The phase-controlled converter gives controllable dc voltage at the output terminals from ac supply to power dc loads. Depending upon the amount of power handled and application requirement there are two types of converters namely, semi or half-controlled converter and fully-controlled converter.
Using a half-controlled converter a positive dc output voltage with unidirectional current is obtained. Whereas in the case of fully-controlled converters the dc output voltage is both positive and negative i.e., the output voltage is reversed, with the unidirectional current. Let us see the difference between a semi or half-controlled converter and a fully-controlled converter.
Comparison Between Half Controlled and Fully Controlled Converter :
| Comparison | Half Controlled Converter | Fully Controlled Converter |
|---|
| Circuit | | |
| Power Switching | In this, converter-controlled devices are SCR’s and diodes. | In this, converter-controlled devices are only SCR’s. |
| Quadrant Operation | It performs only one quadrant operation i.e., forward motoring (I quadrant) operation. | It performs two-quadrant operation i.e., forward motoring (I quadrant) and reverse braking (IV quadrant) operation. |
| Input Power Factor | High power factor. | Low power factor. |
| Control of Output Voltage | Limited control. | Wider control. |
| Average Output Voltage Vout | | |
| Inversion Mode (α > 90°) | Not possible | Possible |
| Freewheeling | Internal freewheeling exist. | External freewheeling diode required. |
| Application | Preferred for heating and lighting control. | Preferred in dc motor drives. |
Key Differences Between Half Controlled and Fully Controlled Converter :
The following are the features of half-controlled converters over full-controlled converters,
- The cost of a half-controlled converter is less when compared to a full-controlled converter, as the number of thyristors being used is half of those being used in full-controlled converters.
- The output voltage of a half-controlled converter has low ripples when compared to that of a full-controlled converter. So, the requirements and the cost of the filter circuit reduces.
- Power factor is high in case of half-controlled converter because in the case of full-controlled converters, the energy stored in load inductance is feedback to the supply during the periods 0 to α, π to (π + α), 2π to (2π + α), and so on. Whereas, in the case of half-controlled converters, during these periods, free-wheeling action takes place, which results in the utilization of the stored energy in the load inductance. Hence, the power factor improves.
- In a half-controlled converter, the average dc output voltage will always be positive whereas, in the case of full-controlled converters it will be positive for 0 < α < 90° and negative for 90° < α < 180°.
- In half-controlled converters, the output voltage can be continuously controlled from its maximum value to zero by varying the firing angle from 0 to 180°. While in the case of full-controlled converters, the range of firing angle is limited (i.e., 0 < α < 90) and so, the output voltage cannot be controlled continuously as in the former case.
