THREE-PHASE CIRCUITS

THREE-PHASE CIRCUITS

There are two types of system available in electric circuit, single phase and three phase system. In single phase circuit, there will be only one phase, i.e the current will flow through only one wire and there will be one return path called neutral line to complete the circuit. So in single phase minimum amount of power can be transported. Here the generating station and load station will also be single phase. This is an old system using from previous time.

Three phase circuit is the polyphase system where three phases are send together from the generator to the load. Each phase are having a phase difference of 120°, i.e 120° angle electrically. So from the total of 360°, three phases are equally divided into 120° each. The power in three phase system is continuous as all the three phases are involved in generating the total power. The sinusoidal waves for 3 phase system is shown below:

The generated voltages are 120◦

apart from each other.

Phase sequences:

left:  abc or positive sequence,

right: acb or negative sequence.

all electric power is generated and distributed in three-phase, at

the operating frequency of 60 Hz (or ω = 377 rad/s) in the United States

or 50 Hz (or ω = 314 rad/s) in some other parts of the world.

Example:

If Vab = 400 V in a balanced Y-connected three-phase generator, find the phase voltages, assuming the phase sequence is:

(a) abc (b) acb

Terms and Naming Conventions

Phase

describes or pertains to one element or device in a load, line, or source. It is simply a "branch" of the circuit and could look something like this .

Line

refers to the "transmission line" or wires that connect the source (supply) to the load. It may be modeled as a small impedance (actually 3 of them), or even by just a connecting line.

Neutral

the 4th wire in the 3-phase system. It's where the phases of a Y connection come together.

Phase Voltages & Phase Currents

the voltages and currents across and through a single branch (phase) of the circuit. Note this definition depends on whether the connection is Wye or Delta!

Line Currents

the currents flowing in each of the lines (I_a, I_b, and I_c). This definition does not change with connection type.

Line Voltages

the voltages between any two of the lines (V_ab, V_bc, and V_ca). These may also be referred to as the line-to-line voltages. This definition does not change with connection type.

Line to Neutral Voltages

the voltages between any lines and the neutral point (V_a, V_b, and V_c). This definition does not change with connection type, but they may not be physically measureable in a Delta circuit.

Line to Neutral Currents

same as the line currents (I_a, I_b, and I_c).

• REVIEW:
• The conductors connected to the three points of a three-phase source or load are called lines.
• The three components comprising a three-phase source or load are called phases.
• Line voltage is the voltage measured between any two lines in a three-phase circuit.
• Phase voltage is the voltage measured across a single component in a three-phase source or load.
• Line current is the current through any one line between a three-phase source and load.
• Phase current is the current through any one component comprising a three-phase source or load.
• In balanced “Y” circuits, line voltage is equal to phase voltage times the square root of 3, while line current is equal to phase current.