Three phase power

Cecil's Columns/Staff Reports
1

Recent Straight Dope Classic on electrical power

Not that I would suggest that Cecil is wrong: not for a moment. But I feel that he may have glossed over some of the intricacies of three phase power. And if his story of two phases of opposite polarity is correct then the US uses a system that is very different from my corner of the globe.

Where I am (Australia and New Zealand) the supply is 240V at 50 Hz. The 240V is an average taken over one cycle: root mean square (RMS) to be precise. Which means that the peak amplitude of is around 339 volts. Not that this makes a whole lot of difference. It is just worthwhile remembering that the voltage is continually changing.

Electrical substations supply three phase to the streets for residential supply. For most household appliances a single phase is used. High power appliances such as stoves, some heating and air-con systems, pool heaters and the like, the appliance is wired across two phases that are 120° apart. This results in a supply of 415 volts (RMS) (as well as a phase shift of -30 degrees). These appliances are generally permanently wired and there is no chance of plugging them in to the wrong kind of socket. The third phase in the supply is used for cut-rate electrical supply. This is mostly water heating and night-store heating systems. The supply companies are able to turn this on and off as needed to even out daily fluctuations in demand.

For all three phases in use, the return current is via the neutral wire which effectively mops up the imbalance in the draw between the three phases. One assumes (but I don’t know the details) that different streets will be connected to the supplied phases in different ways in order to minimise the phase imbalance at any given time.

Of course if you really want it you can be supplied with three phase which is useful for industrial-type motors, arc welders and so on. But generally you wouldn’t bother with the expense.

There are some exceptions to this system. I know of at least one isolated community where the neutral is dispensed with. The whole community (sone 20 houses) is connected to a single phase and the return is earthed. Needless to say, the supply fluctuates a bit and black-outs and brown-outs are not unusual.

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It is (almost–I’ll get to that), and it does. In the vast majority of residential installations in the US, homes are supplied with 240 VAC, center-tapped and this center tap is connected to Earth ground at the service entrance and serves as the neutral return for the inside wiring. The voltage between each leg and the center tap is 120 V and taking the leg-to-leg voltage gives us our 240 V for driers and stoves. However, we say the polarity of one leg to the neutral is the same as the polarity from the neutral to the other leg (think of batteries in a flashlight all oriented in the same direction). We don’t speak of the two legs having opposite polarity. Cecil knows this, of course, but in wording the column for the laity, his language got a little careless.

In the few areas where residential power is supplied as two legs of a three-phase source, the line-to-ground voltage is the familiar 120 V, but the line-to-line voltage is 208 V, as the column correctly states.

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Or, if I may, alternating.

:smiley:

4

Huh, I didn’t know anyone had actually implemented something like this. It’s a very good idea.