Page 246 - Handbook of Electrical Engineering

230 HANDBOOK OF ELECTRICAL ENGINEERING

transformer for this relay has a primary current rating of 150 amps. Assume only one of the main
transformers is operating. Let the route length of the motor cable be 250 metres and assume that the
volt-drop in the cable is 15% during the starting period.
Find a suitable XLPE insulated cable for the motor and ratings for its fuses. Plot the results
on a log-log graph.

Solution:
The first step is to find the lowest fuse rating that can be used for the motor.
As a preliminary guide the fuse rating should be not less than approximately 1.3 times the
motor full-load current, sub-section 7.4.
The full-load current I flm of the motor is,

Rated power
I flm = √
3 Line voltage × Power factor × Efficiency
160000
= √ = 245.4 amps
3440 × 0.93 × 0.92
Hence the lowest fuse rating would be just above 1.3 × 245.4 amps i.e., 355 amps is a standard
rating. However, this may not be adequate to withstand the long starting time.

Since the cable volt-drop is significant it is necessary to revise the starting time duration and
current from the data given by the manufacturer of the motor.
The motor receives a reduced terminal voltage of 85% during starting. Consequently the
starting current I stm is also reduced to 85% of its design value,

I stm at reduced voltage = 0.85 × 245.4 × 6.75 = 1408 amps instead of 1656.5 amps.

The torque developed by the motor is proportional to the square of its terminal voltage; hence
throughout the starting period the torque applied to the driven machine will be reduced to 0.85 × 0.85
times its design amount. This reduction will apply to nearly the whole of the starting period. As a
reasonably accurate approximation the revised starting time t str can be given as,

t str = t st × (1.0 − V ) −2
= 15.0 × (1.0 − 0.15) −2
= 20.76 seconds, round up to 21 seconds.

Where t st is the designed starting time at 100% terminal voltage and V is the known volt-drop
at the motor.

Hence the shape of the time-current curve for the motor will be elongated towards the fuse
curve. At the end of the starting time the current falls rapidly to its full-load value, hence the curve
has a corner point P m , see Figure 9.7. There needs to be a margin between the corner point P m of
the motor and the fuse operating time point P f at the value of the reduced starting current, so that
the manufacturing tolerances do not interact and cause the fuse to operate. Assume a suitable margin