Page 440 - Basic Electrical Engineering

kVA sin ϕ = kVAR (kilo Var) (4.13)

4.7 COMPARISON BETWEEN STAR CONNECTION AND DELTA CONNECTION

As mentioned earlier, the three windings of a generator can be connected

either in star or in delta. Same is the case with transformers. Three-phase
electrical loads and the windings of three-phase motors can also be connected
in star or in delta.

The relationship between voltages, currents and their phase relationship
along with some other related factors have been compared and are presented

in a tabular form.

Star Connection Delta Connection

1. Line current is the same as phase current, i.e., 1. Line current is × the phase current, i.e., I L
I = I Ph = I Ph
L

2. Line voltage is the phase voltage, i.e., V = 2. Line voltage is the same as phase voltage, i.e.,
L
V = V
V Ph L ph

3. Total power = V I cos ϕ 3. Total power = V I cos ϕ
L L
L L
I
I
4. Per phase power = V Ph Ph cos ϕ 4. Per phase power = V Ph Ph cos ϕ
5. Three-phase three-wire and three-phase four-
5. Three-phase three-wire system is possible
wire systems are possible
6. Line voltages lead the respective phase 6. Line currents lag the respective phase currents
voltages by 30° by 30°

Example 4.1 A 400 V, three-phase, 50 Hz power supply is applied across
the three terminals of a delta-connected three-phase load. The resistance and

reactance of each phase is 6 Ω and 8 Ω, respectively. Calculate the line
current, phase current, active power, reactive power, and apparent power of
the circuit.

Solution:

The load is delta connected. Hence