Phase Controlled Converters
Table of Contents
Phase Controlled Converters
Energy line commutated from AC to DC is converted by a Phase controlled converters. It is used to transform fixed-frequency, fixed-voltage AC power into variable DC voltage output, in other words. It is written as
- Fixed Input − Voltage, frequency and AC power
- Variable output − DC voltage output
A converter’s AC input voltage typically has a fixed RMS root-mean-square and fixed frequency. A variable DC output voltage is obtained because the converter uses phase-controlled thyristors. By changing the phase angle at which the thyristors are triggered, this is made possible. The load current’s waveform is thus obtained as pulsating.
During the input supply half cycle, the thyristor is in forward bias and is switched ON via the application of sufficient gate pulse trigger. Current starts to flow once the thyristor has been switched ON, that is, at a point ωt=α to point ωt=β. The moment the load current drops to zero, the thyristor switches OFF as a result of line natural commutation.
There are a number of power converters(Phase Controlled Converters) that utilize natural commutation. These include −
- AC to DC converters
- AC to AC converters
- AC voltage controllers
- Cycloconverters
2- Pulse Converter
To produce pulses for pulse width modulation converters that are carrier based, a 2-phase pulse converter, also referred to as a level 2 pulse width modulator PWM generator, is used. This is accomplished by using level-two topology. The IGBTs and FETs found in three different types of converters are controlled by this block for control purposes.
- 1 arm single phase half bridge
- 2 arms single phase full bridge
- 3 arms three phase bridge
In a 2-pulse converter, the reference input signal is contrasted with a carrier. The pulse is equal to 1 for the upper device and 0 for the lower device if the reference input signal is greater than the carrier. Uni-polar or bi-polar pulse width modulation must be used to control a device with a single-phase full bridge 2arms2. Each of the two arms is independently controlled in unipolar modulation. Internally, a 180° shift in the initial reference point generates a second reference input signal.
The lower switching device in the second single phase full bridge device is in a similar state to the upper switch in the first single phase full bridge device when the bi-polar PWM is applied. Bi-polar modulation produces less fluctuating voltage while unipolar modulation produces smoother AC waveforms.
3-Pulse Converter
Take a look at a three-phase, three-pulse converter where each thyristor is in the conduction mode during the third supply cycle. A thyristor enters conduction for the first time at a 30° angle with respect to the phase voltage.
Its operation is explained using three thyristors and three diodes. When the thyristors T1, T2 and T3 are replaced by diodes D1, D2 and D3, conduction will begin at angle 30° in respect to the phase voltages uan, ubn and ucn respectively. Therefore, the firing angle α is measured initially at 30° in reference to the phase voltage corresponding to it.
Similar to how an inverter works, where power moves from the DC side to the AC side, a thyristor can only allow current to flow in one direction. Controlling the firing angle also affects the voltage in the thyristors. This is achieved when α = 0 possible in a rectifier. Thus, the 3-pulse converter acts as an inverter and a rectifier.
6-Pulse Converter
A six-pulse bridge controlled converter connected to a three-phase source is shown in the figure below. In this converter, p = 2m, or the number of pulses is equal to the number of phases. Two bridges of the six-pulse converter can be combined using the same converter configuration to create a converter with twelve or more pulses.
Two diodes will conduct simultaneously if commutation is unavailable. Two diodes must also be placed at opposite legs of the bridge in order to achieve a voltage drop across the load. Diodes 3 and 6 cannot, for instance, both be ON at the same time. As a result, the line voltage VL from the three-phase source is combined with the voltage drop across the DC load. It is important to remember that the converter is used more frequently the more pulses there are. Additionally, the converter is used less frequently the fewer pulses there are.