What's new
What's new

Is anyone using a digital phase converter?

Those are interesting. I think Johnoder has one, he is a member on this board. They are especially good for variable loads on the RPC. If a person were to balance a regular RPC to provide power to say a 7hp lathe and a 3hp grinder you might put capacitors at each machines control panel separately to achieve optimum balance (no big deal really) for that specific machine. While the phase perfect does that automatically (for a price) as machines are added or subtracted from the load.
 
Is this one of those posts in the wrong section?
 
TMD,
Search through the Phase converter area and you will find quite a bit of discussion on the PP units.

I myself, am completely in love with mine. :D
I went for the 20HP version and it runs my whole shop. Sweet!


Happy Hunting.

ARB
 
Looking at the block diagram in their "technical overview", I am confused as to why these things are all that much better than an RPC or VFD.

http://www.phaseperfect.com/images/phasew1.gif

They clearly show that all the fancy circuitry actually generates only ONE phase, with the other 2 simply being passed through the unit from the input legs, just as with an RPC. In a typical 240/120 single phase service, these 2 legs will be 180 electrical degrees out of phase, whereas in true 3-phase power they would be only 120 degrees apart. The output will NOT be real 3-phase power, but 2 single phase legs with a generated third leg presumably 90 degrees out of phase with either of the other 2. Probably fine for most machine tool applications, but hardly "BETTER THAN UTILITY THREE-PHASE!", as the website claims.

Yes, it will automatically compensate for changing loads, but fundamentally it produces the same kind of "bastard" output as an RPC. A true electronic converter would operate more like a VFD, rectifying the incoming single phase power to DC, then using 3 oscillators/amplifiers 120 degrees apart to generate the outputs. Such units ARE manufactured, and are more commonly used in frequency conversion applications, such as generating 400 Hz 3-phase for bench testing aircraft electronics. A typical unit is shown here:

http://www.novaelectric.com/frequency_converters/cabinetsec.php
 
I got the 10 HP one PP makes, which was close to $2675 with shipping. Perfect phase relationships? Probably not. But since it is WAY better than the latest perfectly balanced version of my homegrown RPC from a noise standpoint, I would do the same again instantly, or maybe even the 20HP mid size one. I made a raised bracket to sit it on that incorporates a filter box to keep Houston's assorted critter bugs/lizards out of the electronics. When it is warm ambient the twin fans come on, otherwise it is virtually silent. That is worth a whole lot to me (no noise boxes like radios in this shop). The Fanuc CNC (AC servos) seems to think it is peachy keen three phase. Plus since I have visited the Black Hills a few times, I like to think it was made in just the right place.

It does have a high leg - the mfg leg on mine is a little over 200V to ground. Something to watch out for. I came close to frying a fan in the CNC.

John
 
"Yes, it will automatically compensate for changing loads, but fundamentally it produces the same kind of 'bastard' output as an RPC. A true electronic converter would operate more like a VFD, rectifying the incoming single phase power to DC, then using 3 oscillators/amplifiers 120 degrees apart to generate the outputs. Such units ARE manufactured, and are more commonly used in frequency conversion applications, such as generating 400 Hz 3-phase for bench testing aircraft electronics."

But, those are frequency and phase converters, not just phase converters.

Where phase conversion, only, is required, not frequency conversion as well, then the Phase Perfect device produces ... er ... perfect three-phase power from single-phase.

(And, the Phase Perfect can do so with 100 percent power factor, which most frequency/phase converters cannot do).

An examination of the "phasor" diagram of a phase converter would prove my assertion beyond doubt.

I've posted these diagrams, and discussed these at length many times before. I'll not do so here.

Check the archives of this Forum.


"It does have a high leg - the mfg leg on mine is a little over 200V to ground. Something to watch out for. I came close to frying a fan in the CNC."

207.8 volts to line-to-ground is nominal for the high-leg of a 240 volt three-phase system, utility or Phase Perfect.

If your premises is more like those found towards the end of a highly-loaded distribution line, where, say 230 line-to-line would be found, then the nominal high-leg would be 199.2 volts, also utility or Phase Perfect.
 
I have been wrestling with this question- whether to build or buy.

Noise seems to be an issue with DIY RPCs. Personally, I couldn't give a fat rats. The "digital" units might create "cleaner" output, and they switch in capacitors relative to load automatically. Hovever, for what they cost, I could build a RPC and buy that Bridgeport as well!

Besides, building this stuff is half the fun.

I'm sure that if a way could be devised to switch in the capacitors relative to load then one could build a DIY "digital" RPC. Might be a use for that old Pentium 1 computer gathering dust in the cupboard....

The big question is: "digital" RPCs are 400% the price of a DIY unit. Are they 400% better?
 
"I'm sure that if a way could be devised to switch in the capacitors relative to load then one could build a DIY 'digital' RPC."

Dropping back to basics, a phase converter, rotary or digital, is providing a source/sink of volt-amperes reactive (VARs), in response to the load conditions, transient and steady state.

A rotary is usually stuck with a "one size fits all" solution, and the use of real capacitors, whereas a digital can be a "best fit" solution, with the use of (digitally) simulated capacitors.

Implicit in the digital is the ability to compensate for transient load changes, which occur within a cycle, as well as long term load changes, such as powering on a machine, with equal facility.

Also, a digital can present a unity power factor load to its source, whereas a rotary almost always presents a lower if not very low power factor, usually lagging, to its source.

Strategies for distributing the source of leading VARs, the capacitors, have been discussed here previously.

Essentially, this means adding the required VARs, that is, the capacitors, on the load side of each machine's main disconnect.

The makers of Phase Perfect attempted to market a lower cost digital, which they dubbed "Phase Simple", but it was quickly withdrawn from the market.

Apparently, a digital of the type hypothesized by the earlier writer, namely one which is conceptually a fixed frequency VFD, was not a success.

If you have any expensive CNC equipment, a digital, such as a Phase Perfect, may be the best solution.

Otherwise, a rotary, even a DIY one, may be the most cost effective solution.
 
My concept of a computer controlled RPC would involve sensing of the voltage on the three legs and switching in actual capacitors in various combinations to adjust the output voltage relative to the load. Obviously this would create a very complex device which may not be worthwhile from an economic perspective. It is a bit of a pie-in-the-sky idea but theoretically possible for someone with lots of resources- ie time and money.

However the concept would not be wholly "digital" but rather a combination of digital and analogue ie a PC with I/O card/s, with the relays on the I/O card/s switching in contactors, which control the combinations of capacitors to adjust the output voltage.

To do this properly I might need 400 contactors and 10,000 capacitors to make it work, and might not be worth the trouble, but if I ever run out of things to do (not likely!) I might have a go.
 
What is to be said about the power requirments of firing up a 40HP motor vs. Digital converter in a not so ideal power situation like residential/shop use?
Or, how about the dangers of hooking up a RPC/Homemade RPC to a new $80K CNC machine.
These are the issues I have been wrestling with that has me lean towards the direction of a Phase Perfect unit.
 
"Or, how about the dangers of hooking up a RPC/Homemade RPC to a new $80K CNC machine."

Or an $80K manual lathe, such as the Monarch 10EE.


"These are the issues I have been wrestling with that has me lean towards the direction of a Phase Perfect unit."

If the machine incorporates a Regenerative Drive, it is not possible to employ an RPC.

Alternatives include a Phase Perfect, and, of course, utility three-phase.

Also a diesel generator set "stationary engine".

An RPC is a good source of simulated three-phase power, but it is not a good sink for three-phase power.

And, what a Regenerative Drive does in order to effect its version of Dynamic Braking is to convert kinetic energy from the spindle motor, which is dc, into PWM three-phase ac, which two of the RPC's three phases can absorb and can dissipate, but which the RPC's "manufactured" phase cannot absorb and cannot dissipate.

Attempting to do so will destroy one or more of the "power blocks" within the Regenerative Drive.

OTOH, a single-phase Regenerative Drive has no such problem, however such a drive should never be operated from the RPC's "manufactured" phase, as is also true of other drive types.

So, the general rule-of-thumb is never connect a Regenerative Drive to an RPC system.
 
A misconception is being propagated here that will only continue to confuse folks.

Both an properly balanced RPC and a Phase Perfect generate a "high leg" that is approx. 200V above *ground*. There is nothing improper or assymetric or unbalanced about that. What you need to wrap your mind around is that the 3 phase output of the RPC or PP is not rotating around *ground* but around a virtual neutral that is offset around 40V from ground.

Viewed from that offset neutral reference point, the RPC and PP output is a true symmetric 120-degree separated 3 phase, with 240V between each phase.

Part of this bit to wrap your mind around is that with either an RPC or a PP, it is not safe to just pick one of the 3 legs at random and connect a 120V load between the leg and the utility neutral (grounded conductor).

To be safe with an RPC, you always need to identify the two utility legs and connect any 120V loads between them and the utility neutral, and any 240V 1ph loads between the two utility legs. What the typical RPC user observes is a control circuit that gets powered from the generated leg and works flakey or doesn't work at all, because when a load motor starts up it drops the leg too low for a contactor to hold.

To be safe with the PP, you must do the same with the 120V loads, but for 240V 1ph loads for example control transformers, unlike an RPC, with the PP you can safely connect the load between any 2 of the 3 legs. One advantage of the PP is that the output has a true surge capacity, it does not droop like an RPC and cause problems with control circuits powered from the generated leg.

In general, in machinery connected to any form of 3 phase power, control circuitry and single phase accessories such as lights and pumps should never be powered from a leg-to-neutral connection, and certainly never from a leg-to-ground connection. That is why you find 500VA or 1KVA control transformers used to power 120V accessory loads. The intent, among other things, is to safely power the 120V loads independent of whatever oddball utility, RPC or other 3 phase source is powering the machine.

Bob
 
"To be safe with the PP, you must do the same with the 120V loads, but for 240V 1ph loads for example control transformers, unlike an RPC, with the PP you can safely connect the load between any 2 of the 3 legs."

Single-phase loads, of any kind, anywhere, should be connected to the L1 and L2 lines, never to the B phase, whether an RPC or a PP.

In the PP case, its B phase is indeed a little safer than the B phase of an RPC, but not that much safer than one can state as an absolute that it is always safe to connect any 240 volt single-phase load to any two phases of a PP.
 
They clearly show that all the fancy circuitry actually generates only ONE phase, with the other 2 simply being passed through the unit from the input legs, just as with an RPC. In a typical 240/120 single phase service, these 2 legs will be 180 electrical degrees out of phase, whereas in true 3-phase power they would be only 120 degrees apart. The output will NOT be real 3-phase power, but 2 single phase legs with a generated third leg presumably 90 degrees out of phase with either of the other 2. Probably fine for most machine tool applications, but hardly "BETTER THAN UTILITY THREE-PHASE!", as the website claims.
It was my understanding that the purpose of using a Phase Perfect over a VFD (argument being that both use at least one simulated phase) was that the PP can better handle regenerative cases than the VFD because 2 legs are still attached to the utility and not going through all the sensitive digital circuitry. Then of course, it can dynamically respond to load changes unlike an RPC. I've not used one, however, to test my theory. If you have a high-dollar cnc control it might be worthwhile. Just running manual machines -- probably not.

Chad
 








 
Back
Top