AVK COSIMAT C2.1 Analogue AVR - Analogue Automatic Voltage Regulator for AVK Alternators | OEM Replacement

AVK COSIMAT C2.1. This is a Analogue Automatic Voltage Regulator from the AVK 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 4-6 A class, field resistance window Matched to the exciter field, 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. Self-excited, shunt fed. 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 spare is being held, hold the drawing with it. A board without its wiring diagram costs a shift of production time to install.

Sensing. Single-phase or three-phase sensing depending on variant. 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. 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.

Power stage and field drive. Power input From the main stator, 50 / 60 Hz, output 63 Vdc class. Field current rating 4-6 A class, forcing capability Short-term forcing current available for load steps, 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. 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.

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. 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.

Frequency behaviour. Rated for 50 / 60 Hz, with Under-frequency protection. 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 trim potentiometer is not a calibration device. If rated voltage is not reachable near the middle of its range, the sensing tap is wrong.

Paralleling. Reactive droop 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. 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.

Protection built into the regulator. Over-excitation, loss of sensing. 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. 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.

What it fits. AVK and compatible brushless 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. 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 Cosimat is AVK's analogue regulator, and the N, N-plus and C2.x suffixes mark successive revisions rather than different electrical ratings. They are largely interchangeable on the same alternator, but the terminal layout changed between revisions, so the wiring has to be checked against the drawing supplied with the 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 COSIMAT C2.1 that certainty comes from the combination of 4-6 A class continuous field current and the Standard analogue response response, and it disappears the moment the field resistance drifts outside the window the board was set up for.

Mounting orientation and cooling. Most AVR boards are convection cooled and assume the heatsink fins are vertical. Mounting a COSIMAT C2.1 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.

What to record. Write down the sensing tap used, the field resistance cold, the field current at no load and at full load, and the voltage trim position. Those four numbers turn the next service visit from an investigation into a comparison.

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 4-6 A class 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 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.

Before the order is placed. Confirm the alternator frame, the measured field resistance, the sensing voltage at rated speed, the excitation system (shunt, AREP, auxiliary winding or PMG) and the mounting footprint. Five answers, no guessing. Every wrong regulator order we see traces back to one of those five being assumed rather than measured.

Where this regulator does not belong. A COSIMAT C2.1 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.

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 COSIMAT C2.1 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.