Many homeowners and general handy workers know at least the basics of single-phase residential wiring and polyphase power. But three-phase work is mostly the domain of professional electricians who work in commercial and industrial settings where there are three-phase services, branch circuits, and motors. When one has learned the basic circuitry and equipment, actually three-phase wiring is easier than single phase because the conductor sizes, box and conduit fill and terminations are all less problematic.
Some individuals refer mistakenly to single-phase systems as “two-phase” because in 240-V circuits there are two phase conductors carrying separate phase currents that are 180° apart. That is actually a single phase. Two-phase electrical systems were widely used in the early Westinghouse-Tesla epoch. They used two circuits with voltage phases differing by one-quarter of a cycle, 90°. The usual configuration was two wires for each phase, so it took four wires to convey power. Occasionally three wires, one of them common to the two phases, were used. In this scheme, the common wire had to be larger.
Two-phase electrical systems persisted throughout the early decades of the twentieth century. At present, the two-phase power is mostly obsolete. In the U.S., commercial two-phase service remains in place only in Philadelphia, Pa., where many buildings in the old section are wired for two-phase power.
The first generators at the huge Niagara Falls hydro plant were two-phase. They were installed in 1895 and consisted of offset windings. Taking advantage of the enormous water power available at Niagara Falls, the Edward Dean Adams Power Station was the first large ac generating facility in North America.
The two-phase ac operated at 25 Hz. A 25-mile transmission line delivered 11 kV to Buffalo, N.Y. The transmission line was built by General Electric and the generating equipment by Westinghouse. A key figure in this mammoth project was Charles Steinmetz. Working as an independent contractor, he was instrumental in resolving heat problems that had caused generator prototypes to burn out prematurely. The ubiquitous Nikola Tesla originally designed the 25-Hz two-phase ac system.
Almost everyone on the planet understands that for a two-wire circuit to work, both conductors must connect from the power source to the load. But when it comes to three-phase, most non-electricians profess ignorance. Actually, once you understand the basics, three phase is easier than single phase, because for a given load, the conductors are smaller, making for easier terminations and less conduit and box fill. For a given horsepower, the ubiquitous three-phase induction motor is less expensive and easier to wire. To change the direction of rotation, reverse any two of the three supply wires.
Most motors over five horsepower are three phase. Industrial settings may also have many small fractional-horsepower three-phase motors.
In higher-power motors and sensitive electrical equipment such as variable frequency drives, any discrepancy in voltage-to-ground or current-under-load can cause a variety of problems, the end result being unwanted temperature rise. To check for such difficulties begin, with the load disconnected, by measuring the voltage at the input terminals and ultimately back to the entrance panel. The measured variation should not be more than 3%, except in the special case of the high-leg delta configuration. Unbalance can be caused by unequal single-phase loads connected upstream, in conjunction with undersized conductors. The remedy is usually rebalancing loads and/or installing larger conductors.
Fully loaded, current in the three-phase conductors can be measured using a clamp-on ammeter. Again, the mismatch should not exceed 3%.
Depending on how the three wires are connected, it may happen that voltage variations in the power supply and impedance variations in the load will either reinforce or cancel out one another. The goal is to have these differences cancel out rather than to reinforce. To this end, roll the connections: Switch 1 to 2, 2 to 3, and 3 to 1. Take care not to reverse any two legs, which in a motor would reverse the direction of rotation. Take clamp-on ammeter readings and use the combination that results in least variation.
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