LEROY SOMER R448 Analogue AVR - Analogue Automatic Voltage Regulator for LEROY SOMER Alternators | OEM Replacement

LEROY SOMER R448. This is a Analogue Automatic Voltage Regulator from the LEROY SOMER range, supplied as a direct replacement for the same model number in the field. The figures that matter are printed in the table above and repeated here so the page can be read on its own: field current rating 5 A rated, field resistance window Matched to the exciter field, accuracy +/-0.5 per cent, working range -40 to +70 degC. Everything else on this page exists to explain how those four numbers decide whether the unit fits a particular alternator.

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. Where a board has been superseded, the replacement usually fits the same footprint but not the same terminal numbering.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.

Sensing. 5 VA isolating-transformer 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 95-520 Vac, 50 / 60 Hz, output Up to 100 Vdc. Field current rating 5 A rated, forcing capability 8 A for 10 s, 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. Counterfeit regulators are a real problem on the widely copied boards. Buy from a source that will state where the board was made.

Accuracy and response. +/-0.5 per cent steady-state accuracy, Fast 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. 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.

Frequency behaviour. Rated for 50 / 60 Hz, jumper selected, with Under-frequency protection with an adjustable knee. 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. 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.

Paralleling. Reactive droop via P1. 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 regulator is a closed loop with a slow plant attached, and the plant is a large inductance. That is why the stability adjustment exists and why it is set after, not before, the voltage is correct.

Protection built into the regulator. Over-excitation fold-back, 8 A 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. A regulator is a closed loop with a slow plant attached, and the plant is a large inductance. That is why the stability adjustment exists and why it is set after, not before, the voltage is correct.

What it fits. Leroy Somer LSA 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. 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.

Field notes on this model. The R448 sits between the R438 and the R449 in the Leroy Somer analogue range. It shares the isolated 5 VA sensing and the AREP / PMG jumper of the R438, so the two are often interchangeable on the same alternator, but the field current limit and the frequency knee are set for a different frame size. Match the regulator to the alternator frame rather than to what happens to be on the shelf.

Where this regulator does not belong. A R448 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.

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.

When the trim runs out of range. If full rotation of the voltage trim still will not bring the machine to rated voltage, the sensing tap is wrong. Moving one tap changes the reference by a factor of about two, so the correct tap is the one that puts rated voltage near the middle of the trim range rather than at either end.

The forcing margin. A regulator that can only just hold rated voltage at rated load has no forcing margin left, and the machine will dip on every load step. The R448 is specified with a minimum field resistance of Matched to the exciter field; staying comfortably above that minimum, rather than exactly at it, is what leaves the forcing margin that makes a set feel solid rather than merely adequate.

Do not overlook the obvious. Before condemning a regulator, confirm the engine is actually at rated speed. A set running five per cent slow will show low voltage on any board, and replacing the AVR will not fix a governor.

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.

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 R448 the first thing to check is whether the 5 A rated 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.

Field cable sizing. The field circuit carries DC at low voltage and high current, so volt drop matters more than ampacity. Size the cable so that the drop at 5 A rated stays under about one volt, and torque the terminals to the manufacturer figure. A field circuit that loses half a volt in the cable is a regulator that cannot reach full forcing when the load steps.