Lebw4993 generator systems

APPLICATION AND INSTALLATION GUIDE

GENERATOR SYSTEMS

Contents

Generator Systems................................................................. 1
Generation System Basics................................................... 2
Main Generator Components ........................................... 2
Magnetic Field and Voltage ............................................. 4
Phase and Voltage ......................................................... 5
Generator Design ............................................................... 6
Rotor ............................................................................ 6
Amortisseur Windings................................................. 6
Salient Pole Rotor....................................................... 6
Non-Salient Pole Rotor ................................................ 6
Permanent Magnet Rotor ............................................ 7
Stator ........................................................................... 7
Windings ................................................................... 9
Insulation Systems ....................................................... 12
Rotor Insulation........................................................ 12
Stator Insulation Combinations .................................. 13
Insulation Life .......................................................... 14
Coil Connections .......................................................... 17
Generator Features and Attachments ............................. 19
Bearings .................................................................. 20
Space Heaters ......................................................... 20
Ingress Protection (IP)............................................... 20
Physical Data............................................................... 21

Nameplate ............................................................... 22
NEMA & IEC Design Considerations ............................... 23
Temperature Rise ..................................................... 23
Maximum Momentary Overloads................................ 25
Maximum Deviation Factor........................................ 25
Telephone Influence Factor (TIF) ................................ 25
Single Frequency Weighting Short Circuit
Requirements........................................................... 26
Overspeed ............................................................... 26
Generator Performance Characteristics ............................... 27
Rated and Per Unit ....................................................... 27
Efficiency ................................................................ 27
Fault Current/Short Circuits ........................................... 28
Reactance ............................................................... 31
Transient Reactance (X’d).......................................... 32
Sub-Transient Reactance (X”d)................................... 34
Synchronous Reactance (Xd) ..................................... 36
Negative Sequence Reactance (X2) ............................ 37
Zero Sequence Reactance (Xo) .................................. 37
Harmonics & Distortion..................................................... 39
Miscellaneous Terms ........................................................ 43
Overspeed Capability ................................................ 43
Heat Dissipation ....................................................... 43
Derating .................................................................. 43
Generator Limits .............................................................. 44

Exciter & Regulator Characteristics and Performance ........... 45
Excitation System ........................................................ 45
Self-Excitation ......................................................... 46
Permanent Magnet Excitation .................................... 46
Voltage Regulation ....................................................... 46
Performance ............................................................ 49
Specifications .......................................................... 50
Frequency Sensing ................................................... 51
Type ....................................................................... 52

Foreword
This section of the Application and Installation Guide generally describes
Generator Systems for Caterpillar® engines listed on the cover of this
section. Additional engine systems, components and dynamics are addressed
in other sections of this Application and Installation Guide.
Engine-specific information and data is available from a variety of sources.
Refer to the Introduction section of this guide for additional references.
Systems and components described in this guide may not be available or
applicable for every engine.

Information contained in this publication may be considered confidential.
Discretion is recommended when distributing. Materials and specifications
are subject to change without notice.

CAT, CATERPILLAR, their respective logos, “Caterpillar Yellow” and the
POWER EDGE trade dress, as well as corporate and product identity used
herein, are trademarks of Caterpillar and may not be used without
permission.

©2008 Caterpillar®
All rights reserved.

Application and Installation Guide Generator Systems

Generator Systems
Electricity is a useful source of energy because it is versatile; much more
versatile than mechanical energy. It can be used for a variety of tasks, such
as lighting, heating and rotating electrical machinery and it can be used in a
variety of locations, such as offshore oilrigs, natural gas fields, remote areas
and urban confines.
Caterpillar generators convert the mechanical energy of an engine, or
prime mover, to electricity. The well-proven, innovative designs of Caterpillar
generators have lead to several highly reliable lines of generators used in
electric power generation applications worldwide. All Caterpillar electric
sets use AC generators. The AC generator, also called an alternator,
converts mechanical energy to electrical energy and acts as a voltage
source for the load.
This module describes basic concepts involved in the various Caterpillar
generator designs. It addresses basic electrical generation concepts,
generator design and performance as well as voltage regulation.
It is important to note that the information in this section applies, primarily,
to synchronous generators. The term synchronous describes the relationship
between the engine rpm and the generator output frequency; they are
exactly proportional.

SECTION CONTENTS

Generator System Basics ...... 2 Generator Performance
• Main Generator Components Characteristics................... 28
• Magnetic Field & Voltage • Rated & Per Unit
• Phase & Voltage • Fault Current / Short Circuits
Generator Design ................ 6 Harmonics & Distortion ....... 40
• Rotor Miscellaneous Terms .......... 44
• Overspeed Capability
• Stator
• Heat Dissipation
• Insulation Systems
• Derating
• Coil Connections
Generator Limits ................ 45
• Generator Features &
• Rated & Per Unit
Attachments
• Fault Current / Short Circuits
• Physical Data
Exciter & Regulator
• NEMA & IEC Design Characteristics................... 46
Considerations • Excitation System
• Voltage Regulation

All rights reserved. Page 1

Generator Systems Application and Installation Guide

Generation System Basics
There are three basic requirements Essentially the process of
for the generation of voltage. They generating voltage goes in the
are magnetism, motion and following order. The exciter provides
conductors. DC current to the rotor windings. DC
When a coil moves relative to current through these wires creates
a magnetic field, a voltage is magnetic flux. Magnetic flux
produced; generation systems are generates an AC voltage in the
based on this concept. When a nearby stator windings when there
conductor cuts through a magnetic is relative motion between the two.
field, a current is produced in that The regulator then senses this
conductor. These two concepts are output and controls the exciter
very closely connected. Keep in current.
mind that it makes no difference if In Caterpillar generators, the rotor
the magnetic field is stationary and (the source of the magnetic field)
the conductor moves or whether rotates inside a stationary armature
the conductor is stationary and the called a stator. One reason for using
magnetic field moves. The important a stationary armature and a rotating
aspect is that there is relative magnetic field is the difficulty of
motion. taking 3-phase current from a
The simplest generator consists rotating armature. The rotor is
of a loop of wire rotating between rotated by a prime mover. In the
two permanent magnet poles. case of Caterpillar generator sets,
the prime mover is usually an
Note: In any generator set
engine.
installation, the frame of the
generator must be positively The rotor contains magnetic poles
connected to an earth ground with windings wrapped around them
or to the hull of the vessel. to form coils. These coils are called
field coils or field windings because
Main Generator Components they create a magnetic field when
An AC synchronous generator excited with a DC current. Typically,
is significantly more complex than the generator field windings contain
the simple generator of a wire loop many turns.
rotating between two permanent A magnetic field radiates out from
magnets. An AC synchronous the rotor as lines of magnetic flux.
generator consists of four main As the rotor rotates, so does the
components and/or systems: magnetic field. When this moving
• Field (rotor) magnetic field comes across a stator
• Armature (stator) winding, an AC voltage is produced.
The magnetic field is strongest at
• Exciter the center of the north and south
• Automatic Voltage Regulator poles where the lines of magnetic

Page 2 All rights reserved.


flux are concentrated. Therefore, synchronous or no-load speed in
the closer a pole is to a stator revolutions per minute (rpm).
winding, the higher the voltage 120 x f
produced in that stator winding. rpm =
Number of poles
It is important to note voltage is
a function of flux change per time,
not only the proximity to the field. If 50 Hz is desired from a four-pole
generator, the generator must be
The symmetrical design of the driven at 1500 rpm. A six-pole
generator ensures that the rotor generator is driven at 1000 rpm.
poles extend over equal arcs and
that the magnetic flux density The generated frequency of 50 Hz
distribution is similar across all is entirely a function of the driven
stator windings. speed.

Magnetic poles refer to the 120 x f
1000 rpm =
magnetic north and south and are 6
the points where the magnetic field f = 50
is strongest on the rotor. The relationship between
the number of poles and the
synchronous speed is shown in
Table 1. These calculations are
figured by taking the fundamental
frequency of 50 or 60 Hz and
dividing it by the number of pole
pairs. It is then multiplied by 2π
to get the synchronous speed
into rad/s, that is then converted
into rpm.
Synchronous Speeds
60 Hz 50 Hz
Poles rpm Poles rpm
2 3600 2 3000
Figure 1
4 1800 4 1500
6 1200 6 1000
A pole relates to the number of
8 900 8 750
magnetic poles developed in the
12 600 12 500
rotating field. Magnetic poles in
a four-pole generator are arranged Table 1
north-south-north-south around the
circumference of the rotor, as Regardless of the number of
shown in Figure 1. The number of pole pairs, the rotor moves 360
poles (north-south-north-south) and mechanical degrees in one
the desired frequency (cycles per revolution. In electrical degrees,
second or hertz) determine the however, each pole pair rotates
360 mechanical degrees. In other



words, electrical degrees are the as high as 250V is often used in
mechanical degrees times the larger generators, while smaller and
number of pole pairs: medium sized generators seldom use
Mechanical Degrees x Number of Pole Pairs voltages higher than 125V.
In a four-pole generator (two pole Because it is so difficult to extract
pairs), each pole pair moves 360 high voltage from a rotating
mechanical degrees, so the total armature, the magnetic field of the
electrical degrees moved is: main generator is rotated rather than
the armature. A rotating armature
360 x 2 = 720
generator will be used as an exciter
The following illustrates electrical in Caterpillar generators.
degrees in terms of number of poles:
Generator output voltage depends
2-poles = 360° electrical on the following:
4-pole = 720° electrical • Speed of relative motion
6-pole = 1080° electrical between magnetic field and
8-pole = 1440° electrical stator conductors
The main armature, or the stator, • Strength of magnetic field
remains stationary. The stator • Number of series turns in the
consists of the stator core, and stator windings
its own windings called stator The strength of the magnetic field
windings, or armature windings. The is proportional to the current flowing
stator may also include exciter field through the field coils. As the
coils when used with a self-excited current rises, the magnetic field
field arrangement. The stator grows stronger.
windings are placed in slots along
The speed of relative motion
the inside of the stator. The stator
between the magnetic field and the
usually contains a large number of
stator windings depends on the
slots. The rotor magnetic field cuts
rotational speed of the rotor (engine
across the stator windings as it
rpm). As the rpm rises, so does the
rotates inside the stator. As a result,
speed (V) of relative motion. The
voltage is produced in these
higher the speed of relative motion,
windings.
the greater the generators output
The stator voltage is the generator capabilities.
output that is supplied to the load.
Voltage can be adjusted by
Magnetic Field and Voltage arranging the stator windings in
The magnetic field is induced in coils and varying the number of
the main generator by a DC current turns, or times the windings are
from the exciter through low voltage wound around the stator. More
field or rotor windings. These voltage can be induced by increasing
windings are low voltage compared the number of turns and less voltage
to the stator windings. DC voltage is induced by using fewer turns.
Consequently, stator windings can



be arranged with the optimum Three-Phase Generator
number of coil turns to produce
the required output voltage.

Phase and Voltage
Phase, voltage and the stator
core are all inter-dependent. The
calculated design of the stator core
and winding distribution enables a
generator to provide the appropriate Figure 3
output voltage.
In three-phase generation, three
The phase voltage of a generator
phases of voltage are produced
is directly linked to the voltage
120° apart. However, to make the
output of that generator. The
connection a symmetrical 3-phase
type of voltage induced is partially
connection, a phase coil must have
dependent on the number of phases
an equal and opposite winding 180°
in a generator.
away. Figure 4 illustrates this
A single-phase generator will relationship; Phase coil B has an
generate a voltage sine wave when equal and opposite winding at phase
the rotor completes one cycle (one coil –B. The result is that the three
360° revolution). Refer to Figure 2. phases and their opposing windings
are actually 60° apart.
Single Phase Generator

Figure 2

A three-phase generator consists
of three coils equally spaced around
the stator and connected in a wye
Figure 4
(Y) or Delta (∆) configuration.
Therefore, three voltages can be
produced consecutively with a 120°
phase difference. Refer to Figure 3.



Generator Design
Generators are constructed in Amortisseur Windings
various ways to satisfy different Amortisseur windings, also known
load and customer requirements. as damper windings, are special
At Prime power ratings, SR4B conducting bars laid into a squirrel
generators are limited to 105°C cage configuration. They are set in
(221°F) temperature rise over an the notches in the rotor and then
assumed 40°C (104°F) ambient and are shorted on the end by either a
may be used on a continuous duty shorting ring or shorting laminations.
(24 hours per day) basis. They are used to increase machine
stability. This is due to the windings
SR5 generators are limited to a
and rotors relationship to the
125°C (257°F) temperature rise
synchronous speed. If the rotor is
over the same conditions.
at synchronous speed, there is no
At Standby power ratings, SR4B induced voltage through the
generators are limited to 130°C amortisseur bars, hence there is no
(266°F) temperature rise over an interaction. Conversely, if there is
assumed 40°C (104°F) ambient a differential between the rotor and
and are used many times for back-up synchronous speed, a voltage will
power needs, and are rated on a be induced in the windings. This
temporary use basis. voltage produces a current flow and
SR5 generators are limited to a in turn creates a magnetic field.
150°C (302°F) temperature rise The interaction of the two magnetic
over the same conditions. fields in turn cause a torque that will
The rating is expressed in kilowatts correct this speed difference
(kW) at 0.8 power factor. A resulting in speed and torque
common ratio, kW divided by stability.
0.8 permits calculation of kilovolt- Salient Pole Rotor
amperes (kVA) on all generators. A rotor configured with the
individual rotor poles protruding from
Rotor the center of the rotor is known as a
The rotor is defined as any rotating salient pole rotor.
winding or element of a generator.
It can be described as an assembly The rotating salient pole field
of thin magnetic steel laminations, arrangement is affected by
tightly compressed and then riveted, reluctance torque and is used for
bolted or welded together to form a engine-driven generators on most
magnetic path. It is around this core machines in the 20 kVA (16 kW)
that the field windings, or coils of and larger sizes.
conducting material, are wound Non-Salient Pole Rotor
around or inserted. A non-salient pole rotor, or
cylindrical rotor, is typically made
from solid cylindrical material and



usually has grooves cut into the pole thick steel stampings/ laminations
faces to place the windings. stacked together. Stacking machines
A rotating non-salient pole field, use automatic welding to assure
or cylindrical rotor, isn’t affected correct stator skew, stack pressure,
by reluctance torque and it is mainly and slot alignment. Stator stampings
used for large, steam turbine-driven are usually stamped from special
generators. The non-salient silicone steel.
configuration usually has grooves The stator core is subjected to
cut into the pole faces to place the an alternating magnetic flux which
windings. induces currents, called “eddy
currents”, in the core that create
Permanent Magnet Rotor
losses. Creating the core from
A permanent magnet (PM) field
stacked stampings rather than from
eliminates the need for an exciter
one solid piece of steel inhibits eddy
and therefore is very cost effective
currents, reducing losses. The
in smaller size generators. The
silicone added to the steel stampings
disadvantage to this is that its flux
and an oxide coating help to inhibit
density (field strength) is constant
eddy currents.
and voltage regulation is poor.
However, if it is to be used in an In addition to eddy current losses
application with a steady load and in the stator core, there are
minor fluctuations, a PM field can hysteresis losses. One definition of
be very effective. hysteresis is the failure of a property
that has been altered by an external
A rotating armature, stationary
agent to return to its original value
field generator is used mainly in
when the cause of the alteration is
small, low-voltage machines. A
removed. Figure 5 illustrates
common use for this generator is as
hysteresis.
an exciter for brushless generators.
For additional discussion of PM The B-H curve in Figure 5 shows
used in larger generator excitation that unmagnetized iron starts at zero
systems, refer to Exciter & Regulator and proceeds to saturation as the
Characteristics and Performance- magnetomotive force (MMF)
Permanent Magnet. increases. In simpler terms, the
unmagnetized iron becomes
Stator magnetized. As MMF is reduced to
The stator is defined as any zero again (or as the iron loses its
stationary winding or element of a magnetism), a residual B’ results.
generator. The stator core is where The residual B’ means that the iron
the usable electricity is generated. does not entirely lose its magnetism
The windings carrying the usable when MMF returns to zero. The
electricity are placed in the stator same curve loops around as MMF
core slots. becomes negative then positive
The stator core is made up of again from an AC exciting current.
hundreds of 0.0185 in. to 0.025 in. Since the hysteresis loop results in



internal domain realignment, energy of attributes, one of which is the
is expended. Losses are proportional physical size of the stator core. The
to the area of the loop created due inside diameter (called the air gap
to the work expended in following diameter) and the inside core length
through the cycle of magnetization are among a number of attributes
and demagnetization. related to the kVA output
The inside stator diameter and core
length are used to calculate the
Dˆ2L value (which determines the
kVA output of the generator).
The air gap is the space between
the pole face and stator lamination
teeth. The air gap area is the ID of
the stator minus the OD over the
pole face divided by 2 multiply by
the length of the stator. The flux
density of the air gap (Bg) is then
the total flux (ft) divided by the
Figure 5 inside core area.
The magnetic flux in the air gap is
Figure 6 illustrates the core loss created by the field windings on the
curve for M-43, 24 gauge, hot rolled rotor. The rotor laminations are not
steel shows how core loss increases necessarily made of the same
with increasing induction at 50 Hz material as stator laminations
and 60 Hz. The core losses because the flux in rotor laminations
measured in this curve include both is unidirectional, always flowing out
eddy currents and hysteresis. of the north and into the south
magnetic poles.
Core Loss Curve Different loads require different
types of voltage. Generators are
designed with various combinations
of slot, conductor, and winding
types to provide the specific kind of
voltage required by the generators’
loads.
Stator stampings contain one of
two kinds of winding slots. The
semi-closed slot accepts only a
random winding, meaning one wire
Figure 6 at a time is fed into the slot. The
open slot accepts a random winding
The output capability of a or a form-wound winding. The entire
generator is measured by a number coil is placed into the slot, rather



than being inserted wire by wire. called a phase sheet, which
Double-layer winding consists of increases the creepage distance,
two coil sides per slot, resulting in separates each coil group.
the same number of coils as slots.
Shown in Figure 7 are both a Lap Routed Winding
random wound, semi-closed slot
cross-section and a random wound,
open slot cross-section.

Stator Windings

Figure 8

Lap winding is a double-layer
winding. There are two coil sides per
Figure 7 slot, resulting in the same number of
coils as slots. Surge rope and tape
Windings are added to end turns to hold the
Stator winding processes can be end turns stationary during surges,
divided into two categories; the motor starting, and transients. The
routing of the coils and the type of surge rope, which is made with a
winding. The routing of the coils strong fiberglass and epoxy, also
refers to the pattern in which the reduces deformation of the end
windings are applied to the coil. turns when they are stressed. The
The type of winding refers to the tape additionally serves to protect
shape of the winding material. the coil end turns from abrasion
between coils. Figure 8 shows a
Routing Styles schematic of a lap wound coil.
There are three coil routing styles.
They are lap, wave, and concentric. Concentric windings are wound
The most common styles are lap into loops within larger loops. This
and concentric. All SR4B and SR5 results in the simplest mechanical
generators are lap wound. assembly and the least amount of
copper. This configuration however,
Lap routed windings are loops experiences slightly higher levels of
routed across one another and harmonics. This winding style is the
wound into a double diamond shape. most economical and allows an
An entire coil group can be wound entire coil group to be wound at
at one time, so connections are once and machine inserted. Like lap
made only at the coil group, not winding, connections are made at
at each coil. An insulation material the coil groups, not at each coil.



Concentric winding is a single-layer rectangular wires which can be
winding. There is one coil side per placed very close to one another,
slot, resulting in twice as many slots resulting in the maximum amount of
as coils. Phase sheets are placed copper, least amount of spaces, and,
between the phases to provide extra therefore, superior efficiency and
insulation between coil groups. durability. While concentric windings
Figure 9 shows a schematic of a and lap windings may be either
concentric wound coil. random or form wound, form
windings are usually impractical for
Concentric Winding generators under 250 kW because
of inadequate spacing.
The two winding types are shown
in Figure 10 and Figure 11.

Random Winding

Figure 9

Coil Pitch
When a coil is wound 180
electrical degrees, the voltages on Figure 10
the conductors on either side are
equal in magnitude and opposite in Random wound coils are wound
direction and are said to be a full with round copper magnet wire that
pitch coil. Sometimes a coil is is insulated with a moisture resistant
wound less than 180° and is said to heavy polyester film and amide-imide
be a fractional pitch coil. The reason overcoat. Containing multiple
for this is to reduce certain harmonic conductors per turn, the coils
factors that will be discussed later consist of one or many turns each
in this guide. SR5 generators are and are wound in a double diamond
2/3-pitch. shape. Random wound coils are cost
Winding Types effective and are suited for low
Winding types can be random or power ratings, standby applications,
formed. Random windings utilize clean environments, and low
coils of round wire. Material costs operating hours.
are lower, but this method leaves The random wound stator is
spaces between the individual wires. treated using a dip and bake
Formed windings utilize square or process. The stator is dipped and



totally submerged into a polyester retarding moisture and contaminant
resin. Polyester resin is used for its buildup. Finally, along with the
superior heat dissipation, flexibility, normal dip and bake process, an
voltage breakdown strength, and extra epoxy dip and bake is done.
moisture resistance. The resin
penetrates the spaces between the Formed Winding
conductors and leaves a uniform
resin build. The resin is then cured
by a bake cycle. By holding the
conductors together, the resin
prevents vibration and premature
failure. Once the polyester resin is
cured, the stator is dipped and
baked using an epoxy resin. The
epoxy adds extra protection against
moisture, chemicals, and other
environmental factors. If requested Figure 11
and/or necessary, additional dips and
Form wound coils are pre-formed
bakes maximize resin buildup and
with rectangular copper magnet wire
environmental protection. As many
and taped with multiple layers of
as 2 to 4 dips and bakes are
fused double daglass over heavy
possible. To further protect against
film polyester with an amide-imide
abrasion and moisture, asphalt
overcoat. Daglass is a polyester
epoxy can also be applied to the
and glass insulation material. The
lead end. All random wound stators
coils are taped to add mechanical
are sprayed with red sealer to help
strength. The number of tape layers
retard voltage tracking and to seal
varies depending on the voltage
all parts from rust and corrosion.
applied to the coil and the dielectric
Tracking is caused by contaminants
strength (ground insulation) needed.
like salt water, which can get
Additional protection can be added
trapped on the coil end turns.
if required because of environmental
Surface currents then develop and
factors. Mica tape is used for ground
carbonize or “track” the resin
insulation because of its insulating
surface.
properties, such as: high dielectric
Coastal insulation protection is strength, high breakdown voltage,
sometimes added to random high insulation resistance, and
wound windings for increased excellent protection against
environmental protection. The end moisture, chemicals, and other
turns are the most susceptible to environmental factors.
moisture and contaminant buildup,
Open stator slots allow the coils
so glass tape is added to the end
to be placed in the slots without
turns to aid in extra uniform resin
separation of the wires within the
retention capability. The tape also
coils. Coils are connected either by
makes coil end turns smoother,



groups or by individual coils with Insulation Systems
both crimp connections and The insulation system is a complex
soldering. Connections are taped combination of insulation materials
and sleeved to assure a good seal that are carefully selected by the
against environmental factors. generator designer to prevent
Because form wound coils are pre- undesirable electrical flow.
formed, air space exists between Generators consist of three types
each coil in the coil head, allowing of material; copper wire, iron, and
better cooling. Each coil head is also insulation. To function properly,
braced with double surge ropes and the individual copper conductors in
other blocking between coils to stator windings must be completely
prevent coil movement during surges insulated from neighboring
and other transients such as sudden conductors in the same coil and
load changes and switching. from the surrounding iron (referred
Form wound stators are Vacuum to as ground).
Pressure Impregnated (VPI) for Insulation must prevent electrical
added protection from the breakdown between components,
environment. VPI is a process where but it must also be used as sparingly
air is removed causing the resin and as possible. Less insulation means
other coatings to adhere more tightly more space available for copper and
to the windings. This process was better heat dissipation. More copper
developed specifically for form and better heat dissipation improves
wound windings. maximum output from the generator.
After the connecting and brazing Due to the added stress put on the
stages of the insulation process, material in higher voltage machines,
the entire stator winding and core higher quality insulation or more
assembly is vacuum pressure insulation must be used.
impregnated with a special polyester Rotor Insulation
resin varnish. Each Caterpillar rotor is precision
Manufacturing costs for layer wet wound. This means the
concentric/random windings are conductors are hand-placed precisely
lowest, but the more expensive in rows, layering one row on top of
lapped/formed windings provide the other. Each layer of conductors
increased durability and electricity is brushed with high bond strength
quality. Caterpillar generators have epoxy before the next layer is added
been designed in accordance with to assure a good seal between
the expected application; Medium conductors.
output prime and high output Since the field coils employ a low
standby use lapped/random. High voltage, field coil insulation problems
output prime power generators use are mostly mechanical. As the rotor
lapped/formed windings. rotates, centrifugal force is applied
to the field coils causing the coils to



bow out. When centrifugal force is Windings are coated with various
broken down into its components, layers of insulation. Insulation
there is a vertical force (FV) and a materials can be used individually
side force (FS). The side force or in combinations and may be
causes the coils to bow out. comprised of acrylics, asphalts,
To prevent field coils from bowing epoxies, melamines, phenolics,
out due to centrifugal force, wedges polyamides, polyimides, polyesters,
are placed in-between the poles with polyethylenes and silicones. Possible
their two sides holding the coils insulation systems include:
tightly against the poles. Extra • Magnet wire with a polyester
wedges and bracing are sometimes and amide-imide overcoat.
added for extra assurance that rotor • Nomex™-mylar™-nomex™
materials do not move. ground insulation. (Mylar
The purpose of the Amortisseur or is a DuPont polyester film.)
damper windings (discussed earlier) Table 2 shows examples of system
is to prevent rotational oscillations voltages that may be encountered.
in the rotor. These windings provide However, the list is by no means
a motoring effect and produce a complete. In the US, system
torque in the rotor working against voltages are based on 120V with
such oscillations, thereby providing multiples of that voltage.
superior voltage stability. This is
Nominal System Identical System
particularly important during parallel Voltages* Voltage
operation where generators carrying
120 110, 115, 125
uneven loads can result in voltage
120/240 110/220, 115/230
and operational instability which
translates to added stress on the 208Y/120 199Y/115
insulation. Damper windings will 240 220, 230
minimize these oscillations. Damper 480 440, 460
windings also dampen oscillations 600 550, 575
occurring from short-circuit and 2400 2200, 2300, 2500
engine pulses.
4160Y/2400 3810, 4000
Tests are performed on the rotor
to assure that there is no material Table 2
movement in the rotor. A prototype
spin test runs at 125% rated speed Caterpillar system voltages
for two hours at 170°C (338°F). are divided into three classes to
differentiate between types of
Stator Insulation Combinations generators. The classes are Low
Stator insulation is categorized into Voltage, Medium Voltage and High
classes based on their ability to Voltage.
withstand heat for a specified period
• Low Voltage — A class of
of time. Class H is temperature rated
nominal system voltages of
at 180°C (356°F) and Class F at
1 KV or less
155°C (311°F).



• Medium Voltage — A class exceed the damage curve (like a
of nominal system voltages fuse), currents above the damage
greater than 1 KV, but less curve will cause insulation to
than 100 KV become brittle, carbonized, and
• High Voltage — A class of cracked. Another threat is the
nominal system voltages equal differential expansion of materials.
to or greater than 100 KV and Copper expands significantly and
equal to or less than 230 KV iron expands, relatively, little with
high current. These materials,
Caterpillar may supply generators expanding at different rates, cause
up to 15 KV in some applications. stress and stress cracking in the
Applications above 15 KV are used insulation. Conductors may also
for transmission voltages and are lose strength, fracture, and/or melt.
used in vary large power plant
However, insulation can safely
generators or they are specifically exceed its rated temperature by
developed for step up transformers. 30° to 50°C (86° to 122°F) if the
Generator armature windings overheating does not last too long.
must withstand a test voltage
of 1000 volts plus two times the Typical Damage Curve
rated voltage of the machine.
The field windings must withstand
1500 volts rms.
A Megger test is used to measure
insulation resistance with respect
to ground. A measurement of
100 meg-ohms is common at the
time of manufacture. Megger
readings of one meg-ohm per
1000 volts of generator rating are
generally considered acceptable. Figure 12
Larger generators are sometimes
required to pass tests associated Insulation Life
with insulation coordination for The more current the stator
lightening and voltage surges. windings can accommodate, the
This requirement falls under Basic greater the generator output. “Q”
Impulse Insulation Levels (BIL) and represents ampere conductors per
the generator must be tested under inch of armature circumference.
surge conditions. There are, however, limitations
to ampere conductors per inch of
Figure 12 shows a typical damage
armature circumference, temperature
curve. Any generator operation with
rise being the most important.
currents above this curve should be
avoided. Although a generator may Temperature rise is the limitation
not suddenly fail when currents of the insulation to withstand the



heat produced when current runs Maximum
Class Operating Material
through the stator windings. The Temperature
thermal expansion of the wires A 105°C (221°F)
Cotton, Silk, Paper,
Suitably Impregnated
from this heat plays a part in this Glass, Asbestos,
limitation. B 130°C (266°F) Mica. Suitably
Bonded
Since the late 1950’s, plastics Glass, Asbestos,
Mica. Suitably
have been used as the primary F 155°C (311°F) Bonded with materials
insulation for generators. As that permit 155°C
operation.
late as 1960, British standards Silicone Elastomer,
specifications referred to insulation H 180°C (356°F)
Mica, Glass,
Asbestos. Bonded
classes by material. Since the with silicone resins
1960’s, however, insulation
Table 3
classes have been referred to by
the maximum temperature limit of
The bar graphs in Figure 13 show
the materials in each class. Refer insulation class temperature limits
to Table 3. measured by resistance or
thermometer and by embedded
detector. Notice the inclusion of
the hot spot as a separate factor
in the resistance graphs.

Figure 13

Ambient temperature and method 40°C (104°F), although marine and
of measurement further subdivide some others prefer between 45°C
the maximum temperature limits (113°F) and 50°C (122°F).
shown in the table. Ambient When measuring stator windings
temperature is usually considered by resistance, a hot-spot allowance



is added. The hot-spot is the and prevent excess bearing heat
location in stator windings with the effects.
highest temperature. A hot-spot is Thermal endurance is the ability
located approximately in the center of the insulation to withstand heat.
of each stator slot and Figure 14 shows typical thermal
approximately in the center of each endurance curve bands for the
field winding. different insulation classes. The
An embedded detector measures curve bands predict insulation life in
the temperature limit of the stator hours versus the temperature of the
windings. Because the embedded windings. There are many test
detector is located at the hot spot, points outside these bands; the
the measured temperature already points shown are merely
includes the hot spot. Note that an representative. Note that this type of
embedded detector can measure the graph is used to compare insulation
temperature limit of the stator systems. These curve bands do not
windings and is usually a standard predict actual design life of a
factory test. Because the rotor is machine. For example, one CANNOT
rotated, however, an embedded assume that the design life of a
detector cannot test the field machine is any given number of
windings; they must be tested by hours because the insulation life is
resistance. a given number of hours. The curve
There are two basic types of bands only COMPARE insulation
embedded detectors; Resistive systems.
Temperature Devices (RTD) and Note: For every 10°C (18°F)
thermocouples. increase in temperature, insulation
An RTD is an electronic sensing life is halved.
device that varies resistance with a
change in temperature. This change Sample Thermal Endurance
in resistance is then measured and Curve Band
converted into a temperature
reading. The RTD delivers more
of an average reading than the
thermocouple.
The thermocouple is another type
of temperature sensing device. It is
typically used in random wound
machines due to its sturdier design.
The thermocouple delivers a reading
at a precise point, usually the hot- Figure 14
spot. Both devices can be placed in
windings and the machine’s There are several indications that
bearings. They can show increases thermal deterioration is occurring in
in winding insulation temperature winding insulation.



• Loss in weight and thickness • Moisture on creepage surfaces
• Increase in stiffness or • Blocked ventilation passages
brittleness • Abrasives in the cooling air
• Increase in density
Coil Connections
• Non-uniform shrinkage with
The design of the coil connections
cracks penetrating from the
in a generator and the way a load is
surface
connected to a generator determine
• Reduced tensile strength the level and type of output voltage
• Less resistance to moisture from that generator.
penetration Figure 15 shows different types
• Reduced dielectric strength of connections for one phase of a
• Serious decrease in resistance 4-pole, three-phase winding. The
phase consists of four pole-phase
Insulation tends to breakdown at groups; one for each pole in the
higher voltages due to the added phase. Each coil could have any
electromagnetic field force. Silicon number of turns, limited only by
Controlled Rectifier (SCR) loads that the space within the slot. The
of high harmonic content can also pole-phase groups can be connected
seriously affect insulation. in series, 4-parallel, or 2-series, 2-
Heat is not the only cause of parallel.
winding insulation deterioration.
Damage to insulation can occur
during winding due to improper
handling, careless winding,
insufficient resin coverage, or
insufficient cure time. Therefore,
tests are conducted after every
winding process is completed.
Additional causes of insulation Figure 15
failure include the following.
• Conducting contaminants; Different types of connections
like dirt and chemicals induce different voltages. Therefore,
the type of connections used in a
• Mechanical damage from generator depends on the voltage
shock, vibration, foreign required by the load. Series
objects and stress connections produce high voltage
• Surge voltages generated in and parallel connections produce
the load or in the line low voltage. The series and series-
• Operation at abnormal parallel connections are often
voltage, current or power combined into the same generator
factor so the generator can be connected
to produce high or low voltage. This
• Loosened coils and wedges



dual voltage capability is usually Some applications demand a
identified; for example, a generator wye connection rather than a delta
labeled “120/240-277/480V” can connection and vice-versa. For
be connected for either 120/240V example, the delta connection is
or 277/480V. 120V and 277V sometimes used to obtain single-
are line-to-neutral voltages (single- phase 120-240 volts, 3-wire, along
phase), while 240V and 480V are with three-phase, 3-wire on the
line-to-line voltages (single-phase same generator. Figure 16 shows
or three-phase). an example of a wye and a delta
Caterpillar generators have 4, 6, connection.
10, or 12 line leads or outputs.
These can be connected into a low
voltage wye configuration (coils
parallel) or into a high voltage
wye configuration (coils in series).
Generators with 12 leads can be
connected in delta configuration.
Larger generators use more than one
wire per line lead. This feature eases
the problem of forming very heavy
conductors inside a limited space
for terminal connections.
All leads are identified. If more
than one wire is used per line lead,
a line lead number identifies each Figure 16
of these wires. Thus, on smaller
In a wye connection, the terminal
generators, there would be only one
voltage is 1.73 times the terminal-
wire marked T1. However, on larger
to-neutral voltage (represented by
generators, there will be two or
the letter “V” in Figure 16). In the
more wires marked T1. These are
same generator, a delta connection
to be connected together to form
would have the same terminal-to-
one lead.
neutral voltage as the wye
The first voltage listed in a rating connection for its terminal voltage.
like 220/440 volts is the line-to-line However, the delta line current (I)
voltage at the specified frequency would now be 1.73 times the wye
when the generator leads are line current (I).
connected for the low voltage.
A wye connection can be
The second voltage listed is the
structured in the two ways shown in
line-to-line voltage at the specified
Figure 17; a fixed neutral connection
frequency when the re-connectable
or a broken neutral connection.
line leads are series connected for
The broken neutral allows you to
high voltage.
reconfigure from a wye to delta
and vice-versa. It also provides



differential protection by allowing however, will only provide
the machine to monitor the currents approximately 57% of the original
into and out of individual phases. three-phase rating.

Open Delta

Figure 17

The zigzag connection is
sometimes used to get an
alternative voltage from a generator.
For example, the connection Figure 19
shown in Figure 18 can provide
The lead terminal numbering
120-208V from a generator wound
system follows the pattern shown
for 120-240V.
in Figure 20. This system is used
Zigzag Connection for both three-phase generators and
motors.

Figure 18 Figure 20
Several single-phase connections
are possible for 2-wire or 3-wire
Generator Features and
loads. These connections are Attachments
normally used only on smaller Several attachments and features
generators at or below 250 kW. can also be configured to assist
An open delta, as shown in Figure in meeting generator requirements.
19 is reconfigured from an original These include bearings,
delta from three-phase to single- environmental protection devices
phase use; this type of connection, and space heaters.



Bearings protection from water. Table 4 lists
Bearings are devices that permit the levels of protection for solid
smooth movement between objects coming in contact with the
surfaces. In a generator, they are machine. Table 5 lists the levels of
used to hold the rotor in place and protection from water. These tables
allow that smooth rotation. only offer brief descriptions; each
A single bearing system is used in level of each category has specific
the smaller generators with lighter definitions and qualifications.
weight rotors. It is necessary to A standard Caterpillar SR5
have a two bearing system when generator is rated for IP23. The first
the weight of the rotor exceeds digit, 2, indicates the machine is
the limits of the engine rear main protected against solid objects
bearing. In a traditional two bearing greater than 12 mm. The second
system, the generator is not digit, 3, indicates protection from
attached directly to the engine’s spraying water.
flywheel housing. This brings about
alignment difficulty due to the
Solid Object Ingress Protection
separation of the two machines. (first digit of IP designation)
A close coupled two bearing system First
Description
alleviates these original alignment Digit
problems by attaching the generator 0 Non-Protected Machine
to the flywheel housing. Machine protected against
solid objects greater than
Space Heaters 1
50 mm (approximately the size
Space heaters are electrical of a hand)
resistance heaters that are often Machine protected against
used to help prevent condensation solid objects greater than
2
12mm (approximately the size
and moisture absorption during of a finger)
nonuse periods. They are application Machine protected against
specific and their operation time can solid objects greater than
3
vary due to this application. 2.5 mm (approximately the
size of a hand tool)
Ingress Protection (IP) Machine protected against
Ingress Protection (IP) refers to solid objects greater than
4
the degree of protection an 1 mm (approximately the size
enclosure provides for a machine. of wires)
Machine protected against
This is protection both for persons 5
dust
in contact with moving parts and
for the protection of machines Table 4
against the harmful effects due to
the collection of water and dust.
An IP designation is typically two
digits, where the first digit is the
level of protection from solid objects
and the second digit is the level of



Inertia
Water Ingress Protection Inertia data is essential to studies
(second digit of IP designation) of transient response, such as the
Second
Description effect of a larger generator with
Digit more rotor weight and greater mass
0 Non-Protected Machine moment of inertia. When the
1 information is given to an engineer,
dripping water
it must be correctly identified.
2 dripping water when machine These figures list the mass
is tilted up to 15° moment of inertia in System
3 International (SI) units and English
spraying water
units. Other symbols used to
4 designate inertia of rotating
splashing water
Machine protected against machinery are WK2, WR2, and GD2.
5
water jets The standard inertia presentation
Machine protected against below is globally recognized and can
6
heavy seas
be readily converted to other inertia
7
effects of immersion designations.
Table 5 Force x Distance x Time2
N•m sec2 = Newton x meter x Second2
Two types of IP protection are lb•in. sec2 = lb x in. x Second2
totally enclosed water cooled
(TEWAC) and totally enclosed air To convert to moment of inertia
cooled (TEAC). (WK2) in SI units:
Multiply N•m sec2 by 9.803
Physical Data m/sec2.
Rotor Weight Multiply lb•in. sec2 by 2.683 to get
Rotor weight indicates the total inertia (WK2) in lb•ft2, the common
weight of the generator rotor. When English unit designation.
used with the formula for inertia, the
Center of Gravity
flywheel effect of the generator can Center of gravity is information
be found. Engineers may use these that gives the location of the
figures when generator loads are generator center of weight in three
sudden or cyclic. planes. With the total generator
Stator Weight weight, the information can be used
Stator weight is given in kilograms to determine the center of gravity
and pounds and gives an indication of an assembly consisting of engine,
of the capacity of a generator. generator, radiator, and base.
Engineers generally assume that a Combined calculations require
heavier stator contains more reducing all values to moments
working copper and iron and (lb•in. or kg•mm). These are
therefore has greater capabilities. summed algebraically in each plane
of reference.



Nameplate • Maximum ambient
The nameplate is an engraved temperature for which the
metal plate or printed film on the generator is designed
side of a motor/generator. The plate • Insulation system designation
contains the minimum required (if the armature and field use
information needed to identify its different classes of insulation
functionality. systems, both shall be given,
National Electrical Manufacturers that for the armature being
Association (NEMA) Requirement given first)
The following is required ** Applies to the exciter in the
nameplate information as set by case of a brush-less machine.
National Electrical Manufacturers
Association (NEMA). International Standards Organization
(ISO) 8528 Requirement
a. Manufacturer’s type and Generating sets shall bear the
frame designation following rating plates as set by the
b. Kilovolt-ampere output International Standards Organization
c. Power factor (ISO) 8528. The nameplate for the
set must provide the following
d. Time rating
information.
e. Temperature rise*
a. The words “Generating set
f. Rated speed in rpm ISO 8528”
g. Voltage b. The manufacturer’s name or
h. Rated current in amperes mark
per terminal c. the serial number of the set
i. Number of phases d. the year of manufacture of
j. Frequency The set
k. Rated field current** e. The rated power, in kilowatts,
l. Rated excitation voltage with the prefixes COP, PRP or
LTP, in accordance with ISO
Additional information may be 8528-1:1993, clause 13
included such as:
f. The performance class in
m. Enclosure or IP code accordance with ISO 8528-
n. Manufacturers name, mark, 1:1993, clause 7
or logo g. The rated power factor
o. Manufacturer’s plant location h. The maximum site altitude
p. Serial number or date of above sea-level, in meters
manufacture. i. The maximum ambient
* As an alternate marking, temperature, in degrees
Temperature Rise can be replaced Celsius
by the following information. j. The rated frequency, in hertz



k. The rated voltage, in volts
NEMA & IEC Design
l. The rated current, in amperes Considerations
m. The mass, in kilograms. The following are design
The rating and class of output of considerations as set in the NEMA
the generator shall be combined as code book.
follows. Temperature Rise
a. Where a continuous rating The observable temperature rise
based on duty type S1 is under rated load conditions of each
stated, the rated output shall of the various parts of the
be followed by the marking synchronous generator, above the
“BR” (basic continuous temperature of the cooling air, shall
rating), e.g. Si=22 kVA BR; not exceed the values in the table
b. Where a rating with discrete below.
constant loads based on duty The temperature of the cooling air
type S10 is stated, the basic is the temperature of the external air
continuous rating based on S1 as it enters the ventilating openings
shall be marked as mentioned of the machine, and the temperature
immediately above. In addition rises given in the Table 6 are based
the peak rated output shall be on a maximum temperature of 40°C
shown followed by the (104°F) for this external air.
marking “PR” (peak
continuous rating), the
maximum running time of
500 h per year and the factor
TL, e.g. Si=24 kVA PR 500 h
per year, TL=0,9.



Temperature Rise
Method of Temperature Rise, Degrees C°
Item Machine Part Temperature Class of Insulation System
Determination A B F H
a. Armature Windings
1. All kVA ratings Resistance 60 80 105 125
2. 1563 kVA and Embedded
70 90 115 140
less detector**
3. Over 1563 kVA
a) 7000 volts and Embedded
65 85 110 135
less detector**
Embedded
b) Over 7000 volts 60 80 105 125
detector**
b. Field Winding Resistance 60 80 105 125
c. The temperature attained by cores, amortisseur windings, collector rings, and
miscellaneous parts (such as brushholders, brushes, pole tips, etc.) shall not
injure the insulation or the machine in any respect.
* For machines which operate under prevailing barometric pressure and which are designed not to exceed the specified
temperature rise at altitudes from 3300 feet (1000 meters) to 13000 feet (4000 meters), the temperature rises, as
checked by tests at low altitudes, shall be less than those listed in the foregoing table by 1 percent of the specified
temperature rise for each 330 feet (100 meters) of altitude in excess of 3300 feet (1000 meters).
** Embedded detectors are located within the slot of the machine and can be either resistance elements or
thermocouples. For machines equipped with embedded detectors, this method shall be used to demonstrate conformity with
the standard (see 20.63).
NOTES:
1—Temperature rises in the above table are based upon generators rated on a continuous duty basis. Synchronous
generators may be rated on a stand-by duty basis (see 22.85). In such cases, it is recommended that temperature rises not
exceed those in the foregoing table by more than 25°C under continuous operation at a stand-by rating.
2—Diesel engine specifications often call for machines which are suitable for 10-percent overload for 2 hours out of any
24 consecutive hours of operation. Generators having a corresponding overload capability are sometimes required. In such
cases, it is recommended that the generators and their excitation systems be designed to deliver 110 percent of kVA at
rated power factor, frequency, and voltage with temperature rises under rated load conditions not exceeding those given in
the above table.
3—Temperature rises in the foregoing table are based upon a reference ambient temperature of 40°C. However, it is
recognized that synchronous generators may be required to operate at an ambient temperature higher than 40°C. The
temperature rises of the generators given in the foregoing table shall be reduced by the number of degrees that the ambient
temperature exceeds 40°C. (Exception—for totally enclosed water-air-cooled machines, the temperature of the cooling air
is the temperature of the air leaving the coolers. Totally enclosed water-air-cooled machines are normally designed for the
maximum cooling water temperature encountered at the location where each machine is to be installed. With a cooling
water temperature not exceeding that for which the machine is designed.
a. On machines designed for cooling water temperatures from 5°C to 30°C — the temperature of the air leaving the
coolers shall not exceed 40°C.
b. On machines designed for higher cooling water temperatures — the temperature of the air leaving the coolers shall be
permitted to exceed 40°C provided the temperature rises of the machine parts are then limited to values less than those
given in the table by the number of degrees that the temperature of the air leaving the coolers exceeds 40°C.)
Table 6



Maximum Momentary Overloads TIF According to the 1960 Single
Synchronous Generators shall be Frequency Weighting
capable of carrying a 1-minute Frequency TIF Frequency TIF
overload with the field set for normal 60 0.5 1860 7820
rated load excitation in accordance 180 30 1980 8330
with Table 7. 300 225 2100 8830
360 400 2160 9080
Armature Current, % of 420 650 2220 9330
Synchronous Speed rpm
Normal Rated Current
540 1320 2340 9840
1801 and Over 130
660 2260 2460 10340
1800 and Below 150
720 2760 2580 10600
Table 7 780 3360 2820 10210
900 4350 2940 9820
It is recognized that the voltage 1000 5000 3000 9670
and power factor will differ from the 1020 5100 3180 8740
rated load values when generators 1080 5400 3300 8090
are subjected to this overload 1140 5630 3540 6730
condition. Also, since the heating 1260 6050 3660 6130
affect in machine winding varies 1380 6370 3900 4400
approximately as the product of the 1440 6650 4020 3700
1500 6680 4260 2750
square of the current and the time
1620 6970 4380 2190
for which this current is being
1740 7320 5000 840
carried, the overload condition will
1800 7570
result in increased temperatures and
a reduction in insulation life. The Table 8
generator shall therefore not be
subjected to this extreme condition Shall not exceed the values in
for more than a few times in its life. Table 7.
It is assumed that this excess kVA Rating of Generator TIF
capacity is required only to 125 to 4999 150
coordinate the generator with the 5000 to 19999 75
control and protective devices. 2000 and Above 70
Table 9
Maximum Deviation Factor
The deviation factor of the open
When specified, the residual
circuit line-to-line terminal voltage
component TIF based on the
of synchronous generators shall
weighting factors given shall not
not exceed 0.1.
exceed the values in Table 10.
Telephone Influence Factor (TIF) These residual components apply to
Telephone Influence Factor (TIF) those voltage ratings 2000 volts or
shall be measured at the generator higher.
terminals on open circuit at rated
voltage and frequency. When
specified, the balance TIF is based
on the weighting factors in Table 8.



kVA Rating of Generator TIF (residual) values which give an
125 to 4999 150 integrated product, equal or
5000 to 19999 75 less than 40 for salient pole
2000 and above 70 machines and 30 for air
Table 10 cooled cylindrical rotor
machines.
Single Frequency Weighting Short b. The maximum phase current
Circuit Requirements is limited by external means
A synchronous generator shall be to a value which does not
capable of withstanding, without exceed the maximum phase
damage, a 30-second, three-phase current obtained from the
short circuit when operating at rated three-phase fault.
kVA and power factor, at five
percent over voltage, with fixed Overspeed
excitation. The generator shall also Synchronous generator shall be so
be capable of withstanding, without constructed that, in an emergency
damage, any other short circuit at its not to exceed two minutes, they will
terminals of 30 seconds or less withstand without mechanical injury
provided: overspeeds above synchronous
speed in accordance with the values
a. The machine phase currents
in Table 11.
under fault conditions are
such that the negative phase Overspeed, Percent of
Synchronous Speed rpm
Synchronous Speed
sequence current (I2), 1801 and Over 20
expressed in per unit of stator 1800 and Below 25
current at rated kVA, and the
Table 11
duration of the fault in
seconds (t) are limited to



Generator Performance Characteristics
220
Vpu = = 0.917
Rated and Per Unit 240
“Per unit” values are expressed as
a decimal fraction of some whole 100
Spu = = 0.833
value. Twenty percent, 20% or 120
0.20 indicates part of some whole
value. Per unit is a pure number. It And from these base numbers you
has no label such as volts, amperes can calculate current and impedance
or ohms. Per unit is often bases.
abbreviated as P.U. or p.u. Sbase (240 V)2
Ibase = = = 50A Zbase =
The per unit system eliminates the Vbase 240 V
use of cube roots that are inherent
to three-phase calculations. The Vbase (240 V)2
Vbase = = = 4.8Ω 12kVa
per unit basis also provides a Ibase Sbase
comparable value system for all
generators, transmission lines, Efficiency
motors and related measures so Efficiency is the percentage of
they look, more or less, alike. engine flywheel horsepower that
kVA, volts, amps, power factor, is converted into electrical output,
resistance and reactance are all or in other words, power out of the
figures converted to per-unit values. generator divided by power in.
Per-unit calculations are defined as 100% efficiency is a theoretical level
the actual value of the figure divided that assumes no losses in heat,
by the base value. It is customary to windage or friction.
select voltage and apparent power
base values; once these are done, all output power
other base values can be calculated. Efficiency =
input power
Refer to “Reactance” for more per
unit calculations. output
Efficiency =
To convert a circuit quantity to output + losses
a per unit value, divide by the base
value, which is usually the rated input - losses
Efficiency =
value. For example, the following input
equations show the per unit values
for a 240/120-V, 120 kVA generator Generator efficiencies are
that is operated at 100 kVA with published at 100% of rated.
220 volts. Percent generator efficiency is
shown in the following formula.
% Gen. kVA x PF x 100
=
Efficiency (kVA x PF) + gen. losses + exciter losses



In discussion of generator generator to provide adequate fault
efficiency, generator losses must current to the short in order to give
be considered. These include the a protective device, such as breakers
following. and fuses, enough time to react.
• Armature Winding — The loss Permanent Magnet (PM) generators
based upon phase currents are able to sustain fault current due
and phase resistances. to their excitation characteristics
while self excited (SE) generator
• Field — The loss due to field
may have trouble providing this
current and field resistance. current for enough cycles. Refer to
• Core Loss — Iron losses, the section entitled “NEMA Design
hysteresis, and eddy currents Considerations” for official standards
due to the flux variation in the on short circuits.
armature core. Caterpillar IE system (Independent
• Stray Load Losses — Iron Excitation) also has short circuit
losses and eddy current losses current capabilities. It is designed
in the copper due to fluxes such that the short circuit current
varying with load and capabilities can be disabled during
saturation. parallel operation with the utility and
• Friction and Windage Loss — fully implemented during stand alone
Loss of the power used to operation for maximum performance.
overcome bearing friction and The IE excitation system relies on
windage. There is a power residual magnetism for voltage build-
used against the friction up. The immunity to non linear
between the bearings and the loads, excellent motor starting and
rotor that creates a friction the short circuit capabilities, are
loss. The power necessary to standard IE features not available on
move the rotor poles through Self Excited generators.
the air and to drive cooling air When a generator experiences a
through the generator creates sudden load increase, such as a
windage loss. starting a motor, output voltage and
• Exciter Loss — All losses speed of the genset dip for a short
similar to those experienced time. This is called a transient dip
by the main generator. These and is illustrated in Figure 21. When
include field, core, stray load, the load is thrown off, an overshoot
friction and windage losses. occurs. Without proper generator
design, this overshoot could
Fault Current/Short Circuits potentially cause a short-circuit.
Short circuits generally occur when
electric current is diverted from its
intended path to ground or another
line of the generator. When short
circuits occur, it is necessary for a



offset the short circuit current for a
Transient Dip very short time.

Figure 21

Each phase of a generator has
the following impedances: Figure 22
• Equivalent Armature A three-phase fault is a short
Reactance between all three phases of the
• Armature Leakage Reactance output. A line-to-line fault is a short
• Armature Resistance between two of the three phases,
and the same concept goes for line-
• Synchronous Reactance
to-neutral and line-to-ground faults.
• Synchronous Impedance A more detailed explanation of faults
• Voltage Proportional to is covered under “Power Systems.”
Excitation Requirements Figure 23 illustrates a short circuit;
• Line Current Variable definitions can be found on
Page 32 of this section.
• Phase Voltage
The vectors shown in Figure 22
demonstrate that to maintain
constant voltage at the terminals,
the excitation requirements will
vary according to the power factor.
Therefore, excitation requirements
are greatest at lagging power factors
and less at leading power factors.
There may also be a direct current
component to short circuits. In most Figure 23
short circuits, the ratio of reactance
to resistance is high. If a fault Stator Resistance
occurs at zero voltage on the sine Stator resistance refers to the
wave in such a circuit, the current internal resistance of the generator
will not be symmetrical. It will that is developed by the stator
contain a DC component that will copper and winding configuration.


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