- A power electronic converter is indeed useful when its output voltage is controllable. Being able to control the output voltage allows for various applications and ensures the converter operates as desired.
- Different techniques are employed to control the output voltage of different types of converters. Some common control techniques include time ratio control for DC choppers, pulse width modulation (PWM) control for inverters, and control of switching angles for phase-controlled rectifiers.
- For phase-controlled rectifiers, the control technique involves controlling the switching angle of the thyristors. However, the previously mentioned techniques may not be suitable if the converter has more than one thyristor, which is often the case.
- In the context of switching angle control for phase-controlled rectifiers, there are several schemes employed, including:
- Cosine wave crossing control: This method utilizes a cosine wave derived from the input supply through an integrator. The cosine wave is compared to a DC reference signal, and trigger pulses are generated accordingly. The firing angle is related to the intersection of the cosine wave and the control signal.
- Ramp comparator control: In this scheme, a ramp signal is compared to a control voltage, and trigger pulses are generated based on the comparison. The firing angle is determined by the intersection of the ramp signal and the control signal.
- Digital firing scheme: This scheme involves using digital techniques to control the firing angle. It may utilize microcontrollers or digital signal processors to generate the trigger pulses based on a control algorithm.
- Equidistant pulse firing scheme: In this scheme, equidistant pulses are generated to trigger the thyristors. The firing angle is determined by the time delay between the trigger pulses.
- The cosine control scheme is a technique used for controlling the output voltage of phase-controlled rectifiers. It employs a cosine wave obtained from the input supply through an integrator. The cosine wave is then compared against a DC reference signal, and trigger pulses are generated accordingly.
- The basic principle behind cosine control is similar to that of a dual converter, where the output voltage ratio is equal to the cosine of the firing angle. This means that the rectifier can be thought of as an amplifier with a linear transfer characteristic.
- The cosine firing scheme for thyristors in single-phase converters provides a linear transfer characteristic between the average output voltage (V0) and the control voltage (Ec). This scheme improves the closed-loop response of the converter systems.
Cosine firing scheme for triggering thyristors
Waveforms for cosine firing scheme
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