MARATHON PM100A Analogue AVR - Analogue Automatic Voltage Regulator for MARATHON Alternators | OEM Replacement

MARATHON PM100A. This is a Analogue Automatic Voltage Regulator from the MARATHON 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 3-4 A class, field resistance window 15 ohm, accuracy +/-1.0 per cent, working range -40 to +60 degC. Everything else on this page exists to explain how those four numbers decide whether the unit fits a particular alternator.

Excitation architecture. PMG or shunt fed depending on variant. 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. Single-phase sensing with voltage 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. If the machine will not hold voltage at all, disconnect the field and measure the sensing voltage first. That one reading separates a sensing fault from a field fault.

Power stage and field drive. Power input From the main stator or PMG, 50 / 60 Hz, output 63 Vdc class. Field current rating 3-4 A class, forcing capability Short-term forcing current available for load steps, field resistance window 15 ohm. 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. An alternator that has been rewound needs its field resistance re-measured, because a rewind commonly moves it by ten to twenty per cent in either direction.

Accuracy and response. +/-1.0 per cent steady-state accuracy, Standard analogue 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. An alternator that has been rewound needs its field resistance re-measured, because a rewind commonly moves it by ten to twenty per cent in either direction.

Frequency behaviour. Rated for 50 / 60 Hz, with Volts-per-hertz roll-off. 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. 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.

Paralleling. Reactive droop via 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. 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.

Protection built into the regulator. Over-excitation shutdown. 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. Conformal coating is the difference between a board that survives a coastal site and one that corrodes quietly in eighteen months.

What it fits. Marathon small and mid-size 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. 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. This is the analogue regulator used on Marathon's smaller alternator frames and as a service replacement on older sets. It is a straightforward board with three adjustments, and the most common field failure is a dried-out electrolytic rather than a semiconductor, which is why a fifteen-year-old unit that has lost regulation often comes back to life with a re-cap.

Where this board earns its place. The PM100A is normally the answer when a MARATHON 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.

Power and sensing are not the same circuit. The power input on a PM100A 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.

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.

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 PM100A is specified with a minimum field resistance of 15 ohm minimum; 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.

Voltage high and unresponsive. If the machine runs to maximum excitation regardless of the trim, disconnect the field and see whether the voltage collapses. If it does, the regulator has failed shorted; if it does not, the field is being fed from somewhere else, usually a shorted thyristor or a mis-wired paralleling circuit. Do not run the machine at full excitation for more than a few seconds while diagnosing, because the winding will not survive it.

New, old stock or refurbished. Discontinued regulators are available as new old stock, which is genuine and works, and as refurbished, which is a gamble on how the board was stored. Ask which one is being quoted. A board that has spent ten years in a damp warehouse has aged electrolytics even though it has never been installed.

The retrofit case. Most PM100A sales are retrofits, not new build. The machine is ten to twenty years old, the original board has failed, and the drawing is gone. In that situation the useful evidence is the exciter field resistance measured with a meter, the residual voltage measured at the AVR terminals with the machine turning at rated speed and no load, and the nameplate kVA. Those three readings will identify a compatible board even when the original part number cannot be found anywhere.

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 PM100A 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.