I am having some trouble understanding three-phase alternating current. I realize that most houses are not three-phase but single phase. Would that not mean that at some point when the flow of electricity switches direction it will stop and the electrical motor would stop under single phase?

Also, I also realize that most transmission lines are three-phase but again houses are single-phase. Does the three-phase transmission towers split off into three separate single-phase transmissions towers?

If anyone can provide me with any of this information it would be great.


Also is the wye or delta connection inside the alternator? Like the magnetic poles are the alternator symbols?


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    $\begingroup$ Household power is delivered at 60Hz or 50Hz, depending on where you are in the world. In either case, the zero-crossing is a rather small amount of time. Motors, being mechanical beasts, have mass, and therefore inertia. During the brief period when current (and therefore torque) is zero, the inertia of the moving parts keeps it turning. True, the mechanical load will cause it to slow down a bit, but when the current builds back up, so does the torque, which speeds it back up again. $\endgroup$ – Anthony X Apr 12 '14 at 22:13
  • $\begingroup$ Yes but since it is a new direction wont the motor need to stop before it changes directions? Or am I missing something? $\endgroup$ – jpell Apr 12 '14 at 22:18

Three-phase has two main reasons to exist:

  • Driving N. Tesla's polyphase induction motors used throughout industry.

  • Reducing the total cost of metal in cross-country power lines: w/single-phase lines, more metal would be needed to transfer the same rate of kilowatts.

You're right: lighting as well as AC motors will briefly turn off at 120 times per second (that's for USA 60Hz line frequency.) For this reason, metal-vapor streetlights and older inductor-ballast fluorescent tubes have significant "hum" modulation in their light output. This strobe effect can be very visible when your eyes sweep across strings of LED-based xmas lights. The hum is greatly reduced with incandescent lamps, since the filament doesn't cool down significantly during the millisecond-long low point in the 60Hz wave.

And you'll probably hear a 120Hz sound when using high-speed carbon-brush motors under high mechanical loads, such as electric mixers and carpentry routers. These high-RPM products can't use AC induction motors which are naturally limited to 1800RPM by the 60Hz drive frequency. The 120Hz variation ends up in the torque output of high-speed brush motors.

Single-phase induction motors should have much less 120Hz mechanical buzz, since they're essentially 2-phase motors with magnetic field torque which rotates rather than oscillates. Either capacitance or inductance is used to give the motor a second electromagnet pole having shifted phase.

  • $\begingroup$ The capacitive/inductive phase-shifted pole is only important at startup. Once a 2-phase motor is running, the extra pole isn't necessary. Capacitor-start motors typically switch it out after the motor achieves a threshold RPM. The key point is that the motor's mechanical inertia carries it through the zero crossings. $\endgroup$ – Anthony X Apr 13 '14 at 0:17
  • $\begingroup$ #Anthony_X you're right: I was thinking of shaded-pole induction motors, not the more common ones with the centrifugal switched start-capacitor. $\endgroup$ – wbeaty Apr 13 '14 at 19:31

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