Indirect Excitation Systems (brushless) UNITROL^® series 

The ABB brushless excitation products and systems consist of a voltage regulator with power unit, the alternating current machine and the rotary diodes for converting the alternating current generated by the exciter machine into the direct current required by the main machine.

The Excitation Converter - Automatic Voltage Regulator (AVR) - send DC current to the exciter (attachted to the main generator). The exciter will rectify the current through rotating diodes on the main machine shaft.

• Indirect excitation current range: 1 - 400 A 
• Exciter respnse limited by the exciter machine
• Field discharge with natural time constant
• Size of exciter machine increases with the speed
• No slip rings (less maintenance and dust)