Leroy-Somer — Checking the windings and rotating diodes using separate excitation
Testing & diagnosis · Alternator windings and rotating diodes
From a Leroy-Somer service manual — check it applies to your exact model before use.
Before you start
- Make sure the alternator is disconnected from any external load and that the terminal box connections are fully tightened.
Procedure
- Stop the unit, disconnect and isolate the AVR wires.
- Assembly A: connect a 12 V battery in series with a ~50 ohm - 300 W rheostat and a diode on exciter field wires (5+) and (6-).
- Assembly B: connect a Variac variable power supply and a diode bridge on both exciter field wires (5+) and (6-).
- Run the unit at its rated speed.
- Gradually increase the exciter field current (rheostat or variac) and measure output voltages on L1 - L2 - L3.
- Check the excitation voltage and current at no load.
Tools required
- 12 V battery Amazon
- Rheostat ~50 ohm - 300 W Amazon
- 1A diode Amazon
- Variac variable power supply Amazon
- Diode bridge Amazon
Tool links are Amazon searches (affiliate links); manufacturer special tools may only be available from a dealer.
Specs & values
- Output voltage balanced within 1% for the rated excitation level
Source: Leroy-Somer service manual. Steps and values are as documented; always cross-check against the official manual and any later revisions for your exact serial number before servicing.