Introduction

Typical Subpan (Partial View)
(1) Time delay relay. (2) Slave relay. (3) Starting motor magnetic switch. (4) Circuit breaker. (5) VR4 voltage regulator. (6) Series boost (attachment).

The VR4 is one of the voltage regulators available for SR4 generators. The VR4 is only used on standby SR4 generators which are: self-excited, 10/12 lead, and rated up to 400KW. Follow all safety procedures and warnings in the SR4 generator manuals.

VR4 regulator (5) is located on side two of the subpan. The subpan is mounted above the generator.

Operation

VR4 Regulator
(7) Terminal strip. (8) Rheostat (voltage level). (9) Fuse. (10) Rheostat (voltage gain).

The purpose of the VR4 regulator is to keep generator output voltage at a rated value. There are two controls on the VR4 that are standard.

* Voltage level rheostat (8) provides a no load voltage adjustment of approximately + 10 to - 25%.

* Voltage gain rheostat (10) provides an adjustment to compensate for engine RPM droop with load. Rheostat (10) can be adjusted to keep the same voltage at no load and full load, even when there is a small change in frequency (rpm).

The rheostats are multiple (30) turn. The adjusting screw on the rheostat does not have a fixed stop. To ensure the lowest position, turn the rheostat 32 full turns counterclockwise from the current position. To ensure the highest position, turn the rheostat 32 full turns clockwise from the current position.

SR4 Generator Schematic (Self-Excited 10/12 Lead)
(5) VR4 regulator. (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).

VR4 regulator (5) senses generator output voltage between two phases. Lines (12) function as sensing lines by tapping into generator output. These lines are connected to terminals 20 and 24 on the VR4. Terminal 22 is also connected but is not used on the VR4. Like other SR4 generator regulators, the VR4 regulates voltage based on volts per hertz.

Leads (12) also function to provide AC power for excitation to the VR4 regulator. The regulator rectifies the AC voltage and controls the excitation current to exciter stator (15).

Fuse (9) limits the field current in the case of an internal short circuit or exciter field fault.

NOTE: VR4 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. The approximate recommended minimum operating speed is:

Four pole generator ... 1200 rpm

Six pole generator ... 750 rpm