LEROY SOMER R238 Analogue AVR - Analogue Automatic Voltage Regulator for LEROY SOMER Alternators | OEM Replacement

What the R238 is. A Analogue Automatic Voltage Regulator built by LEROY SOMER for brushless excitation systems. On paper the specification is a list; in service it is a set of limits. The current the board will hold indefinitely is 2-4 A class depending on variant; the field resistance window it will drive without exceeding its output voltage is Matched to the exciter field; the tightest voltage band it can maintain is +/-1.0 per cent; and the ambient range in which all of that remains true is -40 to +70 degC.

Excitation architecture. Self-excited, shunt or AREP 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. The cheapest diagnostic tool in excitation work is a DC ammeter in the field circuit. It turns a vague symptom into a number within seconds.

Sensing. Single-phase sensing with 0-110 / 0-220 / 0-380 V 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. 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.

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 depending on variant, 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. 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.

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. Load sharing between parallel sets is set up with an instrument, not by ear. A droop adjustment made by listening to the sets will be wrong at every load other than the one it was set at.

Frequency behaviour. Rated for 50 / 60 Hz, with Frequency-sensitive roll-off on most variants. 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. Any AVR that has been in service for more than ten years is running on borrowed time, and the failure always happens at the least convenient moment.

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. Never flash a field through the regulator. Disconnect the board, flash the field directly, then reconnect. Flashing through the AVR is how a working board becomes a failed board.

Protection built into the regulator. Over-excitation protection, fuse. 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. 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.

What it fits. Leroy Somer LSA 35-44 alternators. Lifecycle status: Legacy, replaced by R438 and R449 on new build. 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. The measurement that settles most arguments is the field current at full load, taken with a clamp meter, compared against the rating on the board.

Field notes on this model. The R2xx range is the older generation of Leroy Somer analogue regulators and it is still the most common AVR on LSA alternators built before the AREP era. The last two digits tell you the build: the sensing tap set, the field current class and whether a frequency roll-off is fitted all change across the range, so the model number has to be matched exactly when ordering a spare.

The retrofit case. Most R238 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.

Mounting orientation and cooling. Most AVR boards are convection cooled and assume the heatsink fins are vertical. Mounting a R238 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.

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.

Series resistance is not a fix. Adding a resistor in series with the field to bring it inside a regulator window costs forcing current exactly when the machine needs it most. If the field resistance is outside the window, change the board, not the circuit.

Voltage low under load only. This is the classic field current limit symptom. Measure the field current at full load and compare it with the 2-4 A class depending on variant figure for the R238. If the board is at its limit, either the board is too small for the alternator or the field resistance has risen. A rewind, a corroded terminal or a long cable run will all produce the same reading.

Warranty and what it covers. A one year warranty on a regulator is meaningful only if it covers a field-side failure and not just a dead-on-arrival check. Ask whether a board that fails after six months in service is replaced without argument, because that is the failure mode that actually happens.

Standby versus prime power. A standby set runs a few hundred hours a year and its regulator spends most of its life cold; a prime power set runs continuously and its regulator spends its life at operating temperature. The R238 is rated for -40 to +70 degC, which is fine in both cases provided the terminal box has any ventilation at all. What changes between the two duties is the capacitor ageing inside the board, not the semiconductor, so a prime power machine should have the regulator inspected on a schedule rather than on failure.

Documentation that survives the job. Photograph the terminal block before disconnecting anything, and write the sensing tap and the field resistance on the inside of the terminal box lid. Ten years later that note is the only thing that will let the next engineer choose a replacement without a full survey.