Page 141 - Handbook of Electrical Engineering

INDUCTION MOTORS 123

Table 5.12. Limits to motor ratings due to
system voltage
Motor power Appropriate system
rating (kW) line voltage (volts)
Up to 250 LV e.g. 380 to 440
150 to 3000 HV e.g. 2400 to 4160
200 to 3000 HV e.g. 3300 to 7200
1000 to 15,000 HV e.g. 6600 to 13,800

cross-sectional area of the conductors becomes large. The typical kW limits for various voltages are
given in Table 5.12, see also IEC60034 part 1 clause 29.

5.7 EFFECT OF THE SYSTEM FAULT LEVEL

Motors are controlled by circuit breakers or contactors. With high voltage motors it is necessary
to ensure that the main terminal box and the terminals inside can withstand the effects of a major
three-phase fault inside the box. This applies especially to motors that are to be used in a hazardous
area. As a guide to the level of safeguard, Table 5.13 may be used.
When contactors are backed up by fuses it is possible to reduce the fault levels considerations.
The current versus let-through-time characteristics of the fuses need to be studied if the above fault
levels are to be reduced.

5.8 CABLE VOLT-DROP CONSIDERATIONS

The conductor size and length of the motor feeder cable need to be chosen carefully and the following
points should be considered:-

• Normal running current.
• Starting current.
• Ambient temperature.
• Laying cables in air or buried in the ground.
• Laying cables vertically or horizontally.

• Derating factors for grouping cables.

Table 5.13. Correspondence between system voltage and fault
level at the motor terminals

System line System peak fault System fault level
voltage (volts) current (kA pk ) (MVA)
3300 to 4160 85 to 110 150 to 250
6600 to 7200 70 to 110 350 to 500
11,000 to 13,800 65 to 80 500 to 750