Once you’ve decided to purchase a generator set,
there are several considerations you must keep in mind
when choosing which set to buy, where to install it and
how to install it. This guide will help you make informed
decisions during the selection and installation process.
Choosing the right set is not diffi cult if you take the
time to analyze your requirements carefully. You will
also need to know a few terms and have a basic
understanding of the different types of generator sets
and their operating principles.
Installation requires expert assistance and a strict
adherence to local codes and regulations. We
recommend that you have a contractor do your
installation or, at the very least, have him provide
professional advice.
STAND-BY OR PRIME?
The fi rst determination you will need to make is
whether you will require stand-by or prime power.
Simply stated, prime power is required when you have
no other source of power. A stand-by set steps in and
picks up designated loads when your main power
supply is not available.
GAS OR DIESEL?
There are three main components to a generator set:
A diesel or gas “engine” which drives an electrical
“generator end” and is monitored/governed by various
“controls.”
Engines are either spark ignited (gas, natural gas,
propane) or compression ignited (diesel). Diesel
engines are better for heavy duty and last longer.
Diesel fuel is also less combustible, making it safer to
handle and store.
OPERATING SPEED
Electric equipment is designed to use power with a
fi xed frequency: 60 Hertz (Hz) in the United States
and Canada, 50 Hertz in Europe and Australia. The
frequency output of a generator depends on a fi xed
engine speed. To produce 60 Hz electricity, most
engines operate at 1800 or 3600 RPM. Each has its
advantages and drawbacks.
1800 RPM, four pole sets are the most common.
They offer the best balance of noise, effi ciency, cost
and engine life. 3600 RPM, two pole sets are smaller
and lightweight, best suited for portable, light-duty
applications.
FEATURES & BENEFITS TO LOOK FOR
• Engine block. For long life and quiet operation we
recommend four cycle, liquid cooled, industrial duty
diesel engines.
• Air or liquid cooling. Air cooled engines require a
tremendous amount of air and may require ducting.
They’re noisy too. Liquid cooling offers quieter
operation and more even temperature control.
• The fuel system should be self venting. Engine
speed should be governed by a mechanical or
electronic governor. It is best to have an on-engine
fuel fi lter with a replaceable element.
• Intake and exhaust. Time and money savers
include a large, integral air cleaner with replaceable
fi lter element and a residential muffl er which is built
into the exhaust manifold. This saves the need for an
additional muffl er.
• The lubrication system should have a full fl ow,
spin-on oil fi lter with bypass.
• DC electrical system. Standard 12 volt system
should include: ♦ starter motor and battery charging
alternator with a solid state voltage regulator ♦ quick
disconnection plug-in control panel with hour meter
♦ pre-heat switch and start/stop switch ♦ safety
shutdown system to protect the engine in case of
oil pressure loss or high water temperature ♦ DC
system circuit breaker.
• AC generator should have a 4 pole revolving fi eld.
An automatic voltage regulator will provide “clean”
power.
• A steel skid frame keeps everything in one piece
and eases installation. Vibration mounts isolate
engine vibration for smooth, quiet operation.
• Finally, every set should be test run under load
and include a complete set of operator’s and parts
manuals.
Page 3
WHAT SIZE SET WILL I NEED?
Sizing is the most important step, nothing is more critical in
your choice of a generator. A set that is too small won’t last,
will smoke and can do damage to your electrical equipment.
If it is too large, the engine will carbon up, slobber fuel and
run ineffi ciently. We recommend that a generator set never
run continuously with less than 25% load. 35% to 70% is
optimum.
Additional factors which may affect effi cient operation of your
generator are high altitude and high air temperature. These
conditions will lower generator output. Consult your supplier
for de-ration information.
ESTIMATING YOUR LOAD
To estimate your electrical load, total the wattage of all the
equipment you’ll operate at one time. The wattage needed
to run a given piece of equipment is usually listed on its
nameplate. If only amperage is listed, use this formula to
fi gure wattage:
Amps x Volts = Watts (Single Phase)
Amps x Volts x 1.73 = Watts (Three Phase)
In addition to load requirements, it is important to consider
motor starting load. Starting a motor requires up
GENERATOR TYPES & FEATURES
Generator sets produce either single or three phase power.
Choose a single phase set if you do not have any motors
above fi ve horsepower. Three phase power is better for motor
starting and running. Most homeowners will require single
phase whereas industrial or commercial applications usually
require three phase power.
Three phase generators are set up to produce 120/208 or
277/480 volts. Single phase sets are 120 or 120/240. Use the
low voltage to run domestic appliances and the high voltage
for your motors, heaters, stoves and dryers.
Regulation is how closely the generator controls its voltage
output. Closer regulation is better for extended motor life.
An externally regulated generator has an automatic voltage
regulator and holds a ±1% to 2% voltage tolerance.
Temperature rise is a measurement of the increase in heat
of the generator windings from no load to full load. What
it tells you is the quantity of copper in the generator. The
lower the temp-rise, the more copper and the better the
quality. A 105°, or lower, temp-rise is recommended for both
commercial and residential prime power sets.
to fi ve times more wattage than running it. Selecting a
generator which is inadequate for your motor starting needs
may make it diffi cult to start motors in air conditioners or
freezers, for example. In addition, starting load causes
voltage dips, which is why the lights dim when a large motor
is started. These voltage dips can be more than annoying.
They can ruin delicate electronic equipment such as
computers.
A reliable method for factoring both running and starting
wattage is to take the running wattage of your largest
motor and multiply by ten. Then add the running
wattages of all the smaller motors as well as the
wattage of all the other loads. This will add up to
your total load. Next, determine how much of the
load will be operating at any one time. This is your
running load. Note: If a motor can be wired up at
different voltages, for example 120 volt or 240 volt, it
is usually more effi cient to wire it at the higher voltage.
ENGINE ACCESSORIES AND CONTROLS
After you determine the generator size you will need, make
a list of optional and installation equipment you require.
For noise abatement, we recommend a muffl er, if one is
not built-in, and an exhaust elbow. A good primary fuel
fi lter/water separator is a must to protect your engine’s fuel
system. You will also need a control panel with gauges to
monitor your set (1) – see drawing at right. Stand-by sets
may require a block heater to keep the coolant/water mix at
an adequate temperature for easier starting.
Page 4
AC SWITCHGEAR AND CONTROLS
Switchgear can be as simple or complex as you want or can
afford. Of course, as complexity increases, so does cost.
Balance and a good electrical advisor are the keys here.
The diagrams at right illustrate basic confi gurations for prime
power and stand-by systems.
All generator systems require a circuit breaker (2) and
a distribution panel (3). The ciruit breaker protects the
generator set from short circuit and unbalanced electrical
loads. The distribution panel divides and routes the
connected loads and includes circuit breakers to protect
these loads.
Stand-by systems also require a main circuit breaker between
the utility source and the transfer panel (4). The transfer
panel switches power from the utility to the gen-set and back
so that both aren’t on at the same time.
Auto-start, auto-transfer systems are available but are costly.
Your supplier or contractor can help you determine what you
will need.
BASIC PRIME POWER SYSTEM
DISTRIBUTION
PANEL WITH
CIRCUIT
BREAKERS
GEN-SET
GEN-SET
CIRCUIT
BREAKER
BASIC STAND-BY POWER SYSTEM
UTILITY
SOURCE
GEN-SET
MAIN
CIRCUIT
BREAKER
GEN-SET
CIRCUIT
BREAKER
TRANSFER
PANEL
DISTRIBUTION
PANEL WITH
CIRCUIT
BREAKERS
INSTALLATION
Our fi rst recommendation is: Let a licensed contractor do
it. He has the tools, the know-how and an understanding of
governing regulations and local codes. His expertise will save
you money in the long run.
This diagram shows a stand-by
installation and includes optional
equipment. Many installations are
not nearly as complex as this one.
Let your dealer help you design a
system to meet your requirements
and budget.
1. DC Control Panel
2. Generator AC Circuit
Breaker
3. AC Distribution Panel
4. Transfer Panel (stand-by
only)
5. Cooling Air Inlet
6. Air Outlet
7. Exhaust System
8. Exhaust Flex
9. Exhaust Thimble
10. Fuel Tank
11. Cooling Air Outlet Duct
12. DC Battery
13. DC Battery Charger
(stand-by only)
If you are a dedicated do-it-your-selfer, do your homework
before tackling the job and obtain the proper permits required
by your local jurisdiction. While all gen-sets have some
basic requirements, each brand and model has special
idiosyncrasies. Also, it is extremely important to have all
relative codebooks for reference and to adhere to them
strictly. Most important of all, your system must be inspected
before getting it up and running.
LOCATION
Where do you put it? Wherever you choose, be sure the
following elements are present:
• Air inlet for combustion and engine
cooling (5).
• Outlets for exhaust (7, 8, 9) and
hot cooling air (6).
• Fuel, battery and AC electrical
connections.
• Rigid, level mounting platforms
(many sets are already mounted on a
steel skid base).
• Open accessibility for easy service.
• Isolation from living space. Keep noise and
exhaust away from occupied areas.
• Space and equipment to extinguish a fi re.
• Minimize the possibility of fi re danger.
Remember, gen-sets move on their vibration mounts. Allow
clearance to compensate and use fl ex-joints on all lines and
connections.
Page 5
EXHAUST SYSTEMS
The exhaust system (7) may need to be covered with
insulated material to prevent fi re from contact with
combustible materials, to reduce the heat radiated from the
exhaust and to ensure personal safety. Some insulation
materials are best left to professionals with the proper
equipment. Keep the piping away from combustible materials
including walls.
A seamless, stainless steel fl exible joint (8) must be used
between the generator set and the exhaust system to prevent
metal fatigue.
Don’t use the exhaust manifold to support the exhaust
system, the weight can cause manifold failure. Exhaust pipe
hangers are readily available.
(the smaller the space the generator runs in, the higher the
room temperature is likely to be), smaller spaces may require
ducting. Other factors which will affect the room temperature
include generator size and the outside air temperature or
climate.
In an inside installation, increasing these vent sizes may
cool the room down to acceptable levels. If this doesn’t
provide suffi cient cooling, ducting may be required to ensure
“positive” air fl ow. Stated simply, positive air fl ow is cool,
clean air in – hot air out, as opposed to circulating hot air
inside the room.
Generator cooling fans move moisture as well as air. Moist
air is corrosive to a genset’s copper windings. Make sure air
inlets are positioned to minimize moisture intake.
FUEL SYSTEM
Extreme care should be taken in designing and installing the
fuel system to prevent fi re danger. Fuel lines should have
as few connections as possible and be routed to prevent
damage. Keep lines away from hot engine or exhaust
components. The lines should be no smaller than the inlet
and outlet on the engine. Support fuel lines with clamps, as
needed to help prevent metal fatigue from vibration.
The fuel tank (10) should be level with or below the set to
prevent siphoning in the event of a line failure. Remember
to check the lift capacity of the engine fuel pump and make
sure to stay within its limits. If the set is higher, an auxiliary
fuel pump may be required. To prevent water ingestion, fuel
should be drawn out of the top of the tank with the pick-up
extending to no more than two inches from the bottom.
Fuel storage tanks must have leakage protection. Above
ground tanks are recommended due to EPA regulations.
Check your local codes before installing a tank to make sure
it is EPA approved. The safest tanks are double walled with
alarms. These alarms are simple and well worth it to prevent
a possible fuel spill.
If the tank is mounted above the generator set, use a fuel
shut-off valve. This will allow you to work on the fuel system
wihout the fuel siphoning out. It will also allow you to cut-off
fuel fl ow in the event of line breakage.
A high quality, fuel/water separator fi lter should be mounted
as close to the generator set as possible.
Because of its explosive nature, gasoline fuel systems
have special requirements, see your supplier for complete
information.
AIR
The generator set needs air for combustion and cooling.
The engine is cooled by a radiator and an engine fan. The
generator is cooled by an internal fan. The room, or space, in
which the generator operates should not exceed 100°F. We
recommend keeping it under 85°F if possible. All installations
require an intake for cool, clean air and an outlet vent for hot
air.
Since the size of the space affects the room temperature
DC CONTROL PANELS AND BATTERY
Mount your control panel wherever it is most convenient.
Mounting it on a wall isolates it from engine vibration. Dual
remote panels give you the added convenience of operating
your set from two locations. Wire harness plug-ins are
available on some sets. Simply plug one end of the harness
into the set and the other into the control panel. Harness
extensions are also available.
Protect the panel from moisture. Route the harness in dry,
protected wire raceways.
Check your manufacturer’s recommendation for battery
and battery cable sizes. Stand-by sets often have a battery
charger which keeps the starting battery fully charged and
assures quick emergency starting.
AC CONNECTIONS
Connecting the generator to your electrical distribution
system is a job for a qualifi ed, licensed and bonded
electrician who knows local building codes.
BEFORE STARTING UP
Once you are fi nished with the installation, you should call
your supplier or electrician again. Arrange to have him come
and inspect the work and start your set. He will be able to
catch any mistakes that may have been made and either fi x
them for you or tell you how to do it yourself. 30% to 40%
of all generator problems can be attributed to installation
problems that weren’t caught because no one did a proper
pre-start inspection. Those numbers prove that the inspection
is well worth the time and money spent.
FOR ADDITIONAL INFORMATION
• Unifi ed Building Code
• NFPA Pamphlets on generator and electrical power
systems.
• Emergency/Stand-by Power Systems by Alexander Kusko
Page 6
Electrical
BASE ELECTRICITY
Electricity or electrical power was not utilized as a
major form of work producing energy until the late
nineteenth century. The existence of electricity is
not, however, a nineteenth century or modern day
discovery. The ancient Greeks, in fact, unknowingly
discovered electricity in observing that a piece of rough
amber would attract and pull tiny fl akes of wood and
feathers toward it. The word “electricity” is itself derived
from the Greek defi nition of amber.
Through the centuries man continued his studies of
the mysteries of electricity. Long before anyone ever
heard of electrons or even imagined that the atom
existed, certain men had observed and recorded some
of the basic laws of electricity. Even with the recent
development of the electron theory, these basic laws
remain relatively unchanged and still serve as vital
contributions to our understanding of electricity. Since
acceptance of the electron theory has advanced our
understanding of the fundamentals so greatly, a review
of this theory is imperative to further study of electricity.
TYPES OF ELECTRICAL CURRENT
Electrical energy used today is commonly generated
in either the form of direct current produced by
chemical action and through electromagnetic induction
or alternating current which is also produced by
electromagnetic induction.
Before proceeding in the discussion of types of currents
we need to know a little about the operation of a
simple generator. Generators utilize a form of magnetic
induction to create fl ow of electrons.
A simple generator consists of a coil or loop of wire
arranged so that it can be rotated in circular motion
and cut through a magnetic fi eld consisting of North
and South poles. Referring to the illustration, Figure 3, we can see that current alternates according to the
armature’s position in relation to the poles. At 0˚ and
again at 180˚ no current is produced. At 90˚ current
reaches a maximum positive value. Rotation to 270˚
brings another maximum fl ow of current only at this
FIGURE 3 - OPERATION OF A SIMPLE GENERATOR
Page 7
position current has reversed its polarity and now
fl ows in the opposite direction. All generators produce
alternating current in the armature. DC generators are
therefore basically AC alternators modifi ed to produce
direct current by addition of devices which cause fl ow
to be unidirectional.
DIRECT CURRENT
Electrons in direct current always fl ow in a single
direction. Current created through chemical action
by an automobile battery, for instance, produces a
smooth, constant fl ow of electrons all going in the same
direction.
A DC generator also produces a unidirectional fl ow
of electrons, however, a ripple or variation in intensity
is evident in its current. This is due to the fact that a
DC generator utilizes only the positive alternation of
the alternating current. Apparently this current would
pulsate from zero to maximum value and return to zero
at regular intervals. This is not the actual case since
devices are used to smooth out these pulsations so
that current is held at a high maximum value with only
slight variation in intensity.
ALTERNATING CURRENT
With alternating current on the other hand, the
electrons fl ow fi rst in one direction then reverse and
move in the opposite direction and repeat this cycle
at regular intervals. This reversal is due to a principle
of electromagnetic induction. A wave diagram or so
called “sine” wave of alternating current shows that
the current goes from zero value to maximum positive
value, reverses itself again to return to zero. Two
reversals of current such as this is referred to as a
cycle. The number of cycles per second is called hertz.
Page 8
FIGURE 4 - DIRECT CURRENT WAVE FORMS
FIGURE 5 - AC REVERSES POLARITY AND DIRECTION OF FLOW
ELECTRICAL UNITS
FIGURE 5-A - ALTERNATING CURRENT SINE WAVE
In the study of electricity and electrical circuits, it
is necessary to establish defi nite units to express
qualitative values of current fl ow, voltage (difference in
potential) and resistance. The standard electrical units
area as follows:
AMPERE - UNIT OF CURRENT FLOW
The rate of electron fl ow in a circuit is represented by
the ampere which measures the number of electrons
fl owing past a given point at a given time, usually in
seconds, )One ampere, incidentally, amounts to a little
over six thousand-million-billion electrons per second.)
The rate of fl ow alone is not, however, suffi cient to
measure electric energy. For example, a placid stream
may fl ow the same gallons per minute as water gushing
out of a fi re hydrant. Relating this to electricity, we can
have the same amount of current in this electricity,
however it is obvious that the difference in potential or
voltage must be greater in the smallest wire to obtain
the same number of amperes. To measure electric
energy accurately, we have to know both the rate of
fl ow and the voltage which causes the fl ow.
VOLT - UNIT OF ELECTROMOTIVE FORCE (EMF)
The volt is the measurement of the difference in
electrical potential that causes electrons to fl ow in an
electrical circuit. If the voltage is weak, few electrons
will fl ow and the stronger voltage becomes, the more
electrons will be caused to move. Voltage, then, can
be considered as a result of a state of unbalance and
current fl ow as an attempt to regain balance. The volt
represents the amount of emf that will cause current to
fl ow at the rate of 1 ampere through a resistance of 1
ohm.
OHM - UNIT OF RESISTANCE
In all electrical circuits there is a natural resistance
or opposition to the fl ow of electrons. When an
electromotive force (emf) is applied to a complete
circuit, the electrons are forced to fl ow in a single
direction rather than their free or orbiting pattern.
Utilization of a good conductor of suffi cient size
will allow the electrons to fl ow with a minimum of
opposition or resistance to this change of direction
and motion. Resistance within an electrical current
is evident by the conversion of electrical energy into
heat energy. The resistance of any conductor depends
on its physical makeup, its cross sectional area, its
length and its temperature. As the temperature of a
conductor increases, its resistance increases in direct
proportion. One ohm expresses the resistance that
will allow one ampere of current to fl ow when one volt
of electromotive force is applied. Resistance applies
to all DC circuits and some AC circuits. Other factors
affect rate of fl ow in most AC circuits. These factors are
known as reactance and are described later.
OHM’S LAW (MEASURING UNITS)
In any circuit through which a current is fl owing, three factors
are present.
a) The potential difference (volts) which causes the current to
fl ow.
b) The opposition to current fl ow or resistance of the circuit
(ohms).
Page 9
FIGURE 6 - ELETRICAL UNITS
c) The current fl ow (amperes) which is maintained in the
circuit as a result of the voltage applied.
A defi nite and exact relation exists between these three
factors thereby the value of any one factor can always be
calculated when the values of the other two factors are
known. Ohm’s Law states that in any circuit the current will
increase when the voltage increases but the resistance
remains the same, and the current will decrease when
the resistance increases and the voltage remains the
same. The formula for this equation is Volts=amperes x
ohms (E=IR).
To use this form of Ohm’s Law, you need to know the
amperes and the ohms, for example, how many volts are
impressed on a circuit having a resistance of 10 ohms and a
current of 5 amperes? Solution E=5 x 10 = 50 volts.
The formula may also be arranged to have amperes the
unknown factor, for example, Amperes = volts divided by
ohms.
To have ohms the unknown factor, arrange the formula in this
MEASURING UNIT - SYMBOLEQUATIONSRELATION OF UNITS*
CURRENT FLOW - AMPERES = I
DIFFERENCE ON POTENTIAL - VOLTS = E
RESISTANCE - OHMS = R
AMPERES =
VOLTS = AMPERES X OHMS
OHMS=
manner. Ohms = volts divided by amperes.
The circle diagram provided can be used as an aid to
remembering these equations. To use this diagram, simply
cover the unknown factor and the other two will remain in
their proper relationship.
WATTS - UNITS OF POWER
We measure electric power in watts. One watt is equal to
a current of one ampere driven by an emf of one volt. For
the larger blocks of power we use the term kilowatt for one
thousand watts. There is a defi nite relationship between
electric power and mechanical power. One horse power equals
seven hundred and forty-six watts of electrical energy. (746)
Since power is the rate of doing work, it is necessary to
consider the amount of work done and the length of time
taken to do it. The equation for calculating electrical power
is P = E x I or Watts = Volts x Amperes. Using this equation
to fi nd the power rating of a 120 volt, 30 ampere generator,
we would come up with the following: P = 120 x 30 = 3,600
watts. The power equation can also be expressed in different
VOLTS
OHMS
VOLTS (E)
OHMS
VOLTS
AMPERES
AMPS
(I)
(R)
* When two values are known, cover the unknown to obtain the formula.
Page 10
CURRENT FLOW IN A CIRCUIT IS DIRECTLY PROPORTIONAL TO THE VOLTAGE AND
INVERSELY PROPORTIONAL TO THE RESISTANCE.
WATTS - THE MEASURING UNIT OF ELECTRICAL POWER
EQUATIONS
WATTS = VOLTS x AMPERES
WATTS (P)
VOLTS
(E)
AMPS
(I)
AMPERES =
VOLTS =
WATTS
VOLTS
WATTS
AMPERES
forms. We can use it to fi nd amperes when watts and volts
are known. An example of this would be: Amperes = Watts
divided by Volts. This equation is used frequently in fi guring
the current of any DC electric plant or any appliance such as
electric heater or light bulb rated in watts. We can combine
the ohm equation with the watt equation to form other useful
equations in determining power factor of circuits.
REACTANCE IN AC CURRENT
In DC the only opposition to current fl ow to be considered is
resistance. This is also true in AC current if only resistance
type loads such as heating and lamp elements are on the
circuit. In such cases the current will be in phase with the
voltage - that is, the current wave will coincide in time with
the voltage wave. Voltage and current are seldom, however,
in phase in AC circuits due to several other factors which are
inductive and capacitive reactance.
Inductive reactance is the condition where current lags
behind voltage. Magnetic lines of force are always created
at right angles to a conductor whenever current fl ows with-in
a circuit. An emf is created by this fi eld only when current
changes in value such as it does constantly in alternating
current. This magnetic fi eld induces electromotive forces
which infl uences current to continue fl owing as voltage drops
and causes voltage to lead current. If a conductor is formed
into a coil, the magnetic lines of force are concentrated in the
center of the coil. This greater density causes an increase in
magnetically induced emf without increasing current. Coils,
therefore, cause inductive reactance. This condition is also
caused by an induction motor on the circuit which utilizes the
current’s magnetic fi eld for excitation.
Capacitive reactance is, on the other hand, the condition
where current leads the voltage. Capacitance can be thought
of as the ability to oppose change in voltage. Capacitance
exists in a circuit because certain devices within the circuit
are capable of storing electrical charges as voltage is
increased and discharging these charges as the voltage falls.
FIGURE 9 - REACTANCE SINE WAVES
Page 11
POWER FACTOR
Unity power factor applies to the circuits where current
and voltage are in phase. This is also referred to as
a power factor of 1. The true power (watts) of a unity
power factor circuit is easily calculated as a product of
amperes times volts (divided by 1000 for KW).
When out of phase conditions prevail, as is the usual
case in AC circuits, the product of amperes times
volts reveals the apparent power of the circuit rather
than the true power. KVA represents kilovolt-amperes
and describes apparent power while KW is used to
describe true power in AC circuits with inductive or
capacitive reactance. An analogy relating mechanical
work to electrical power may help explain the reason
for apparent and true power ratings of reactance type
AC circuits.
Referring to Figure 10, we see an airplane towing
a glider. Assume that the tow plane must , for some
reason, pull the glider in Position A. In this position, the
tow cable is at an angle of 45˚. The force applied by the
tow plane is then at an angle to the direction of motion
of the glider.
It is obvious that more force must be exerted in Position
A to do the same amount of useful work that would be
accomplished in Position B where no angle exists and
force and motion are in the same direction.
A situation similar to that shown in the foregoing
analogy presents itself in inductive or capacitive AC
circuits. In these circuits more power must be supplied
than can actually be utilized because an angle similar
to the one in the analogy exists between voltage and
current. Since current either leads or lags voltage by
a number of degrees in time, they never reach their
corresponding maximum values at the time within
these circuits.
Referring to the 45˚ inductive reactance sine wave
illustrated in Figure 10-A, we see that at point B (or
90˚ in time) voltage has reached its maximum value
while current has approached but not quite reached
its maximum value. If we calculate the power in the
circuit at this point (or any other point for that matter)
the product of volts times amperes will not indicate
the actual or true power for while voltage is at its peak
value, current is at less than its maximum value. In
other words, this reveals only the apparent power.
To determine the true power, the number of degrees
that current is out of phase with amperes must be
applied as a correction factor.
This correction factor is called power factor in AC
circuits and it is the cosine of the phase angle. The
cosine of any angle is usually listed in math and
electrical handbooks. The cosine of the angle of 45˚
would be 0.707 or electrically a power factor of 0.707.
The triangular representation shown in Figure 10A can be used to fi nd the apparent (KVA) and true
(KW) ratings of a 240 volt, 55 ampere, single phase
generator. Since KVA is the product of volts times
amperes, KVA in this case will equal 240 x 55 divided
by 1000 or 13.2. The triangle shows an angle of 45˚
Page 12
FIGURE 10 - MECHANICAL WORK - POWER FACTOR
FIGURE 10-A - POWER FACTOR DETERMINED BY DEGREE VOLTS “OUT OF PHASE” WITH
AMPERES
between volts and amperes. The power factor would be
the cosine of this angle or 0.7.
The true power of this generator can now be calculated
as the product of KVA (13.2) times power factor (.7).
The true power of the generator will, therefore, be
9.24 KW. At .8 power factor, this same generator could
be rated at 10.56 KW so we see that the higher the
power factor - the greater the real power (KW) of the
generator.
Normally the rating of a single phase AC generator is
stated at “unity” power factor for pure resistance type
loads. This rating is also frequently stated at .8 power
factor for 3 phase generators to accommodate average
reactance type loads. The power factor rating of a
generator must at least match the power factor of the
load applied. In most cases, it is not safe to assume
that a load is, in fact, average and that the generator’s
.8 power factor rating is suffi cient to carry the load. The
actual power factor of the load should be determined.
There are numerous ways in which the power factor
of a circuit can be determined, however, a discussion
of the various methods becomes too involved to
adequately cover in our study of basic electricity.
Page 13
AN APPROACH TO PRACTICAL GENERATORS
Practical AC generators are of the rotating fi eld
type. The magnetic fi eld of the rotating fi eld poles is
generated by many turns of wire which are supplied by
direct electrical connection to the exciter armature (in
brushless generators).
The stator, or armature, is constructed of stacked
laminations with many slots in which the coils of
wire lay. Since a single turn of wire could not be long
enough to generate the voltages required, many turns
are wound together and distributed in the slots in
such a manner that the voltages generated are added
together by connecting the coils in series. In order to
generate voltages in various phase relationships, the
wires in a given section of the armature are grouped
together for each phase as shown in fi gure 3 and fi gure
4 for single phase and three phase respectively.
All of the possible reasons for the distribution of coils
among several slots could not be covered here, nor can
the effects of this distribution be discussed completely.
However, the shape and value of the output voltage
wave depends upon this distribution.
Single-phase and three-phase generators.
So far we have been discussing the single phase
generator, that is, a generator with one winding. It may
have two or more groups of coils, but it is still single
phase if the voltages across the two groups of coils
reach their peak at the same time.
Some generators have two windings of which one
reaches its peak at the time the other reaches zero.
This is a two phase generator. There are very few
applications for a two phase systems. Therefore, this
brief note will be all of the discussion on this type.
Of the systems we will cover, the one using three
windings is most common. These three windings are
so placed that three separate voltages are generated.
This is called a three-phase generator. The three
voltages are equal in value and 120 electrical degrees
apart as shown in fi gure 5. The three windings may
be connected in a triangle as in fi gure 6. This is called
a Delta (∆) connection, or they may be connected in
a Wye (Y) as shown in fi gure 7, with one end of each
winding connected. The three-phase system makes
more effective use of the iron and copper than a singlephase system, to the extent that in most diesel engine
driven generator sets, a single-phase generator will
weigh more than a three-phase generator of the same
output rating.
Page 14
∆
Figure 3 - Single phase groupingFigure 4 - Three phase grouping
FIGURE 10-A - MECHANICAL WORK - POWER FACTOR
Figure 5 - Three-phase voltage or current
Generally the Wye connection is used in preference
to the Delta connection because the neutral can be
grounded and also to prevent circulating current which
can occur in the Delta connection. Delta connections
are used in preference to Wye connections when you
want 240 volts three-phase and also 120/208 volts
single-phase.
If the central or neutral point of a Wye connection
is connected to a line, the circuit becomes a threephase, four wire system. The three-phase, four
wire Wye connection shown below in fi gure 8 gives
120/208 volts, and accommodates both lights and
motors, without the use of lighting transformers. This
connection is commonly used for low-voltage networks.
In a three-phase system if the voltage is mentioned
without any reference to whether a Delta or Wye
system is used or, in a Wye system, whether line-toline or line-to-neutral voltage is meant, the reference
is almost always to be taken to mean the line-to-line
voltage.
You are advised to note that in the Delta connection
the line-line voltage = phase voltage. In the Wye
connection line-line voltage = phase voltage x √3 or
(1.732 x phase voltage).
Figure 6 - 3-phase 3-wire delta system
Figure 8 - 3-phase 4-wire wye system
Figure 7 - 3-phase 3-wire wye system
Page 15
Principles of Generator Operation
Through residual magnetism of the exciter
stator or main rotor, the generator produces
the start voltage to fi re up the automatic
voltage regulator, (AVR). AC from the main
stator is fed to and sensed by the AVR. Which
in turn supplies DC to the excitor stator.
During no load operation this is around 8 to
12 vdc.
This magnetizes the exciter stator............
The exciter rotor spins inside the exciter stator
fi eld breaking the lines of fl ux, thus absorbing
as AC. Then the AC is fed through a rectifi er
system to convert to DC............
This DC is then fed up the shaft to the main
rotor, magnetizing the rotor.......
And the main stator absorbing the lines of
fl ux, produces AC regulated by the AVR to the
proper output.
When a Permanent Magnet Generator (PMG
or W-Series generator), is used, the difference
in the operating system is the AVR gets its
power from the PMG, not the generator stator.
Only the sensing for the AVR comes from the
generator stator. This design creates better
voltage control for motor loads, SCR loads,
and provides 300% short circuit protection.
Page 16
Output
Main Stator
Shaft
Main
Rotor
Exciter
Stator
Principle of Generator Operation
AVR
PMG
Stator
Rotating
Rectifi er
Exciter
Rotor
PMG
Rotor
Page 17
Output
Main Stator
Shaft
Main
Rotor
Page 18
Principle of Generator Operation
AVR
Exciter
Stator
Rotating
Rectifi er
Exciter
Rotor
Generator-Drive Engines
FREQUENCY
Drive engines for AC generators must run at a speed that
generates the proper electrical frequency. The speed
at which an engine runs to produce the desired output
frequency is the synchronous speed.
Common synchronous speeds for utility loads are:
Engine Speed Generator Frequency
Poles
3600 RPM 2 poles 60 Hz
3000 RPM 2 poles 50 Hz
1800 RPM 4 poles 60 Hz
1500 RPM 4 poles 50 Hz
1200 RPM 6 poles 60 Hz
The synchronous speeds for aircraft support diesel
generators are:
Engine Speed Generator Frequency
Poles
3000 RPM 16 poles 400 Hz
2400 RPM 20 poles 400 Hz
2000 RPM 24 poles 400 Hz
1846 RPM 26 poles 400 Hz
GOVERNOR DROOP
Droop is the speed change when an engine goes from full
load to no load at wide open throttle. Lugger engines are
set with a maximum governer droop of 5% at 1800 RPM,
and 7% at 1500 RPM. The formula for droop (%) is:
FREQUENCY REGULATION
Frequency critical circuits must have an engine that
runs at constant speed. This cannot be achieved with
the standard mechanical governors on the generator
drive engines. A “zero droop” or “isochronous” governor
maintains a constant engine speed at any load.
Isochronous operation on a Lugger diesel engine requires
a fuel injection pump with a customer provided add-on
electronic governor or an electronically controlled engine.
Frequency regulation is a result of the engine governor
droop. Adjusting frequency requires an engine governor
adjustment. Electrical specifi cations always specify
frequency regulation.
GOVERNOR STABILITY
Stability is determined by how well an engine’s governor
maintains a constant speed with a steady load. The
fl uctuation with mechanical droop governors is ±0.5% or
about ±8 RPM. Isochronous governor systems should
provide a fl uctuation of ±0.25% or less.
Mechanically governed generator-driven engines may
surge when governor droop adjustment is less than 5% @
1800 rpm (7% @ 1500 rpm). Governor stability is affected
by the governor droop adjustment. Adjusting a mechanical
governor to reduce droop will make the governor less
stable throughout the operating range. This reduction in
stability can cause “hunting” or “surging” of the engine.
Part load operation also allows unburned fuel to gather
in the engine exhaust and lube systems. This type of
operation can result in unsightly leakage from the exhaust
system, as well as increased maintenance costs. An
oversized engine will more likely have these problems. A
generator set operates best from 50% to 90% of full rated
load. Long term operation at less then 30% of full rated
load is not recommended.
(No Load RPM - Full Load RPM) x 100
Full Load RPM
At 5% droop, an 1800 RPM generator-driven engine at
a full load speed of 1800 RPM would go to 1890 RPM at
no load. This falls within the normal frequency band of 60
Hz to 63 Hz, which is acceptable for pumps, fans, motors,
general lighting and utility power.
VOLTAGE REGULATORS
External voltage regulators control the output voltage of
the generator by controlling the fi eld excitation current.
Internally regulated generators are used for special
purpose applications and are not adjustable.
The simplest manual and mechanical regulators use
rheostats (variable resistors) to adjust the fi eld excitation
current to the generator. Systems with little or no variation
in load, or systems that don’t require close voltage
regulation, may use this type of voltage regulation. Manual
and mechanical regulators are inexpensive, but have
unacceptable performance for most electrical systems.
Mechanical regulators can hold the voltage regulation to
Page 19
±4%. No regulation is available with manual control.
Transistorized and Silicon Controlled Rectifi er (SCR)
voltage regulators provide analog control of the fi eld
current. Variations in load are sensed by the regulator
which adjusts the fi eld excitation current to regulate
voltage.
Digital or microprocessor controlled regulators sense
engine and generator operating conditions, and make
appropriate adjustments in fi eld current and voltage based
on logic programmed into the microprocessor.
Any voltage regulator (transistorized, SCR or digital) that
can adjust the fi eld current in response to a load change is
called an Automatic Voltage Regulator or AVR. AVR’s can
maintain the voltage within ±2% of nominal voltage, and
some hold to ±0.5% or better.
TRANSIENT RESPONSE
When load is applied to an AC generator set, the engine
speed drops until the governor can recover. The time it
takes to recover the voltage and frequency to the normal
bandwidth is called, recovery time. Recovery times are
infl uenced by many factors including engine, generator,
and voltage regulator design.
The operational requirements of the electrical system
are determined by the type of load on the system. Light
bulbs are not affected by voltage or frequency changes
other than a change in brightness (brown-out) when the
voltage drops. However, when electric motors run below
rated frequency, they overheat. If the voltage drops too far,
motor controller relays may drop out and knock the motor
off line.
AVR’s are designed to drop voltage when a sudden load is
applied. This drop, called voltage dip, reduces the load on
the engine and allows for quick recovery times. Dropping
the voltage also reduces the load on the motors and
reduces motor heating problems. Voltage dips of up to
35% are acceptable for most utility load systems. Voltage
sensitive circuits may tolerate voltage dips of up to 20%.
To improve the recovery time, AVR’s for diesel generator
sets may incorporate a Volts/Hz adjustment that drops
voltage and frequency while the engine is picking up
the load. Loss of frequency regulation for a few seconds
does not cause problems for typical utility loads. Volts/Hz
regulators designed for turbocharged engines have a delay
to allow for turbocharger recovery before applying the
load. This gives quicker overall response than loading the
engine before the turbocharger can respond to the load
change. AVR’s designed for naturally aspirated engines do
not have this delay feature.
provide response times in the 4-second to 5-second range
when going from no load to full load with a maximum
voltage dip of 35%. Better performance can be achieved
by lowering generator output levels, applying the load in
steps or with high performance voltage regulators.
CYCLIC IRREGULARITY
When the engine fi ring pulses are spaced further apart
than one electrical cycle or 1 Hz, the electrical wave form
may be distorted. This can cause problems for certain
types of electronic equipment. Cyclic irregularity is most
likely when the number of engine cylinders is less than the
number of poles in the generator.
OVERSPEED PROTECTION
Most customers assume a runaway engine to be the
cause of overspeed problems in a diesel generator set.
This is seldom the case. A runaway engine is unlikely.
A generator set is more likely to overspeed due to the
introduction of regenerative power into the electrical bus.
This drives the generator as a motor and overspeeds the
unit. When this occurs, the engine governor drops the
fuel rate to the idle setting. For overspeed protection, the
generator set assembler can provide an overspeed trip
which would cut off fuel to the engine and shut down the
generator. The trip should be set at 15% to 20% above
rated engine speed.
BALANCED THREE-PHASE LOAD
Generators should have the resistive and inductive loads
balanced on each phase. A phase imbalance of more than
5% will cause unstable voltage regulation. This problem
cannot be corrected with engine or generator adjustments.
The distribution circuits should be rearranged until balance
can be achieved.
DC GENERATORS
DC Generators are occasionally used for special purpose
equipment or more typically to repower old units. Since
there is no frequency in a DC electrical system, it is much
simpler to operate in parallel. DC generator drive engines
use droop governors and do not need to be synchronized.
The load is balanced with fi eld excitation adjustment.
The use of AVR’s has improved the response
characteristics of generator sets so that engines with
high BMEP ratings can carry larger electrical loads. With
modern AVR’s, which incorporate Volts/Hz adjustment, the
Lugger diesel prime mover engines can be expected to
Page 20
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