GENERAL DESCRIPTION 

The SX440 is a half-wave phase-controlled thyristor type Automatic Voltage Regulator (AVR) and forms part of the excitation system for a brushless generator. 

In addition to regulating the generator voltage, the AVR circuitry includes under-speed and sensing loss protection to ensure safe, reliable control of the generator. Excitation power is derived directly from the generator terminals. Positive voltage build-up from residual levels is ensured by the use of efficient semiconductors in the power circuitry of the AVR. 

The AVR is linked with the main stator windings and the exciter field windings to provide closed-loop control of the output voltage with load regulation of +/- 1%. In addition to being powered from the main stator, the AVR also derives a sample voltage from the output windings for voltage control purposes. In response to this sample voltage, the AVR controls the power fed to the exciter field, and hence the main field, to maintain the machine output voltage within the specified limits, compensating for the load, speed, temperature, and power factor of the generator. 

A frequency measuring circuit continually monitors the generator output and provides output under-speed protection of the excitation system, by reducing the output voltage proportionally with speed below a presettable threshold. 

A manual adjustment is provided for the factory setting of the under-frequency roll-off point, (UFRO). This can easily be changed to 50 or 60 Hz in the field by push-on wire link selection. 

Overvoltage caused by open circuit sensing terminals is avoided by sensing loss detection circuitry which reduces the generator terminal voltage to a safe fixed level. 

Provision is made for the connection of a remote voltage trimmer, allowing the user fine control of the generator's output.