GENERATOR SELECTION CONTENTS - Diesel Generator

SECTION 4522
January 1990
Page 3
This system required that all 6 ends of the 3 phase motor are brought to terminals. Through two contractors or a
changeover switch, the windings are initially connected in ‘star’; then usually after a preset time delay or when the
motor has run up to a steady speed, the windings are reconnected into ‘delta’. This is the normal running condition at
full line volts.
This mean that:-
(a) the starting voltage is reduced to 1/√3 of V L since V L = √3V line to neutral
(b) starting current is also reduced to 1/√3 of the D.O.L. value since I α T M
(c) starting kVA is reduced to 1/3 of the D.O.L. value since kVA α V L ²
(d) starting torque is reduced to 1/3 of the D.O.L. value since T M α V L ²
Assuming, as with the D.O.L. case:-
per unit over load capability = 2.5 p.u.
D.O.L. starting current = 6.0 x 1 M
motor efficiency = 90%
Then the value of the ratio of S can be calculated from
kW genr. = _______6________ kW M
3 x 2.5 x 0.9
ratio S = 0.889
Note: For values of S less that 1.0, this implies that the a.c. generator kW rating can be less than the motor kW rating
for starting purposes. This is true, but of course the a.c. generator would be unable to supply the motor with its full
load continuously rated steady state power! In other words, values of S less than 1.0 are not practical.
Auto Transformer Starting