Page 573 - Handbook of Electrical Engineering

566 HANDBOOK OF ELECTRICAL ENGINEERING

The starting impedance Z msp is:-

Phase voltage
Z msp =
Starting current
4000
= √ = 5.35 ohms per phase
3 × 431.64
The starting resistance R msp (parallel branch) is:-

Z msp 5.35
Z msp = = = 21.4 ohms per phase
cos φ s 0.25
The starting reactance X msp (parallel branch) is:-

5.35
Z msp
X msp = = = 5.526 ohms per phase
sin φ s 0.9682
Hence the per-unit motor starting resistance is R msppu :-

R msp 21.4
R msppu = = = 0.80 pu per phase
Z msp 26.75
And the per-unit motor starting reactance is X msppu :-

5.526
X msp
X msppu = = = 0.2066 pu per phase
Z msp 26.75
Where, R msppu and X msppu are parallel components representing the motor during the
starting condition. Convert this impedance to the system base at the motor system voltage of
4181.8 volts.

(base kVA) (motor rated voltage) 2
R ms + jX ms = (R msppu + jX msppu ) 2
(motor kVA) (system base voltage)
(0.8 + j0.2066)(3125)(4000) 2
=
3
(598.08 × 10 )(4181.8) 2
= 0.38244 + j0.9875 pu
i) Summary of the results thus far.
The data to be used in the per-unit circuit diagram in Figure G.2 are:-
Generator R g = 0.2pu
X g = 0.25 pu
SWBD parallel load R og = 3.4722 pu
X og = 7.1676 pu
Transformer R c = 0.00635 pu
X c = 0.05446 pu
MCC parallel load R ol = 6.2498 pu
X ol = 10.085 pu