Points to remember in three phase circuits

POINTS TO REMEMBER IN THREE PHASE CIRCUITS

1. Phase sequence:

It is the order in which the alternating quantities attain their maximum values.

2. For the clockwise rotation of the magnetic field, the phase sequence is called positive phase sequence ie., RYB.

3. For the anti-clockwise direction of rotation of magnetic field, it is negative phase sequence ie., RBY.

4. Balanced supply system:

If the voltages in the phases are equal in magnitude and displaced by 120°, the system is called a balanced supply system.

5. 3-phase balanced load :

If the loads in the 3-phase are equal in magnitude (in ohms) and same angle (Both magnitude and nature) it is called balanced load system.

Example: Take star - connected load system. Let ZR = 10/450. If Zy = ZB = 10/45°. Then it is balanced load system.

6. 3-phase unbalanced load:

If atleast one of the loads is different from other two loads the load is called unbalanced load. For Example, Take star -load. If ZR = Zy = 10/30° and ZB = 10-30°. It is unbalanced. The magnitudes of the impedances may also be different.

7. In a balanced supply system, the phasor sum of all the three voltages = 0, at any instant.

8. In the star connection of supply, the common point is junction of three similar ends of the three phases.

9. In the Delta - connection, the 3-phases are connected end to end.

10. In the star - connection, at the neutral point, the current flowing through the neutral conductor is obtained by applying KCL.

11. In a star - connection, the line current = phase current, the line voltage √√3 times phase voltage.

12. In a delta - connection, line voltage = phase voltage and line current = √3 times phase current.

13. In a 3-phase circuit, total power = W = √3 E_L I_L cos ϕ

 [Note: ϕ is the angle between phase voltage and phase current. It is not the angle between E, and IL]

14. To solve a problem on unbalanced Delta load supplied by balanced supply, find each phase current and then apply KCL to get the line currents. It is like solving single phase circuits.

15. In the two wattmeter method to measure 3-phase power, load can be either balanced or unbalanced. The connection can be either Delta or star.

16. In the two wattmeter method of measurement of three phase real power, connections are done as below:

The current coils of the wattmeters are connected in two different lines. The potential coil other ends are connected commonly to the third line where there is no connection of current coil.

17. In the two wattmeter method, if one wattmeter shows negative reading, the inference is that the power factor is less than 0.5.

18. In the above method, (a) If one wattmeter shows zero reading, the power factor is 0.5.

(b) If both wattmeters indicate positive readings, then power factor is more than 0.5.

19. In the two wattmeter method, assuming both readings to be positive, total power is (active) = W2 + W, and reactive power = √3 (W₂ - W₁).

20. If the current coil of single phase wattmeter is connected in one line and its voltage coil is of connected between other two lines, then the meter needs only reactive power.