LEROY SOMER R438 Analogue AVR - Analogue Automatic Voltage Regulator for LEROY SOMER Alternators | OEM Replacement

What the R438 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 5 A rated; the field resistance window it will drive without exceeding its output voltage is Matched to the AREP or PMG; 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, jumper ST9 selects the source. 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. 5 VA through an isolating transformer: the 0-110 V terminals cover 95-140 V, 0-220 V cover 170-260 V, 0-380 V cover 340-520 V. 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. 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.

Power stage and field drive. Power input 95-520 Vac, 50 / 60 Hz, taken from the main stator or from the AREP winding, output Up to 100 Vdc into the exciter field. Field current rating 5 A rated, forcing capability 8 A for 10 s, field resistance window Matched to the AREP or PMG. 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. 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.

Accuracy and response. +/-0.5 per cent steady-state accuracy, Normal or fast, selected by jumper ST2, with 500 ms in fast mode, 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. 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.

Frequency behaviour. Rated for 50 / 60 Hz, jumper ST3, with Load Acceptance Module fitted as standard: 13 or 25 per cent voltage dip selected by ST10, knee at 48 / 58 Hz or 65 Hz by ST11. 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. 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.

Paralleling. Reactive droop via potentiometer P1 with an optional 1 A 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. The most common cause of intermittent excitation loss is a crimp terminal that was never properly made. Wiggle the harness with the machine running before replacing anything.

Protection built into the regulator. Electronic over-excitation protection: after 10 s of overload the field current folds back to about 1 A and the regulator must be powered down to reset. F1 fuse 8 A slow-blow at X1 / X2. 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. 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.

What it fits. Leroy Somer LSA alternators with AREP or PMG excitation. 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 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 R438 is the workhorse Leroy Somer regulator, and the reason it survives in harsh sites is the Load Acceptance Module. When engine speed drops on a big load step, the LAM pulls terminal voltage down by 13 or 25 per cent, which cuts the kW the alternator is asking for and lets the engine recover instead of stalling. Jumper ST9 tells the regulator whether it is fed by an AREP winding or a PMG, and getting that jumper wrong is the most common installation error on this board.

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 R438 that certainty comes from the combination of 5 A rated continuous field current and the Normal or fast, selected by jumper ST2, with 500 ms in fast mode response, and it disappears the moment the field resistance drifts outside the window the board was set up for.

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.

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.

Temperature drift of the field. Copper rises about 0.4 per cent in resistance per degC, so a field that measures 12 ohm cold will measure around 15 ohm at a 70 degC winding temperature. A regulator set up on a cold machine and never re-checked will therefore be running at a different operating point every time the season changes, and the symptom is a voltage that is correct in winter and low in summer.

Symptom first, board second. A set that will not build up voltage has one of four causes: no residual magnetism, a broken sensing circuit, a failed regulator, or a field circuit open. Test them in that order, because the first two are free to check and the third is not. Flashing the field with a battery through a resistor will separate a residual problem from a regulator problem in under a minute.

Counterfeits. Where a board is widely copied, buy from a source that will state the country of manufacture and stand behind it. A counterfeit regulator usually works for a month and then fails on the first cold morning, and by then the machine has a reputation problem rather than a parts problem.

Where this board earns its place. The R438 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.

The terminal box is a hostile place. Engine vibration, radiant heat from the exhaust and conductive dust all arrive at the same panel. Mount the R438 on the resilient side of the box if there is a choice, keep it off the hottest wall, and make sure the lid actually closes over the wiring rather than pinching it. A regulator that is mechanically stressed will eventually crack a solder joint, and the resulting fault looks exactly like a failed semiconductor.