2 phase? What is this symbol?

[Tony Shea]

I guess I'm one of those people who thinks terminology matters.

In the US residential power distribution system, you have a transformer that is center tapped on the secondary. The input to that transformer is single phase and the output is single phase (center tapped). It is absolutely not two phase.

Also, in the three phase system, if you take only two of the three wires, you have single phase. In order to get additional phases, you have to use all three wires, and that will give you 3-phase. When you have three wires, you can take any two and you'll have single phase between those two. There are three possible combinations of the wires (1-2, 1-3, 2-3) and you'll have single phase between each of those three combinations. Those three single phases will be 120 degrees apart.

The fact that in the residential power system you have a center tap line does not create another phase. You have 240 volts across the two "outside" connections on the secondary and 120 volts between either of the "outside" wires and the center tap. But it's all single phase.

When you connect power to a 240 volt load, you use just two wires and it's only possible to have a single phase between two wires.

Please stop this nonsense about residential power systems being two phase. It absolutely is not.

Mike

THANK YOU MIKE!!! Lets cut the crap and end the debate. Mike is exactly correct. The 240V coming into a house is a SINGLE 240V phase with 1 sign wave. Just because there is a center tap for 120V loads does not make it 2 phase. I think it's crazy that a European company would list a 2 under the phase section of the nameplate on a single phase machine. This is a complete misunderstanding of Electricity and worries me a bit about their engineers. I could go on for about an hour why this is all craziness but Mike's post sums it up perfectly.

Having said that, can you find the motor nameplate? I would love to see what the motor says. I would have a hard time believing that the motor has a 2 listed under the phase section of the nameplate. If it does, I would probably trash the thing....j/k

[Rod Sheridan]

Absolutely incorrect.

One 180 degree phase displacement is single phase, and requires 2 wires

2-phase requires 3 wires and has a 90 degree phase displacement

3 phase power requires a minimum of 3 wires and has a 120 degree phase displacement.

Regards, Rod

[Eric Arnsdorff]

This is getting a bit side tracked with the terminology.

Clearly residential "single phase" power has 2 "hot wires" providing AC voltage/power. The actual AC voltage referenced to the center neutral tap does have 2 different sin waves of AC voltage (they are shifted by 180 degrees and you can distinctly see the 2 different sin waves). It is not the same sine wave. If it were the voltage difference between them would be zero. Some of you define phase by the resulting difference between the two having a single AC sin wave. This is true too. But it is produced from 2 lines having a sine wave 180 degrees phase shift between them (or any other out of phase angle). Just like typical 3 phase has a voltage between each pair but is shifted 120 degrees. And yes the accepted terminology is to call using the voltage difference between the two wires as single phase. The single coil of a standard transformer is actually producing a polarity difference as the voltage difference is generated. It isn't the same as the utility producing 3 sin waves out phase. But it is still 2 sin waves out of phase when you reference it to a center tap neutral. We could go on discussing if 3 phase delta loads have 3 single phase loads or if 3 phase wye loads have 3 single phase loads.

This is where it all breaks down with the terminology. What is being counted when you have 3 phase power. If I have 3 phase supply that isn't connected to anything then it really isn't any of the terminology as best I can tell from these arguments. Or is it that 3 phase has to be generated from the source with 3 different devices which are mechanically operating out of phase with each other? Therefore, anything that isn't generated this way can't be called 2 phase or 3 phase, etc.

You can shift phases with transformers as well. They are called phase shifting transformers (you could probably have guessed that from the name). And by the way you can generate additional voltage phases by using inductors and capacitors. But if anything that has AC voltage between 2 legs that is a single sin wave then everything is single phase.

It doesn't matter!

The manufacturer put a symbol on this motor with what is being called 2 phase here - I don't see it actually specify 2 phase on the label. This isn't typical but it was done. Being upset by it doesn't change it.

If I had this item it would cause me to look closer at it to make sure. But it does appear to mean the 2 hot wires providing the AC sinusoidal specified voltage. There is a difference for some loads if it is 2 hot wires or a line and neutral. In addition, some things need both hot wires and the neutral. It's all about the load. I read this indication as saying it is expecting you to connect it to 2 hot AC wires out of phase with each other (in phase would result in the motor not running). I also suspect this was done from someone who was putting a motor for American power have the 2 hot wires used to make 240 VAC at 60 Hz. This motor may not operate from the residential voltage in the country of manufacture and they were simply trying to indicate it differently. That's just my speculation on why. And it's ok.

Maybe we could write a thesis on this label.

[Ken Fitzgerald]

For some reason I cannot edit my post to add this....edit button is gone

Anyway, here is the plate in question....it is exactly like Erik's plate, it lists as ~2 I typically understand this box to indicate phases, maybe I'm wrong. Regardless, is this is single or 3 phase machine??? As I mentioned it's a 9hp motor so I would tend to think 3 phase, and it is advertised as 3 phase, but the plate confuses me. What do you think??

Attachment 475887

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[Dan Friedrichs]

This is getting a bit side tracked with the terminology.

The whole point of this thread was that the incorrect use of "2 phase" confused the OP and left him unclear what type of power the saw actually used. So the value in using the correct terminology is evident. Had SCMI marked the machine using the correct terminology (1 phase), confusion would have been avoided.

The single coil of a standard transformer is actually producing a polarity difference as the voltage difference is generated. It isn't the same as the utility producing 3 sin waves out phase. But it is still 2 sin waves out of phase when you reference it to a center tap neutral.

The choice to use the center tap as the reference is arbitrary. We could choose to "ground" one of the "hot" legs, and then the output from the center tap and remaining "hot" leg would be:
Vct(t)= 170 cos (t)
Vh2(t) = 339 cos (t)

Now there is no polarity difference, just an amplitude difference. Would you argue this is "2 phase"? No - changing the amplitude is not the same as changing the phase.

By similar logic, what if we added an additional (second) center tap? Would that make this a "3 phase" transformer? No - each tap would just produce a different amplitude, but all signals would be in phase.

This is where it all breaks down with the terminology. What is being counted when you have 3 phase power. If I have 3 phase supply that isn't connected to anything then it really isn't any of the terminology as best I can tell from these arguments. Or is it that 3 phase has to be generated from the source with 3 different devices which are mechanically operating out of phase with each other? Therefore, anything that isn't generated this way can't be called 2 phase or 3 phase, etc.

A simpler way to understand it is to consider one of the key advantages of 3-phase power: ability to produce a field that rotates with time. With single-phase power (which include a center-tapped transformer), the lack of a phase shift means there is no ability to spatially rotate a magnetic field. This is why single-phase motors require a starting mechanism (e.g. start capacitor and centrifugal switch).

So a general requirement for polyphase power is that it provides a rotating field. This enables a polyphase motor to start without a separate mechanism to induce rotation. If a separate mechanism (start winding, centrifugal switch) is required, the system is not polyphase.