AVK cosimat n+ Analogue AVR - Analogue Automatic Voltage Regulator for AVK Alternators | OEM Replacement

What the cosimat n+ is. A Analogue Automatic Voltage Regulator built by AVK for brushless excitation systems. On paper the specification is a list; in service it is a set of limits. The current the board will hold indefinitely is 4-6 A class; the field resistance window it will drive without exceeding its output voltage is Matched to the exciter field; the tightest voltage band it can maintain is +/-1.0 per cent; and the ambient range in which all of that remains true is -40 to +60 degC.

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. Field resistance is a temperature-dependent number. A reading taken on a cold morning and a reading taken after an hour at load are both correct and both different.

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. 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. 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, 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. 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 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. 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.

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. 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 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.

Where this board earns its place. The cosimat n+ is normally the answer when a AVK 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.

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 n+ 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.

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.

Series resistance is not a fix. Adding a resistor in series with the field to bring it inside a regulator window costs forcing current exactly when the machine needs it most. If the field resistance is outside the window, change the board, not the circuit.

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 cosimat n+ is specified at +/-1.0 per cent; a drift of a couple of per cent over a fifty degree swing is normal for an analogue board and is not a fault.

Counterfeits. Where a board is widely copied, buy from a source that will state the country of manufacture and stand behind it. A counterfeit regulator usually works for a month and then fails on the first cold morning, and by then the machine has a reputation problem rather than a parts problem.

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 cosimat n+ 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.