Handbook of Electrical Engineering

CABLES, WIRES AND CABLE INSTALLATION PRACTICES 237

Full-load currentItxflof the transformer

=

Stx

√

3 V

=

250000. 0

√

3 × 415

=347.8 amps

The 100% impedanceZIpuof the transformer

=

V

√

3 Itxfl

=

45

√

3 × 347. 8

=0.6889 ohms/phase

Therefore the 4.5% leakage impedance

Zsec=|Rsec+jXsec|= 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 currentIflmrof the motor

=

Sm

√

3 ×VEfficiency×Power factor

=

37000

√

3 × 415 × 0. 92 × 0. 85

=65.82 amps

The 100% impedanceZmrnof the motor

=

V

√

3 Iflmr

=

415

√

3 × 65. 82

=3.64 ohms/phase at a power factor of 0.85

The starting impedanceZmsof the motor

=

Zmm

Starting to running current ratio

=

3. 64

7. 00

=0.52 ohms/phase at a power factor of 0.45

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 timetdisis 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

(Is) at 5.0 seconds.

From (9.7)

Amin=

Is

√

5

K

=Is× 0. 01564