A generating set that cranks, starts and holds rated speed but produces no output voltage has a fault in the excitation circuit. The engine side is working. Something between the exciter and the main rotor field is not.
Work through the causes in order of likelihood and cost. Replacing the AVR first is the most common wasted purchase in this trade, because a failed rotating rectifier diode presents almost identically.
1. Check for residual magnetism
A self-excited alternator needs a small amount of residual magnetism in the rotor to begin building voltage. Without it the AVR has no output to sense and no excitation to amplify, so nothing happens.
Residual magnetism is lost when a set stands unused for a long period, after the rotor has been removed, or following a heavy reverse-current event.
Flashing the field restores it. With the set stopped, apply a 12 V battery briefly across the exciter field with the correct polarity, through a diode to prevent reverse current. Restart the set. If output builds and holds, the fault was residual magnetism alone.
2. Confirm engine speed
Voltage is proportional to speed. A set running at 1350 rpm instead of 1500 rpm will produce low output and may trip under-frequency roll-off on regulators that have it, such as the SX440.
Check frequency at the terminals: 50 Hz output requires 1500 rpm on a four-pole machine, 60 Hz requires 1800 rpm. If speed is low, the fault is a governor or fuelling problem, not an electrical one.
3. Test the rotating rectifier
The rotating rectifier converts the exciter's AC output into DC for the main rotor field. Six diodes sit on two heatsink plates, with a surge suppressor across them.
- A short-circuit diode loads the exciter and collapses output
- An open-circuit diode produces low, unstable output that hunts on a slow cycle
- A failed surge suppressor often shows visible damage
Isolate the set, disconnect the diode leads and test each diode on a multimeter's diode range. A healthy diode conducts one way only. Replace the complete set rather than a single diode — the others share the same thermal history and a partial replacement usually fails again within months.
4. Check the AVR sensing leads
Before condemning the regulator, confirm it can actually see the output. A broken or disconnected sensing lead means the AVR reads zero volts, drives excitation to maximum, and either produces nothing or trips over-voltage.
Check continuity on the sensing wiring and confirm the fuse in the sensing circuit where one is fitted.
5. Replace the AVR
Only once the above are eliminated. When ordering, the alternator frame and winding number both matter, and so does the excitation type:
- Self-excited machines take an SX-series regulator such as the
SX460orSX440 - PMG-equipped machines take an MX-series regulator such as the
MX341orMX321
These are not interchangeable. An MX341 fitted to a self-excited machine has no PMG supply to drive it and will not function at all.
What to send us
If you would rather have the part identified than guess it, send a photograph of the alternator nameplate showing the frame and winding number, the set make and kVA rating, and a note of what the set does — nothing at all, or low and hunting. That is enough to confirm the correct regulator and diode set before anything ships.
Questions
Why does my generator produce no voltage after standing unused?
Almost always loss of residual magnetism. A self-excited alternator needs a trace of magnetism in the rotor to begin building voltage, and it decays when a set stands idle for months. Flashing the field with a 12 V battery restores it.
Should I replace the AVR or the diodes first?
Test the diodes first. They are cheaper, and a failed rotating rectifier diode produces symptoms nearly identical to a failed AVR. Replacing the regulator while a shorted diode remains in place will not fix the fault.