GRAMEYER GRT7-TH2 50A Analogue AVR - Analogue Automatic Voltage Regulator for GRAMEYER Alternators | OEM Replacement

GRAMEYER GRT7-TH2 50A. 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 50 A continuous, 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. 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.

Sensing. Single-phase sensing, 160-300 Vac or 320-600 Vac. 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 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.

Power stage and field drive. Power input 170-300 Vac single-phase or two-phase, 50 / 60 Hz, output 76.5-126 Vdc. Field current rating 50 A continuous, forcing capability 70 A for 10 s, 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 from 8 to 500 ms, 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 set that runs at reduced speed will show low voltage on any regulator. Check engine speed against the nameplate before touching the excitation system.

Frequency behaviour. Rated for 50 / 60 Hz, with Adjustable 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. 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.

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. 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.

Protection built into the regulator. Under-frequency, 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. 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.

What it fits. Mid and large frame 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. 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.

Field notes on this model. The GRT7-TH2 is the higher-current companion to the TH4. The suffix after TH2 is the rated field current, so a TH2-35A is built to hold 35 A continuously, which means a different power stage rather than a relabelled board. These are used on large frame alternators where a TH4 would run out of field current on a cold load step.

Where this regulator does not belong. A GRT7-TH2 50A 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.

Documentation that survives the job. Photograph the terminal block before disconnecting anything, and write the sensing tap and the field resistance on the inside of the terminal box lid. Ten years later that note is the only thing that will let the next engineer choose a replacement without a full survey.

Paralleling setup. With two sets on the bus, adjust droop until reactive load divides in proportion to machine rating, then check it again at half load. A droop setting that looks right at full load and wrong at half load is usually a CT ratio error rather than a regulator fault.

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.

Intermittent loss of excitation. Intermittent faults are almost always mechanical: a crimp that was never properly made, a terminal screw that was tightened onto insulation, or a board that is flexing. Wiggle-test the harness with the machine running at no load and watch the field current meter.

Documentation. Insist on the wiring drawing for the exact suffix being supplied. Generic family drawings are the reason installations end up with the sensing on the wrong tap, and the resulting fault is indistinguishable from a faulty board.

The retrofit case. Most GRT7-TH2 50A 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.

Documentation that survives the job. Photograph the terminal block before disconnecting anything, and write the sensing tap and the field resistance on the inside of the terminal box lid. Ten years later that note is the only thing that will let the next engineer choose a replacement without a full survey.