Generator brands · Alternators
Leroy-Somer Alternator Repairs for SHUNT, AREP and PMG Machines
Our technicians repair Leroy-Somer alternators where the generator stands in Gauteng, from regulator and rotating diode faults to worn bearings and damp or burnt windings. Most Leroy-Somer generator alternators use one of three excitation systems, SHUNT, AREP or PMG, and that decides how a voltage fault is traced and which regulator can replace the old one. Windings are tested on site, and a rewind is arranged only when drying out will not do.
Established 2006
20 years of experience
24/7 operations
We Arrive Within 1–2 Hours
(usually within 1 hour)
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One-year warranty
Qualified and certified technicians
We Service & Repair All Generator Brands
All generator makes and models
1 kVA–20,000 kVA capability
SHUNT, AREP and PMG compared
| System | What powers the regulator | On a big load step or short circuit |
|---|---|---|
| SHUNT | What powers the regulatorThe main stator output; voltage builds up from residual magnetism | On a big load step or short circuitExcitation falls as the voltage dips, so recovery is slowest |
| AREP | What powers the regulatorTwo auxiliary stator windings: one follows the output voltage, the other rises with load current | On a big load step or short circuitThe load-driven winding boosts excitation, so recovery is quicker |
| PMG | What powers the regulatorA permanent magnet generator on the shaft, independent of the output | On a big load step or short circuitExcitation holds steady whatever the output voltage does, and distorting loads do not upset it |
Which regulator belongs on which machine
Leroy-Somer regulators are the AVRs on these machines (our AVR guide explains what an AVR does), and they are not interchangeable. The R250 suits SHUNT machines and has no inputs for auxiliary windings, so it cannot run an AREP machine. The R438 and R448 are used with AREP or PMG excitation, and digital regulators such as the D350 add more settings and protection.
Analogue regulators are adjusted for voltage, stability and the under-frequency point below which voltage is reduced to protect the exciter. Some also lower the voltage briefly during a large load step to help the engine recover. A digital regulator’s configuration is set to match the machine and the engine, then checked by testing.
Before a replacement regulator goes in
- The alternator nameplate and the old regulator’s model are read and recorded
- The old settings are noted, where the unit still works
- Auxiliary winding or PMG leads are identified and tested
- Rotating diodes and the varistor are tested, so a shorted part cannot overload the new unit
- Sensing is matched to the voltage and to how the windings are connected
A wrongly wired regulator can fail as soon as the set starts.
Reconnectable windings and sensing
Many Leroy-Somer machines have stator windings with six or twelve output leads that can be linked for different voltages, or for single-phase output. If the links change and the regulator’s sensing does not, it holds the wrong voltage. With single-phase sensing, an overloaded phase that the regulator does not watch can sit low while the voltage it does watch looks correct, so single-phase loads should be spread evenly, as our single and three-phase guide explains.
Diagnosis
How we trace a Leroy-Somer voltage fault
- 01
Confirm the symptom
No voltage, low, high or unstable, measured at the alternator terminals at rated speed, not at a socket in the building.
- 02
Check the excitation supply
The regulator’s supply from the stator, auxiliary windings or PMG, and its output to the exciter field.
- 03
Test the rotating parts
Rotating diodes, varistor, exciter armature and main rotor winding.
- 04
Test the windings
Insulation and winding resistance on stator, rotor and exciter, before choosing drying out, repair or a rewind.
- 05
Step the load on
Load is added in steps while we watch how far the voltage dips and how fast it recovers.
Wear and tear
What we find on older Leroy-Somer machines
Diode bridge and varistor: Running but no power
One open diode leaves the regulator working flat out and the voltage falling away as load rises. A shorted varistor can stop voltage building at all.
Bearings
A rumble, rising temperature or grease escaping from the rear bearing housing comes before real damage. Changed in time, the bearing is the only part that needs replacing.
Damp windings: Generator rewinding
Machines in humid plant rooms or near wash bays absorb moisture, and low insulation readings are the first sign. Many dry out well; badly tracked or burnt windings need rewinding.
Heat in the terminal box: Voltage problems
Loose output lugs and links discolour terminals and can overheat a regulator mounted beside them.
Independent service
We Service & Repair All Generator Brands
Onsite Generator Repairs is an independent generator specialist. We repair, service and maintain Leroy-Somer equipment as part of our work on every make of generator.
Manufacturer warranty claims go through the manufacturer’s own network. If your generator is still under warranty, check the warranty terms before arranging work by anyone else.
Brand names and trademarks belong to their respective owners. Onsite Generator Repairs is an independent service and repair company.
Service areas
Where we work in Gauteng
FAQ
Frequently asked questions
Can I fit an R250 regulator to an AREP alternator?
No. The R250 is designed for SHUNT excitation. An AREP machine needs a regulator made for AREP, such as an R438 or R448, depending on the machine.
Why does the voltage dip badly when a motor starts?
Motor starting draws a large current. SHUNT machines lose excitation as the voltage dips, so they recover more slowly than AREP or PMG machines. We check the load against the rating and the regulator settings.
My alternator gave no voltage after months standing idle. What happened?
SHUNT and AREP machines rely on residual magnetism to start building voltage, and a set that stood unused for months, for example during a long break in load shedding, can lose it. A technician restores it by flashing the field, after checking the diodes and regulator, because faults there look the same. See AVR repairs.
Can Leroy-Somer alternators be rewound?
Yes. Rewinding is considered when insulation tests fail or windings have burnt; we test on site and arrange it. See generator rewinding.
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