Looking for book recommendations on Electrification / Electric Grid history.

There are scans of many early Electrical Engineering books on Archive.org. that you can read or download to read later. Single phase was perfect for simple lighting, or universal motors. Two phase drove larger motors, but toot four wires, while three phase only needed three which results in a lower distribution cost, along with improved ability to balance loads. DC was difficult to transfer long distances. Voltage conversion required motor/generators to step it up or down. They were not only inefficient when compared to a transformer, they needed maintenance. I recently had my 7200/120-0-120V pole pig fail. It had been in service since 1964.

Thank you for the pointer.

Understood.

I see many references to two-phase using four wires. However I'm quite certain that I have seen a reference to two-phase (with 90 degree delay) using three wires. I believe this configuration used a common conductor. I think this was in the context of a sewage pump. I'll have to do some more digging.

I agree that three wires for three phase is 25% better than four wires for two phase. But I need to reconcile what I'm remembering with the sewage pump.

Is this because there's another phase to spread different loads across? Because it seems to me like you could ""balance things (as long as there are enough of them) across any number of phases.

Agreed. Which is what surprises me about some of the HVDC transmissions that I'm seeing done in and across large swaths of China.

Agreed.

50+ years seems like a good run to me. Definitely not something that any motor / generator that I've ever heard of could do without maintenance.

The two phase drawings that I've seen were five wire, with two center tapped transformers with their center taps tied to ground to prevent a wild leg situation. In a three wire configuration, the minimum voltage on either high leg would be at least twice that of a five wire installation. That requires better insulation in the transformers, motors and switch gear.. All of these raise the costs to install, and maintain.

Two-phase with 90 degree phase difference is actually four-phase.

The US distribution of 120 - 0 - 120V is two-phase.

To produce a rotating field in a motor, at least 3 phases are needed.

no, it is two-phase. Sin/Cos, I/Q

no, it is a single split phase

no, you just need two, a normal stepper motor is a two phase motor

when you have two or more phases you can generate any other phase by linear combination

The AIEE / IEEE supports (or supported) your statement.

I found a good article in March / April 2004 edition of the IEEE power & energy magazine, "the first polyphase system - a look back at two-phase power for ac distribution". (I have no idea why all but the IEEE moniker is in lower case, but it is, so I copied it.)

""""As the use of ac motors expanded during the 20th century, the problem of providing both 115 V for lighting and 230 V for motor use from two-phase distribution systems became significant. One solution was the adoption of a two-phase, five-wire system in which center taps on both phases were connected together to create a neutral. This, then, resulted in a "star" configuration (analogous to the three-phase "why" connection) and, technically, was a four-phase system. As such, 115 V (single-phase) for lighting was available for any of the four phase wires to the neutral, while 230 V (two-phase) was available for motors for the four phase wires themselves."""

The operative portion being "This ... technically, was a four-phase system."

I agree that it appears to be two opposing phases from the consumer's point of view. However it is produced by a single phase from the utility company. Thus it is a single phase feed from the utility companies point of view.

Then there is the entire discussion around are the 0° and 180° a /single/ phase (with opposing reference points) or not. Often "straight line" / "single vector" comes into technical discussions ~> debates.

Whatever you call the quintessential residential service, it is decidedly different than what the utility company calls two-phase, where the phases are 90° out of phase with each other.

My understanding is that at least two, or more than one, are needed. There is no requirement for three phases.

Multi-phase is all about maximizing power transfer at least cost. Three phase just about reduces the required copper to one half that of single phase transmitting the same power. That's a BIG savings.

"In a three-phase system feeding a balanced and linear load, the sum of the instantaneous currents of the three conductors is zero. In other words, the current in each conductor is equal in magnitude to the sum of the currents in the other two, but with the opposite sign. The return path for the current in any phase conductor is the other two phase conductors." <-- show this.

"Constant power transfer and cancelling phase currents are possible with any number (greater than one) of phases, maintaining the capacity-to-conductor material ratio THAT IS TWICE THAT of single-phase power. However, two phases results in a less smooth (pulsating) current to the load (making smooth power transfer a challenge), and more than three phases complicates infrastructure unnecessarily."

"As compared to a single-phase AC power supply that uses two conductors (phase and neutral), a three-phase supply with no neutral and the same phase-to-ground voltage and current capacity per phase can transmit three times as much power using just 1.5 times as many wires (i.e., three instead of two). Thus, the ratio of capacity to conductor material is doubled.[5] The ratio of capacity to conductor material increases to 3:1 with an ungrounded three-phase and center-grounded single-phase system (or 2.25:1 if both employ grounds of the same gauge as the conductors)."

Ignore that dumb irrelevant phasor math headache in the article.

Power transmission is an example of economics inducing the best science for the job. Without it you either end up with a stupidly overpriced installation, or an installation of very limited extent due to cost. As it was in U.S., rural areas went without electrification until the 1930s because not enough customers per mile to pay for it.

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In the US, it's called "split phase", a form of single phase.

The key difference between single phase and polyphase is that in single-phase, the delivered power pulsates at twice the fundamental frequency, 50 or 60 Hz. With two phase, one phase is the Sin[t] voltage waveform and the other is Cos[t] voltage waveform.

Power is proportional to the square of voltage, so the delivered power is Sin[t]^2+Cos[t]^2, which equals unity regardless of t. The power in each phase pulsates, but their sum is constant.

Yes.

Two or more phases (polyphase) is necessary and sufficient.

Joe Gwinn

What you are describing is called "split phase" in the states. Technically it is not polyphase....(IIRC) j

The only requirement is economic; a two-phase electrical distribution system with centertap GND takes five conductors. A three-phase electrical distribution system takes four conductors.

At some point (like a generator) there might be two untapped or floating windings for the 'four-phase' system, which is why 'two-phase' became a way of describing that system.

Many servos used two-phase four-wire setup, but at low power so the added wires were not a cost driver.

Not really. It was always well-known how to use shared wires. But with two-phase, the common wire needed to be larger than the other two wires.

Joe Gwinn

You aren't likely to find an engineer that works on power systems that agrees with either. Or an electrician that works on 3-phase.

So single phase motors don't work? Single phase motors have field rotation, but it is in either direction (both?). They have a winding only used to start the motor that has a shifted phase - 2-phase start. They run on single-phase. (Some have reduced power to the 'start' winding while running.)

------ There is also corner grounded 3-phase that is sometimes used. Could be an irrigation pump.

And also ungrounded 3-phase - a single fault will not shutdown critical processes. Probably always requires an alarm for voltage imbalance.

Also quite well known - 240V high-leg delta. One of the 240V transformers is center-tapped, which is grounded and is a neutral for

240/120V. The high-leg is not used with the neutral (could be at 208V)

Delta systems can be powered with 2 transformers - open delta. Current is out of phase with voltage and current rating has to be derated.

And for relatively small 3-phase transformers like 480 to 208/120

2-cores can be Scott/T connected. The 2 cores are true 2-phase.

The original generator(s?) at the Ames/Tesla Niagra plant was 2-phase connected to Scott connected transformers to produce 3-phase. Engineer was someone named Scott.

Also single phase induction motors. Invention of these motors made AC systems much more practical. Tesla had patents on about every variation of induction motor.

Things have changed drastically since the 'good old days'. Conversion of DC transmission back to AC now uses semiconductor or other devices. There is a DC transmission line from N Dakota (next to a coal mine) to a conversion station near Minneapolis (1978). Conversion equipment looks like something out of Buck Rogers.

DC transmission avoids capacitive/reactive currents and skin effect, and can tie regions that are not synchronized (there are 4 major separate systems in the US).

From the fount of all knowledge:

You probably know that Edison developed DC systems (Pearl Street Station NYC). Tesla had patents on most aspects of AC systems and Westinghouse developed those systems. There developed the "War of the Currents".

Westinghouse won the first major battle, powering the 1893 World's Fair. Westinghouse won the final battle with a contract for the Adams Power Plant at Niagra Falls (In a case of industrial malfeasance the empty Transformer House is all that is left of the Adams Plant.)

Other articles that may be of interest (I haven't read any of them):

Adams/Tesla

For single-phase loads on 3-phase wye the neutral currents cancel and if there are equal resistive loads on all 3 phases the neutral current is zero.

Electronic loads generate harmonics (EU has limits, does the US?) Some of the harmonics add instead of cancel, and the neutral current can be larger than any of the phase currents.

My limited / academic understanding of 3-phase is that if the three phases are equal, neutral will effectively be zero. However I believe this is highly dependent on power factor and if the loads are capacitive / resistive / inductive. I think that an ideal situation is that the capacitive and inductive components are approximately equal and cancel each other, thus making the load appear as mostly resistive. I /think/ that this is largely within a given phase and that ideally all three phases as resistive as possible and preferably equal to each other.

I have no idea. I'll watch other replies to learn more.

I naively wonder if electronic loads / harmonics is referring to capacitive load that I was describing above.

Perhaps, but it takes two phasors to describe that split phase, and three for classical 3-phase.

There's usually a lossy inductive winding, and you don't see those in anything much bigger than a sewing machine motor. Shaded-pole motors, too, use inductive windings to generate a magnet field phase that doesn't match the input phase.

Inductors, like motors, store energy in their magnetic field. Supply current increases the energy storage, then the stored energy goes back into the supply. This happens twice in each AC cycle. The current drawn by the motor increases because of this added reactive current (low power factor). But since the flow is in and out, power is drawn and returned and no real power is used. But the higher current results in higher resistive losses in the wire. That is not a huge loss for a consumer, but is a large loss in the long lines of a utility company. The generators also have to supply a higher current. But the higher current does not result in real power and does not register on the utility watt-hour meter. Utilities are not pleased by their cost and, for industries, may install a second VARh (VA-reactive-hour) meter that measures the reactive 'power', and then add a 'VAR penalty' to the utility bill. This can make correcting the power factor (power factor correction capacitors - capacitors store energy in an electric field) real cost effective for the industry. Utilities also add power factor correction capacitors to their systems.

Fully correcting the inductance (power factor = 1) makes the inductor and capacitor resonant - may not be a good idea.

Balancing the current between phases allows you to use all of the power that is available and keeps the phase voltages close to equal.

Resistors, inductors and capacitors, and combinations (linear), result in a sinusoidal current waveform. Rectifiers and many other loads distort the current waveform so it is no longer a sinusoid. If it is distorted there are harmonic current components. The 180, 360, 540, ... Hz current components (triplens) in the phases (for 3-phase) add instead of canceling and the neutral may have to be larger.

might be interesting

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may also be interesting

The 2 phasors are along the real axis. One leg is just the opposite polarity of the other (at any instant).

3-phase - 2 of the phasors involve the imaginary axis.

You can get 2 phases from a single phase transformer? Discussion of "2 phase" winds up confused with "2-phase".

Single phase motors, which I described (and you deleted), are very common (probably at least half of the induction motors made). And readily available up to at least 5 HP.