Northern Lights Power Generation Reference Manual

Technical

Reference Manual

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TABLE OF CONTENTS

Northern Lights Selection Guide ............................................................ 3-6

Basic Electrical ..................................................................................... 7-13

Practical Generators ........................................................................... 14-15

Principles of Generator Operation ...................................................... 16-18

Generator Drive Engines .................................................................... 19-20

Startup Procedure for Generator Sets ......................................................21

Paralleling Procedures, Basic ..................................................................22

Engine Power Ratings ........................................................................ 23-24

Engine Noise ...................................................................................... 25-28

Fuel System ....................................................................................... 29-32

Lubrication System ............................................................................. 33-35

Cooling System .................................................................................. 36-37

Coolant Tech. Bulletin L423 ................................................................ 38-40

Alignment and Vibration ...........................................................................41

Torque-to-yield ..........................................................................................42

Electrical Formulas, Data, and Tables ................................................ 43-51

Time and Speed Table ..............................................................................52

Electrical Terminology ......................................................................... 53-59

Marine Terminology ............................................................................ 60-61

Industrial Terminology ......................................................................... 62-67

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Selection & Installation Guide

for Prime and Stand-By Power Generator Sets

CHOOSING THE RIGHT GENERATOR SET

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.

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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.

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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

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.

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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

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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

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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.

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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).

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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 - SYMBOL EQUATIONS RELATION 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.

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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

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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˚

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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.

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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.

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∆

Figure 3 - Single phase grouping Figure 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

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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.

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Output

Main Stator

Shaft

Main

Rotor

Exciter

Stator

Principle of Generator Operation

AVR

PMG

Stator

Rotating

Rectiﬁ er

Exciter

Rotor

PMG

Rotor

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Output

Main Stator

Shaft

Main

Rotor

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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

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±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

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