If you are measuring the voltage of a three phase power system you take only two phase in account then why there is a need for third phase?

Answer 1

Answer Take all phases into account. Voltage is measured between two phases of the three phases at one time, so what this means is this...first you read voltage between line 1 and line 2...then you read voltage between line 2 and line 3...and then from line 1 to line 3. Each phase of a 3 phase system is 120 degrees from the other in a 360 degree pattern. It takes all 3 phases to start a 3 phase motor but can run on two. If a 3 phase motor tries to start on two phases it is refered to as single phasing and can damage the motor.

Another Answer

First of all, let's get the terminology correct. The wires that join a three-phase load to its supply are called 'LINE conductors', not 'phase conductors'! This is very important. Phases, which are normally inaccessible, are either the generator windings, the transformer windings, or the individual loads, connected to the line conductors -this can make measuring phase voltages very difficult unless you can access the interior of these machines/loads.

The voltage of a three-phase system is normally defined in terms of its line voltage, not its phase voltage, so one normally measures its line voltage by connecting a voltmeter between any two line conductors or terminals. As the line voltages are determined by the supply system, all line voltages should be the same, regardless of which line conductors you choose to place the voltmeter between.

The important thing, however, and this is something your voltmeter will NOT tell you, is that the three line voltages are out of phase with each other -each lagging its predecessor by 120 electrical degrees. And this is important, because it is the phase displacement between these voltages, not the magnitude of each voltage, that allows -for example- a three-phase motor to self-start.

Answer 2

The Power supplied to your house is 2 phase. the phases are 180degree out of phase (180 * 2 = 360 = one full cycle) so that when one phase peeks at 120Volts, the other phase dips at - 120Volts. The difference of voltage between the phase is a total of 240Volts. Some major appliances make use of this 240V to operate.

In commercial and industrial environments, there are typically 3 phases provided. the phases are 120degrees out of phase (120 * 3 = 360 = one full cycle). The three phases provided simplify the use of electrical motors(fans, compressors, elevator motors, etc.) as each phase overlaps the other to maintain a constant current to motors making the voltage supply more efficient.

A 4 phase system would require the phases to be at 90degrees out of phase (90 * 4 = 360 = one full cycle). The consequence would be that two pairs of phases would cross 0 Volt levels at equal times; thus reducing the overall efficiency of such arrangements. Not to mention that extra wiring would be necessary to carry the fourth phase.also its not ecnomically best because conductor size cost increses 25% more than 3 phase. (MSI)