Introduction

SR4 Generator (Partial View)
(1) VR3 regulator. (2) Terminal box. (3) Droop rheostat location. (4) Power transformer PT2.

This manual covers the VR3 voltage regulator as it is used on 4/6 lead, self-excited SR4 generators. This manual must be used in conjunction with service manual Electric Set Generators SENR7958 and Operation and Maintenance Manual SEBU6150. Follow all safety procedures and warnings in the above generator manuals.

VR3 regulator (1) is located in generator terminal box (2).

Operation

VR3 Regulator
(5) Terminal strip (for attachments). (6) Rheostat (voltage level). (7) Rheostat (voltage gain). (8) Terminal strip (for sensing and power). (9) Fuse cover. (10) Fuses.

The purpose of the VR3 regulator is to keep generator output voltage at a rated value. There are two controls on the VR3 that are standard.

* Voltage level rheostat (6) provides a no load voltage adjustment of approximately +10 to -25%.

* Voltage gain rheostat (7) provides an adjustment to compensate for engine RPM droop with load. Rheostat (7) can be adjusted to keep the same voltage at no load and full load, even when there is a small change in frequency (rpm).

Rheostats (6) and (7) are multiple turn. They do not have a fixed stop at the end of travel, but have an override ratchet.

SR4 Generator Schematic (Self-excited 4/6 lead)
(1) VR3 regulator. (4) Power transformer PT2. (11) Lines - 3 phase output voltage. (12) Lines - sensing voltage and AC power. (13) Main stator. (14) Main rotor. (15) Exciter stator. (16) Exciter rotor. (17) Lines - VDC excitation voltage.

VR3 regulator (1) uses all three phases to sense generator output voltage. Lines (12) function as sensing lines by tapping into generator output and connecting to terminals 20, 22 and 24 (AC input) on terminal strip (8). Like other SR4 generator regulators, the VR3 regulates voltage based on volts per hertz. The VR3 offers an approximate 8 volts per hertz characteristic.

Wires 20 and 24, of lines (12), are used to provide AC power to step-down power transformer PT2 (4). PT2 will step-down the AC voltage to 240 VAC for excitation to the VR3 regulator. The regulator rectifies the AC voltage and controls the excitation current to exciter stator (15).

Two fuses, in secondary output wires 30 and 31 of PT2, limit the field current in the case of an internal short circuit or loss of sensing signal.

NOTE: VR3 regulators include an overcurrent shutoff to protect generator components. If the unit operates for an extended period of time at low rpm, the overcurrent shutoff may trip. It will reset automatically when the generator is completely shutoff. For 60 Hz generator operation, the recommended minimum operating speed is:

Four pole ... 1200 rpm

Six pole ... 800 rpm

NOTE: VR3 regulators are sensitive to A-B-C phase rotation. Connections must be as shown in this manual; T1 to 22, T2 to 24, and T3 to 20.

NOTE: If it is required to provide a shutoff for the VR3 regulator, connect a SPDT switch (rated 20 amp, 240 volt) in:

Line 30 - for generator sets with a power transformer only.

Line 24 - for generator sets with sensing and power transformers.