LEROY SOMER R120 Analogue AVR - Analogue Automatic Voltage Regulator for LEROY SOMER Alternators | OEM Replacement

LEROY SOMER R120. This is a Analogue Automatic Voltage Regulator from the LEROY SOMER range, supplied as a direct replacement for the same model number in the field. The figures that matter are printed in the table above and repeated here so the page can be read on its own: field current rating 2-4 A class, field resistance window Matched to the exciter field, accuracy +/-1.0 per cent, working range -40 to +70 degC. Everything else on this page exists to explain how those four numbers decide whether the unit fits a particular alternator.

Excitation architecture. Self-excited, shunt fed. That single sentence decides most of the regulator's behaviour in a fault: whether the field survives a close-up short circuit, whether the machine builds up from residual voltage on its own, and how much the terminal voltage dips when a large motor starts on the same bus. Counterfeit regulators are a real problem on the widely copied boards. Buy from a source that will state where the board was made.

Sensing. Single-phase sensing with voltage taps. The sensing circuit is the regulator's only view of the outside world, so its wiring matters as much as its specification. A voltage error introduced by a loose terminal or a long run of untwisted cable is indistinguishable, from the regulator's point of view, from a genuine voltage error on the alternator. Conformal coating is the difference between a board that survives a coastal site and one that corrodes quietly in eighteen months.

Power stage and field drive. Power input From the main stator, 50 / 60 Hz, output Up to 90 Vdc. Field current rating 2-4 A class, forcing capability Short-term forcing current available for load steps, field resistance window Matched to the exciter field. These are not independent numbers: the output voltage ceiling and the field resistance together set the maximum current the board can actually deliver, and the continuous rating is only reachable at the low end of the resistance window. Vibration kills more regulators than heat does. If a board has failed twice in the same machine, look at the mounting before looking at the supplier.

Accuracy and response. +/-1.0 per cent steady-state accuracy, Standard analogue response, thermal drift Low thermal drift. In practice the accuracy figure is the least interesting of the three. What an operator notices is recovery after a load step, which is governed by response and by available forcing current, and what a maintenance engineer notices after two years is drift, which is governed by the reference circuit and the ambient temperature inside the terminal box. Terminal boxes are hot, dirty and vibrating. Anything mounted inside one should be judged on how it behaves in year eight, not on how it reads on the bench.

Frequency behaviour. Rated for 50 / 60 Hz, with Not published for this model - ask us for the nameplate data. The frequency-sensitive function exists to protect the prime mover, not the alternator: as engine speed falls the regulator reduces excitation so the alternator stops demanding torque the engine cannot supply. Without it, an overloaded engine is dragged down until it stalls, and the alternator is blamed for a fault that belongs to the fuel or governor system. The trim potentiometer is not a calibration device. If rated voltage is not reachable near the middle of its range, the sensing tap is wrong.

Paralleling. Reactive droop via CT. On a single set this circuit does nothing. On two or more sets it is the only thing that stops the machines from exchanging reactive power in a slow oscillation, and it is the first place to look when a parallel installation shows a circulating current that nobody can explain. A set that runs at reduced speed will show low voltage on any regulator. Check engine speed against the nameplate before touching the excitation system.

Protection built into the regulator. Over-excitation protection. These functions protect the alternator and the regulator, not the installation. Downstream protection, loss of mains and reverse power all belong to the controller, and a site that relies on the AVR for them is a site that will eventually trip in a way nobody can explain. Take a photograph of the terminal block before removing anything. Wiring drawings for older machines are frequently wrong, and the photograph is the only reliable record.

What it fits. Small and mid-size Leroy Somer alternators. Lifecycle status: Legacy or current depending on variant. Both halves of that sentence matter at order time. A board that fits perfectly but has been discontinued is a different commercial decision from one that fits and is in current production, and the price difference rarely reflects the difference in risk. A set that runs at reduced speed will show low voltage on any regulator. Check engine speed against the nameplate before touching the excitation system.

Field notes on this model. This is one of the compact analogue regulators used on smaller Leroy Somer alternators and on older machines that predate the R438. Field current is modest, so it suits alternators up to a few hundred kVA. Because it is shunt fed, build-up depends on residual voltage, and a machine that has been standing for months may need to be flashed before it will excite.

The retrofit case. Most R120 sales are retrofits, not new build. The machine is ten to twenty years old, the original board has failed, and the drawing is gone. In that situation the useful evidence is the exciter field resistance measured with a meter, the residual voltage measured at the AVR terminals with the machine turning at rated speed and no load, and the nameplate kVA. Those three readings will identify a compatible board even when the original part number cannot be found anywhere.

Field cable sizing. The field circuit carries DC at low voltage and high current, so volt drop matters more than ampacity. Size the cable so that the drop at 2-4 A class stays under about one volt, and torque the terminals to the manufacturer figure. A field circuit that loses half a volt in the cable is a regulator that cannot reach full forcing when the load steps.

Paralleling setup. With two sets on the bus, adjust droop until reactive load divides in proportion to machine rating, then check it again at half load. A droop setting that looks right at full load and wrong at half load is usually a CT ratio error rather than a regulator fault.

Measure, do not assume. Nameplate field resistance figures are nominal at 20 degC and are frequently wrong after a rewind. Take the measurement with a four-wire meter if one is available, because a two-wire reading on a low resistance winding includes the lead resistance and can be off by twenty per cent.

Voltage drifts with ambient temperature. If the voltage is correct cold and low hot, the thermal drift of the regulator and the temperature coefficient of the field are adding up in the same direction. The R120 is specified at +/-1.0 per cent; a drift of a couple of per cent over a fifty degree swing is normal for an analogue board and is not a fault.

Documentation. Insist on the wiring drawing for the exact suffix being supplied. Generic family drawings are the reason installations end up with the sensing on the wrong tap, and the resulting fault is indistinguishable from a faulty board.

Where this regulator does not belong. A R120 is a voltage regulator, not a protection relay and not a governor interface. If the site needs loss-of-mains detection, reverse power protection or synchronising, those functions live in the controller, not here. Expecting the AVR to carry them is how installations end up with a set that holds voltage beautifully and still trips the site on a mains failure.

Mounting orientation and cooling. Most AVR boards are convection cooled and assume the heatsink fins are vertical. Mounting a R120 flat on its back inside a sealed box shortens its life even though the ambient temperature reads acceptable, because the local air around the thyristor is far hotter than the box average.