GRAMEYER GRT8-TH1 100A Analogue AVR - Analogue Automatic Voltage Regulator for GRAMEYER Alternators | OEM Replacement

Short version. The GRT8-TH1 100A is a Analogue Automatic Voltage Regulator, high current. It supplies the exciter field, senses the alternator output and corrects the difference several times a second. Its rating is 100 A continuous, its field resistance window 6-50 ohm, its accuracy 0.5 per cent, 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. 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. 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. 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. Take a photograph of the terminal block before removing anything. Wiring drawings for older machines are frequently wrong, and the photograph is the only reliable record.

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 100 A continuous, forcing capability 140 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. 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.

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. Where a board has been superseded, the replacement usually fits the same footprint but not the same terminal numbering. Read the drawing, do not read the old wiring.

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. A regulator rated for a minimum field resistance of a few ohms will not drive a high resistance field to full current, however good its specification looks on paper.

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. Where a board has been superseded, the replacement usually fits the same footprint but not the same terminal numbering. Read the drawing, do not read the old wiring.

What it fits. Large frame alternators, 1000 kVA and above. 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. Take a photograph of the terminal block before removing anything. Wiring drawings for older machines are frequently wrong, and the photograph is the only reliable record.

Field notes on this model. The GRT8-TH1 is Grameyer's high-current range, built for alternators whose exciter field takes tens of amps. The suffix is the continuous field current, so the board has to match the alternator frame. Putting a 10 A board on a machine that needs 130 A will not simply under-perform, it will fail. These boards carry a substantial heatsink and are physically much larger than a TH4.

Matching the board to the duty, not to the label. Two GRAMEYER alternators of the same kVA can need different regulators if one is specified for motor starting and the other for a resistive load. The GRT8-TH1 100A is specified at 0.5 per cent static regulation accuracy, which is the number that matters when the load is a mix of drives and lighting. For a purely resistive load even a looser board is invisible to the customer, and paying for accuracy that nobody measures is the most common way money is wasted on a rebuild.

Field cable sizing. The field circuit carries DC at low voltage and high current, so volt drop matters more than ampacity. Size the cable so that the drop at 100 A continuous stays under about one volt, and torque the terminals to the manufacturer figure. A field circuit that loses half a volt in the cable is a regulator that cannot reach full forcing when the load steps.

Commissioning sequence. Start with the machine at rated speed, no load, and the field disconnected. Confirm the sensing voltage at the AVR terminals is inside the window the board expects, then connect the field with the voltage trim at minimum. Bring the trim up slowly and watch the field current, not the terminal voltage, because the terminal voltage will look correct long before the field current has stabilised on a cold winding.

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 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 GRT8-TH1 100A is specified at 0.5 per cent static; 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.

Where this board earns its place. The GRT8-TH1 100A is normally the answer when a GRAMEYER 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.

The terminal box is a hostile place. Engine vibration, radiant heat from the exhaust and conductive dust all arrive at the same panel. Mount the GRT8-TH1 100A on the resilient side of the box if there is a choice, keep it off the hottest wall, and make sure the lid actually closes over the wiring rather than pinching it. A regulator that is mechanically stressed will eventually crack a solder joint, and the resulting fault looks exactly like a failed semiconductor.