LEROY SOMER D700 Digital AVR - Digital Automatic Voltage Regulator for LEROY SOMER Alternators | OEM Replacement

What the D700 is. A Digital 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 5-8 A class; 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 +/-0.5 per cent; and the ambient range in which all of that remains true is -40 to +70 degC.

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. 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.

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. 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.

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. Field resistance is a temperature-dependent number. A reading taken on a cold morning and a reading taken after an hour at load are both correct and both different.

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. 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.

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. Where a board has been superseded, the replacement usually fits the same footprint but not the same terminal numbering. Read the drawing, do not read the old wiring.

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. 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, 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. 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.

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. 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.

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 board earns its place. The D700 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.

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.

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 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 D700 is specified at +/-0.5 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.

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.

What the customer is actually buying. When a regulator is quoted as a replacement, the buyer is not purchasing a circuit board. They are purchasing the certainty that the set will hold voltage on the next load step. On the D700 that certainty comes from the combination of 5-8 A class continuous field current and the Digital, configurable response, and it disappears the moment the field resistance drifts outside the window the board was set up for.

Power and sensing are not the same circuit. The power input on a D700 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.