Measurement of 3-Phase line and Phase Quantities

240

Aim: To practically determine the relation between line and phase quantities of both current and voltage and compare them with their respective theoretical values.

Apparatus Required

Sl.NoApparatusSpecificationQty
1.Voltmeter0 – 600V2
2.Ammeter0 – 5A2
3.3 Phase Lamp Load ————–1

Theory

In a single-phase system, there are two wires which carry power from source to destination.

One among them is called ‘phase’ and the other is ‘Neutral’.

Neutral has a potential difference of 0 Volts.

The phase has a voltage that keeps on swinging between a positive and negative maximum with reference to neutral.

In a symmetric 3-phase supply system, three conductors carry alternating current of same frequency and voltage amplitude relative to a common reference but a phase difference of one-third of a cycle between each.

The common reference is usually connected to ground and often to a current-carrying conductor called the neutral.

Due to the phase difference, the voltage on any conductor reaches its peak at one-third of a cycle after one of the other conductors and one-third of a cycle before the remaining conductor. This phase delay gives constant power transfer to a balanced linear load.

As compared to a single-phase AC power supply that uses two conductors (phase and neutral), a three-phase supply with no neutral and the same phase to ground voltage with the same current capacity per phase can transmit three times as much power using just 1.5 times as many wires.

Line voltage refers to the voltage measured between any two line conductors in a balanced three-phase system.

Phase voltage refers to the voltage measured across any one component in a balanced three-phase source or load.

Wye-connected sources and loads always have line voltages greater than phase voltages, and line currents equal to phase currents.

Delta-connected sources and loads always have line currents greater than phase currents, and line voltage equal to phase voltage.

Procedure

a) Star Connection

  • connect the circuit as shown in figure 1
  • Switch on the power supply using TPST(Triple pole single throw) switch
  • Note down the line voltage, line current, phase voltage and phase current from the respectively connected meters

b) Delta Connection

  • Connect the circuit as shown in figure 2
  • Repeat the steps 2 and 3 from the procedure above

Observations and Calculations

Circuit diagram

Star Connection

Tabular Column

Star Connectionline to line VL(Volts)line to neutral Vph (Volts)line IL (Amps) line Iph (Amps)
Practical
TheoreticalVph = VL / √ 3Iph = IL

Delta Connection



Delta Connectionline to line VL(Volts)line to neutral Vph (Volts)line IL (Amps) line Iph (Amps)
Practical
Theoretical Vph = VLIph= IL / √ 3

Result

Star ConnectionDelta Connection
Line voltage, VL
Phase Voltage, Vph
Line Current, IL
Phase Current, Iph

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