63 Frequency Converters 2.7 Control Circuit and Methods The control circuit, or control card, is the fourth main component of the FC. The three hardware components dealt with so far (rectifi er, intermediate circuit and inverter) are nearly always based on the same principles and components regardless of the manufacturer. In the majority of cases, these components are standard, nearly always purchased from the same external manufacturers. The control circuit design stands in contrast to these, as the area where the FC manufacturer concentrates all its acquired knowledge. In principle, the control circuit has four main tasks: • Controlling the FC semiconductors.
The semiconductors determine the anticipated dynamic characteristics or accuracy • Exchanging data between the FC and peripherals (PLCs, encoders) • Measuring, detecting and displaying faults, conditions and warnings • Performing protective functions for the FC and motor Using microprocessor technology, with single or dual processors, it is possible to increase control circuit speeds using ready-made pulse patterns that are stored in memory. As a result, there is a signifi cant reduction in the number of calculations required. With this type of control, the processor is integrated into the FC and is always able to determine the optimum pulse pattern for each operating stage.
There are a variety of control methods available for determining the dynamic characteristics and response time in the event of a change in reference or torque as well as the positioning accuracy of the motor shaft. In general, the basic functions of a FC can be summed up as follows: • Rotating and positioning the rotor • Open or closed-loop speed control of the AC motor • Open or closed-loop torque control of the AC motor • Monitoring and signalling operating states Categorising the various voltage-source FC’s available on the market (according to the form of control), at least six diff erent types can be identifi ed:
• Simple (scalar) without compensation control • Scalar with compensation control • Space vector control • Open loop fl ux (fi eld-oriented) control • Closed loop fl ux (fi eld-oriented) control • Servo-controlled systems