3-Phase Electrical System

Asked and answered many many times here, search this site if you want the big story.

But one more time...

Poly phase power provides the smoothest application of power. 3 phases is the optimal delivery of polyphase power; 3 wires. 2 phase takes at least 4 conductors (although there is a version that uses 3) but is less continuous in it's power delivery, 4 phase would take 8 conductors, 6 phase would take 12 conductors etc. etc.

Also perform a search of the internet regarding the history of 3-phase supply (and motors and generators) for a much more detailed historical account.

As I was taught many years ago three phase power is the most efficient system to transmit electricity, requiring the least number of conductors per kilowatt.

Two phase systems were popular early in the development of electrical systems, however two phase required three conductors (As does three phase), or four conductors.

I encountered a two phase system still in operation in 1965 in Hartford CT, I don,t know if the system is still operating, or where the two phase power was generated.

As you suggest other polyphase systems have been tried, and six phase systems are employed on large cruise ship propulsion systems.

I sailed as an "Ocean Ranger" on large cruise ships in Alaska summer of 08' I was surprised that the propulsion systems aboard all (7) ships I sailed on employed diesel electric propulsion systems using cycloverters employing two seperate cycloverters per propulsion motor, operating at 30 degree phase difference, with a maximum speed frequency of 18 HZ. Essentially the 15 MW synchronous motors operated as six phase stepper motors. The reason explained to me was that the motors operated in six phase mode to produce greater torque with less noise. Also for reliability the propulsion system could suffer a casualty of one cycloverter and the propulsion motor continue to operate in three phase mode referred to as "Half Motor" mode.

Interestingly I found that most engineers on board did not understand how the cycloverters worked, when I remarked about reading the nameplate data from a propulsion motor as max HZ 18, I was told "Everything aboard this ship is 60 HZ. On AMSTERDAM I got to speak to a young Dutch 3rd assistant engineer about the system, he laughed a bit, and agreed that few of the engineers aboard understood the operation of the cycloverters, he revealed in the conversation that as a cadet he had written a paper on the cycloverter system operation, and provided me with an electronic copy of the cycloverter systems employed on the Holland American lines ships. A very enlightening document of how variable speed by frequency conversion was done without double conversion (AC to DC to variable frequency/voltage AC.) I suggest you search cycloverter information to understand this control approach.

Regards CEKM

Hi CEKM,

Appreciated reading your exstensive knowledge of 3-phase Electrical System.

Perhaps, if you have the time, please advise your views the advantages or disadvantages, that I read about, China now constructing High Voltage DC electrical power transmission lines for 'trucking' large amounts of High Voltage Direct Current over long distances instead of 3 phase power.

BP

The subject of HV DC links verses HV AC has been covered in a few threads on CR4 previously so also try a site search for more information.

An internet search will also bring up numerous links with helpful documentation, as will wikipedia.

BP,

A high voltage DC transmission system from the Pacific Northwest US where power is abundant to the Los Angeles area has existed for many years. Driving along I5, the transmission towers with two conductors can be seen, The system terminates at a conversion station in the San Fernando valley just about where the California Aquaduct enters the first reservoir.

The advantage of using high voltage DC is that higher voltages can be used for the transmission and therefore more power delivered than with AC, consider peak voltage of sine wave AC, and RMS voltage, (.707 of peak), and the additional power advantage becomes apparent due to the losses due to corona at high voltages.

Regards CEKM

That's Great, All Electrical people of Entire World are on same plateform without any difference of opinion for adopting 3 phase system.

Others must learn a lesson from us.

This is from the internet search.

This describes the first "polyphase" (more than one phase) system developed for the distribution of alternating current (ac) power. This two-phase system was subsequently rendered obsolete, however, by the superior three-phase system that is now universally used throughout the world. Today, the large-scale generation, transmission, and distribution of electric power is by means of the three-phase ac system; that is, three individual single-phase voltages and currents having a 120° phase relationship to each other and intermingled on three wires (excluding a neutral). The three-phase system has been adopted because it provides for a constant rather than pulsating power flow to motors, and because it is an efficient system as far as the amount of copper required per kilowatt transmitted. The theoretical complexity of the three-phase system, however, delayed its complete acceptance in the early days of electric power system development.

During the early 1890s, understanding the behavior of simple single-phase ac was enough of a challenge. It was not until Charles P. Steinmetz, the legendary General Electric scientist, developed the concept of the use of the "j" operator (unity magnitude at a 90° phase angle) and complex numbers for ac circuit calculations that the behavior of voltages and currents in ac circuits and machines was truly understandable. Likewise, it was not until the introduction of what eventually came to be known as "symmetrical components," during the early 20th century, that the calculation of three-phase voltages and currents became relatively straightforward. This technique utilized an "a" operator that was of unity magnitude at a 120° phase angle (–0.5 + j0.866). This operator was of significant value since, in a balanced three-phase system, the voltages and currents are at 120° phase relationships to each other.

Where a two-phase transmission with separate circuits is used, then if the separate circuits are wound on different armatures, each can be regulated to give a constant voltage at the receiving end. This is the case, for instance, in the large dynamos built by the Westinghouse Company for use at the World's Fair in Chicago . The difficulty due to the uneven loading of the circuits is specially marked in the case of the three-phase system, and it is one of the principal objections that have been urged against the employment of this system for purposes of distribution.

It had already been realized, however, that the three-phase configuration was superior for transmission from the point of view of efficiency. Thus, special phase-changing transformers were designed by Charles F. Scott of Westinghouse in order to step up the two-phase generated voltage at Niagara Falls to 11,000-V, three-phase for transmission to Buffalo , New York . The General Electric Company was awarded the contract to build the phase-changing transformers and so was licensed by Westinghouse to utilize the connection developed by Scott for this purpose.

At Buffalo , some of this three-phase power was used for rotary converters that supplied 110/220-V dc power for the Edison distribution system downtown.

Bridge,

Thanks for the enlightenment on two phase and evolution to three phase. I was always curious about the history and evolution, though never exerted the energy to research it, your response was very interesting.

I ran across two phase still in use in 1965 at the Gray manufacturing facility in Hartford CT when sent on a call due to a boiler burner motor failure, the motor had to be rewound of course because two phase motors were then obsolete.

I again encountered two phase at the Aetna Insurance building in Hartford when a two phase to three phase converter had to be installed for new three phase equipment. Likely this converter is/ was of the transformer type mentioned in your message about the Niagara transmission conversion, as the converter I encountered was a static converter, not a motor- Alternator rotary converter.

I was informed that the conversion creating the third phase was not a perfect 120 degree conversion due to the inability of the phase shifting of the transformer scheme could not provide a true 120 degree phase shift, but close enough to be usable.

Regards CEKM

3-phase system is universilly used because the bigger machines in use these days can be supplied with 3-phase but not 4-phase or 6-phase.If you supply a bigger machine with a phase system which is higher than it specified phase, the machine will get blown up.Hence 3-phase system.

The only "Silly/ stupid" question is the one not asked.

Regards CEKM