What are the disadvantages of single phase AC supply as compared to three phase supply?

What are the disadvantages of single-phase AC supply as compared to three-phase supply?

Right Answer is:

All of the above

SOLUTION

Disadvantages of single-phase AC supply

  • Single-phase power transmission requires a live and a return wire, so power is lost due to the current flowing through two wires for each (there is one) driving electromotive force, while in three-phase power transmission the sum of all currents at either end is zero, and there is no return of current in the case of symmetrical loading, which means a return conductor is available “for free.”
  • A balanced three-phase, three-wire circuit with equal voltages uses 75% of the copper required for conductors compared with single-phase supply circuits.
  • As the industrial loads increase in size and require more power single-phase supply cannot meet this demand.
  • Due to the same current return in the second wire, single-phase voltages drop much more quickly than do three-phase voltages.
  • Single-phase supply produces a breathing magnetic field in single-phase motors, which means these motors (split motor and capacitor motor) cannot self-start without an additional coil, called a starting coil, capacitor, and centrifugal switch. A three-phase supply produces a revolving magnetic field, which in turn causes self-start torque in three-phase motors.
  • Three-phase motors are less expensive and usually lighter and smaller than single-phase motors of the same horsepower rating. There is also a wider choice of enclosures available than for single-phase motors.
  • Some single-phase loads generate a spectrum of current harmonics causing excess conductor heat, power loss, and low efficiency, while the effect of current harmonics when a three-phase supply is used is light due to three-phase transformer connections (delta connection) resulting in less loss and high efficiency.
  • Interrupting the single-phase source causes power loss for the loads, while in a three-phase power supply with a special transformer connection (open delta). the loss of one phase can be recovered by the other two phases, which means loads continue to have supply power at a reduced rate.
  • With respect to PF correction, only the capacitor bank method is available for single-phase sources, while in three-phase sources three methods for power correction are available. Furthermore keeping the same kVAR single-phase capacitor banks incurs a much greater cost compared to three-phase capacitor banks.
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