Page 584 - Handbook of Electrical Engineering

CALCULATION OF VOLT-DROP IN A CIRCUIT CONTAINING AN INDUCTION MOTOR 577

Hence the terminal voltage of the generator V gs is:-

V gs = I gs Z ogs
= 0.8040 − j0.0115 pu, which has a magnitude of 0.8040 pu.

This is nearly equal to V ms .

The voltage V gs is within 1.5% of the rigorous case but too optimistic for the motor voltage.
However, most of the volt-drop is due to the generator impedance in either case and so once
some cases have been screened in this way then the more accurate method may be applied to the
serious cases. Since the result is optimistic it therefore requires a safety margin of 2% to 5% to
be added when this method is used. The percentage volt-drops can be calculated as follows:-
• Generator and motor terminal volt-drop in percent.

V mo − V ms
Volt-drop at starting% = × 100%
V mo
1.0 − 0.8040
= × 100% = 19.6%
1.0
which is about 4% better than the rigorous case.

u) Formular method based on kVA rating
The simplification of the power system can be generalised by using a formular method. The
simplified system can be represented by Figure G.3, where:-

Z g is the source impedance, e.g. generator transient reactance.
Z m is the motor impedance (Z mr for running and Z ms for starting).
is the standing load impedance, Z o in Figure G.3.
Z l

Figure G.3 Reduced equivalent circuit for calculating the volt-drop in a 500 kW HV motor.