25-08-2017, 09:32 PM
AC to DC Converters
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Introduction
Single phase uncontrolled rectifiers are extensively used in a number of power electronic based converters. In most cases they are used to provide an intermediate unregulated dc voltage source which is further processed to obtain a regulated dc or ac output. They have, in general, been proved to be efficient and robust power stages. However, they suffer from a few disadvantages. The main among them is their inability to control the output dc voltage / current magnitude when the input ac voltage and load parameters remain fixed. They are also unidirectional in the sense that they allow electrical power to flow from the ac side to the dc side only. These two disadvantages are the direct consequences of using power diodes in these converters which can block voltage only in one direction. As will be shown in this module, these two disadvantages are overcome if the diodes are replaced by thyristors, the resulting converters are called fully controlled converters.
Resistive-Inductive load
the circuit diagram and the waveforms of a single phase fully controlled halfwave rectifier supplying a resistive inductive load. Although this circuit is hardly used in practice its analysis does provide useful insight into the operation of fully controlled rectifiers which will help to appreciate the operation of single phase bridge converters to be discussed later.
Operation in the continuous conduction mode
As has been explained earlier in the continuous conduction mode of operation i0 never becomes zero, therefore, either T1T2 or T3T4 conducts. Fig 10.4 shows the waveforms of different variables in the steady state. The firing angle of the converter is α.
Operation in the discontinuous conduction mode
So far we have assumed that the converter operates in continuous conduction mode without paying attention to the load condition required for it. In figure 10.4 the voltage across the R and L component of the load is negative in the region π - θ ≤ ωt ≤ π + α. Therefore i0 continues to decrease till a new pair of thyristor is fired at ωt = π + α. Now if the value of R, L and E are such that i0 becomes zero before ωt = π + α the conduction becomes discontinuous.