GRAMEYER GRTA-200 Analogue AVR - Analogue Automatic Voltage Regulator for GRAMEYER Alternators | OEM Replacement

GRAMEYER GRTA-200. This is a Analogue Automatic Voltage Regulator from the GRAMEYER 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 on GRTA-4, field resistance window 6-50 ohm, accuracy 0.5 per cent, working range -20 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. 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.

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. 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.

Power stage and field drive. Power input 170-300 Vac, 50 / 60 Hz, output 76.5-126 Vdc. Field current rating 3-4 A class on GRTA-4, forcing capability Short-term forcing current available for load steps, field resistance window 6-50 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. 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.

Accuracy and response. 0.5 per cent steady-state accuracy, Adjustable, 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. The most common cause of intermittent excitation loss is a crimp terminal that was never properly made. Wiggle the harness with the machine running before replacing anything.

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. If two identical sets behave differently on the same load, swap the regulators before swapping anything else. The result tells you which side of the machine the fault is on.

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. 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.

Protection built into the regulator. Over-excitation. 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. 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.

What it fits. 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. 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.

Field notes on this model. The GRTA series is the compact end of the Grameyer range, used on smaller alternators and as a service replacement where a TH4 is physically too large. Field current is modest, so it is not a substitute for a TH4 on a large frame even though the terminals look similar.

Where this regulator does not belong. A GRTA-200 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.

Power and sensing are not the same circuit. The power input on a GRTA-200 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.

Load step test. Apply and remove at least half rated load and watch the recovery. A healthy regulator pulls the voltage back inside one per cent within a few cycles. If the recovery is slow but the final voltage is correct, the problem is response, not accuracy, and on an analogue board that usually means the field circuit is seeing more resistance than it was set up for.

Measure, do not assume. Nameplate field resistance figures are nominal at 20 degC and are frequently wrong after a rewind. Take the measurement with a four-wire meter if one is available, because a two-wire reading on a low resistance winding includes the lead resistance and can be off by twenty per cent.

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 3-4 A class on GRTA-4, higher on GRTA-200 figure for the GRTA-200. 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.

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 GRTA-200 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.

Mounting orientation and cooling. Most AVR boards are convection cooled and assume the heatsink fins are vertical. Mounting a GRTA-200 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.