Section 2 Operation; Fast-Responset Ii Concepts; Short Circuit Performance - Kohler 40COZ/33CFOZ Service Manual

Marine generator set

Table of Contents

2.1 Fast-Responset II Concepts

The generator excitation system uses a permanent

magnet exciter with a silicon controlled rectifier (SCR)

assembly that controls the amount of DC current fed to

the generator field. This type of system uses a voltage

regulator that signals the SCR assembly through an

optocoupler. The voltage regulator monitors generator

output voltage and engine speed and signals the SCR

assembly to turn on or off accordingly through the

optocoupler. The optical coupler consists of a light

emitting diode (LED) mounted on the stationary end

bracket and a photo transistor mounted on the rotating

shaft. The photo transistor picks up the infrared signal

from the LED and signals the SCR assembly to turn on

or off, depending upon the need, as dictated by the

voltage regulator. See Figure 2-1.

The voltage recovery period of this type of generator is

several times faster than the conventionally wound field

brushless generator because the generator set does not

have to contend with the inductance of the exciter field.

The

generator

set

characteristics than the static-excited machine because

it is not dependent upon the generator set output voltage

for excitation power. Possibly the greatest advantage of

this type of machine is its inherent ability to support

short-circuit current and allow system coordination for

tripping downstream branch circuit breakers.

The generator set systems deliver exciter current to the

main field within 50 milliseconds (0.050 seconds) of a

change in load demand.

2.2 Short Circuit Performance

When a short circuit occurs in the load circuit(s) being

served,

output

voltage

momentarily rises to 600--1000% of the generator set's

rated current until removal of the short.

assembly sends full exciter power to the main field. The

generator then sustains up to 300% of its rated current.

Sustained high current causes correspondingly rated

load circuit fuses/breakers to trip.

breaker kit shuts down the excitation system in the event

of a sustained heavy overload or short circuit.

TP-5737 5/01

also

has

better

recovery

drops

and

amperage

The SCR

The safeguard