R438 Voltage Regulator Caterpillar

The current limit for the generator excitation is adjusted with the "P5" potentiometer. The maximum setting corresponds to the excitation current that is required in order to maintain a three-phase short circuit current of approximately three times the rated current for 10 seconds. Turn "P5" clockwise in order to increase the current limit for excitation. Turn "P5" counterclockwise in order to decrease the current limit for excitation. The current limit should be adjusted in order to match the rating on the generator and the application for the generator. Current that is adjusted too high may be damaging to the generator or the components for the generator. Current that is adjusted too low may cause undesired tripping of the R438 under certain transient conditions. The generator must be shutdown and restarted in order to reset this type of trip. A static method of adjusting the current limit may be used in order to adjust the current limit to the actual operating power requirements of the generator. This method may be safer for the generator set.

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The high voltage that is produced by an operating generator set can cause severe injury or death. Before performing any maintenance or repairs, ensure that the generator will not start. Place the engine control switch in the "OFF" position. Attach "DO NOT OPERATE" tags to all starting controls. Disconnect the batteries or disable the starting system. Lock out all switchgear and automatic transfer switches that are associated with the generator.

Static Adjust of the Current Limit

Follow the procedure below in order to perform a static adjustment of the current limit:

Illustration 1	g01742013
Static setup for the "R438" Automatic Voltage Regulator (AVR) (1) "P5" current limit adjustment (2) Step-down transformer with 120 primary and a of 48 to 96 VAC secondary (3) Double-Pole Single-Throw (DPST) switch for the R438 supply voltage (4) DPST switch for the R438 voltage sensing circuit (5) "P2" voltage adjustment

1. Shut down the generator set. Lock the generator with a safe lockout procedure.

2. Disconnect the power supply wires for the R438 from terminals "X1","X2" and "Z1","Z2".

3. Disconnect the wires for voltage sensing from the "0", "110V", "220V" and "380V" terminals.

4. Connect the secondary of the transformer (2) to terminals "X1" and "X2". Refer to Illustration 1.

5. Connect a DPST switch (3) to the primary of the transformer (2) .

6. Connect a DPST switch (3) to the voltage sensing terminals "0" and "110". Refer to Illustration 1.

7. Wire both of the DPST switches to a 120 VAC power source. The 120 VAC source must be switched off and back on as required. Ensure that the 120 VAC source is switched off before it is wired to the circuit.

8. Install a 10 AMP DC ammeter in series with the exciter field.

9. Ensure that "P5" is fully counterclockwise.

10. Ensure that both DPST switches are closed.

11. Apply power to the circuit by switching on the 120 VAC source.

12. If the ammeter does not display a current, adjust "P2" clockwise in order to obtain a steady current reading.

13. Adjust "P5" clockwise until the required maximum current is obtained.

    Note: Ensure that the current is no more than 8 Amperes.

14. Switch the 120 VAC power source off for at least 10 seconds. Reapply the 120 VAC power to the circuit.

15. Verify that the current limit setting returns to the correct value.

16. Switch off the 120 VAC power source.

Testing the R438 Internal Protection

Note: Ensure that the generator is safely locked out.

Perform this test with the circuit in Illustration 1. Follow this procedure in order to test the over current protection of the R438 in a simulation of a short circuit on a generator:

1. Switch on the 120 VAC power source.

2. Open switch (4). The excitation current that is viewed on the ammeter should rise to the value of the current limit that was set in Step 13 of "Current Limit Static Adjust". The current should remain stable for 10 seconds. The current should then drop to approximately 1 amp.

3. Reset the R438 by opening DPST switch (3) for 10 seconds and then close switch (3) again.

4. The R438 exciter current should return to the current limit level.

5. Remove power from the R438.

6. Remove the test wiring from the R438 and rewire the R438 to the generator.

Note: It is necessary to readjust the "P2" voltage adjustment to the correct voltage before returning the generator to service.

Excitation Shutdown Switch

Under certain conditions it may be desired to shut off the output for the exciter from the R438 AVR. For example, when drying the windings of a generator, it would be necessary to shut off the output for the exciter. Follow the wiring diagram for the type of exciter that is used for the generator in order to configure a switch for the exciter.

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NOTICE
The excitation circuit for the Automatic Voltage Regulator (AVR) must not be left open when the machine is running. Damage to the AVR will occur if it is operated with the exciter circuit left open.

Illustration 2	g01742033
Shunt excitation (6) Single Pole Single Throw (SPST) switch with a contact rating of 16 A 250 VAC

Illustration 3	g01742035
AREP and PMG excitation (7) DPST switch with a contact rating of 16 A 250 VAC

Note: The excitation shutdown switch may be also used in order to reset the internal over current protection.