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3 PHASE CIRCUIT ANALYSIS

See how three phase power is produced. A step by step presentation that enlighten us on the true underlying principles in the operation of a three phase power.

In electrical engineering, three-phase electric power systems have at least three conductors carrying voltage waveforms that are 2Ï€⁄3 radians (120°, 1⁄3 of a cycle) offset in time. In this article angles will be measured in radians except where otherwise stated.

Following are the requirements that must be satisified in order for a 3-phase system or circuit to be balanced
  • All 3 sources are reprensented by a set of balanced 3-phase variables
  • All loads are 3-phase with equal impedances
  • Line impedances are equal in all 3 phases
Having a balanced circuit allows for simplified analysis of the 3-phase circuit. In fact, if the circuit is balanced, we can solve for the voltages, currents, and powers, etc. in one phase using circuit analysis. The values of the corresponding variables in the other two phases can be found using some basic equations. This type of solution is accomplished using a "one-line diagram", which will be discussed later. If the circuit is not balanced, all three phases should be analyzed in detail.

While the previous chapter dealt with a single-phase (1-Phase ) AC that is transmitted through a transmission
line (consisting of a pair of wires) to a load, attention now turns to a three-phase (3-Phase ) AC power system, in which three AC sources operate at the same frequency but with different phases. A 3-Phase AC power system has the following advantages over a 1-  Phase AC power system:

Split-phase power systems achieve their high conductor efficiency and low safety risk by splitting up the total voltage into lesser parts and powering multiple loads at those lesser voltages, while drawing currents at levels typical of a full-voltage system. This technique, by the way, works just as well for DC power systems as it does for single-phase AC systems. Such systems are usually referred to as three-wire systems rather than split-phase because “phase” is a concept restricted to AC.

These lab exercise require a 3-Phase voltage source that can output 12V p-p per phase and a current of at least 100mA rms per phase. When making measurements it is important to know if the power supply and test equipment share a common ground or if they are isolated. Battery operated instruments can be considered isolated.  Some instruments that use a power transformer may be isolated.  In some cases an isolation transformer may be used.
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