SteveR wrote: ↑
Thu Jan 27, 2022 3:01 am
Hi Harold, No, not 4 wire - just the regular power that comes in to our house. We have 2 hot legs and a ground/neutral. Hot-Hot is ~230V (220?) and Hot-Neutral is ~115 (110?). To me it makes sense to call it two phase. My introduction was years ago when the neighborhood pole mounted transformer blew up. Only half of the lights in the house were working until they replaced the transformer because each phase runs about 1/2 of the main panel.
Harold is correct. Household power in the USA is single-phase, so-named because if scoped across the hot legs a single sine wave will be observed. The neutral is a grounded center tap on the transformer secondary and therefore is not a reference leg for determining number of phases.
If the power source is two-phase, connecting the inputs of a dual-channel ’scope to the hot legs would reveal two sine waves exactly 180 degrees out of phase. Similarly, if one has access to a multichannel ’scope and connects it to the three hots of a three-phase line, one will see three sine waves exactly 120 degrees out of phase with each other.
True two-phase power never caught on here, as three-phase is more economical of infrastructure—only three conductors are required for transmission, versus four for two-phase power. This characteristic became very important early in the history of commercial power generation, as loads could be hundreds of miles from the generating station. Clearly, a three-wire distribution system stretching several hundred miles has a cost advantage, both in installation and maintenance, when compared to a four-wire system.
High-leg, delta-connected three-phase services have a grounded center tap on one of the three secondaries in the transformer, conventionally between phases L1 and L2. Measured from L1 or L2 to neutral (the center tap), the nominal voltage is ~120. Measured across L1/L2 it would be ~240. Measured from L3 (the high leg, aka “wild leg”) to neutral, the output would be approximately 208 volts (120 × √3). Due to that “odd” voltage, the wild leg connection is rarely used with single-phase loads.
Another three-phase supply scheme is wye-connected, which is what is at my shop. In this arrangement, all three secondaries have a common connection, so when schematically drawn out, the secondaries appear to be in the shape of the letter Y
. The usual voltages provided by the wye connection are 208 between any two phase hot legs, e.g., L1 and L2, and 120 volts between any phase hot leg and the common center connection. The latter is the grounded neutral.
As 120 volts can be obtained between any phase and neutral in a wye setup, loading on the transformer tends to be more balanced than with the delta arrangement in an installation with a lot of single-phase consumption, assuming the wiring is properly laid out. This characteristic results in more closely-matched voltages between phases (which is very beneficial to motor life) and less harmonic distortion being kicked back into the line. The downside is current draw by three phase motor loads at any given horsepower will be higher than on a delta setup, since the somewhat lower voltage has to be made up with higher current to maintain motor power.