Single Phase & Three Phase Power Electric Circuits & System

By | October 4, 2016

Single Phase power circuits

Single phase electric power or circuit refers to the distribution of alternating current electric power using a system in which all the voltages of the supply vary in corresponding exactly. Single-phase distribution is used when loads are mostly used for the purpose of lighting and heating which can also be for few large electric motors as well.  Single-phase supply connected to an alternating current electric motor does not produce a revolving magnetic field therefore single-phase motors need additional circuits for starting, and such motors are uncommon above 10 or 20 kW in rating. Single-phase comes to the home with two wires: active and neutral. The neutral wire is connected to earth (water pipe, earth stake, etc.) at the switchboard.



Three-phase power circuits vs 1-phase system

Almost all electric power generation and most of the power transmission in the world is in the form of three-phase AC circuits. A three-phase AC system consists of three-phase generators, transmission lines, and loads. There are two major advantages of three phase power systems over a single-phase system i.e. more power per kilogram of metal form a three-phase machine and power delivered to a three-phase load is constant at all time, instead of pulsing as it does in a single-phase system.

Advantages of 3 phase power system over 1-phase system can be summarized as below:

  • Higher power/weight ratio of alternators. A three phase alternator is smaller and lighter that a single phase alternator of the same power output. Hence, it is also cheaper.
  • A three phase power transmission system requires less copper or aluminium to transmit the same quantity of power of a specific distance than a single phase system.
  • Three phase motors are self-starting due to the rotating magnetic field induced by the three phases. On the other hand, a single phase motor is not self starting, it requires a capacitor and an auxiliary winding.
  • In Single phase systems, the instantaneous power(power delivered at any instant) is not constant and is sinusoidal. This results in vibrations in single phase motors. In a three phase power system, though, the instantaneous power is always the same.
  • Three phase motors have better power factor compared to single phase motors.
  • Three phase power supply can be rectified into dc supply with a lesser ripple factor


Three-phase power was first conceived by Nikola Tesla. In the early days of electric power generation, Tesla not only led the battle concerning whether the nation should be powered with low-voltage direct current or high-voltage alternating current, but he also proved that three-phase power was the most efficient way that electricity could be produced, transmitted, and consumed. Three-Phase Circuits There are several reasons why three-phase power is superior to single phase power.

The horsepower rating of three-phase motors and the KVA (kilo-voltamp) rating of three-phase transformers is about 150% greater than for single-phase motors or transformers with a similar frame size. An AC generator designed to develop a single sinusoidal voltage for each rotation of the shaft (rotor) is referred to as a single-phase AC generator. If the number of coils on the rotor is increased in a specified manner, the result is a Polyphase AC generator, which develops more than one AC phase voltage per rotation of the rotor.


In general, three/ 3 phase power systems are preferred over single-phase power systems for the transmission of power for many reasons.

  • Thinner conductors can be used to transmit the same kVA at the same voltage, which reduces the amount of copper required (typically about 25% less).
  • The lighter lines are easier to install, and the supporting structures can be less massive and farther apart.
  • 3 phase power equipment and motors have preferred running and starting characteristics compared to single-phase power systems because of a more even flow of power to the transducer than can be delivered with a single-phase supply.
  • In general, most larger motors are three phase powered because they are essentially self- starting and do not require a special design or additional starting circuitry.

Relationship of Voltages and currents in 3 Phase Power

There are two types of connections in 3 phase power circuits. Y and ∆ also known as star and delta circuits. Each generator and each load can be either Y- or ∆-connected. Any number of Y- and ∆-connected elements may be mixed in a power system.



Three phase power Connections

Both the three phase power source and the three phase load can be connected either Wye or DELTA configurations. In this case you have 4 possible connection types.

  • Y-Y connection
  • Y-Δ connection
  • Δ-Δ connection
  • Δ-Y connection

Balanced Δ connected load is more common. Y connected sources are more common. Three phase power sources are usually Wye connected and three phase loads are Delta connected. There is no neutral connection for the Y-∆ system.


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