3 Phase and Motors - a brief explanation

Ok this is getting confusing, misleading,off target and misinformed in IMHO. I don't wish to further confuse but yes a 3 phase motor can "run" on single phase BUT ONLY IF IT"S UP AND RUNNING before going to single phase. That's how static phase convertors works. A static phase convertor uses a bank of capacitors to simulate a third phase but it drops it off after the motor comes up to speed or amperage. It then "runs" at 2/3 capacity at full speed. BUT single phase electric will never START up a 3 phase motor without a static phase convertor, a rotary phase convertor, a VFD( or evidently as witnessed in a previous thread a new electronic 3 phase generator) and and will in fact burn up in a instant if tried, that's why they invented and market 3 phase monitor protectors. Rotary phase convertors work by using a addtional electric motor to spin to produce the third phase leg. Neither static or rotary as far as I know can be made to be produce variable speed three phase. Now with that said it's EXTREMELY unlikely you will ever see 120/240 3 phase. That's because it would be a delta transformer with one high leg. You will only see that in some very ancient commercial bldgs. Because you can only use 2/3 the power available for 120 loads. Two legs to neutral will measure 120 while the high leg to neutral will read, depending on primary voltage, as high as 277. What is normal now adays is 277/480 volt 3 phase or 120/208 3 phase both of which are Y type transformer which when metered between phases will read the high voltage and when metered from phase to neutral will read the lower voltage.

Now with all that said, for knife makers, in a nut shell only the most basic (shaded pole) single phase motors can be made variable speed, think ceiling fans, window fans. Heavy duty single phase motors can be slowed down with pulleys but there's a loss of efficiency and performance. Most if not ALL households DO NOT HAVE 3 Phase and couldn't afford it if it was available. HOWEVER with VFD (Variable FREQUENCY DRIVES) often called VSP (Variable SPEED DRIVES) home owners can use standard single phase power to produce 3 Phase power
to use 3 phase motors AND have variable speed. It does this through complicated electronics to vary the hertz not the voltage. Basically using 120 volt single phase using a VFD you can power up to a 1 1/2 hp 3 phase motor, but with 240 v single phase pushing a VFD you can power 2hp and larger 3 phase motors.
Now we could go on and explain motors. They are not all the same (3400 speed/ 1780 speed) but that's another thread all together. But here's a link that explains the difference's which I agree with completely. http://www.northridgetool.com/resources/the_motor_matters.html
 
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Thanks LCoop.
My article is not meant to be a all exclusive explanation, but a guide to people who have no understanding of the subject. Obviously, with extra parts and conditions, many things can be made possible with motor. But the long and short is that a three phase motor needs three phase power, and a single phase motor needs single phase power. Any attempt to run them on the opposite power will likely end in smoke.
 
Thanks for all this info. I've been sort of sniffing around this stuff as I am putting together a new grinder. I put three phase into my front barn which is only a couple of hundred yards off the highway. Don't remember what it cost but I needed it to run a gang saw which has a massive old 15 HP motor. Now my well pump 5hp just gave up after 15 years of service so I was able to replace it with a 3 phase pump. Will be interesting to see if I actually get a drop in my electric bill since that well is the main item on that meter. My well and pump guy is chattering away about VFD controls as the latest thing for controlling 3 phase pumps instead of the old pressure switch.. All pretty cool stuff but I'm always concerned about things that I can't fix with a welder and a crescent wrench.

I do believe I will take the plunge, though, and use a 3 phase motor and a VFD to control sped and create three phase in my shop. I imagine the power company would sock it to me again to bring the 3 phase the extra distance from my front barn to my shop.
 
TonyRV2 said:
... The two 120V potentials however are 180 degrees out of phase from one another simply by virtue of the fact that you're approaching the neutral centertap from oppisite sides of the secondary. This is akin to having two automotive batteries in series and calling their common connection ground..... + - + -....the center - and + would be connected to one another. If this is also my ground reference, then from the outside positive terminal I would measure +12V to ground. From the outer - terminal I would measure -12V to ground. At any instant in time, the two DC voltages are of opposite polarity. The same is true of the two 120V sources relative to the transformers centertap. The 120V sources are still however single phase.
Click to expand...

180 degrees out of phase = in phase :) It's common to refer to the center-tapped system as "split phase" or 180 out of phase. As you note, we happen to use the center tap ("neutral") and tie that to ground. So on a dual trace scope the hot legs look like opposite polarity. If we referenced ground to one end of the transformer secondary and looked at the center and other hot leg, we'd see them "in phase", meaning same polarity, just at different voltage.

Some info on standard vs. inverter (VFD) rated motors: http://motors.automationdirect.com/Information/compare.html

Even if an older 3-phase motor is run at standard voltage and frequency, it may not withstand the higher voltage spikes produced by a VFD.
 
This is an excellent thread, guys. Thanks for all the involved typing! I have a little bit to add about converters.

I have a 10 hp rated Phase Perfect. It is not static or rotary, it is its own thing- a digital phase converter.

All of the different methods of phase conversion have their merits. VFD changes speed, and rotary can be built and is the traditional workhorse in many shops. However neither will work well to drive a 3 phase welder. VFD's can be harmed by the nature of the load, and rotaries, while being less delicate, often will not generate 3 phase of sufficient quality for a welder's input.

This is where the digital converter shines. It puts out a nicely balanced synthetic 3 phase power, and is robust enough that driving a welder will not hurt it. I have a big old Airco CV-300II 300 amp wire feed welder that runs beautifully on my Phase Perfect. I like having it as well, because my VFD will put out only up to 3hp, and the digital converter will fill demands beyond that up to 10 hp- say, if I got a power hammer upgrade. (Here's hoping.)

Also as noted before, the Phase Perfect is much more efficient. Even if I had a rotary converter that would drive my welder fine, it would have to be pretty massive and I'd have to upgrade the power supply to my shop.

I have a 3hp static Phase converter that sits on a shelf gathering dust- I got it with a machine and I doubt I'll ever use it. Not sure what place they have in today's market.
 
Good post. I can't help but point out that US residential power is 2-phase, not single-phase - although I know it is common practice to refer to it as single phase.

3 phases, equally spaced, plus neutral = 3 phase (makes perfect sense)
2 phases, equally spaced, plus neutral = single phase (??? makes no sense)

Just as a bicycle can't be best described as "single wheel," residential power that is composed of two equally opposed phases can't be best described as "single phase."

http://www.oempanels.com/what-does-single-and-three-phase-power-mean
 
Patrick,

There is single phase and polyphase. Polyphase is two or more phases. In the late 1800's, there was work done on 2-phase and 3-phase generators and motors. 3-phase won.

What we have in the U.S. for residential service is ONE phase off a 3-phase system. Run through a step-down transformer with a center-tapped secondary winding. Referencing "neutral", or ground, the two hot legs appear to be opposite in polarity. You can say they are 180 degrees out, but they aren't. There is no phase offset. 0 is the same as 180 or 360. They both peak and cross zero in sync with each other. I might be able to get 2-phase if my neighbor was on a different primary tap.

Putting two 12V batteries in series is a good example. Does that make a 24V system, or is it a +/- 12V system?
 
USMCPOP said:
180 degrees out of phase = in phase :) It's common to refer to the center-tapped system as "split phase" or 180 out of phase. As you note, we happen to use the center tap ("neutral") and tie that to ground. So on a dual trace scope the hot legs look like opposite polarity. If we referenced ground to one end of the transformer secondary and looked at the center and other hot leg, we'd see them "in phase", meaning same polarity, just at different voltage.....
Click to expand...

Oh contrare! 360 degrees out of phase = in phase :) All voltages are measured with respect to some reference, a reference determined by those that built the thing. The two 120V sources are only 180 degrees out of phase by virtue of the centertap which has been established as the reference and which in most places, when suitable, is earth grounded. I believe we're saying the same thing, just a bit differently.
 
Everything is plugged into RS-232 or USB ports and digitally plotted on graphs and flow charts today. In my day, we took resistance probes and hooked a live signal to an oscilloscope and after syncing it to the scope actually "looked" at the wave. That made understanding the wave structure and the reference to 0° pretty easy. I know they are now digital and have LCD displays, but they are more computer and less an electronics machine now days. Kids are used to symbols and emoji, and the trace on a flat screen isn't "real".

I left one of my old antique oscilloscopes - a several hundred pound black behemoth with probably 100 knobs and an 8" round CRT - in the shop at my old house. One day my grandson asked me, "Grandpa, does that old TV set in the workshop still work?" Seems they are dinosaurs today, and seems no one recognizes them .... sort of like the rest of us dinosaurs.
 
How can anything be out of phase when we're talking single phase? The polarity appears opposite simply because the center tap was chosen as a reference. You can look at it as +120V and -120V, or just 240V.
 
USMCPOP said:
How can anything be out of phase when we're talking single phase? The polarity appears opposite simply because the center tap was chosen as a reference. You can look at it as +120V and -120V, or just 240V.
Click to expand...

Imagine a 4-phase power system. 4 legs, each 120v. Leg 1 peaks at 0°, leg 2 peaks at 90°, leg 3 peaks at 180°, and leg 4 peaks at 270°. According to your logic above, you would call this a 2-phase system, correct? What about 6, 8, 10, 12 - or any even number of opposing phases?

Now imagine that we make a slight change to household power. Instead of leg 1 peaking at 0° and leg 2 peaking at 180°, we'll make leg 2 peak at 179°. Have we magically gone from single phase to two phase by making a 1° adjustment in cycle timing?
 
I think we are drifting with minutia. This thread was intended to be a basic explanation of the subject to help those with little understanding. All the last posts have been more on semantics, and could make things less clear.

Suffice to say that 120VAC single phase is attained by one of either legs of a center-tapped 240VAC single phase transformer. The two legs are 180° opposed ( or out of phase) to each other when referenced to the center-tap. Three phase is three legs off the main generator and the three legs (true phases) are 120° opposed.
 
For purposes of clarification, here is a single phase AC sine wave. Two hot wires 180 degrees out of phase with each other.
S-phase.JPG


And here is 3 phase sine wave. Three hot wires, 120 degrees out of phase with each other.
3-phase.JPG


I always draw pictures like these, when explaining to someone the difference in the two, and why 3 phase motors and welders run smoother.
 
Two wires, one phase. Electrons flow out on one wire, back on the other. Then the polarity reverses.

There is only one winding on the transformer secondary, even if it is center tapped.
 
USMCPOP said:
Two wires, one phase. Electrons flow out on one wire, back on the other. Then the polarity reverses.

There is only one winding on the transformer secondary, even if it is center tapped.
Click to expand...

True that there is only one winding, but don't confuse the number of windings with the relative phases with respect to ground. These are two distinct and separate issues. I could duplicate a center tapped secondary by putting to equal transformer secondarys in series with one another....there, now we have two distinct windings doing the same job. A center tapped secondary can be modeled in this way. It is only relative, and I repeat the word RELATIVE to the center tap that we've chosen to represent the circuit's reference that the two 120V sources are 180 degrees phase shifted from one another. If the reference were on one extreme end of the secondary, which would only yield one 240V circuit an one 120V circuit, these two circuits would be considered as being in phase. Again, its all relative to the circuit reference. This sort of thing comes up in electronics all of the time. In certain types of control circuitry and amplifiers its desirable to generate a signal thats 180 degrees out of phase with the original signal.....you guessed it...a center tapped transformer is often used to do the job. Sorry Stacy.....I held off as long as I could. ;-)
 
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