Wiring three phase: Oxtools/BarZ approach

[extropic]

@rabler

Reading your reply, I realize I didn't ask the question well enough.

In your reply, when you say "You really can’t define an angle without two phases", does that mean the two hot (120V) legs of 240V single phase service are not "two phases"?

 

[Dabbler]

In a delta 3phase system each of the two terminals make a single phase sine wave 240V RMS there are 3 ways to connect to a 3 phase source, and each of those sources are 120 degrees apart. In a Y 3 phase system each of the sources measure 208 volts RMS.

There is no other 'geometry' here.

There is no practical use for using two of the 3 phases BTW. I cringe when people refer to 240 V as "two Phase" based on the number of wires.
Please, just don't.

 

[ahazi]

What you are considering is where the neutral is referenced to. In standard three phase, the neutral is often considered to be the center of the triangle. So 120 degrees between legs relative to neutral. In high leg delta 3 phase (google it for pictures), the neutral is the center of one side of the triangle. So two legs are 180 and one is 90 from neutral, but it is still an equal lateral triangle, 240 volts between any two legs. 3 phase motors don’t use neutral, so they don’t care what neutral is referenced to. RPCs produce 3 leg delta, which is why you measure line to line voltage but get funky results for line to neutral.

Three leg delta is also fairly common in industrial use. It delivers the classic 3 phase, plus 240v and 120v single phase. Otherwise you get 3 phase with 208V leg to leg and 120V leg to center of the triangle neutral. A 240V 3 phase center to neutral would give something odd like 138V. At least I think that’s right, I’m doing geometry in my head so my math is not guaranteed.

Thanks! You are right. I ignored the fact that the neutral is not used and it is only 2 floating legs so it is easy to create a 3rd leg that has the proper phase relationship.

I grew up and did early tinkering many years ago with 240/380 volt systems. Each house received 3 phase power so the notion of rotary phase converter was non existent. The phases always aligned properly in regard to neutral in Y configuration. Power plugs were almost always 5 pin with 3 phases, neutral and ground. Ground was never tied up to neutral as this seems to be a US standard of tying the power transformer CT at the pole to ground.

 

[rabler]

@rabler

Reading your reply, I realize I didn't ask the question well enough.

In your reply, when you say "You really can’t define an angle without two phases", does that mean the two hot (120V) legs of 240V single phase service are not "two phases"?

The two legs of 120V are indeed 180 degrees out of phase. In my mind that is indeed two phases, described mathematically be two different angles, or two different vectors. But, they are generated by a center tapped transformer. On the high voltage side there is only one voltage, generating both 240 and/or splitting that into two sets of 120v. So power guys think of it as single phase. My background is more signal related so I would say its one phase of 240 or two phases of 120. This gets to be almost a religious argument, depending on background. Convention in nomenclature is to count phases on the powerline side.

 

[rabler]

In a delta 3phase system each of the two terminals make a single phase sine wave 240V RMS there are 3 ways to connect to a 3 phase source, and each of those sources are 120 degrees apart. In a Y 3 phase system each of the sources measure 208 volts RMS.

There is no other 'geometry' here.

@Dabbler Not completely true. See the wikipedia article on high leg delta for another example. Admittedly this is a variation of the delta configuration, but it is an important difference when a site uses both single and three phase.

Voltage is just a function of the number of windings on the transformer. You can generate either one of those delta or y configs from the same high voltage lines, or 440V, or …
Further, a transformer in itself doesn’t have a ground reference. It floats unless a ground is established. Where that ground is established is makes a difference, and that can be different at two different customers on the same high voltage three phase utility line, as long as those two customers are on different transformers.

Delta and Y are really terms applicable to how three phase power is used in a motor or three phase transformer windings. Since a power utility has to supply power to more than three phase motors, it’s oversimplifying to think those are the only configurations.

 

[ahazi]

Interesting discussion.

Reminds me of the methods to generate higher voltage with autotransformer (110 volt to 220 volt for example). The transformer needs to be sized only to half the load, the other half is directly from the source.

This explains how the rotary phase converter motors are much smaller than a similar sized 3 phase motor as it only needs to generate 1 phase that is 1/3 of the load.

 

[StevSmar]

I wondered what would happen if you forgot to turn the rotary phase converter on. Wouldn’t you be single phasing your three phase motor (and it would draw a lot of current)?

Perhaps if you connected one leg of the machines controls to the derived leg, then if that phase wasn’t present then either there would be no power available to the controls or the fuse of the controls would blow? (I don’t know what voltage the derived leg sits at when it’s not connected to the phase converter. It seems likely it will just float and can’t provide power?)

(Also, if the derived “neutral point” is not near zero, wouldn’t this also mean the insulation of the devices to ground needs to be higher too? And therefore the machines insulation voltage would also need to be confirmed?)

(I haven’t thought about this much since seeing the video(s (Oxtools and Keith Fenner), except for thinking I’d be very likely to forget to turn the rotary phase converter on eventually…)

 

[Dabbler]

oversimplifying to think those are the only configurations.

Agreed.

In the specific answer to the original question as to the notion that single phase is not 'in sync'. I am attempting to explain that each of the phases is a complete supply in itself. Many people are lured by the phase diagrams into false assumptions. Using 120v/240V systems here in N America further complicates people's understanding of what is a very simple system.

I could have written pages about the configurations I have seen in industrial settings. And then there's phase matching and distribution issues that are actually complex. We could talk for days!

I appreciate your clarification!

---

BTW the first Rotophase I installed: it was to repair the installation that was done by a "Master Electrician" who was supposed to be trained in both industrial and residential installations. He had mis-wired the capacitors in the converter (in those days, late 70s, the internals were not completely wired - they were adapatable to the installation re delta versus wye motors) He had ruined 3 of the 6 capacitors when testing. They literally burst.

I came in because it was I who recommended the electrician. Here's me early 20s - no electrician's ticket, no training, never wired 3 phase before... I looked at the manual, wired up the new capacitors ($700 worth), checked everything, and it worked for over 40 years. When the inspector came in, we told him about the incident, what I did to mitigate it, and he passed the installation. Here in Canada, your insurance is dependent on that inspection. This was in an industrial bay running a bunch of 3 phase equipment.

The business was sold and closed down 3 years ago. I'm using that same Rotophase today.

 

[rabler]

I wondered what would happen if you forgot to turn the rotary phase converter on. Wouldn’t you be single phasing your three phase motor (and it would draw a lot of current)?

Perhaps if you connected one leg of the machines controls to the derived leg, then if that phase wasn’t present then either there would be no power available to the controls or the fuse of the controls would blow? (I don’t know what voltage the derived leg sits at when it’s not connected to the phase converter. It seems likely it will just float and can’t provide power?)

(Also, if the derived “neutral point” is not near zero, wouldn’t this also mean the insulation of the devices to ground needs to be higher too? And therefore the machines insulation voltage would also need to be confirmed?)

(I haven’t thought about this much since seeing the video(s (Oxtools and Keith Fenner), except for thinking I’d be very likely to forget to turn the rotary phase converter on eventually…)

Yes, that should prevent you from being able to start the motors on many machines. But connecting controls to the derived leg is generally bad for two reasons: the derived leg is less well regulated and usually the controls are what are going to be more sensitive to the voltage fluctuation. Also, RPC's generally start better with no load on the generated leg, and the control circuits are the most likely to put a load on that leg with the end machine not fully on. I've retrofitted my three phase lathe with a bright indicator light across the controls leg to show that it is on so I know to throw it's internal disconnect switch, although my RPC is set up so that when it is off, all three legs are disconnected.

If your RPC isn't running, your three phase motors generally won't start at all, so you'd know something was wrong. Hopefully you'd turn the machine off relatively quickly.

 

[Dabbler]

that should prevent you from being able to start the motors on many machines.

In my installations, there is a 3 phase contactor that does not engage until the phase converter is up to speed, - about 2 seconds. That way, all the legs are dead until all legs are available. I think of this as a safety issue.

In one shop with a home made converter. the guy turned on his lathe. All you could hear was a loud 60 cycle buzz. He said "oops" turned off his lathe , and then turned on his phase converter... and his lathe ran fine.