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Inspection and Operation of Marathon MP Series Generator

Jan. 03, 2020

Inspection of Marathon MP Series AC Generator

Through the generator, AVR and auxiliary devices are solid and well insulated. If the generator was stored for long time it might be moistened, and should be inspected carefully before operation.

b. Measure the insulating resistance of windings against the ground by ohmmeter, if the value is less than 2 MΩ in cold condition, the generator should be dried. Caution: At measuring, the electronic elements and capacitor should be shorted, and the AVR, should be disconnected to avoid damaged.

c. Make sure all connecting joints are tight and contact good.

d. Be sure the frame is properly grounded, and ground wire should be same as the size of lead wires.

e. Make familiar with the rated data of nameplate before operation.

1. Voltage adjustment

In general, the generator was carefully inspected and tested in operation before it left the factory, and the normal voltage has been corrected, so it is not necessary to adjust. If necessary, the setting potentiometer of voltage may be adjusted again. If the external potentiometer is used for remote control, it also can be adjusted to set the voltage.

2. AVR Wiring

The wiring diagram of SE350 and APR125-5 are also connected to the 190V-240V, but there is a plug wire that should be put in the "50Hz" or "60Hz" position.

3. Operation

After inspection and adjustment have been made, and no trouble is found,, the generator can run normally and generate the electricity. In order to keep generator run well, please remember follow in notices:

a. The generator output must be disconnected from the load, then start the prime mover.

b. Check the speed and adjust to the rated RPM, the voltage of generator will get to normal value, then close the main circuit breaker to the load. The genset may be changed under load, and frequency may be lower than the normal frequency, check and adjust the speed as necessary.

c. Before shut down the genset, remove load and turn off the main circuit breaker. This is especially important, or the loss of residue magnetism can result.

d. If starting up the generator for first time or the generator stored for long time,  generator may be loss of the residue magnetism, and voltage can not build up, should charge to F +, and F - with DC 12V voltage, the "+" touch to F, and "-" touch to F - for 15 -20 seconds. Caution: the generator must be still during charging the magnetism, or AVR may be damaged.

e. Three phase AC generator should be operated at balanced load. Should pay more attention to the load and the balance of current to prevent the generator loaded on the single phase or serious unbalancing condition, or the AVR, and generator can be damaged.


SYMPTOM: Generator produces no voltage or abnormal voltage

Regulator fuse blown

Check all wiring. If there is no problem, replace bad fuse and start the unitagain.

Voltmeter damaged

Verify the panel meter with multi-meter or other meter that is known to be accurate.

Defective voltmeter

Check the meter in time, replace bad meter.

Loose the connecting plug of AVR

Check the connecting plug, make sure all joints tight and contact good.

Surge suppressor short circuit

Check the surge suppressor and eliminate the short circuit or replace bad one.

Defective diodes

Take down the diodes and test them by multimeter. Replace the bad diodes.

lost residue magnetism

Connect a 12V battery to the exciter field, the "+" of battery to the F + (red lead), and the "-" to the F - (black lead) for abort 12 to 20 seconds. Caution: The generator should be stand still during flashing. 

Incorrect connect

Check carefully, and correct the wiring refer to the wiring diagram.

Exciter field breaker

Clean, twist and weld the broken wire. Wrap this place with insulating material.

Loose joints or poor contact

Clean and tight the joints again.

Armature winding of generator short circuit

Find the failure place, clean, weld and wrap it.

Armature winding of exciter breaker or short circuit 

Locate the failure and replace the winding.

Incorrect speed

Verify speed with tachometer or frequency meter.

AVR protective shutdown circuits are operating

Correct the problem and adjust AVR. Refer to AVR manual.

AVR inoperative

Adjust or replace the AVR. Refer to AVR manual.

SYMPTOM: Voltage is fluctuating

Incorrect speed

Verify speed with tachometer or frequency meter.

Unstable speed

Verify governor stability.

AVR stability

Adjust AVR stability. Refer to AVR manual.

Defective/loose connecting 

Inspect all wiring for loose or dirty connections.

Defective diodes, surge suppressor, or generator windings

Test generator with constant excitation (12V battery test)

Remote voltage adjust rheosat (if used)

Check operation of rhostat. Refer to AVR manual.

Defective AVR

Replace the AVR. Refer to AVR manual.

Defective bearing of worn bearing carrier causing an uneven air gap

Replace worn bearing. Check bearing carrier for wear. Replace as necessary.

SYMPTOM: Generator overheated


Measure amperes and verify that the load does not exceed the nameplate rating of the generator.

Armature winding of generator short circuit

Replace the damaged winding.

Ventilation obstruction

Clean dust and dirt in the generator.

Armature winding of exciter short circuit

Replace the bad winding.

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