LEROY SOMER R250 Analogue AVR - Analogue Automatic Voltage Regulator for LEROY SOMER Alternators | OEM Replacement

What the R250 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 trim potentiometer is not a calibration device. If rated voltage is not reachable near the middle of its range, the sensing tap is wrong.

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. 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.

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. Where a spare is being held, hold the drawing with it. A board without its wiring diagram costs a shift of production time to install.

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. 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.

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. 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, 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. An alternator that has been rewound needs its field resistance re-measured, because a rewind commonly moves it by ten to twenty per cent in either direction.

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 trim potentiometer is not a calibration device. If rated voltage is not reachable near the middle of its range, the sensing tap is wrong.

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.

Matching the board to the duty, not to the label. Two LEROY SOMER alternators of the same kVA can need different regulators if one is specified for motor starting and the other for a resistive load. The R250 is specified at +/-1.0 per cent regulation accuracy, which is the number that matters when the load is a mix of drives and lighting. For a purely resistive load even a looser board is invisible to the customer, and paying for accuracy that nobody measures is the most common way money is wasted on a rebuild.

Wiring practice that prevents most call-backs. Run the sensing leads as a twisted pair, separate from the field cables, and land them on the terminals the drawing names rather than on the ones that happen to be nearest. On a R250 the sensing input draws very little current, which is exactly why it is vulnerable: a loose terminal adds resistance that the regulator reads as a voltage error and corrects by pushing the field up, so the machine runs hot and high for no visible reason.

Bench test before installation. Where a spare board is available, test it on the bench with a variable AC supply into the power input and a resistor bank as the field load. A board that cannot hold a steady output into a resistor will not hold it into an exciter either, and finding that out on the bench costs nothing.

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 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 R250. 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.

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 board earns its place. The R250 is normally the answer when a LEROY SOMER machine has lost regulation and the original board is either unavailable or priced like a small car. What decides the fit is not the brand name on the cover but three numbers: the field resistance window, the continuous field current, and the sensing voltage the machine actually delivers at the AVR terminals. Get those three right and the regulator behaves; get any one of them wrong and the symptom usually looks like a mechanical fault rather than an electrical one.

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.