MagnaPLUSGeneratorTroubleShootingGuide(SP0023).docx12/27/2018TRS3
Step 2 : Constant Excitation Test
SAFETY NOTE – Constant Excitation test MUST be done with the gen-set running.
Conduct constant excitation test, only if Exciter Stator passed all tests in Step 1.
Disconnect exciter stator leads F+ & F- from the voltage regulator – see Figure 2 – and
observing polarity, connect to a 12V battery. The gen-set starting battery can be used as the
source of constant excitation.
Run the get-set, no-load with all breakers switched off. Measure output voltage at output
terminals: A – B, B – C, and C – A shown in Figure 2. A well-charged battery should give nearly
full output voltage – refer to specifications table by model – possibly a bit more. Voltage should
be balanced between all three phases.
Possible Constant Excitation Test Results
Results Possible Corrective Action
Output voltage is balanced and within
expected range. ( -10% – +15% ) Voltage Regulator has failed and must be replaced.
Output voltage is well below expected
range.
One or more diodes have failed.
Replace Rectifier Assembly is preferred action.
ALL diodes may be replaced individually.
Output Voltage ≤100V Multiple component failures. Replace generator.
Output Voltage is Unbalanced Main stator has failed. Replace generator.
Step 3 : Check Main Stator Windings
The main stator windings are very low resistance which makes them difficult to field test. Set
meter to the lowest possible Ohms scale and measure resistance at output terminals: A – B, B –
C, and C – A shown in Figure 2. You must have good continuity and should have a balanced
resistance reading between phases. Refer to Specifications Table for resistance by model.
If you have NO continuity or resistance readings vary significantly – anything higher than about
1 ohm for most models, you have a failed main stator winding.
Set meter to the highest possible Ohms scale. Check for continuity between output terminals
and the ground terminal. Resistance should be greater than 5 megOhms or infinite. If you have
continuity or < 5 megOhms resistance between the main stator leads and the ground terminal
the main stator is faulted to ground.
Failed main stator windings cannot be repaired. Replace generator.
If testing STEPS 1 - 3 have not found any faults or defects, move to the opposite drive end of the
generator to check the diodes on the rectifier assembly. You will need to remove the back cover to gain
access to the rectifier assembly. It is not necessary to remove the exciter rotor.
