-phase motor, but you lose the power benefits and smoothness of real 3-phase.

That may be the case but it's working just fine on my lathe. I have a 3 phase model and although I will eventually change it to variable speed with a VFD I just wanted to get it running before putting in the effort to convert it. Plus, I like that I can use it to build a rotary converter down the line if I want to.

John

 

[graham-xrf]

Regular single phase motors (no VFD)
Single phase motors with a capacitor have the windings physically at 90° to each other. The single phase AC is applied to one of the windings. To contrive a second phase suitable for the other winding, the current is put through a capacitor. This capacitor initially charges up, and from then on, AC current through it is shifted through 90° where this time, the "degrees" means electrical phase timing degrees. The neutral is common, and the direction is changed by swapping the connection that gets the capacitor with the one that gets the direct line.

The AC power phase shift capacitor is special, basically constructed as two polarized capacitors in series, back-to-back, so as not to deteriorate, and the better to handle the high voltage.
- - - - - - - - - - - -- - -
VFDs
"VFDs" are now now no longer simple frequency changers, but use pulse-width modulated high frequency currents. They are capable of delivering high torque, even at low RPMs. On simple motor drive setups, they can sense the motor speed from the drive currents. On high precision servo control, they use shaft encoders to get the shaft position feedback.

The three phase case is easier to immediately appreciate, but for single phase motors, VFDs are able to synthesize power waveforms which surge on and off into the 90° spaced windings in the same way as would have been contrived with a phase-shift capacitor. Do not simply connect up a VFD to a single phase motor without losing the capacitor. If ever there was a time to RTFM, this is it!

@Dabbler is right in that one should not confuse external capacitors on VFD setups with the kind of capacitor used with conventional single phase motors. Capacitors, along with large inductors, all at the input to the VFD, are filters to stop high frequency power currents, with nasty harmonic content contaminating with the mains waveform through the rest of the building, possibly damaging other kit, or modifying their lifetimes. (Think lighting, computers, etc.) They can cause the Father and Mother of all EMC radiated RF interference.

On the output side, there are often capacitors as part of waveform modifiers, and snubber networks to tame high voltage switching transients, especially when starting or stopping. You may also find a power resistor to aid deceleration control energy dumping. Except in truly ghetto installations, expect the motor wires to have a grounded screen, properly bonded to the provided terminal at the VFD.

 

[JimDawson]

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.