GE EX2100 - Manual (Page 27)

EX2100 User’s Guide GEH-6632 Chapter 2 Functional Description •••• 2-7
Cooling Fan Assembly
The SCR bridge assembly is cooled with forced air. From two to six overhead fans
are used, depending on the bridge rating and redundancy requirements. The fans are
powered by single-phase 115 V ac supplied by the customer. In redundant
applications, a fan may be replaced while the exciter is running.
Main Dc Contactors. 41A or 41A/41B (Optional)
The main dc contactor (at the output of the power conver sion module) provides a
disconnect between the power conversion module and the generator field. The
contactor picks up when the running mode is selected and no fault exists in the
excitation. The contactors are normally actuated using pilot relays on the EXTB
board driven by the controller. The auxiliary contacts from the contactor are routed
back through the EXTB board as feedback signals.
Free Wheeling Diode De-excitation
De-excitation, the dissipation of the field current after the dc contactor opens, can be
done with a free wheeling diode. This diode is connected from the generator field
negative lead (anode) to the positive lead (cathode). The reverse voltage causes
current to flow through the diode, and the field resistance causes the current decay.