STAR OR WYE (Y)-CONNECTED SYSTEM
This is the method of connecting the separate phases of a three-phase system. This system is obtained by joining together similar ends, either the start or the finish; the other ends are joined to the line wire. The common point N is called the neutral or star point, at which the similar ends are connected. The voltage between any line and the neutral point, i.e., the voltage across the phase winding, is called the phase voltage, and the voltage between any two outer lines is called the line voltage. The neutral point is connected to the earth.
pic sources: JB Gupta
The arrowheads on emfs and currents indicate the positive direction. Positive directions of emfs are taken from the star point outwards. The three phases are numbered as 1,2,3 or a,b,c or R, Y, B.usually, it is numbered as R, Y, B i.e the three natural colors red, yellow, and blue. There are mainly two phase sequences namely RYB and RBY. RYB is taken as positive and RBY as negative.
The emf induced in 3 phases is shown above. In star connection, there are two windings between each pair of outer, and due to the joining of similar ends together the emf induced in them are in opposition. The potential difference between the two outer known as line voltage, is the phasor difference of phases emfs of the two phases.
The potential difference between outer R and Y or the line voltage E RY is the phasor difference of phase emfs ER and
since the phase angle between phasors
From phasor diagram
similarly potential difference between outers Y and B or the line voltage
and potential difference between routers B and R or the line voltage
since in a star-connected system, each line conductor is connected to a separate phase, the current flowing through the lines and phases are the same.
phase voltages and currents in a balanced three-phase circuit is written as:
where ψ is the phase angle between phase voltage and phase current.
Total instantaneous power,
The sum of the three-second harmonic oscillating terms which have a progressive phase differences of 120 degrees is Zero. The instantaneous power in a 3-phase balanced system is constant and equal to three times the average power per phase.
Total power (p)=√3 *line voltage*power factor
Total apparent power
In a balanced star-connected system
- Line voltage is 120° apart.
- Line voltages are 30° ahead of the respective phase voltages.
- Line voltages are √3 times phase voltages.
- Line currents are equal to phase currents.
- The angle between line currents and the corresponding line voltages is(30° ±ψ);+ve for lagging currents and -ve for leading currents.
where cosψ is the angle between the respective phase current and phase voltage (not between line current and line voltage).
- In a balanced system, the potential of a neutral or star point is zero because potential at a neutral point or star point.
MESH OR DELTA (Δ) connected system
From the above figure, when the starting end of one coil is connected to the finishing end of another coil, a delta connection is obtained. The directions of emfs in the coil have been taken as positive from start end to finish end. The current phasors are shown in the above figure:
since the phase angle between phase current phasors I YR and -I BR is 60°,
Assuming balanced load
The phase current in each winding is equal and let it be equal to Ip.
Similarly line current
since in a delta-connected system, only one phase is included between any pair of line outers,potential difference between the line outers, called line voltage and is equal to phase voltage.
i.e. Line voltage EL=phase voltage Ep
i.e Total power output=√3 *line current*power factor
Total reactive power, Q
In a balanced delta-connected system:
- Line currents are 120° apart.
- Line currents are 30° behind the respective phase currents.
- Line currents are √3 times phase currents.
- Line voltages are equal to phase voltages.
- The angle between line currents and the corresponding line voltages is(30° ±ψ);+ve for lagging currents and -ve for leading currents. same as in the star system.
- where cosψ is the angle between the respective phase current and phase voltage(not between line current and line voltage).
- In a balanced system, the resultant emf in the closed circuit will be Zero.
There will be no circulating current in the mesh if no load is connected to the lines.
Comparison between STAR and DELTA systems
|1||Similar ends are joined together.||Dissimilar ends are joined together.|
|2||Possible to carry a neutral wire to the load. only star connected system can give a 3-phase 4-wire arrangement. hence star connected system can be used for lighting as well as power load ie. for an unbalanced load.||Phase voltage is equal to line voltage|
|3||Phase current is equal to line current i.e Ip=IL||Phase current is equal to 1/ times line current|
|4||Not Possible to carry neutral wire to the load.||Not Possible to carry neutral wire to the load.|
|5||The neutral point of a star-connected system can be connected to the earth, so relays and protective devices can be provided in the star-connection system for the protection of the system against ground faults.|| Delta connections are mostly used in transformer for operating small low voltage 3-phase motors and are suitable for rotary convertors. |
Most of three phase induction motors are delta-connected.
Conversion of balanced load system from STAR TO DELTA AND VICE VERSA
Any balanced load system can be replaced by an equivalent delta-connected system or vice-versa because of the relationship between phase and line voltages and currents.
For example, a balanced star-connected load having an impedance of magnitude Z with a power factor cosψ in each phase can be replaced by an equivalent delta-connected load having an impedance of magnitude 3Z and power factor cosψ in each phase.
For a balanced star-connected load,
For the same line in delta-connected system values of voltage and current as in case of a star-connected system
comparing (i) and (ii) we get,