GRAMEYER GRT7-TH4 R6 7A Analogue AVR - Analogue Automatic Voltage Regulator for GRAMEYER Alternators | OEM Replacement

Position in the range. The GRT7-TH4 R6 7A sits in the GRAMEYER regulator family carrying the designation Analogue Automatic Voltage Regulator. Its place is defined by the field current it can hold, 5 A rated on TH4 and TH4 E, against the field resistance it can drive, 6-50 ohm at 20 degC, at 0.5 per cent accuracy over -20 to +60 degC. Where a machine falls outside either of the first two figures, a different member of the same family is the correct answer rather than a workaround.

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. 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.

Sensing. 160-300 Vac or 320-600 Vac single-phase sensing. 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 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.

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 5 A rated on TH4 and TH4 E, forcing capability 7 A for 10 s on the 5 A builds, 10 A for 10 s on the 7 A builds, field resistance window 6-50 ohm at 20 degC. 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 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.

Accuracy and response. 0.5 per cent steady-state accuracy, Dynamically 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. 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.

Frequency behaviour. Rated for 50 / 60 Hz, with Adjustable under-frequency protection by trimmer. 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. 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.

Paralleling. Reactive droop, 5 A CT for parallel operation with a PAR/5 controller. 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. Load sharing between parallel sets is set up with an instrument, not by ear. A droop adjustment made by listening to the sets will be wrong at every load other than the one it was set at.

Protection built into the regulator. Under-frequency, over-excitation, EMI filtered input. 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. 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.

What it fits. Leroy Somer, Stamford, Marelli and Mecc Alte alternators with 6-50 ohm fields. 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 GRT7-TH4 is the most widely fitted Grameyer board and it comes in four build variants: TH4 at 5 A rated, TH4 P at 7 A rated, TH4 E at 5 A rated with a +/-9 Vdc analogue input, and TH4 PE at 7 A rated with the analogue input. The analogue input is what lets the regulator follow a power factor controller or a remote voltage trimmer, so the E and PE versions are the ones specified on paralleled and marine installations. All four carry CSA approval.

Where this board earns its place. The GRT7-TH4 R6 7A 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.

Fusing. Fit the fuse the manual specifies and nothing larger. A bigger fuse does not protect the regulator better, it protects it later. On a GRT7-TH4 R6 7A the fuse is there to clear a field short before the thyristor is destroyed, and a 20 A fuse in place of an 8 A one turns a cheap board replacement into a rewind.

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.

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 5 A rated on TH4 and TH4 E, 7 A rated on TH4 P and TH4 PE 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 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 GRT7-TH4 R6 7A 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.

New, old stock or refurbished. Discontinued regulators are available as new old stock, which is genuine and works, and as refurbished, which is a gamble on how the board was stored. Ask which one is being quoted. A board that has spent ten years in a damp warehouse has aged electrolytics even though it has never been installed.

Where this regulator does not belong. A GRT7-TH4 R6 7A 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 GRT7-TH4 R6 7A 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.