Here is a theoretical question for the electricians out there:
In discussing how to set up my new table saw with a 230-volt motor, I had two people (who generally know their stuff) tell me that anything with more than one hot lead could not be single phase. The motor name plate (as well as the specs on a lot of saws) say just that: single phase, 230 volts. (Some are dual voltage, 115/130, single phase.)
I would like to explain this to my two colleagues. How do I?
Thanks
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-Groucho Marx
Replies
i have both a air compressor and a large table saw that are 110 /220 usable single phase and they both work on single phase duh i dont have 3 phase hydro
if your pals are the best you can fid for tech nical info time to move to a new hood
move to a new hood
Maybe not that bad, yur friends jus dont know the proper defunisuns.
OK, very simply, in 120/240, (or 115/230, old nameplate?) one 120 V leg is 180 degrees phase shifted from the other, but it is still termed single phase in the power industry.
If you have 120/208, then you have 3 phase, where the 208 is the difference between 2 phases (of 3) that are 120 degrees phase shifted from each other. 208 = sqrt(3)*120 if you know basic trig and draw out a vector diagram of 3 voltages 120 deg phase difference.
Hope that helps and keeps your friends happy.
In my barely educated, non-electrician opinion: your connection is 2 110v feeds/hot and 1 neutral (plus a ground). The 2 110v feeds come off opposite sides of the CB panel. Each side of the panel is fed by a different supply from the POCO. So in essence 220v in residential can be seen as 110v 2 phase.
Electricinas and EE's; How f-ed up on my perception of this?
Your PUCO supplies a single phase high voltage. The transformer supplies a low voltage (220) volt output, but the center tap is grounded, so you end up with 2 hot wires with 110 volts referenced to ground that are opposite in polarity.
So it's still single phase, there is no real phase shift, just a voltage inversion.
so the AC sine wave for for both 110v is the same?
They are in phase but inverted, so when one is positive, the other is negative. They are not the same.
"They are in phase but inverted, so when one is positive, the other is negative. They are not the same."Youse guys (not just Mark) are all wrong on this inverted, 1/2 cycle off stuff.The waveforms are exactly the same! This is why it is single phase. The two waveforms are added together to make a bigger waveform, i.e. more voltage.Think of two transformers instead of a center tapped transformer. Each secondary has an output voltage waveform. Both waveforms are the same in time and voltage. You can wire the two secondaries in series two ways. One way the waveforms add together and you have twice the voltage. This would be the 120 and 240 example. The other way they cancel each other out, and you have 0 volts out! This would be the one is positive while the other is negative example. Think two "D" cells in series in a flashlight, one way the bulb lights, the other way the bulb darks.Frank DuVal
Correctly stated. Thanks.
The waveforms in 3-phase are exactly the same -- they're all 60Hz sine waves. It is the nature of sine waves that inversion and translation by 180 degrees are equivalent and indistinguishable. Either terminology is technically correct.
If your view never changes you're following the wrong leader
Good explanation Frank, very simple to picture.
A 120/240 system is 2 phase, a 240 volt or 120 volt system is single phase.To measure phase you need 3 connections. In this case the 3rd is the neutral.Phase is the angle between two different voltages. If you have a dual channel scope and connected one input from one leg to neutral (120) that would be the reference. The other input is from the other leg to neutral. You would see 2 traces 180 degrees apart.The engineer would say that was 2 phase, but has not practical purpose other than that you can combine them to get 240.When you talk about 240 you only have the single voltage and nothing to reference it agains then you have single phase.Thing of a very basic motor with a permanate magnet rotor and single coil around the outside. When the voltage reaches a peak one end side of the stator will be magnetic north and will atrack the south end of the stator. 1/120 of second later the stator will reverse polarity.But the motor does not know which way to flip so it just hums. If you tried to use the "2 phases" of a 120/240 system you would have the same problem as both coils would peak at the same time.With a 3 phase system you have 3 voltages, 120 degrees apart. So first one coil peaks, then the next one 120 mechanically around the stator and 1/360 second later. So the rotor moves in that direct. Then 3 rd phase 120 degrees from the second.For single phase motors a second coil is added and a capacitor is series to generate a second phase that is delayed from the first one.There where some much older systems that did supply a true 2 phase power and there was 90 degrees between the two phases.The basic rule is that if there is only 2 power connections it has to be single phase..
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
"A 120/240 system is 2 phase, a 240 volt or 120 volt system is single phase"NO! And you are usually right....120/240 is a single phase systemTwo Phase is an antiquated system, last used near Boston, where the waveforms are 90 degrees out of phase with each other.Three Phase is common in this country and the waveforms are 120 degrees out of phase with each other.A 120/240 system is single phase, as no waveforms are out of phase with each other. They must be in phase to add perfectly! i.e. 120 plus 120 equals 240. You may note on a 120/208 three phase system that 120 plus 120 equals 208.Frank DuVal
Did you read my WHOLE message?"A 120/240 system is single phase, as no waveforms are out of phase with each other. They must be in phase to add perfectly!"So you are saying that one leg of the 120 is IN PHASE with the other 120 leg.If that was the case then you could connect the two legs togehter. Get a 2 chanel scope and measure look at the two legs.They will be 180 degrees apart.That is more than one phase.I also explained (maybe poorly) how that 180 degree apart voltages won't operate a poly phase motor..
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Bill,I did read your whole message, I was just correcting the incorrect statement. And this may be a fault of our English language, but yes, the two waveforms, either hot to neutral, are in phase! It is just using the neutral conductor as the reference is making one look "out of phase".If you take the dual trace scope and attach the ground reference lead to one of the hot conductors (use an isolated ground scope or battery scope), channel A input to the neutral and channel B input to the other hot conductor, you will see the two waveforms are in phase! Trace A will be half the height of trace B, but they are IN PHASE!Frank DuVal
If you take the dual trace scope and attach the ground reference lead to one of the hot conductors (use an isolated ground scope or battery scope), channel A input to the neutral and channel B input to the other hot conductor, you will see the two waveforms are in phase! Trace A will be half the height of trace B, but they are IN PHASE!
Ah Frank, look again, by hooking channel A to neutral you are inverting that phase on the scope which is why you see the 2 waveforms 'in phase'.
If you put a diode from each side of 120/240 with neutral as common, you get a full wave rectified waveform - if 'in-phase', you would only get 1/2 wave rectification.
I read a pretty good analogy: if you're standing by the side of a interstate, look left and all the traffic will be coming towards you. Look right and all the traffic is moving away from you. But it's really all moving in the same direction, i.e. in phase.
This is what the scope sees with the common neutral point (you standing by the highway).....it shows the waveforms as being 180 shifted. To view it properly, you need to invert one side of the signal.
Edited 10/28/2007 10:37 am ET by edlee
Note that, for a general waveform (not a sine wave), being 180-degrees shifted IS NOT the same as being inverted.
If your view never changes you're following the wrong leader
edlee, This is a great analogy!Frank DuVal
Glad you guys don't design airplanes<G>
The discussion began as to why a center tapped service (most US homes) was considered single phase and not two phase.Yes, it is a common circuit to use a center tapped secondary to two diodes to get a full wave rectified DC. By definition, a full waveform of a single phase, getting DC power out of both the positive and negative going parts of the same waveform. But, if you look at the two anodes with a scope, you will still see only one waveform. Hence, single phase. Just because you can change your ground reference does not make more phases! Everything on a 120/240 volt typical home is single phase, just different voltage potentials.In my scope hookup, I am not inverting the waveform. Grounding the center tap appears to invert the waveform, but only to ground reference. To see why it is considered single phase I went back to the original circuit and ungrounded the center tap.Frank DuVal
As I said earlier, technically 240V service can be considered two-phase, but a lot of folks get emotional when you use that term.
If your view never changes you're following the wrong leader
If you have a dual channel scope and connected one input from one leg to neutral (120) that would be the reference. The other input is from the other leg to neutral. You would see 2 traces 180 degrees apart.
There was a long discussion about this topic in general, including some about 'scope connections , on another forum. The conclusion was that there are two ways to hook up your scope to this circuit, one shows 180 shift, one shows no shift.
So then it becomes more about terminology than physics. You can view it that you're adding two waves that are the same and getting 240v, or you're subtracting 2 waves that are shifted 180 degrees, and getting 240v.
Ed
Edited 10/28/2007 10:03 am ET by edlee
"So then it becomes more about terminology than physics."Partially. That is why I gave the difference between what an engineer would call it and an electrican."If you have a dual channel scope and connected one input from one leg to neutral (120) that would be the reference. The other input is from the other leg to neutral. You would see 2 traces 180 degrees apart."The only way to do that is to use the one of the hot legs as the common. Then you would be comparing one 120 volt signal with a 240 volt signal. So while they are inphase they are much different voltages..
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Your friends are wrong.
What allows there to be two 'hots,' and for them to combine to make 240 volts, is that they are out of time with each other .... by exactly 1/2 cycle. Kind of like a revolving door ... one hot is going 'in' as the other is coming 'out.'
Otherwise ... look at the motor nameplate ...and BELIEVE it. The nameplate will say it needs 120/240 Single phase ... NOT 3 phase.
Or, you can point to the lone transformer on the pole outside ... and ask your experts just how you can get three phase from a single transformer.
They are not out of time, they are just inverted. If they were out of time there would be two phases. A single phase high voltage is applied to the transformer, there is no possible way to get a change in time at the output. In fact, if the center tap was not used and one of the outputs was grounded there would be only 220 volts on one side and ground in your panel. No neutral, only one hot.
But note that "inverted" is the same as 180 degrees out of phase.
If your view never changes you're following the wrong leader
Similar, but not the same, because there would have to be a time shift for them to be out of phase. That's why it's called a single phase. I know it is frequently called 180 out of phase, its pretty common terminology. It also leads to misunderstanding.
There is a time shift -- half a cycle. It just looks to the untrained eye like an inverted waveform. ;)(Basically, for a sine wave the two are indistinguishable.)
If your view never changes you're following the wrong leader
I'm not about to quibble over grammar. If the peak of one 'leg' is 1/60 of a second behind the other, 'out of time' works for me!
"If the peak of one 'leg' is 1/60 of a second behind the other, 'out of time' works for me!"If the peak of one waveform is 1/60th of a second behind the other waveform, and the frequency is 60 Hz (cycles per second for us old guys), they are the same waveform, you are just looking at the next cycle!Frank DuVal
1/120th of a second.
If your view never changes you're following the wrong leader
"Single phase power systems are defined by having an AC source with only one voltage waveform. "
That's a quotation fromm this pretty good link to some AC electrical theory:
http://www.allaboutcircuits.com/vol_2/chpt_10/1.html
230V is the voltage from wire to wire -- the motor doesn't care if one is neutral, they're both equally "hot", or it's 200V on one and 30V on the other. They do need to be 180 degrees out of phase, though, or you can't add the voltages (hence two 120V 3-phase leads, which are 120 degrees out of phase, "add up" to 208V).
Your motor will be fat, dumb, and happy on standard 240V, double-hot wiring. (The 10V difference in nameplate rating is inconsequential, and for motors a slight overvoltage is better than a slight undervoltage.)
(Actually, there is some theoretical basis for calling 240V, double-hot service "two phase", but the terminology seems to offend some people.)