LEROY SOMER RS128-0 Analogue AVR - Analogue Automatic Voltage Regulator for LEROY SOMER Alternators | OEM Replacement

Short version. The RS128-0 is a Analogue Automatic Voltage Regulator. It supplies the exciter field, senses the alternator output and corrects the difference several times a second. Its rating is 2-4 A class, its field resistance window Matched to the exciter field, its accuracy +/-1.0 per cent, and it is intended for machines whose field winding sits inside that window. Read the rest of this page as the detail behind those three figures.

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. 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.

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. A voltage that is correct at no load and low at full load is almost never an accuracy problem. It is a current limit, a resistance problem or a cable problem.

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. If two identical sets behave differently on the same load, swap the regulators before swapping anything else. The result tells you which side of the machine the fault is on.

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. If the machine will not hold voltage at all, disconnect the field and measure the sensing voltage first. That one reading separates a sensing fault from a field fault.

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. 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.

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. On a machine that has been standing for months, check residual voltage before condemning anything: a winding that has lost its remanence will not build up on any regulator.

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. A regulator is a closed loop with a slow plant attached, and the plant is a large inductance. That is why the stability adjustment exists and why it is set after, not before, the voltage is correct.

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 regulator is a closed loop with a slow plant attached, and the plant is a large inductance. That is why the stability adjustment exists and why it is set after, not before, the voltage is correct.

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 RS128-0 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.

Power and sensing are not the same circuit. The power input on a RS128-0 can come from a different winding from the sensing input, and on PMG machines it always does. Wire them as two separate circuits, fused separately, and label both. The single most expensive mistake on a PMG set is landing the sensing leads on the PMG supply, which gives the regulator a voltage reference that rises with speed and produces a set whose voltage tracks engine rpm.

Under-frequency check. Run the set at reduced speed and confirm the volts-per-hertz knee actually operates. This test is skipped on most commissioning sheets and it is the one that saves an engine on a real overload, because the knee is what stops the alternator from loading a lugging engine into a stall.

The forcing margin. A regulator that can only just hold rated voltage at rated load has no forcing margin left, and the machine will dip on every load step. The RS128-0 is specified with a minimum field resistance of Matched to the exciter field; staying comfortably above that minimum, rather than exactly at it, is what leaves the forcing margin that makes a set feel solid rather than merely adequate.

Voltage high and unresponsive. If the machine runs to maximum excitation regardless of the trim, disconnect the field and see whether the voltage collapses. If it does, the regulator has failed shorted; if it does not, the field is being fed from somewhere else, usually a shorted thyristor or a mis-wired paralleling circuit. Do not run the machine at full excitation for more than a few seconds while diagnosing, because the winding will not survive it.

Before the order is placed. Confirm the alternator frame, the measured field resistance, the sensing voltage at rated speed, the excitation system (shunt, AREP, auxiliary winding or PMG) and the mounting footprint. Five answers, no guessing. Every wrong regulator order we see traces back to one of those five being assumed rather than measured.

Reading the application backwards. Most regulator problems arrive described as engine trouble. A set that hunts on load, or whose voltage sags for two seconds after a contactor closes, is usually reporting a field current limit rather than a fuel problem. On a RS128-0 the first thing to check is whether the 2-4 A class figure is being reached on a cold morning, because field resistance rises with temperature in the opposite direction to copper and a board that is marginal at 20 degC can be comfortable at 50 degC or the other way round depending on the winding.

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.