1. Review – AC Power  Is a power factor of +0.707 ‘good’ or ‘bad’? Why?  What does a pf of -0.1 mean?  A motor draws 14A from 230V supply at 0.82 pf.  How much reactive power compensation is needed to improve pf to 0.95?  Sketch the power triangles.

2. Three phase systems

3. Goals   Identify at least 3 advantages of 3 phase power over single phase   Explain the difference between line and phase voltages and currents   Apply formulae to solve 3 phase problems   Sketch phasor diagrams of 3 phase voltage and current   Solve problems with balanced 3 phase loads   Solve problems with unbalanced 3 phase loads

4. Recall: One single conductive coil rotates in a constant magnetic field, a sinusoidal voltage is generated. Single phase ac voltage

5. Three phase voltages  The number of coils on the rotor is increased to 3  Separated by Three phase ac generator 120 o

6. Three phase generators  Three induction coils are physically placed 120° apart  The voltage induced across each coil will have  Same peak value,  Same shape and frequency  120 o phase shift Field winding (a rotating electromagnet) Three-phase windings 3-phase outputs StatorRotor Neutral

7. Star connected three phase generator The three terminals denoted N are connected together

8. Line voltages and phase voltages Star connected

9. Delta connected three phase generator The generator coils are connected in triangular configuration with no neutral. VθbVθb

10. Line currents and phase currents Delta connected

11. Three phase loads Star–connected or Delta–connected three phase loads ( (Z a, Z b and Z c represent load impedances)

12. Star-star connected three phase systems

13. Star-delta connected three phase systems

14. Delta-star connected three phase systems

15. Delta-delta connected three phase systems

16. Three Phase Power Balanced three phase loads:

17. Advantages of three phase systems over single phase 1.Thinner conductors can be used to transmit the same kVA at the same voltage.  Less copper required (typically about 25%)  Reduced costs  Reduced losses 2.Allow two voltage levels (V Ph / V LL ) 3.Generate more power than a single phase generator 4.Keep a constant / smooth shaft torque on motor….

18. Constant three phase power Single phase power Three phase power

19. Unbalanced Loads

20. Goals   Identify at least 3 advantages of 3 phase power over single phase   Explain the difference between line and phase voltages and currents   Apply formulae to solve 3 phase problems   Sketch phasor diagrams of 3 phase voltage and current   Solve problems with balanced 3 phase loads   Solve problems with unbalanced 3 phase loads