BASLER APM300 Paralleling Module - Load Sharing Accessory for Parallel Generator Sets | OEM Replacement

Short version. The APM300 is a Paralleling Module (AVR accessory). 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. A voltage that is correct at no load and low at full load is almost never an accuracy problem. It is a current limit, a resistance problem or a cable problem.

Sensing. Takes the AVR sensing and CT signals. 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 measurement that settles most arguments is the field current at full load, taken with a clamp meter, compared against the rating on the board.

Power stage and field drive. Power input Supplied from the AVR control supply, 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. Static accuracy is measured on a resistive load at constant frequency. Real loads and real governors are neither, so treat the figure as a comparison between boards rather than a promise.

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. 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.

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. 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.

Paralleling. Active and reactive load sharing between paralleled generator sets. 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. Counterfeit regulators are a real problem on the widely copied boards. Buy from a source that will state where the board was made.

Protection built into the regulator. Reverse power, loss of excitation and dead-bus protection at system level. 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. Basler DECS and AVC regulators in multi-set installations. 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. Static accuracy is measured on a resistive load at constant frequency. Real loads and real governors are neither, so treat the figure as a comparison between boards rather than a promise.

Field notes on this model. The APM is not an AVR. It is the paralleling module that sits between several AVRs and makes them share kW and kvar instead of fighting each other. If a site has two or more sets running in parallel and the kvar is swinging between machines, the fault is usually in the load-sharing wiring or the APM, not in the regulators.

Parallel operation is a different animal. A single set on an island load only has to hold voltage. Two sets in parallel also have to agree about reactive load, and that agreement is made or broken by the droop circuit, not by the voltage accuracy. If a site is going to parallel, the regulator choice has to be made with the droop method in mind from the beginning, because retrofitting load sharing onto a board that was never specified for it costs more than buying the right one.

Wiring practice that prevents most call-backs. Run the sensing leads as a twisted pair, separate from the field cables, and land them on the terminals the drawing names rather than on the ones that happen to be nearest. On a APM300 the sensing input draws very little current, which is exactly why it is vulnerable: a loose terminal adds resistance that the regulator reads as a voltage error and corrects by pushing the field up, so the machine runs hot and high for no visible reason.

Stability before accuracy. Set the stability adjustment until the voltage stops hunting on a load step, then leave it alone. Chasing the last tenth of a per cent of static accuracy by tightening stability is how a set ends up oscillating at 2 Hz under a resistive load and looking like a governor fault. The APM300 is specified at Not applicable; a real machine on a real load will not do better than that no matter how the trimmer is set.

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.

Voltage low under load only. This is the classic field current limit symptom. Measure the field current at full load and compare it with the Not published for this model - ask us for the nameplate data figure for the APM300. If the board is at its limit, either the board is too small for the alternator or the field resistance has risen. A rewind, a corroded terminal or a long cable run will all produce the same reading.

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.

Parallel operation is a different animal. A single set on an island load only has to hold voltage. Two sets in parallel also have to agree about reactive load, and that agreement is made or broken by the droop circuit, not by the voltage accuracy. If a site is going to parallel, the regulator choice has to be made with the droop method in mind from the beginning, because retrofitting load sharing onto a board that was never specified for it costs more than buying the right one.

Mounting orientation and cooling. Most AVR boards are convection cooled and assume the heatsink fins are vertical. Mounting a APM300 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.