Digital Control of a Three-Phase to Single-Phase Cycloconverter Motor Drive

Abstract

*† ‡ A three -phase to single -phase cycloconverter with a digital control is presented. T he cycloconverter in this case is a thyristor based design and requires care in the timing algorithm of the circuit. This algorithm relies solely on a single “reference” signal generated within the digital control. This type of control algorithm is commo nly known as the cosine wave crossing method (CWCM). Throughout this paper, another implementation of this method will be performed with a digital signal processor. A complete design of this power conversion process will allow for a standard three -phase, AC signal to be converted to a reduced frequency, AC signal. Results of this design will be presented with a motor. I. Introduction YCLOCONVERTERS are used in a wide variety of high power applications today to drive both induction and synchronous motors. Some of these applications include steel rolling mills, cement kilns, ore grinding mills, mine winders, and ship propulsion drives. These applications require a variable frequency and variable magnitude power supply. Specifically, they require a powerful voltage input but at a frequency that has been stepped down. A standard three -phase power source operates at a 60Hz frequency per phase. While this can provide a lot of power, the frequency is much too fast to drive a motor at the slow rate required by the applications mentioned. To accommodate these particular applications, a cycloconverter can reduce the frequency of the input to be applied to loads of any power factor. In other words, a cycloconverter is a power conversion device that converts an AC input at one frequency to AC output at another frequency. The basic structure of a cycloconverter consists of two full bridge circuits assembled back to back with thyristors instead of diodes. If two of these bridges are used, one can be used to rectify the input for a time creating a positive half -cycle and the other can invert the signal for a time creating a negative half -cycle [1] . Controlling the duration of each half -cycle will essentially create an output w ith a slower frequency. Making this outp ut signal sinusoi dal in nature depends on accurately timed phase control to determine when thyristors in the circuit should conduct. To derive the timing needed to trigger these thyristors, a control scheme must be implemented. The scheme applied in thi s design is a commonly used method of control called the cosine wave crossing method (CWCM). This method requires fast and accurate sampling of the three input signals through an analog to digital converter (ADC) and a processor that can quickly compare i ts own self -generated “reference” with these signals. To accomplish this proposed design, a digital signal processor (DSP) from Analog Devices is employed [2 ]. The Analog Devices Motor Control (ADMC) 401 DSP board provides the digital control for the ci rcuit with an ex ecution cycle of 26 Millions of Instruction Per Second (MIPS) and a 16 -bit DSP core. The evaluation board provides an ideal test environment by allowing a computer to download the code to the DSP and also do in -circuit debugging all throug h the serial interface. Also, to achieve the design specifications, the ADMC401 board also possesses an 8 channel, 12 -bit analog to digital converter (ADC). Each phase of the input waveform is transformed down to an acceptable voltage and sampled through this flash based ADC. The software that processes the input signal is writte n in an assembly based language which allows for basic math functions along with a fixed point number system. The sinusoidal “reference” signal is produced with the built in tri gonometric functions. It is then applied to the sampled input phases to determine the exact moment when each thyristor should fire. These gating signals are then produced from the 12 digital input/output (I/O) lines of the ADMC401. With each sequence of timed pulses, a single -phase, reduced frequency signal is produced as the output which completes the power conversion process.