DC Link Converter
DC Link Converter:
Fig: DC link Converter
- The thyristors, or switching devices of transistor family, which are termed as self-commutated ones, usually the former, which in this case is naturally commutated, are used in controlled converters (rectifiers).
- The diodes, whose cost is low, are used in uncontrolled ones. But now-a-days, switching devices of transistor family are used in inverters, though thyristors using force commutation are also used.
- A diode, connected back to back with the switching device, may be a power transistor (BJT), is needed for each device. The number of switching devices in dc link converter depends upon the number of phases used at both input and output.
- The number of devices, such as thyristors, used in cyclo-converters depends on the types of connection, and also the number of phases at both input and output.
- All features of a cyclo-converter may not be available in a dc link converter. Similarly, certain features, like Pulse Width Modulation (PWM) techniques as used in inverters and also converters, to reduce the harmonics in voltage waveforms, are not applied in cyclo-converters.
Advantages of DC link converter:
1. The output frequency can be varied from zero to rated value, with the upper frequency limit, being decided by the turn-off time of the switching devices, which is quite low due to the use of transistors in recent time.
2. The control circuit here is simpler, as compared to that used in cyclo-converter.
3. It has high input power factor, if diode rectifier is used in the first stage. If phase-controlled thyristor converter is used, power factor depends upon phase angle delay.
4. It is suitable for higher frequencies.
Disadvantages of DC link converter:
1. The conversion is in two stages, using two power controllers − one as converter and other as inverter.
2. Forced commutation is required for the inverter, if thyristors are used, even though phase control is used in converter, where natural commutation takes place.
3. The feature of regeneration is somewhat difficult, and also is involved to incorporate in a dc link converter.
4. The output waveform of the inverter is normally a stepped one, which may cause non-uniform rotation of an ac motor at very low frequencies (< 10 Hz).
5. The distorted waveform also causes system instability at low frequencies. This can be reduced by using PWM technique.