Page 253 - Handbook of Electrical Engineering

CABLES, WIRES AND CABLE INSTALLATION PRACTICES 237

Full-load current I txfl of the transformer

250000.0
S tx
= √ = √ = 347.8 amps
3 V 3 × 415
The 100% impedance Z Ipu of the transformer

V 45
= √ = √ = 0.6889 ohms/phase
3 I txfl 3 × 347.8

Therefore the 4.5% leakage impedance
Z sec =|R sec + jX sec |= 0.045 × 0.6889 = 0.031 ohms/phase

or 0.00308 + j0.0308 ohms/phase
b) Find the motor impedances for the starting and running conditions:
Full-load current I flmr of the motor

S m
= √
3 × V Efficiency × Power factor
37000
= √ = 65.82 amps
3 × 415 × 0.92 × 0.85

The 100% impedance Z mrn of the motor
V 415
= √ = √ = 3.64 ohms/phase at a power factor of 0.85
3 I flmr 3 × 65.82
The starting impedance Z ms of the motor

Z mm
=
Starting to running current ratio
3.64
= = 0.52 ohms/phase at a power factor of 0.45
7.00
c) Find the conductor and armouring impedances for various cables that may be suitable.
From Tables 9.23 and 9.24 the impedance data for the circuit temperature and power frequency
conditions are shown below.
d) Since the motor is fixed equipment of the Class 1 type, the disconnection time t dis is 5.0 seconds.
The fuse operating currents at 5.0 seconds are shown in Table 9.31.
e) Find the I-squared-t parameters for the cable conductors with respect to the fuse operating current
(I s ) at 5.0 seconds.
From (9.7)
√
I s 5
A min =
K
= I s × 0.01564