LEROY SOMER R729 AVR Accessory Module - Sensing or Paralleling Support | OEM Replacement

Short version. The R729 is a AVR accessory or sensing module. It supplies the exciter field, senses the alternator output and corrects the difference several times a second. Its rating is Not published for this model - ask us, its field resistance window Not published for this model, its accuracy Not applicable, and it is intended for machines whose field winding sits inside that window. Read the rest of this page as the detail behind those three figures.

Excitation architecture. Not applicable, accessory module. 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. 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.

Sensing. Three-phase voltage detection or paralleling sensing depending on the module. 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. 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.

Power stage and field drive. Power input Supplied from the AVR, output Control signal to the AVR. Field current rating Not published for this model - ask us, forcing capability Short-term forcing current available for load steps, field resistance window Not published for this model. 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. 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.

Accuracy and response. Not applicable steady-state accuracy, Not applicable, 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 cheapest diagnostic tool in excitation work is a DC ammeter in the field circuit. It turns a vague symptom into a number within seconds.

Frequency behaviour. Rated for 50 / 60 Hz, with Not published for this model - ask us for the nameplate data. 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. Conformal coating is the difference between a board that survives a coastal site and one that corrodes quietly in eighteen months.

Paralleling. Reactive droop set-up for parallel operation. 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 cheapest diagnostic tool in excitation work is a DC ammeter in the field circuit. It turns a vague symptom into a number within seconds.

Protection built into the regulator. Module-level protection only. 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. 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.

What it fits. R438 / R449 / R450 regulator family. 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. 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.

Field notes on this model. The R7xx boards are not regulators. R731 is the three-phase voltage detection module, R734 is the paralleling sensing module and R726 is the interface board for external control. They wire alongside an R438-class AVR. If a machine with an R731 fitted reads the wrong voltage, the fault is usually in the module sensing fuses rather than in the AVR itself.

Reading the application backwards. Most regulator problems arrive described as engine trouble. A set that hunts on load, or whose voltage sags for two seconds after a contactor closes, is usually reporting a field current limit rather than a fuel problem. On a R729 the first thing to check is whether the Not published for this model - ask us for the nameplate data figure is being reached on a cold morning, because field resistance rises with temperature in the opposite direction to copper and a board that is marginal at 20 degC can be comfortable at 50 degC or the other way round depending on the winding.

Mounting orientation and cooling. Most AVR boards are convection cooled and assume the heatsink fins are vertical. Mounting a R729 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.

Why resistance matters more than current rating. A regulator delivers a voltage, and the field decides how much current that voltage produces. A board rated at Not published for this model - ask us for the nameplate data into a low resistance field will deliver far less into a high resistance one, because the output voltage is the limit. This is why an AVR that is electrically perfect on paper can fail to excite a particular alternator, and why the resistance figure belongs in the quotation rather than in the manual.

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 R729 is specified at Not applicable; a drift of a couple of per cent over a fifty degree swing is normal for an analogue board and is not a fault.

Part numbers are not interchangeable across suffixes. On this family the suffix carries the field current class, the sensing arrangement or the frequency band, and two boards that look identical in a photograph can be a hundred per cent different in rating. Quote the full model number including the suffix, and where the suffix is unclear send a photograph of the original board with the terminal markings visible.

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 R729 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.

Fusing. Fit the fuse the manual specifies and nothing larger. A bigger fuse does not protect the regulator better, it protects it later. On a R729 the fuse is there to clear a field short before the thyristor is destroyed, and a 20 A fuse in place of an 8 A one turns a cheap board replacement into a rewind.