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

Short version. The GRT7-TH4 PE 7A is a Analogue Automatic Voltage Regulator. It supplies the exciter field, senses the alternator output and corrects the difference several times a second. Its rating is 5 A rated on TH4 and TH4 E, its field resistance window 6-50 ohm at 20 degC, 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. 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. 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. 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.

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. Counterfeit regulators are a real problem on the widely copied boards. Buy from a source that will state where the board was made.

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. 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.

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. 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.

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. 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.

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. 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.

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. If the machine will not hold voltage at all, disconnect the field and measure the sensing voltage first. That one reading separates a sensing fault from a field fault.

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 regulator does not belong. A GRT7-TH4 PE 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.

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.

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.

The forcing margin. A regulator that can only just hold rated voltage at rated load has no forcing margin left, and the machine will dip on every load step. The GRT7-TH4 PE 7A is specified with a minimum field resistance of 6-50 ohm at 20 degC; staying comfortably above that minimum, rather than exactly at it, is what leaves the forcing margin that makes a set feel solid rather than merely adequate.

Spares strategy. Keep one regulator of each family in stock rather than several of one. A failed AVR takes a set off line in seconds and the replacement lead time is measured in days, which is why the cost of the spare is trivial compared with the cost of the outage.

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.

What the customer is actually buying. When a regulator is quoted as a replacement, the buyer is not purchasing a circuit board. They are purchasing the certainty that the set will hold voltage on the next load step. On the GRT7-TH4 PE 7A that certainty comes from the combination of 5 A rated on TH4 and TH4 E, 7 A rated on TH4 P and TH4 PE continuous field current and the Dynamically adjustable from 8 to 500 ms response, and it disappears the moment the field resistance drifts outside the window the board was set up for.

Power and sensing are not the same circuit. The power input on a GRT7-TH4 PE 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.