Permanent Magnet Alternator Concept-40Ekozd And 35Efkozd Models; Short Circuit Performance - Kohler EKOZD Series Service

1.6 Permanent Magnet Alternator
Concept—40EKOZD and
35EFKOZD Models
A permanent magnet alternator is identified with one of
the following designations: 4P_X, 4Q_X, 4R_X, 4S_X,
or 4T_X, and 4UA_. Example: Gen. Model 4S12X. The
first alpha character (S) identifies the alternator family. If
the last character is an X, it denotes a Fast-Responset
X alternator.
The generator set has a rotating-field alternator with a
smaller rotating-armature alternator turned by a
common shaft. The main rotating-field alternator
supplies current to load circuits while the rotating-
armature (exciter) alternator supplies DC to excite the
main alternator's field.
The generator set has a 4-pole, rotating-field with
brushless, permanent magnet alternator excitation
system. The PM system provides short-circuit
excitation current up to 300% at 60 Hz (approximately
275% at 50 Hz) for a minimum of 10 seconds to allow
selective circuit breaker tripping.
The alternator excitation system uses a permanent,
magnet exciter with a silicon controlled rectifier (SCR)
assembly which controls the amount of DC current fed to
the alternator field. This type of system uses a voltage
regulator (located within the Decision-Makerr 3500
controller) which provides a signal to control the SCR
assembly through an optical coupling. The voltage
regulator monitors engine speed and alternator output
voltage to turn a stationary light emitting diode (LED) on
or off, according to engine speed and output voltage.
The LED is mounted on the end bracket opposite a
photo transistor board which rotates on the shaft. The
photo transistor picks up the signal from the LED and
tells the SCR assembly to turn on or off, depending upon
the need, as dictated by the voltage regulator. See
Figure 1-6.
TP-6953 7/19
The voltage recovery period of this type of alternator is
several times faster than the conventionally wound field
brushless alternator because it does not have to
contend with the inductance of the exciter field. It also
has better recovery 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 machine is
its inherent ability to support short-circuit current and
allow system coordination for tripping downstream
branch circuit breakers.
The alternator system delivers exciter current to the
main field within 0.05 seconds of a change in load
demand.
1.6.1

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 the short is removed. The SCR
assembly sends full exciter power to the main field. The
alternator then sustains up to 300% of its rated current.
Sustained high current will cause correspondingly rated
load circuit fuses/breakers to trip.
alternator protection feature serves to collapse the
alternator's main field in the event of a sustained heavy
overload or short circuit.
drops and amperage
The controller
Section 1 Specifications 25