unit 28 - generators-import

8/7/2019 Unit 28 - Generators-Import

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1. NEETS MOD 5 (Intro To Motors And Generators)

2. NSTM Chap 300, Electrical Plant General

3. Electricians Mate

ReferencesReferences


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Enabling Objectives:

1. RETRIEVE or RECOGNIZE information concerning AC andDC generators.

2. RECOGNIZE information pertaining to the construction andcomponents of AC and DC generators.

3. RECOGNIZE the information pertaining to the basicoperation of generators.

4. RECOGNIZE information pertaining to the procedures usedto PERFORM basic generator maintenance.


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Enabling Objectives:

5. CALCULATE values of frequency, number of poles, RPMs,

and line/phase values in wye/delta connections.

6. RETRIEVE OR RECOGNIZE information pertaining to theeffects of changing operating values on generator

performance.

7. CONSTRUCT and TROUBLESHOOT an AC generator.

8. APPLY the safety precautions associated with the use of

test equipment on generators in accordance with NAVY

SAFETY PRECAUTIONS FOR AFLOAT FORCES


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1. AC GENERATOR THEORY

a. A Generator is a machine that converts MECHANICALenergy into ELECTRICAL energy.


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AC Generator Theory (cond)

b. The same magneticinduction generator that

was used to develop an

AC voltage in unit 11.5

(AC waveforms) contains

the basic components ofall magnetic induction

generators.


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c. Three factors necessary to create voltage through magneticinduction, given by the formula:

1. A Magnetic Field (I )

2. A Conductor (K)

3. Relative motion between the two (N)

E = K I N


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d. Recall that as a single loop conductor rotates within a magnetic

field, an AC voltage is induced into the conductor.

1. AC voltage will be delivered to the load through an

assembly of slip rings and brushes


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e. As long as the 3 necessary elements required to producevoltage are present, voltage will be produced regardless of

the arrangement of the elements.

1. A single loop conductor moving within a magnetic fieldwill produce voltage.

2. A magnet, or magnetic field, moving WITHIN a single

loop conductor will produce voltage as well. This method isthe most common arrangement to produce voltage.


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2. AC GENERATOR COMPONENTS

a. The 2 basic parts of induction generators are the fieldwinding and the armature winding.

1. Field Winding- produces the magnetic field, or main field

flux.

2. Armature Winding- where voltage is induced into the

windings or conductors. Supplies power to the load.

3. For relative motion to take place between the conductor

(armature) and field, all generators must have 2 mechanical

parts. A rotor and a stator.


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AC Generator Components (cond)

a. The Rotor is the part that rotates/revolves, and is centered

inside the generator.b. The Stator is the part that remains stationary.

c. In a DC Generator the armature winding is always wound

in the rotor. In AC generators (alternators) the armature may

be either the rotor or stator.

Rotor Stator


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4. Types of Rotor:

a. There are 2 types of rotors used in alternators. Theyare called Salient Pole and Cylindrical/Turbine Driven.

1. Salient Pole Rotor

laminated with protruding

pole pieces on which the

windings are housed.

Low speed rotor, used at

speeds of1200 RPMs or

below.


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Salient Pole Rotor


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2. Cylindrical/Turbine Driven Rotor windings firmlyembedded in slots. High speed rotor, used at speeds of1200

RPMs or greater.


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Cylindrical/Turbine Driven Rotor


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Generator Rotors


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8. Bearings - Both ends of the shaft.Provides ease of rotation and reducesfriction.

7. End Bells Houses the bearings and aligns the rotor shaft.

6. Core Material - Consists of steel laminations slotted to housethe windings. Designed to concentrate lines of flux.

5. Frame Houses and supports other stator components.


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9. Slip Rings and Brushes Serve as sliding contact to provide

a path for current.

a. The slip rings are metal rings, usually steel or brass,

connected to the ends of the rotor conductors. Insulated fromthe rotor and each other.


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Brush TensionDevice/Spring

Brushes

Slipring

Brush holder

b. The brushes serve as a sliding contact to transfer power.They are usually made of carbon and are sturdy enough tocarry current, but are soft enough so as not to damage the sliprings.


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c. The brushes are held against the slip rings by a brush

holder assembly that positions the brushes and appliespressure to ensure good connection.

9. Exciter device that supplies/provides DC potential to start or

maintain the main field flux. May be a separate DCgenerator or a rectifier circuit powered from the generator

itself.


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10. Prime Mover- machine that supplies the mechanical force

(motion) that turns the rotor of the generator.

a. Steam/Gas Turbine - because of the high operating

speed of turbines, the generators are not connected directlyto the turbine shaft, but are instead connected through

reduction gears. Generator speed range from 900 to 1800

RPMs. In rare instance: 3600 RPMs.


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2. Gas/Diesel Engine - engine is directly coupled to the

generator and operates at 600-1800 RPMs.


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3. Electric Motor directly coupled with a generator.


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3. AC GENERATORS TYPES

a. Rotating Armature, Stationary Field Generator (RASF)

1. Output voltage is taken from the armature windingsthrough slip rings and brushes.

2. Limiting factor as to the amount of power supplied: thecurrent carrying capacity (size) of the brushes.

3. Seldom used as Ships Service Generator because of its

low power capability. Used only for light loads.


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b. Rotating Field, Stationary Armature Generator (RFSA)

1. Output voltage is taken from the armature windings.

2. Limiting factor as to the amount of power provided isthe size of the armature windings.

3. Most commonly used type because of its high power

capability.


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RASFRevolving Armature, Stationary Field

Magnetic Field StatorRotor output voltage

RFSARevolving Field, Stationary Armature

Magnetic Field Rotor

Stator Windings output voltage


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4. AC SINGLE PHASE ROTATING-FIELD GENERATOR

a. Generating a single phase EMF.

1. Voltage is induced into the armature due to flux of thefield cutting conductors of armature.

2. Minimum or zero voltage is induced when the field polesare positioned at an equal distance between armaturepoles. (Conductors are in parallel with the magnetic field).

3. Maximum voltage is induced when the field poles arealigned with the armature poles (conductors areperpendicular to the field).

4. One complete turn produces one complete sine wave or

one cycle. (commonly referred to as Hertz).


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Generating a single phase EMF (cond)


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Generating a single phase EMF (cond)


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Single Phase AC Generator


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b. Operation of the AC three Phase Rotating-Field Generator

1. Three phase generators are more commonly used than

single phase generators because of some major advantages.

a. Three phase generators deliver more power than single

phase generators of the same physical size.

b. More efficient use of armature space is possible when

using three phase generators.

2. The armature of a three phase generator consists of 3

single phase windings: Phase A, Phase B, and Phase C.

a. The windings are equally spaced 120 electrical degrees

apart on the armature core.

b. The Field is still DC, requiring only 2 slip rings and

brushes to provide excitation.


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c. Generating a 3 Phase EMF

1. Induced voltages in each phase are equal in magnitude

but 120 electrical degrees apart.


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2. When one induced voltage is maximum, the other two

induced voltages are at 50% of maximum BUT of opposite

polarity (direction).

B

100%

A&C50%


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3. The algebraic sum of all 3 induced voltages is equal to

zero.

Example:

C = 100 Volts

A = -50 Volts

B = -50 Volts

Zero(0) Volts


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d. Three phase Armature Connections: The methods of

connecting the 3 single phase armatures to produce a 3

phase generator are basically SERIES (wye) and

PARALLEL (delta).

(1) Terms:

a. LINE VOLTAGE (E) voltage present at output

terminals of a generator.b. PHASE VOLTAGE (e) voltage induced in each

individual phase of the generator.

(2) WYE connection: connected to form a commonjunction, giving characteristics of a SERIES circuit. The

remaining three leads are connected directly to the bus to

supply power to the load.


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S

S

S

S Start

F - Finish

F

a. Two phases are connected in series between any twoa. Two phases are connected in series between any twooutput terminals; therefore, since current is common in seriesoutput terminals; therefore, since current is common in seriescircuits, the Line current is equal to the Phase current.circuits, the Line current is equal to the Phase current.

I = iI = i


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b. Line voltage (E) is equal to 1.73 x phase voltage (e)b. Line voltage (E) is equal to 1.73 x phase voltage (e)

E = 1.73eE = 1.73ec.c. Line voltage can also be found by VERTORIALLY addingLine voltage can also be found by VERTORIALLY adding

the Phase voltages together.the Phase voltages together.

d. Being a series circuit, if a phase winding opens, thatd. Being a series circuit, if a phase winding opens, thatphase will be lost and one line connection will bephase will be lost and one line connection will beavailable.available.


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A Phase

B Phase

C Phase

WYEConnection

N S


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DELTA Connection

S Start

F - Finish

A

B

C

AF

CF

BF

Cs

As

Bs


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a. Voltage is common in a parallel circuit, therefore line voltage

and phase voltages are equal.

b. Line current (I) is equal to 1.73 x phase current (i) due to the

phase current being 120 electrical degrees out of phase with

each other.

E = e

I=1.73ic. Line current can also be found by VECTORIALLY adding the

Phase currents together.

d. The type of connection used on a particular generator is

generally selected by the manufacturer to obtain the

required operating characteristics and to ensure ease and

simplicity of manufacture and repair in the most economical

way possible.


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H. GENERATOR LOSSES

1. Power Loss Anything that produces heat in the generator

and reduces its efficiency. More power is applied than is

being extracted.

2. Efficiency ratio of output to input power

Pout

Pin%Efficiency =%Efficiency = X 100X 100

3. Friction Losses wind & bearing friction in the rotor to slow

down requiring more power from prime mover.

4. Three Types of Core Losses:a. Hysteresis molecular friction caused by shifting

domains in the core material. To minimize, use low

reluctance material. Example: soft iron, silicon steel.

(Reluctance low opposition to magnetic lines of flux)


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b. Eddy Currents circulating current in the core material.

Reduced by using thin sheets of core material calledlaminations.

c. Copper Losses (I2

R) losses due to resistive properties

of a wire as current flows through it. Reduced by using

less wire as possible.

3 Types of Core Losses: (cond)

5 A t R ti th di t ti f th i fi ld fl


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5. Armature Reaction the distortion of the main field flux

caused by the interaction of the main field and armature field.

a. It is compensated by increasing the DC excitation to the

main field windings.b. The greater the load on the generator or armature

current, the greater the Armature Reaction.


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c. Flemings Left Hand Rule:

1. Thumb1. Thumb motionmotion of conductorof conductor2. Index finger2. Index finger direction of magneticdirection of magnetic fluxflux3. Middle finger3. Middle finger direction ofdirection of currentcurrent


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Exercise 1:

N S NSN S. X .X X

What is the direction

of the rotor?

What side is the

North and Southpole?

What is the direction

of current?

.

6 Motor Action the development of a force that opposes the


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6. Motor Action the development of a force that opposes the

speed of the prime mover. It is base on the principle that

when a current carrying conductor is placed in a magnetic

field, that conductor will tend to move at right angles to that

field.

6 Motor Action (cond)


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6. Motor Action (con d)

No load = No current flow

6 Motor Action (cond)


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6. Motor Action (con d)

a. As the Armature pole approaches the main field pole, an

interaction between the 2 fluxes causes thearmature flux to build up (increase) on the main

field. Both fluxes opposes each other and will

cause the rotor to stop if the rotor were moving too

slowly.

b. To compensate for this loss, increase the speed of the

prime mover.


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I. GENERATOR RATINGS

1. Generators are rated by its ability to dissipate heat generated

by its power losses (i.e. Heat)

2. AC generators are generally rated by the Apparent Power (AP)it can deliver to a load and is measured in Volt-Amp/Kilo-Volt-

Amp.

E x I x 1.73

1000Apparent Power = = KVA

Generator Ratings (cond)


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Generator Ratings (cond)

3. True Power (TP) power that is actually used or consumed by

the resistive load and is measured in KW (Kilowatt). It isDEPENDENT upon the power factor of the load.

E x I x 1.73 X PF

1000

True Power = = KW

4. Reactive Power (RP) power that is stored and returned back

to the source by the reactive components and is measured in

KVAR (Kilo-Volt Amp Reactive). It is the vectorial differencebetween KVA and KW.

Reactive Power = KVA2 - KW2


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J. Methods of controlling the terminal output of Generators

1. Conductor (K) increase/decrease the number of turns will

vary the output voltage, but it is neither practical noreconomical.

2. Speed of the Prime mover (N) increase/decrease speed will

vary output voltage, but will also affect the frequency (F) of

the output voltage.3. Strength of the Magnetic Field (I) increase/decrease strength

of magnetic field will vary output voltage. It is the

PREFERRED method. It is accomplished by using a variable

rheostat (VAR) in series with the field windings.

Methods of controlling the terminal output of Generators


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g p

(cond)

4. The field winding receives excitation from an external DC

source (battery, rectifier, DC generator). When the main fieldincreases, the armature windings are cut with greater force

inducing more voltage into the armature.

5. Induced voltage increase, armature current increase and

armature reaction and motor action increase.



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g p

(cond)

Adding Load Removing Load

Armature CurrentArmature Reaction

Motor Action

Gen. Output Voltage

Prime Mover

Frequency

VAR

Magnetic Field (I)

Gen. Output Voltage (E)



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g p

(cond)

6. Voltage Regulation ability of the generator to maintain a

constant terminal output voltage under varying load condition.

ENL - EFL

EFL% Voltage Regulation =% Voltage Regulation = x 100x 100

a. Voltage control of the AC generator is usually performed

automatically by electronic components and systems, butcan be performed manually. It is controlled by changing

the strength of the magnetic field.

K M th d f t lli th t t f f AC


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1. Factors affecting frequency:

a. Number of poles

b. Speed of rotation most preferred/feasible method

K. Methods of controlling the output frequency of an AC

generator

Frequency (F) =Frequency (F) =P = Number of Poles

N = Speed

2. Formula:

P N

120


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3. Frequency/cycles per second - shows how many North andSouth poles will complete a cycle in 1 second.

4. The standard output frequency of AC generators is 60 cycles

per second, also known as Hertz (Hz).

a Armature Reaction causes Motor Action which slows


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a. Armature Reaction causes Motor Action which slowsdown the primer mover. As the load increases on thegenerator, Armature Reaction and Motor Action increase,slowing the prime mover and lowering the frequency.

b. By increasing the fuel supply to Diesel or Gas Turbineengines, or by increasing the steam flow to Steam Turbines,these prime movers will increase speed of the generator rotor,and return the frequency to the rated value.

L DC G t


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L. DC Generators

DC generators are quickly being replaced with AC generators;

however, some special applications, such as cranes & mine

sweeping systems, require a high amount of DC current.


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DC Generators (cond)


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DC Generators (con d)

3. A DC generator uses a COMMUTATOR in place of SLIP

RINGS. The split slip ring is the commutator which

produces a DC output voltage.

4. COMMUTATION the process of obtaining a DC output

voltage from armature windings that has an AC voltage

induced into it.



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Recall the basic RASF generator, but instead of the 2 slip rings,use one slip ring cut in half with the brushes placed opposite

each other.

a. The Split slip ring is the commutator which produces aDC output voltage.


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BASIC DC GENERATOR

b Size of the brushes


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b. Size of the brushes

limiting factor as to the

amount of power

generated by a DCgenerator.

c. Increasing the number of

commutation

segment/armatureconductors provide a

smoother output voltage.

1. High power DC

generator have several

sets of brushes mounted

on the commutator to

increase the amount of

DC current available.



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5. The same power losses that affect AC generators, affect DC

generators:

a. Friction loss

b. Eddy current loss

c. Hysteresis loss

d. Copper loss

e. Armature Reaction

f. Motor Action

M Generator Maintenance


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M. Generator Maintenance

1. Majority of generator malfunctions are caused by improper /

lack of preventive maintenance on the brush riggingassembly and slip ring / commutator.

2. Brush rigging assemblya. The bottom of the

brush holder should

be between 1/8 1/16 inch from the

surface of the

commutator/slip rings.

1/8-1/16

Generator Maintenance (cond)


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Generator Maintenance (con d)

b. All brush leads should be securely connected to the

brushes and the brush holders. They also should move freely

in their holders but should not vibrate.

c. Replace all brushes that are:

1. Worn or chipped

2. Have damaged leads or pigtails3. Worn 50% of original length/size

4. Metal portion of the brush

is 1/8 inch from contact

with the commutator/sliprings.



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( )

d. Before replacing brushes clean all carbon and grease off

brush rigging.

e. All brushes should be checked for proper seating beforeputting a generator back in operation.

f. SEATING process of shaping a brush until it fits the

contour (shape) of the commutator / slip ring.


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g. Methods of seating the brush:

1. Sandpaper preferred method. Use a fine grade of

sandpaper, about the same width as the slip rings.

Steps:

Generator is secured andtagged out of commission.

Place the sandpaper underthe brush (course sideup). Pull sandpaper underin the direction of rotation.Ensure sandpaper is kepton the surface of slip rings

so as not to round theedges of brushes.Continue process until ittakes the shape of sliprings. Finish by using afiner grade of sandpaper.



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2. Seating stone made of soft abrasive material. Place

max pressure on brush. Generator MUST be running. After

seating, adjust spring tension.



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( )

3. Adjust the brush tension by using a scale and a piece of

paper. (Use 2 - 2.5 psi pressure if a MRC or MTM is not

available.)

4. After the brushes

have been seated,

regardless of

which method isused, clean the

inside of the

generator by using

a vacuum cleaner(suction side) or

lint-free rags.

D t


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Dynanometer



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3. Slip rings / Commutators

a. After 2 weeks ofoperation, an oxide film

which is uniform glaze

brown in color develops

on the surface of the slip

rings / commutator. Thismust NOT be removed.

It prevents excessive

wear of the brushes and

slip rings/commutator.



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b. If generator cannot be secured, use canvass wiper to clean

the surface of slip ring / commutator.

c. If generator can be secured,

use vacuum cleaner and lint-

free rags. Remove any buildup of grease or carbon dust.

d. If the surface becomes pitted, grooved or scratched, it

causes sparking and excessive brush wear. Notify WCS so

the proper corrective action is taken.



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4. Using a multi-meter, the resistance between the phases

should be equal or the same.

5. Megger (Meg-Ohm meter) used to measure ground

(insulation) resistance of generator. Readings should be

infinite or at least 1 megohm or stated on PMS card.

N APPLICATION


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N. APPLICATION

Generator Trainer

Set-up for WYE Connection


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Set-up forWYE Connection

Set-up for DELTA Connection


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Set-up forDELTA Connection

General Safety Precautions


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General Safety Precautions

1. Remove all metal or jewelry prior to entering lab.

2. During lab operation the trainee will NOT make or

break electrical connections on energized circuits.

3. Power is to be secured prior to any changes in circuit

configuration.

Review of Objectives:


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This is what you have learned.

1. RETRIEVE or RECOGNIZE information concerning AC andDC generators.

2. RECOGNIZE information pertaining to the construction and

components of AC and DC generators.

3. RECOGNIZE the information pertaining to the basic operation

of generators.

4. RECOGNIZE information pertaining to the procedures used to

PERFORM basic generator maintenance.



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5. CALCULATE values of frequency, number of poles, RPMs,and line/phase values in wye/delta connections.

6. RETRIEVE OR RECOGNIZE information pertaining to the

effects of changing operating values on generatorperformance.

7. CONSTRUCT and TROUBLESHOOT an AC generator.

8. APPLY the safety precautions associated with the use of testequipment on generators in accordance with NAVYSAFETY PRECAUTIONS FOR AFLOAT FORCES


This is what you have learned.

SUMMARY AND REVIEW


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SUMMARY AND REVIEW

unit 28 - generators-import

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