Phase controlled converter will be converting AC to DC energy (line commutated) which means that it can be used to convert fixed-frequency and fixed-voltage AC power into variable DC voltage output. It can be expressed as
AC input voltage which goes into a converter is usually at a fixed RMS (root mean square) and fixed frequency. The insertion of phase-controlled thyristors in the converter will make sure that a variable DC output voltage is achieved and this can be achieved by changing the phase angle at which the thyristors will be triggered. Consequently, a pulsating waveform of the load current will be obtained.
At the time of input supply half cycle, thyristor will be in forward bias and it will be switched ON using sufficient gate pulse (trigger) application. Current will start flowing once the thyristor is switched ON, i.e. at a point ωt=α to point ωt=β. The moment the load current has dropped to zero, thyristor will be switched OFF as a result of line (natural) commutation.
There are many power converters which make use of natural commutation and they include
Awe will discuss above power converters in the next coming chapters.
A 2-phase pulse converter which is also known as a level 2 pulse width modulator (PWM) generator will be used for generating pulses for pulse width modulation converters which are carrier based. This will be done by using the level-two topology. This block controls switching devices can be used to control purposes such as IGBTs and FETs which come in three types of converters:
Here, the reference input signal in a 2-pulse converter will be compared to a carrier. If the reference input signal is greater than the carrier, pulse will become equal to 1 for the upper device and 0 for the lower device.
For controlling a device with a single-phase full bridge (2 arms), it is important to apply unipolar or bipolar pulse width modulation. In unipolar modulation, one among the two arms will be controlled independently and a second reference input signal will be created internally using a shift in initial reference point by 180°
After applying the bipolar PWM, the state of the lower switching device in the second single phase full bridge will become similar to the upper switch in the first single phase full bridge device. Using a unipolar modulation will be resulting to a smooth AC waveforms whereas the bipolar modulation will be leading to low varying voltage.
Assume a three-phase 3-pulse converter, where every thyristor will be in conduction mode at the time of third supply cycle and the initial time a thyristor will be triggered into conduction which is at 30° in reference to the phase voltage.
Operation of a 3-pulse converter can be explained through three thyristors and three diodes. After replacing thyristors T1, T2 and T3 with diodes D1, D2 and D3, conduction will start at angle 30° in respect to the phase voltages uan, ubn and ucn correspondingly. Thus, the firing angle α will be measured initially at 30° in reference to the phase voltage related to it.
Current flows in one direction through the thyristor which is similar to inverter mode of functioning where power will be flowing from DC side to AC side. Additionally, voltage in the thyristors will be controlled by scheming the firing angle and this can be achieved when α = 0(possible in a rectifier). Therefore, 3-pulse converter will be acting as an inverter and a rectifier.
Below figure displays a six-pulse bridge controlled converter which is connected to a three-phase source. In this converter, the number of pulses will be twice that of phases i.e. p = 2m. By using the same converter configuration, two bridges of the six-pulse can be combined to acheive0 a twelve or more pulses converter.
In the absence of commutation, two diodes will be conducting at any given time. Additionally, to achieve a voltage drop across the load, two diodes should be positioned at opposite legs of the bridge. For example, diodes 3 and 6 cannot be ON simultaneously. Thus, the voltage drop across the DC load will be a combination of line voltage VL from the three-phase source.
Remember that more the number of pulses, the greater the utilization of the converter. Additionally, the fewer the number of pulses the lesser the utilization of the converter.
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