Marathon Electric Magna Plus 280-430 Frame Instruction manual

MAGNAPLUS®GENERATOR
280–430 Frame
Installation, Operation,
and Maintenance Manual
Marathon Electric Mfg. Corp.
A Subsidiary of Regal-Beloit Corp.
P.O. Box 8003
Wausau, WI 54402-8003
Phone: [phone]
Fax: [phone]
www.marathonelectric.com

2
CONTENTS
Safety 2
Receiving and Storage 2
Principles of Operation 3 - 4
Installation 4 - 6
Wiring Connections 6 - 9
Operation 9 - 10
Maintenance 10 - 11
Testing 11 - 12
Service 12 - 14
Troubleshooting 14 - 17
Specifications 18
Parts List & Recommended Spare Parts 19 - 20
SAFETY
PLEASE REMEMBER SAFETY FIRST. If you are not sure
of the instructions or procedures contained herein, seek
qualified help before continuing.
This service manual emphasizes the safety precautions
necessary during the installation, operation, and
maintenance of your MagnaPLUS generator. Each section
of this manual has caution and warning messages. These
messages are for your safety, and the safety of the
equipment involved. If any of these cautions or warnings
are not readily understood, seek clarification from qualified
personnel before proceeding.
Before any service work is done, disconnect all power
sources and lock out all controls to prevent an unexpected
start-up of the generator set driver. Proper grounding
(earthing) of the generator frame and distribution system in
compliance with local and national electrical codes and
specific site requirements must be provided. These safety
precautions are necessary to prevent potential serious
personal injury, or even death.
The hazards associated with lifting or moving your
MagnaPLUS generator are pointed out in the installation and
maintenance sections. Incorrect lifting or moving can result
in personal injury or damage to the unit.
Prior to start-up of the unit ensure that all generator leads
are properly connected to the generator link board located
inside the connection box. Always assume that there will
be voltage present at the generator terminals whenever the
generator's shaft is rotating,and proceed accordingly.
Residual voltage is present at the generator terminals and at
the automatic voltage regulator panel connections even with
the regulator fuse removed. Caution must be exercised, or
serious injury or death can result.
This manual is not intended to be a substitute for properly
trained personnel. Installation and repairs should only be
attempted by qualified, trained people. The cautions and
warnings point out known conditions and situations that are
potentially hazardous. Each installation may well create its
own set of hazards
When in doubt, ask. Questions are much easier to handle
than mistakes caused by a misunderstanding of the
information presented in this manual.
RECEIVING AND STORAGE
RECEIVING AND STORAGE
Upon receipt of the generator, it is recommended that it be
carefully examined for possible shipping damage. The
generator was given to the freight carrier in good condition;
thus, the carrier is responsible for the product from the
factory dock to the destination. Any damage should be
noted on the freight bill before accepting the shipment. Any
claims for damage must be promptly filed with the delivering
carrier.
UNPACKING AND HANDLING
Carefully read all instruction tags shipped with the unit.
When lifting, attach an overhead crane to the lifting lug(s)
on the generator frame. Apply lifting forces in a vertical
direction. When transporting single bearing generators, the
generator’s rotor must be adequately supported to prevent
damage.
WARNING
THE LIFTING LUG(S) ON THE GENERATOR ARE
DESIGNED TO SUPPORT THE GENERATOR ONLY.
DO NOT LIFT ACOMPLETE GENERATOR AND
DRIVER ASSEMBLY BY MEANS OF LIFTING
LUG(S) ON THE GENERATOR. PERSONAL INJURY
OR EQUIPMENT DAMAGE MAY RESULT.
STORAGE
In the event that the generator is not immediately installed
on its prime mover, it is recommended that the unit be
stored indoors in a clean, dry area which is not subject to
rapid changes in temperature and humidity. If the generator
is stored for a long period of time, the generator should be
tested, cleaned and dried as required before being put into
service. See the maintenance section of this manual for
further information. If the unit has been stored in an area
where it has been subject to vibration, it is recommended
that the bearing(s) be inspected and replaced as necessary.

3
PRINCIPLES OF OPERATION
Rotating Assembly
Main Field
(rotor)
(+)
(-)
DC
(in)
Rotating Rectifier Assembly
3 Phase -- Full Bridge
Exciter Armature
(rotor)
3 Phase AC (out)
DC
(in)
(+)
(-)
Automatic
Voltage
Regulator
Exciter Field
(stator)
N
S
PMG Field
(rotor)
PMG
Armature
(stator)
PMG (optional)
L
1
L
2
L
3
Input Power -- Single Phase
(shunt powered regulator)
Sensing Input -- Single Phase
3 phase (optional)
Exciter Field Power
(DC out)
PMG Input Power (optional)
(1 phase, 300/250 hertz)
Main Armature
(stator)
3 Phase AC (out)
FIGURE 1 -- MagnaPLUS Circuit Diagram
FIGURE 2 -- Typical MagnaPLUS Layout Diagram

4
PRINCIPLE OF OPERATION
MagnaPLUS generators are a brushless, self excited,
externally voltage regulated, synchronous AC generator.
The generator is made up of six major components: main
stator (armature), main rotor (field), exciter stator (field),
exciter rotor (armature), rectifier assembly, and voltage
regulator. In understanding the above terminology, note the
following: stators are stationary, rotors rotate, a field is an
electrical input, and an armature is an electrical output.
These system components are electrically interconnected as
shown in figure 1 and physically located as shown in
figure 2.
The generator’s exciter consists of a stationary field and a
rotating armature. The stationary field (exciter stator) is
designed to be the primary source of the generator’s
residual magnetism. This residual magnetism allows the
exciter rotor (armature) to produce AC voltage even when
the exciter stator (field) is not powered. This AC voltage is
rectified to DC by the rotating rectifier assembly and fed
directly to the main rotor (field). As the generator shaft
continues to rotate, the main rotor (field) induces a voltage
into the generator's main stator (armature). At rated speed,
the main stator’s voltage produced by the residual
magnetism of the exciter allows the automatic voltage
regulator to function. The regulator provides voltage to the
exciter resulting in a build-up of generator terminal voltage.
This system of using residual magnetism eliminates the
need for a special field flashing circuit in the regulator. After
the generator has established the initial residual voltage, the
regulator provides a controlled DC field voltage to the exciter
stator resulting in a controlled generator terminal voltage.
Voltage Regulation
In the standard configuration (shunt excited), the automatic
voltage regulator receives both its input power and voltage
sensing from the generator's output terminals (See
Figure 1). With the optional PMG configuration, the
regulator receives input power from the PMG. The regulator
automatically monitors the generator's output voltage
against an internal reference set point and provides the
necessary DC output voltage to the exciter field required to
maintain constant generator terminal voltage. The
generator's terminal voltage is changed by adjusting the
regulator's reference set point. Consult the regulator
manual for specific adjustment and operating instructions.
MOTOR STARTING
When a motor is started, a large surge of current is drawn
by the motor. This starting current is equivalent to the
motors locked rotor or stall current and is 5 to 10 times
normal full load current. When the generator supplies this
in-rush of starting current, the generator voltage dips
temporarily. If the motor is too large for the generator, the
generator’s voltage dips greater than 30 percent. This may
result in the motor starter de-energizing or the motor
stalling. MagnaPlus generators generally supply .3 to .4
horsepower per generator KW in motor starting capability.
For specific data contact Marathon Electric.
PARALLEL OPERATION
All MagnaPlus generators are built with 2/3 pitch main stator
windings and full amortisseur (damper) windings. These
features make the MagnaPlus generators suitable for
parallel operation when equipped with the proper voltage
regulators and voltage regulator accessories. Consult with
the factory for further information relative to parallel
operations.
NONLINEAR LOADING
Solid state electronic control devices (variable frequency
drives, precision motor controls, battery chargers, etc.)
utilize electronic switching circuits (thyristors, SCRs, Diodes,
etc.). These switching circuits introduce high frequency
harmonics which distort the normal wave form of the
generator. This creates additional heat in the generator
windings and may cause the generator to over-heat.
Problems which can occur are not limited to the generator.
Poor wave shape may adversely effect various loads
connected to the generator. Consult Marathon Electric for
further information relative to nonlinear loads.
INSTALLATION
PREPARATION FOR USE
Although the generator has been carefully inspected and
tested in operation prior to shipment from the factory, it is
recommended that the generator be thoroughly inspected.
Check all bolts for tightness and examine the insulation on
lead wires for chafing prior to proceeding with installation.
Remove all shipping tapes, bags, skids and rotor support
blocking. For two bearing units, rotate the shaft by hand to
ensure that it rotates smoothly without binding.

5
WARNING
DISABLE AND LOCKOUT ANY ENGINE CRANKING
DEVICES BEFORE ATTEMPTING TO INSTALL OR
SERVICE THE GENERATOR. FOR ELECTRIC
START SETS, DISCONNECT THE CRANKING
BATTERY. FOR AIR START, DISCONNECT THE AIR
SUPPLY. FOR MOTOR GENERATOR SETS, OPEN
THE POWER SUPPLY TO THE DRIVE MOTOR.
FAILURE TO COMPLY WITH THESE SAFETY
PROCEDURES COULD RESULT IN SEVERE
PERSONAL INJURY OR EQUIPMENT DAMAGE.
NEVER "BAR OVER" THE ENGINE GENERATOR
SET USING THE GENERATOR'S FAN. THE FAN IS
NOT DESIGNED FOR THIS PURPOSE. BARRING
OVER THE SET WITH THE FAN COULD DAMAGE
THE FAN AND RESULT IN PERSONAL INJURY OR
EQUIPMENT DAMAGE.
GENERATOR MOUNTING
Single Bearing Units.
Single bearing units are provided with an SAE flywheel
housing adapter flange and flexible drive discs. Coupling
the generator's shaft to the engine flywheel is accomplished
with special steel drive discs bolted to the shaft. In addition
to the drive discs, there may be a hub spacer, spacer discs,
or a combination of hub spacer and spacer discs inserted
between the drive discs and the shaft to achieve the proper
shaft extension ("G" dimension per SAE J620c). Holes are
provided in the periphery of the coupling discs which
correspond to tapped holes in the prime mover's flywheel.
The outside diameter of the drive discs fit in a rabbet in the
flywheel so that concentricity is assured.
Grade 8 place bolts and hardened washers are
recommended to mount the drive discs to the flywheel. DO
NOT USE SPLIT TYPE LOCK WASHERS. Split lock
washers when biting into the drive disc cause stress risers
which may result in the disc fracturing.
The SAE flywheel housing adapter ring and the engine
flywheel housing are designed to match each other with no
further alignment necessary. Use grade 5 or greater
mounting bolts. MagnaPLUS generator frames are
constructed with two or three bolt holes per foot. The feet
should be shimmed where necessary to obtain solid contact
with the sub-base. With the frame securely bolted to the
engine flywheel housing, there is no side thrust or pull on
the generator frame, thus no real need to secure the feet
with more than one bolt per foot.
GENERATOR MOUNTING
Two Bearing Generators -- Direct Drive
Two bearing generators are provided with a keyed shaft
extension. For direct drive generators, the assembler
furnishes a flexible coupling which is installed between the
driver and the generator's shaft. Aligning the generator and
its driver as accurately as possible will reduce vibration,
increase bearing life, and ensure minimum coupling wear. It
may be necessary to shim the generator feet for proper
support and alignment. Secure the feet of the generator
with grade 5 or greater bolts through the holes provided in
the mounting feet. Consult the coupling manufacturer's
instructions for alignment specifications and procedures.
GENERATOR MOUNTING
Two Bearing Units -- Belt Driven
Two bearing MagnaPLUS generators can be belt driven
provided belts are sized and applied correctly. Please refer
to your supplier of belts and sheaves for correct sizing and
tensioning specifications. A bearing life calculation should
be performed. Marathon Electric recommends a minimum
B-10 life of 40,000 hours. If cog type belts are used, a
vibration may be introduced which could lead to premature
failure of the bearings.
END PLAY TESTING
Refer to the engine manual for recommended end play
specifications and measurement procedures. If end play is
not to specification, it is an indication that the generator
shaft is not moving freely in the assembly, and normal life of
the thrust bearing could be impaired. Probable causes of
this problem are:
1.Improper seating of drive discs in the flywheel resulting
in misalignment.
2. Improper mating of generator frame to engine flywheel
housing resulting in misalignment.
3. Improper "G" dimension per SAE J620c on either the
engine or generator.
TORSIONAL VIBRATION
Torsional vibrations are generated in all rotating shaft
systems. In some cases, the amplitude of these vibrations
at critical speeds may cause damage to either the
generator, its driver, or both. It is therefore necessary to
examine the torsional vibration effect on the entire rotating
system. IT IS THE RESPONSIBILITY OF THE
GENERATOR SET ASSEMBLER TO ASSURE THE
TORSIONAL COMPATIBILITY OF THE GENERATOR AND
ITS DRIVER. Drawings showing pertinent dimensions and
weights of the rotating assembly will be supplied by
Marathon Electric upon request.

6
ENVIRONMENTAL CONSIDERATIONS
The MagnaPLUS generator is designed for heavy duty
industrial applications; however, dirt, moisture, heat and
vibration are enemies of rotating electrical machinery.
Excessive exposure to the elements may shorten generator
life. The temperature of the cooling air entering the intake
openings of the generator should not exceed the ambient
temperature shown on the generator’s nameplate.
Generators intended for outdoor application should be
protected with housings having adequate ventilation.
Although the standard insulation systems are moisture and
humidity resistant, space heaters are recommended for
extreme conditions. If the generator is to be installed in an
area where blowing sand and dust are present, the
enclosure should be fitted with filters. Filters reduce erosion
on the generator's insulation by blocking high velocity
abrasive particles generated by the flow of cooling air
through the generator. Consult the factory for appropriate
filters and generator deratings required.
WIRING CONNECTIONS
Wiring of the generator and accessories should be
done in accordance with good electrical practices.
Follow government, industry and association
standards.
The generator conduit box construction allows cable entry
from multiple sides. A hole saw or other appropriate tool
may be used to provide for conduit entrance. Protect the
interior of the generator from shavings when drilling or
sawing. An approved connector must be used in
conjunction with the conduit. To minimize the transmission
of vibration, it is essential that flexible conduit be used for all
electrical entrance to the generator conduit box.
All MagnaPLUS generators are equipped with link boards
(terminal strips) for both internal and external connections.
All connections made to the studs of the link board should
be made with high quality ring terminals. Ring terminal
sizes are: 6 mm (280 Series Frames) and 10 mm (360 and
430 Series Frames). Torque link board connections to the
following specifications: 280 frame -- 5.4 NM (4 Ft Lb); 360
& 430 frame -- 27 NM (20 Ft Lb).
Refer to the connection diagram supplied with the generator
and / or the proper diagrams shown in this manual. Install
all inter-component and external wiring in accordance with
national and local electrical codes. The neutral in the
following connection diagrams shown below may be either
grounded (earthed) or left above ground potential (floating).
See national and local codes and / or the system
distribution wiring schematic diagram for the proper
connection of the neutral.
The following connection diagrams are shown for
twelve lead generators. Ten lead generators have the
same terminal designations except for leads T10, T11,
and T12. These three leads are internally connected
inside the generator and brought out as a single lead
(T0). Ten lead generators can only be connected in a
wye configuration
HIGH (SERIES) WYE CONNECTION
T
6
T
3
L
3
T
12
T
9
T
5
T
2
T
11
T
8
T
1
T
4
L
1
L
2
T
10
T
7
L - L
L - N
VOLTAGE (HIGH WYE)
Hz L-L L-N
60 480 277
460 266
440 254
416 240
380 219
50 416 240
400 231
380
219

7
LOW (PARALLEL) WYE CONNECTION
L
2
T
7
T
10
T
1
T
4
L
1
L
3
T
5
T
2
T
11
T
8
T
6
T
3
T
12
T
9
L - L
L - N
VOLTAGE (LOW WYE)
Hz L-L L-N
60 240 139
230 133
220 127
208 120
190 110
50 208 120
200 115
190
110
HIGH (SERIES) DELTA CONNECTION
L
1
L
2
L
3
T
1
T
4
T
7
T
10
T
2
T
5
T
8
T
11
T
12
T
9
T
6
T
3
L - L
L - N
`
VOLTAGE (HIGH DELTA)
Hz L-L L-N
60 277 139
240 120
50 240 120
220 110
200
100
LOW (PARALLEL) DELTA CONNECTION
L
1
L
2
L
3
T
1
T
4
T
7
T
10
T
2
T
5
T
8
T
11
T
3
T
6
T
12
T
9
L - L
L - L
VOLTAGE (LOW DELTA)
Hz L-L L-N
60 120 NA
110 NA
50 110 NA
100
NA

8
DOUBLE DELTA -- SINGLE PHASE CONNECTION
T
3
L
1
L
2
T
1
T
4
T
7
T
10
T
6
T
5
T
2
T
12
T
9
T
11
T
8
L - N L - N
L - L
VOLTAGE (DOUBLE DELTA)
Hz L-L L-N
60 240 120
220 110
50
220
110
Note: Single phase KW/KVA ratings are approximately
equal to 50% of the generator’s three phase ratings.
LOW ZIG ZAG -- SINGLE PHASE (PARALLEL) CONNECTION
L
2
T
3
T
6
L
1
T
7
T
10
T
1
T
4
T
2
T
5
T
8
T
11
T
12
T
9
L - N L - N
L - L
VOLTAGE (LOW ZIGZAG)
Hz
L-L
L-N
60 240 120
220 110
50 220 110
200 100
Note: Single phase KW/KVA ratings are approximately
equal to 50% of the generator’s three phase ratings.
HIGH ZIG ZAG -- SINGLE PHASE (SERIES) CONNECTION
L
1
L
2
T
1
T
4
T
7
T
10
T
2
T
5
T
8
T
11
T
12
T
9
T
6
T
3
L - N L - N
L - L
VOLTAGE (HIGH ZIGZAG)
Hz
L-L
L-N
60 480 240
460 220
50 415 208
380 190
Note: Single phase KW/KVA ratings are approximately
equal to 50% of the generator’s three phase ratings.

9
DEDICATED SINGLE PHASE CONNECTION
HIGH VOLTAGE - SERIES CONNECTED
VOLTAGE (DEDICATED)
Hz L-L L-N
60 240 120
220 110
50 220 110
200
100
OPERATION
PRE-START INSPECTION
Before starting the generator for the first time, the following
inspection checks are recommended:
1. A visual inspection should be made for any loose parts,
bad connections, or foreign materials.
2. Bar the set over by hand for at least 2 revolutions to be
sure that there is no interference and that the set turns
freely. If the set does not turn freely, check for
clearance in the generator and exciter air gap.
3. Check all wiring against the proper connection
diagrams, and ensure that all connections and
terminations are tight and properly insulated.
WARNING
MAGNAPLUS GENERATORS MAY HAVE VOLTAGE
PRESENT AT THE LEAD TERMINALS WHEN THE
SHAFT IS ROTATING. DO NOT PERMIT
OPERATION OF THE GENERATOR UNTIL ALL
LEADS HAVE BEEN CONNECTED AND INSULATED.
FAILURE TO DO THIS MAY RESULT IN PERSONAL
INJURY OR EQUIPMENT DAMAGE
4. Verify that all equipment is properly grounded (earthed).
5. Clear the surrounding area of any materials that could
be drawn into the generator.
6. Check all fasteners for tightness.
7. Check all access plates, covers, screens and guards. If
they have been removed for assembly or inspection,
reinstall and check for security.
8. Review all prime mover prestart-up instructions, and
ensure that all recommended steps and procedures
have been followed.
9. Remove any masking materials affixed during painting.
Inspect the generator, prime mover, and any accessory
equipment to ensure that nameplates, and all safety
warning / caution signs and decals provided with the
equipment are in place and clearly visible.
Note: It is strongly recommended that the authority
having jurisdiction over the installation site be
consulted to determine if any additional warning or
caution notices, or additional safety devices are
required by local codes / standards. Any such
required notices or devices should be installed
prior to initial startup.
START-UP
The following procedure should be followed when starting
the generator set for the first time.
1. The generator output must be disconnected from the
load. Be sure that the main circuit breaker or fused
disconnect is in the open position.
2. Open the input power to the automatic voltage
regulator. Remove the fuse or disconnect and insulate
one of the regulator input power leads. (See separate
regulator manual)
3. Verify that all prime mover start-up procedures have
been followed.
4. If the unit is provided with space heaters, ensure that
they are de-energized. In some installations, a set of
auxiliary contacts on the main circuit breaker or transfer
switch will automatically open the space heater circuit
when the generator is connected to the load.
5. Start the prime mover, and adjust it for proper speed.
See generator nameplate.
L
2
L
1
T
1
T
2
T
3
T
4
L - N L - N
L - L

10
6. The purpose of this initial test with the regulator out of
the circuit is to detect any wiring mistakes without
exposing the unit to undue risk. Check all line to line
and line to neutral voltages for balanced voltage. If
voltages are balanced, shut down the set and reconnect
the regulator. If voltages are unbalanced, shut down
the equipment and check for improper wiring. If the
problem persists, consult the factory.
With the regulator de-energized, the residual voltage
should be 10 - 25% of rated value. It is recommended
that this residual voltage and driver RPM be recorded
for use as a future troubleshooting benchmark.
WARNING
THE FOLLOWING TEST MUST BE CONDUCTED BY
QUALIFIED ELECTRICAL PERSONNEL. LETHAL
VOLTAGE MAY BE PRESENT AT BOTH THE
GENERATOR AND VOLTAGE REGULATOR
TERMINALS DURING THIS PROCEDURE. CAUTION
MUST BE EXERCISED NOT TO COME INTO
PERSONAL CONTACT WITH LIVE TERMINALS,
LINKS, OR STUDS. SERIOUS INJURY OR DEATH
COULD RESULT.
7. Start the set and adjust the terminal voltage to the
desired value by means of the regulator voltage
adjustment. If the regulator is equipped with a stability
adjustment, follow the instructions in the regulator
manual to adjust the stability. Again, check all line to
line and line to neutral voltages for balance. It is
recommended practice to record the no load excitation
(DC voltage to the exciter stator), generator terminal
voltage, and driver speed as a benchmark for future
troubleshooting.
8. Close the main circuit breaker to the load.
9. Monitor the generator output current to verify that it is at
or below nameplate value.
10. Check generator speed (frequency) under load. Adjust
as necessary. (Refer to prime mover or governor
manuals)
SHUTDOWN PROCEDURE
There are no specific instructions for shutting down the
generator; however, several good practices should be
observed to prolong equipment life.
1. It is advisable to disconnect all loads (open main circuit
breaker or disconnect) prior to shutdown. This is
especially important if loads can be damaged by low
voltage or low frequency conditions during generator
"coast down".
2. Isolate all conditions that could apply voltage to the
generator terminals while the generator is at rest.
Failure to comply could result in personnel injury or
equipment damage.
3. If the unit is equipped with space heaters, verify that the
heater circuit is energized.
MAINTENANCE
The following maintenance procedures should be followed to
ensure long equipment life and satisfactory performance.
Maintenance intervals will depend upon operating
conditions.
1. Routinely check intake and exhaust air screens to
ensure that they are clean and free of debris. Clogged
intake air screens will reduce cooling air flow and result
in higher operating temperatures. This will reduce
generator life and may result in generator damage.
2. All MagnaPLUS generators are equipped with double
shielded ball bearings lubricated for the life of the
bearing. Every 1,000 hours check the bearing(s) for
smooth, quiet operation. For continuous duty
generators, recommended practice is to replace the
bearing during major overhauls of the engine.
3. Periodically inspect the unit for any buildup of
contamination (dirt, oil, etc.) on the windings. If the
wound components have become coated with heavy
concentrations of oil and grime, the unit should be
disassembled and thoroughly cleaned. This operation
is not one that can be accomplished effectively on site,
but rather one that should be conducted by an
authorized service center equipped with the appropriate
apparatus and solvents necessary to properly clean and
dry the generator.
WARNING
THE FOLLOWING TEST MUST BE CONDUCTED BY
QUALIFIED ELECTRICAL PERSONNEL. LETHAL
VOLTAGE MAY BE PRESENT AT BOTH THE
GENERATOR AND VOLTAGE REGULATOR
TERMINALS DURING THIS PROCEDURE. CAUTION
MUST BE EXERCISED NOT TO COME INTO
PERSONAL CONTACT WITH LIVE TERMINALS,
LINKS, OR STUDS. SERIOUS INJURY OR DEATH
COULD RESULT.
4. Every 2,000 operating hours or in conjunction with
scheduled engine maintenance, check the DC no load
excitation voltage per item #7 in the startup procedure.
Compare this voltage with the value recorded during
initial startup. If this value of no load excitation voltage
is markedly higher than the bench mark reading, it is an
indication of problems in either the exciter, main field,

11
or the rotating rectifier assembly. Ensure that RPM is
the same as initial test.
5. Monitor and record insulation resistance with a 500 volt
mega-ohm meter. The minimum acceptable reading is
2 mega-ohms. If the reading drops below the
minimum, the generator should be cleaned and dried at
an authorized service shop. Consult Marathon Electric
for more information.
DRYING WINDINGS
Generators in service may inadvertently have their windings
exposed to splashing or sprayed water. Units that have
been in transit or storage for long periods of time may be
subjected to extreme temperature and moisture changes
causing excessive condensation. Regardless of the source
of moisture, wet windings should be thoroughly dried out
before operating the unit. If this precaution is not taken,
serious damage to the generator can result. The following
procedures may be utilized in drying the generator’s
windings. The method selected will be influenced by
winding wetness and situation limitations.
Space Heaters
An electric heater may have been supplied with the
generator. When energized from a power source other than
the generator, the heater will gradually dry the generator.
This process can be accelerated by enclosing the unit with a
covering and inserting additional heating units. A hole
should be left at the top of the covering to permit the escape
of moisture. Care should be taken not to overheat various
accessory equipment mounted with the generator.
Forced Air
Another method to dry the generator is to run the set with no
excitation (see startup procedure item #2). The natural flow
of ambient air through the generator will tend to dry the
windings. This method can be accelerated by adding a
source of heat at the air intake to the generator. Heat at
point of entry should not exceed 80 C (180° F).
TESTING
Visual Inspection
Remove covers and look for any obvious problems: burnt
windings, loose connections, broken wires, frayed insulation,
cracked brackets, missing hardware, etc. Check for foreign
objects which may have been drawn into the generator.
Verify that the generator’s air gaps (main rotor and exciter)
are free from obstructions. If possible, rotate the generator
manually to ensure free rotation. Never “bar over” the
engine generator set using the generator fan.
WARNING
THE FOLLOWING TEST MUST BE CONDUCTED BY
QUALIFIED ELECTRICAL PERSONNEL. LETHAL
VOLTAGE MAY BE PRESENT AT BOTH THE
GENERATOR AND VOLTAGE REGULATOR
TERMINALS DURING THIS PROCEDURE. CAUTION
MUST BE EXERCISED NOT TO COME INTO
PERSONAL CONTACT WITH LIVE TERMINALS,
LINKS, OR STUDS. SERIOUS INJURY OR DEATH
COULD RESULT.
CONSTANT EXCITATION TEST
(12V BATTERY TEST)
The generator “no load” voltage is dependent on exciter
input voltage and generator speed. With the generator
operating at rated speed and 12 volts dc applied to the
exciter field, the generators terminal voltage will be near
rated value.
1. Shutdown the generator set and connect a voltmeter on
the generator terminals.
2. Disconnect the regulator’s F+ (F1) and F- (F2) leads
and connect them to a 12V battery. Caution should be
taken to ensure that the battery is not exposed to any
potential arcing.
3. With no load on the generator (main breaker open) run
the generator at rated speed. Measure the generator’s
terminal voltage and compare this value with values
recorded during installation.
If voltage readings are normal, the main generator and
excitation are operating properly. Troubleshooting should
continue with the regulator. If readings are not normal the
problem is in the generator. Continue testing diodes, surge
suppressor, and windings.
Continuity / Resistance Test
The generator has four components which can be checked
using an ohm meter: exciter stator, exciter rotor, main
stator and main rotor. Each of these components are
comprised of various windings which form a complete
electrical path of relatively low resistance. Using an ohm
meter measure the loop resistance of each component.
Compare these measured values with the values listed in
the specification section of this manual. Note that very
small resistance values require precision equipment to make
accurate measurements; however, a standard ohm meter
will provide a good indication of winding continuity.

12
Insulation Test
Insulation resistance is a measure of the integrity of the
insulating materials that separate the electrical windings
from the generator’s steel core. This resistance can
degrade over time or be degraded by contaminants: dust,
dirt, oil, grease, and especially moisture. Most winding
failures are due to a breakdown in the insulation system. In
many cases, low insulation resistance is caused by moisture
collected when the generator is shutdown
Insulation resistance is measured with a megger (mega-
ohm meter). A megger measures insulation resistance by
placing 500 volts between the winding and the frame of the
generator. Caution must be taken to remove all electronic
devices (regulators, diodes, surge protectors, capacitors,
protective relays, etc.) from the winding circuit before
checking the insulation. Winding insulation can be checked
on the main stator, main rotor, exciter stator, and exciter
rotor. Minimum resistance is 2 mega-ohms. If the winding
resistance is low it must be dried (see maintenance section)
or repaired.
DIODE TESTING
If the generator is close coupled to an engine, it may be
necessary to "bar over" the engine in order to gain access to
a given area of the rectifier assembly. NEVER use the
generator's fan as a fulcrum to accomplish this. Use the
engine manufacturer's recommended practice to manually
turn over the engine. To prevent possible injury to
personnel, and damage to the equipment, ensure that the
engine cannot start during this procedure.
Remove the two main rotor leads and the three exciter rotor
leads from the rectifier assembly (figure 4). The rectifier
assembly is now electrically isolated from the generator.
The diodes remain mounted and the diode leads remain
connected to the terminal posts. Using an ohmmeter or a
battery light continuity tester, place one test probe on the
diode lead terminal post. In succession, touch the other test
probe to the lead screw hole in each heat sink. Reverse the
probes and repeat the procedure. You have now tested the
three diodes connected to this terminal post in both the
forward and reverse direction. Repeat the procedure using
the other diode terminal post.
FIGURE 3: DIODE POLARITY
When the positive test probe is connected to the diode's
anode and the negative test probe is connected to the
diode's cathode (forward biased), the diode will switch on
and conduct electricity (figure 3). This is observed by a low
resistance reading when using an ohm meter or the lighting
of the bulb when using a battery light continuity tester.
Reversing the test leads (reverse biased) will result in the
diode switching off and no electricity will be conducted. The
results of these tests should indicate one of three conditions:
1. Good diode: Will have a much greater resistance in
one direction than the other. Typical reverse biased
resistance will be 30,000 ohms or greater, while forward
biased resistance will be less than 10 ohms. The
battery-light tester will have the light "on" in one
direction and "off" in the other.
2. Shorted condition: Ohmmeter reading will be zero, or
very low in both directions. The continuity tester will
have the light "on" in both directions.
3. Open condition: Ohmmeter will have a maximum
(infinity) reading in both directions. Continuity tester
light will be off in both directions.
Diode failure after a 25 hour "run-in" period is generally
traceable to external causes such as a lightning strike,
reverse current, line voltage spikes, etc. All 6 diodes are
essentially in the same circuit. When a diode is stressed to
failure, there is no easy method to determine remaining life
in the other diodes. To avoid possible continued failures, it
is recommended that the entire rectifier assembly be
replaced rather than replacing individual diodes.
SERVICE
GENERAL
The service procedures given in this section are those which
can reasonably be conducted on-site with a minimum
number of special tools and equipment. All service
procedures should be conducted by qualified maintenance
personnel. Replacement parts may be ordered through an
authorized service center or directly from the factory.
FIELD FLASHING
Restoring Residual Magnetism
(not applicable on PMG equipped generators)
To restore residual magnetism to the generator, connect a
12 volt battery to the exciter field while the generator using
the following procedure:
1. Shutdown the generator set. Remove the exciter field
leads F+ and F- from the regulator.
Terminal End
Stud End
Anode
(+)
Cathode
(-) Anode
(+)
Cathode
(-)
Forward Reverse

13
CAUTION:
Failure to remove the exciter field leads from the
automatic voltage regulator during flashing
procedures may destroy the regulator.
2. Connect the F+ and F- leads to the battery’s
corresponding positive and negative terminals. This
should be done using an appropriate length of lead wire
to separate the battery from the point of connection
(batteries may explode when exposed to an electric arc).
After 3 to 5 seconds, remove the F- lead. An inductive
arc should result. If no arc is drawn, repeat the
procedure.
3. Reconnect the F+ and F- leads to the regulator. Restart
the generator and verify that terminal voltage is
developed. If terminal voltage does not develop, repeat
the field flashing procedure and / or consult the trouble
shooting section.
BEARING REMOVAL
Prior to performing this operation, it is suggested that the
alternator's shaft be rotated until two of the main rotor poles
are in a vertical position. Once the bearing bracket is
backed out, the rotor will drop on the main stator core.
Having the rotor in this position will limit the amount of rotor
drop to that of the air gap. Visually inspect the bearing bore
for damage or wear. If worn or damaged, replace prior to
reassemble.
Opposite Drive End Bearing Bracket Removal.
Prior to proceeding with bracket removal, disconnect exciter
field leads F+ and F- from the automatic voltage regulator
and ensure that they are free to move when the bearing
bracket is removed. Remove the bearing bracket retaining
bolts. Using a pair of screw drivers, wedge the bracket off
the frame. After approximately 1/8 inch, the bracket will
clear the locating register on the frame and will drop until
the rotor is resting on the main stator core. Continue to pull
the bracket free from the bearing. Visually inspect the
bearing bore and o-ring (if equipped) for damage or wear. If
worn or damaged, repair or replace prior to reassembly.
Drive End Bearing Bracket Removal,
Two Bearing Units.
Remove any drive arrangement from the generator shaft
extension. Remove the bearing lock ring retaining screws.
There is no o-ring in the drive end bearing bracket. The
shaft extension must be supported before proceeding
further. A hoist and sling, jack, or some other means of
support with a capacity of 2 tons should be used.
Remove the bearing bracket retaining cap screws. Using a
flat bladed screw driver or chisel, pry the bracket back from
the frame. After approximately 1/8 inch, the bracket will
clear the locating register on the frame. Lower the shaft
extension until the rotor is resting on the main stator core.
Continue to pull the bracket free from the bearing. Visually
inspect the bearing bore for damage or wear. If worn or
damaged, sleeve or replace prior to reassembly.
Reassembly note: Before the bearing bracket is seated
against the frame, a threaded rod may be used to help align
the inner bearing cap with the bearing bracket.
BEARING REPLACEMENT
Using a bearing puller, remove the existing bearing. It is
strongly recommended that the bearing be replaced any
time the it is removed from the shaft. ALWAYS install the
same type and size bearing that was supplied as original
equipment. Order by part number from the parts list, and
include the unit serial number and part number when
ordering. Heat the bearing to a maximum of 100oC (212oF)
in an oven. Apply a thin coat of clean lubricating oil to the
press-fit area of the rotor shaft. Using suitable heat
resistant gloves, install the bearing over the end of the shaft
until it seats against the shaft shoulder. The bearing should
slide on the shaft and be seated without excessive force.
Should the bearing bind on the shaft prior to being seated
against the shoulder, a piece of tubing slightly larger than
the press fit area can be used to drive the bearing to its final
position. Using light taps with a soft mallet, apply pressure
to the inner race only.
RECTIFIER ASSEMBLY REMOVAL
The rectifier assembly cannot be removed until the opposite
drive end bearing bracket and bearing have been removed
(see bearing removal procedure). Remove the three exciter
rotor leads from the heat sinks and the two main rotor leads
from the main rotor posts (see Figures 4). Remove the
screws securing the rectifier assembly and pull the
assembly free from the shaft.
DIODE REPLACEMENT
Prior to installing a replacement diode on the heat sink,
apply a thin film of conductive heat sink compound around
the base of the diode (do not coat the threads). When
installing a diode on the heat sink, care should be taken not
to over torque the retaining nut which could cause damage
to the device. Torque to 28 pound-inches. If not damaged,
the existing diode lead wire may be unsoldered from the
failed diode, and resoldered on the replacement.

14
430 FRAME 280 / 360 FRAME
A - Exciter Rotor Lead, B - Main Rotor Lead, C - Red (+) Suppressor Lead, D - Black (-) Suppressor Lead
FIGURE 4: ROTATING RECTIFIER ASSEMBLY
RETURNED GOODS
Contact Marathon Electric Manufacturing Corporation for
authorization before returning any product. We can not be
responsible for any items returned without authorization.
CAUTION
Single bearing generators must have their rotor
assembly properly secured to prevent damage during
transit to the factory, or to an authorized service center.
TROUBLESHOOTING
This section is intended to suggest a systematic approach to
locating and correcting generator malfunctions. The section
is arranged according to the symptoms of the problem. The
steps have been arranged in an attempt to do the easy
checks first and prevent further damage when
troubleshooting a disabled machine.
The first step of troubleshooting is to gather as much
information as is possible from operating personnel and
individuals present during the failure. Typical information
includes: how long the unit had been operating; what loads
were on line; weather conditions; protective equipment that
did or did not function. In addition, information as to the
operating condition of the generator's prime mover is vital.
Has the prime mover been maintaining constant speed? If
not, have there been extended periods of under speed
operation? Has the prime mover experienced an over-speed
condition? If yes, what was the maximum speed, and how
long did the unit operate at that elevated speed?
The generator speed should be maintained at rated
nameplate value during all operating tests. The frequency
of the generator depends upon rotational speed. Most
regulators used with MagnaPLUS generators have built in
under frequency protection such that if the speed is reduced
more than 5%, the voltage will drop off rather rapidly with
further reductions in speed.

15
WARNING
HIGH VOLTAGES MAY BE PRESENT AT THE GENERATOR’S TERMINALS WHEN THE UNIT IS RUNNING.
SOME ACCESSORY EQUIPMENT SUCH AS SPACE HEATERS MAY BE ENERGIZED FROM AN OUTSIDE
POWER SOURCE WHEN THE UNIT IS AT REST. TOOLS, EQUIPMENT, CLOTHING AND YOUR BODY MUST
BE KEPT CLEAR OF ROTATING PARTS AND ELECTRICAL CONNECTIONS. SPECIAL PRECAUTIONS MUST
BE TAKEN DURING TROUBLESHOOTING SINCE PROTECTIVE COVERS AND SAFETY DEVICES MAY BE
REMOVED OR DISABLED TO GAIN ACCESS AND PERFORM TESTS. BE CAREFUL. SERIOUS PERSONAL
INJURY OR DEATH CAN RESULT FROM THESE HAZARDS. CONSULT QUALIFIED PERSONNEL WITH ANY
QUESTIONS.
GENERATOR PRODUCES NO VOLTAGE
CAUSE CHECK AND REMEDY
Voltmeter off or defective Check voltage with a separate meter at the generator terminals.
Incorrect or defective connections Verify generator connections. See drawings supplied with the generator or lead
connection diagrams in this manual. Inspect all wiring for loose connections, open
circuits, grounds, and short circuits.
Loss of residual Flash the field. Refer to field flashing in the service section. If the generator is equipped
with a PMG, field flashing is not necessary -- check regulator fuse and input power from
the PMG.
Defective diodes, suppressor, or
windings Test the generator using the 12 volt battery test as specified in the testing section. If the results indicate generator problems, perform insulation, continuity, and diode tests as
specified in the testing section.
Regulator protection operating Adjust regulator. Consult regulator manual.
Regulator inoperative Adjust or replace regulator. Consult regulator manual.
GENERATOR PRODUCES LOW VOLTAGE, NO LOAD
CAUSE CHECK AND REMEDY
Underspeed operation Check speed using a tachometer or frequency meter.
Voltmeter off or defective Check voltage with a separate meter at the generator terminals.
Incorrect or defective connections Verify generator connections. See drawings supplied with the generator or lead
connection diagrams in this manual. Inspect all wiring for grounds, open circuits and
short circuits.
Loss of regulator power Check regulator fuse and input power. Input power is produced by the generator’s
residual voltage or from an optional PMG.
Regulator adjustment Adjust regulator settings. Consult regulator manual.
Regulator incorrectly connected Review the generator connection diagram or reference the regulator manual.
Defective diodes, suppressor, or
windings Test the generator using the 12 volt battery test as specified in the testing section. If the results indicate generator problems, perform insulation, continuity, and diode tests as
specified in the testing section.
Regulator inoperative Adjust or replace regulator. Consult regulator manual.

16
GENERATOR PRODUCES LOW VOLTAGE WHEN LOAD APPLIED
CAUSE CHECK AND REMEDY
Excessive load Reduce load. The load on each leg should be evenly balanced, and rated current should
not be exceeded on any leg.
Large motor starting or low load
power factor Motor starting currents are too large for the generator. When starting multiple motors, sequence the motors and start the largest motors first. Reduce lagging power factor load.
Driver speed droop or belt slip Check driver. If belt driven, check belt tension. Check under frequency setting on
regulator. Under frequency voltage roll-off may be activated.
Reactive droop If the generator is equipped for parallel operation, some droop is normal as reactive load
increases. When operating as a single unit, the parallel CT can be shorted to eliminate
this effect. Refer to Regulator manual.
Line drop If voltage is proper at generator terminals but low at load terminals, increase external wire
size.
Defective diodes, suppressor, or
windings Test the generator using the 12 volt battery test as specified in the testing section. If the results indicate generator problems, perform insulation, continuity, and diode tests as
specified in the testing section.
GENERATOR PRODUCES FLUCTUATING VOLTAGE
CAUSE CHECK AND REMEDY
Fluctuating engine speed Check engine and governor systems for malfunctions. Check load for fluctuation.
Regulator stability Adjust Regulator stability. Refer to Regulator manual.
Regulator external rheostat Replace defective or worn rheostat. Use shielded cable to minimize electrical noise.
Defective rectifier assembly Check assembly for loose connections. Test the diodes as specified in the test section.
Loose terminal or load connections Improve connections both mechanically and electrically.
Defective regulator Replace regulator.
GENERATOR PRODUCES HIGH VOLTAGE
CAUSE CHECK AND REMEDY
Faulty metering Check voltage with separate meter at generator terminals.
Incorrect connections Verify generator connections. Refer to drawings supplied with the generator or connection
diagrams in this manual.
Regulator adjustments Adjust regulator. Consult regulator manual.
Leading power factor Check the power factor of the load. If power factor is leading, change load configuration.
Excessive leading power factor (capacitors) can cause voltage to climb out of control.
Incorrect regulator connection Verify regulator voltage sensing is connected correctly. Consult regulator manual.
Defective regulator Replace regulator.

17
GENERATOR BUILDS VOLTAGE FROM STARTUP,
THEN GOES TO LOW (RESIDUAL) VOLTAGE
CAUSE CHECK AND REMEDY
Regulator protective circuit
operating Check indicators on regulator. Correct problems and adjust regulator as is required. Refer to regulator manual.
GENERATOR IS OVERHEATING
CAUSE CHECK AND REMEDY
Generator is overloaded Reduce load. Check with ammeter and compare with nameplate rating.
Clogged ventilating screens Clean air passages.
High room temperature or altitude Improve ventilation or reduce load.
Insufficient circulation of cooling air Generator location and enclosure design must provide adequate air flow and minimize
recirculation of hot air.
Unbalanced load The load on each leg should be as evenly balanced as possible and should not exceed
rated current on any one leg.
GENERATOR PRODUCES MECHANICAL NOISE
CAUSE CHECK AND REMEDY
Defective bearing Replace bearing.
Loose or misaligned coupling Tighten, realign, or replace coupling.
Belt slap or loose guards Check belt tensioning. Check belt guard fasteners.
EQUIPMENT RUNS NORMALLY ON UTILITY POWER,
BUT WILL NOT RUN ON GENERATOR SET
CAUSE CHECK AND REMEDY
Distorted voltage waveform Analyze load. Excessive SCR (thyristor) loading will cause distortion. Some equipment
may be sensitive to distorted waveforms. Refer to Marathon Electric..
Improper generator voltage or
frequency Check name plates of devices comprising the load. Compare required voltage and frequency with that of the generator. Adjust driver speed and/or generator voltage as
necessary to match generator output to load requirements.
CAUTION: Compare required voltage,frequency, and KVA with generator nameplate to ensure adequate generator
capacity. If in doubt, consult Marathon Electric for information regarding generator capacity.

18
SPECIFICATIONS
EXCITER FIELD NO LOAD TERMINAL VOLTAGE
MODEL GENERATOR RESISTANCE NO LOAD VOLTS WITH 12 VDC FIXED EXCITATION
STATOR* ROTOR 480 V / 60 HZ HIGH WYE / 60 HZ HIGH WYE / 50 HZ
281PSL1500 4.20 .400 11.0 485 400
281PSL1501 4.15 .400 11.0 490 404
281PSL1502 3.20 .439 9.0 528 435
282PSL1503 2.00 .470 10.4 500 415
282PSL1504 1.51 .512 11.3 490 400
282PSL1505 1.00 .575 10.1 515 415
283PSL1506 .681 .654 11.0 495 400
283PSL1507 .480 .758 12.0 480 390
284PSL1508 .346 .875 12.0 480 375
361PSL1600 .381 .750 11.8 485 400
361PSL1601 .264 .810 12.5 475 385
361PSL1602 .181 .990 14.1 460 370
362PSL1604 .138 1.05 12.2 480 380
362PSL1606 .0980 1.20 10.8 500 405
363PSL1607 .0692 1.37 12.2 475 380
431PSL6202 .0214 .8114 15.1 440 360
431PSL6204 .0477 .6373 13.6 455 385
431PSL6206 .0371 .6793 13.82 455 370
431PSL6208 .0133 .715 12.20 475 390
432PSL6210 .0214 .8114 15.1 440 360
432PSL6212 .0226 .8656 14.1 445 385
433PSL6216 .01215 1.0672 16.2 425 345
433PSL6220 .01214 .9743 15.6 430 350
* Stator resistance measured line to line in a high wye connection.
DEDICATED GENERATOR RESISTANCE EXCITER FIELD
SINGLE PHASE STATOR ROTOR NO LOAD VOLTS / 60 HZ
281PSL1511 1.420 .381 8.3
281PSL1512 1.106 .395 8.1
281PSL1513 .632 .430 8.7
282PSL1514 .436 .450 9.2
282PSL1515 .240 .520 9.7
283PSL1516 .160 .620 13.3
284PSL1517 .0918 .760 12.2
284PSL1518 .0610 .857 16.6
361PSL1611 .0695 .750 17.5
361PSL1612 .0434 .857 16.1
361PSL1613 .0369 .926 13.6
362PSL1615 .0191 1.20 17.0
363PSL1617 .0119 1.35 23.0
431PSL1811 .0248 .516 9.9
431PSL1813 .0129 .615 13.8
432PSL1814 .00931 .643 15.1
432PSL1815 .00723 .852 11.2
MODEL / FRAME SIZE EXCITER RESISTANCE
STATOR ROTOR
281, 282, 283, 284 23.0 .120
361, 362, 363 -- three phase 23.5 .120
361, 362, 363 -- dedicated single phase 23.0 .135
431, 432, 433 -- three phase 20.33 .076
431, 432 -- dedicated single phase 18.0 .105

19
PARTS LIST – SINGLE BEARING
Typical Generator Cross Section
Reference
Number Part Name Reference Number Part Name
1End Bracket (under end cover 360 & 430 frames) 11 Main Stator
2Bearing 12 Main Rotor
3O-ring (280 frame only) 13 Rotor Integral Keyway
4Rectifier Assembly 14 Fan
5Air Intake Screen (280 frame only) 15 Mounting Adapter (SAE)
6Exciter Rotor 16 Shaft
7Exciter Stator 17 Drive Hub
8Link Board (terminal block) 18 Drive Disk (SAE)
9Conduit Box 19 Exhaust Screen (drip cover not shown)
10 Generator Frame 20 Mounting Base
Note: Illustration above is a 280 frame MagnaPlus. Other Frame sizes are typical. Optional PMG not shown.
The generator model and serial numbers are required when ordering parts.

20 SB 504 9/03
PARTS LIST – DUAL BEARING
Typical Generator Cross Section
Reference
Number Part Name Reference Number Part Name
1End Bracket (under end cover 360 & 430 frames) 11 Main Stator
2Bearing (nondrive end) 12 Main Rotor
3O-ring (280 frame only) 13 Rotor Integral Keyway
4Rectifier Assembly 14 Fan
5Air Intake Screen (280 frame only) 15 End Bracket (drive end)
6Exciter Rotor 16 Bearing (drive end)
7Exciter Stator 17 Shaft
8Link Board (terminal block) 18 Key
9Conduit Box 19 Exhaust Screen (drip cover not shown)
10 Generator Frame 20 Mounting Base
Note: Illustration above is a 280 frame MagnaPlus. Other Frame sizes are typical. Optional PMG not shown.
The generator model and serial numbers are required when ordering parts.