Thyristor Convertors

The convertors in dc drive systems are designed to control and supply separately-excited dc machines in applications that demand rapid response, high accuracy and great reliability.

The convertors are available with single-convertor arrangement and in double-convertor arrangement.

Operation. The armature supply unit has a closed-loop current control system which ensures that the armature current follows the set current, reference. The system is based on a current controller, the output signal of which can influence the armature current via the control pulse circuits and the thyristor unit.

The current measurine circuits continuallv supply the current controller with information on the actual value of the armature current, and when this is compared with the reference an error signal is obtained which corrects the output voltage of the controller if necessary.

The controller current includes circuits for adaptive control. This means that the pain of the controller increases if the current is discontinuous. This maintains the response time of the current control system. A typical response time for the current control is 10—15 min.

The current control system includes a buffer amplifier for the actual current value. This makes it possible to connect a meter to displav the armature current, for example. By applying a control signal, the output of the buffer amplifier can be converted, so that the output signal changes sign when the direction of current flow changes in a double convertor.

Convertor Module

Convertor module is used in all systems. It is also available as a non-enclosed function module for the current range 40—1400 A.

The convertor module is a complete unit which has been tested for correct operation. It is made up of thyristor unit and convertor control equipment. The convertor control equipment, in other words the electronic control circuits, consists of circuit boards mounted on a hinged panel. On con-vertor module 40—400 A the panel is supplied fixed in position; for the 650—1400 A module it is supplied separately.

Fig 1. Convertor module

The convertor control equipment is located on the inside of the panel. The permanent basic functions are grouped on one circuit board known as the mother board. A number of optional functions in the form of small circuit boards can be connected to this mother board.

Mounted on the outside of the hinged panel is drive control equipment. The sub-functions are in the form of circuit boards connected to a mother board which provides all the internal connections between the sub-functions. Drive control equipment communicates with the convertor control equipment via a flat cable.

A convertor module can be fitted with all the options available for the convertor control equipment and the drive control equipment. N. B. The open-core transformer for earth fault protection is supplied as a separate unit.

For supply voltages up to and including 500 V, the primary voltage of the auxiliary supply transformer is the same as the supply voltage of the convertor module. Where the supply voltage, is higher, the transformer must be supplied separately with 500 V 60 Hz or 460 V 60 Hz.

For rated currents 230 A and above, an external supply at 110 V 50 Hz or 115 V 60 Hz is required to power and operate a cooling fan.

Fig. 2. Simplified block diagram

Convertors

Presentation. The convertors in dc drive systems are designed to control and supply separately-excited dc machines in applications that demand rapid response, high accuracy and great reliability.

Fig. 1:

a - single convertor; b - double convertor

The convertors are available with single-conyertor arrangement and in double-convertor arrangement.

Single converters. In a single convertor the thyristors are arranged in a six-pulse two-way circuit (see Fig. 1,a).

A single convertor can supply current in one direction only, but its output voltage can change polarity. This determines the rotation and torque direction characteristics of the machine, as illustrated in Fig. 1,a.

The single convertor can operate in the first and fourth quadrants. The direction of torque can only be changed by reversing the field current of the machine.

Double convertors. In a double convertor there are two sets of thyristors connected in antiparallel (see Fig. 1,b). Both current and voltage can change sign, and the machine can be braked and reversed without additional equipment; this is known as four-quadrant drive.