SDCS-FEX-2 / SDCS-FEX-2A Field Exciter Board

SDCS-FEX-2 / SDCS-FEX-2A are mostly used as the excitation control modules for ABB DCS500 and other series of direct current drive systems. They are mainly compatible with the direct current drive systems of the ABB DCS500 series and can be used in conjunction with the armature converters and other components of this series of drive equipment to form a complete DC motor excitation control unit. They can also be used as replacement parts for the old equipment of this series.

The field exciter board SDCS-FEX-2 / FEX-2A consists of a power part and a control board, which connects all components electrically and mechanically

cally to each other. This arrangement has to be mounted inside the armature converter module beside the electronic power supply SDCS-POW-1.

This is intended to be done for DCS modules of type C1, C2, and A5, not for A6 and A7 (C4).

 

The power part is built up with two power modules. Each of the modules consists of one diode and one thyristor, so they are wired up and controlled like a half-controlled bridge.

 

The control is based on a fully digital system. The µ-processor reads all information from the power part, is supplied with all needed voltage levels and control signals via the flat cable X14 by the SDCS-CON-2, and generates the firing pulses for the power part.

 

The range of the single-phase rated AC input voltage is 110 V to 500 V, and the maximum current capability is 16 A. If this field exciter is used for smaller

field current, the control unit automatically selects a lower current range between 3 A to 16 A to get the best resolution.

Control unit

The control unit includes the following main blocks:

- Microcontroller 80C198 for controlling and firing

- Actual DDC measurement with an AC transformer.

- RS-485 interface to the converter's controller board SDCS-CON-x.

 

The software for the field current control is stored in the ROM memory of the 80C198. The control is done by using a PI structure for the current controller. All parameters needed for the control or for scaling reasons (selection of burden resistors) are stored in the non-volatile memory of the armature converter and transferred to the field controller during each initializing process via the RS-485 link. The Node number is always fixed and coded to Node number = 1.

 

The output Iact represents the actual field current, which is measured via the AC transformer, then rectified and transferred into a voltage signal with

burden resistors. The burden resistors, as mentioned before, are adapted by the board itself depending on the setting of the nominal field current

of the motor (see list before). The resulting burden voltage can be measured at test terminals beside X:. The 2.2 KΩ resistor allows a short circuit at the terminals; the external measuring device should have an internal resistance higher than 1MΩ.

 

Power section

Two diode-thyristor modules are arranged as a half-controlled single-phase rectifier. The anodes of the two diodes are not connected directly to each

other as usual; they are now connected to the ends of the four-turn primary winding of the current transformer. The center tap is the negative output of the rectifier. Thus, it is possible to measure the DCt with an AC transformer.

 

A MOV (Metal Oxide Varistor) protects the AC input against voltage spikes from the external source. Another MOV protects the DC output against voltage surges, which can be caused by the field winding of a DC machine.

 

 

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