The main electrical energy forms commonly available are in constant DC (direct current) and constant AC (alternating current). Yet for various applications, the several forms of these two energies are used. For this conversion purpose, several converters like inverters, rectifiers, choppers and cycloconverters are employed. The cycloconverters can in fact transfer AC power of a fixed frequency to the AC power of a different frequency (figure 1). Here, AC-AC conversion process is done with a frequency change. Hence it is also referred as frequency changer. Normally, the output frequency is less than the input frequency. There are no DC stages in between this conversion process. The implementation of control circuit is complicated due to large number of SCRs. Microcontroller or DSP or microprocessor is used in control circuits.
Types of CycloconvertersMainly there are two types according to the mode of operation which are showing below.
- Blocking mode Cycloconverters
- Circulating current Cycloconverters
Blocking Mode CycloconvertersHere, the positive converter will provide the necessary voltage when there is positive load current. At that time, the negative converter will be in the blocked condition. During the negative load current, the negative converter will provide the necessary voltage and at that time, the negative converter will be in the blocked condition. This type of operation is known as blocking mode operation and the converters which use this method of operation are termed as blocking mode cycloconverters. There is no need of intergroup reactors because only one converter is working at a time. Cost and size of these converters are low. These converters are most commonly used.
Circuiting Current CycloconverterIn this type of converter, both converters are in the operating state at a time. Sometimes, the supply will get short circuited when both converters are enabled. The intergroup reactor (IGR) is connected in between the converters to avoid this short circuiting. The circulating current which is unidirectional is present here. Mainly there are two types according to the output frequency which are showing below
- Step-up cycloconverters
- Step-down cycloconverters
Step-up CycloconvertersIt can provide output having the frequency greater than the input frequency by using line commutation.
Step-down CycloconvertersIt provides output having lower frequency than the input frequency by using forced commutation. These types of converters are further classified into following:
Single phase to Single phase CycloconvertersIt consists of two full wave converters that are linked back to back as shown below. For simplicity, the operation of cycloconverter is explained with this type. The operation is as follows. Imagine we have to get one fourth of input voltage as the output. Then during the first two cycles of input voltage, the positive converter will operates and provide the current to the load and the input voltage gets rectified. During the next two cycle the other converter (-ve converter) will operates and provide the current to the load in the opposite direction (figure 3). When one converter is in operation, the other will be in the disabled condition. Therefore, there is no circulating current between these converters.
Three-Phase to Single-Phase CycloconvertersThis type of converter can operates in four quadrants (+V, +I, -V, -I) in two modes (inverting and rectifying). Here; +V, +I are in rectifying modes and -V, -I are in inverting modes. Positive converters will provide positive current and negative converters will provide negative current to the load. At a time, one converter will operate. The circuit is shown below.
Three Phase to Three Phase CycloconvertersThese types of Cycloconvertes are formed by using 3 three-phase to single cycloconverters linked together to the load and is used in AC machine systems. Two basic configurations are wye and delta. Output of these converters can be connected in wye and delta. The phase of output voltages are 120o shifted. The circuit diagram is shown below.
Applications of Cycloconverter
- Cement mill drives.
- Rolling mills.
- Ship propulsion drivers.
- Water pumps.
- Washing machines
- Mine winders.