Reference Voltage Sources
Reference Voltage Sources:
Fig: Block diagram of logic and trigger circuit Fig: Reference voltage generator block diagram
- If the amplitude and frequency of the reference signal is varied, then the amplitude and frequency of the output voltage varies automatically.
- In the case of three-phase to three-phase cyclo-converter, the reference signal does additional function of shifting eOA , eOB & eOC , by phase shift of 1200 . The three-phase variable frequency, variable voltage sine wave reference voltage can be designed in various ways.
- As the frequency of the reference voltage signal is low, normally limited to Hz, one-third of the line frequency of 50 Hz (may be higher (25 Hz) in some case), one of the design approach as given here, is to use a mixer, wherein two signals having frequencies, fc & fd are mixed to obtain frequencies (fc ± fd).
- Then, a low pass filter is used to obtain a signal of required frequency, (fc - fd). An astable multi-vibrator is used to generate a square wave with frequency, (3fd), which is then fed to a three-stage ring counter, whose output is three numbers of three-phase, square wave (ФA , ФB & ФC)of frequency fd, at a phase shift of 1200.
- The fixed frequency oscillator (fc) produces three outputs (Ф1 , Ф2 & Ф3) which may be taken as three-phase. Three mixers − one for each phase, as stated earlier, are used to combine the fixed and variable frequencies.
- The output of each mixer stage is a square wave with half-wave symmetry, consisting of a fundamental and a series of odd harmonics.
- If all higher order higher harmonics are neglected, the output signal has only two frequencies, sum or difference of the fixed and variable frequencies.
- Then, a low pass filter is used to select the low frequency signal (fc – fd) and also eliminate the high frequency one (fc fd).
- Finally, the three reference signals obtained are in the form,
where, Em is the peak of the cosine modulating wave