LEROY SOMER D350 Digital AVR - Digital Automatic Voltage Regulator for LEROY SOMER Alternators | OEM Replacement

Position in the range. The D350 sits in the LEROY SOMER regulator family carrying the designation Digital Automatic Voltage Regulator. Its place is defined by the field current it can hold, 5-8 A class, against the field resistance it can drive, Matched to the exciter field, at +/-0.5 per cent accuracy over -40 to +70 degC. Where a machine falls outside either of the first two figures, a different member of the same family is the correct answer rather than a workaround.

Excitation architecture. AREP or PMG. 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. 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.

Sensing. Three-phase or single-phase RMS sensing with a wide voltage window. 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. 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.

Power stage and field drive. Power input From the main stator, 50 / 60 Hz, output Up to 110 Vdc. Field current rating 5-8 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. 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.

Accuracy and response. +/-0.5 per cent steady-state accuracy, Digital, configurable, 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 regulator rated for a minimum field resistance of a few ohms will not drive a high resistance field to full current, however good its specification looks on paper.

Frequency behaviour. Rated for 50 / 60 Hz, with Programmable volts-per-hertz curve. 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. 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.

Paralleling. Reactive droop, configurable. 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. 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.

Protection built into the regulator. Over-excitation, loss of sensing, over-temperature. 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 rated for a minimum field resistance of a few ohms will not drive a high resistance field to full current, however good its specification looks on paper.

What it fits. Leroy Somer LSA 40-50 alternators. Lifecycle status: In production. 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 cheapest diagnostic tool in excitation work is a DC ammeter in the field circuit. It turns a vague symptom into a number within seconds.

Field notes on this model. The D-series is the digital generation of Leroy Somer regulators. Sensing is three-phase RMS, so the regulator holds voltage on an unbalanced load where an analogue single-phase unit would drift, and the volts-per-hertz curve is set in software instead of with a jumper.

Where this regulator does not belong. A D350 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.

Earthing and shielding. The regulator reference is the machine frame. Bond the AVR mounting plate to the frame with a dedicated conductor rather than relying on the mounting screws, and earth the sensing cable shield at one end only. When a set shows a voltage that wanders slowly with no load change, the cause is usually a circulating current in the shield rather than a failing board.

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.

Rewind caution. After a field rewind the resistance often changes by ten to twenty per cent, which can push a marginal regulator out of range. Whenever a machine comes back from a rewind, re-check the regulator against the new field resistance before returning it to service.

Intermittent loss of excitation. Intermittent faults are almost always mechanical: a crimp that was never properly made, a terminal screw that was tightened onto insulation, or a board that is flexing. Wiggle-test the harness with the machine running at no load and watch the field current meter.

Delivery and packing. Regulators are static-sensitive assemblies and should arrive in a sealed bag with foam, not loose in a box with a heatsink. Inspect on arrival and photograph the packaging before opening if there is any sign of crush damage.

Where this board earns its place. The D350 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.