please any body answer these questions in power electronics

[mohammed elzaq]

Compare between a) diode rectifier Bridge & thyristor rectifier bridge
b) DC-link scheme &cycloconverter

 

[Styx]

Re: please any body answer these questions in power electron

Compare between a) diode rectifier Bridge & thyristor rectifier bridge

if you constantly provide a gate-pulse a thyristor rectifyer acts as a standard rectifier

b) DC-link scheme &cycloconverter

A cycloconverter a a direct AC-conversion topolocy, no DC-link (bulky) capacitor is needed. Treat it as the predeccor to the matrix converter

 

[mohammed elzaq]

thanks Mr.styx
but can you explain more???

 

[Styx]

thanks Mr.styx
but can you explain more???

in what way, yr original question is a bit vague, I could write a Phd abt cycloconverter, but I have a feeling you dont want that.

 

[mohammed elzaq]

i want only the advantage and the disadvantage of the cyclo. and dc link scheme.
thanks

 

[Styx]

Cycloconverter:
Advantage over DC-link

1) no DC-link capacitor due to direct conversion from AC
2) inherent regenerative capability
3) fantastic sinusoidal quality generation on the output for a desired output freq of less tehn 1/3 of supply freq
4) can operate a very high power densities (due to Thrystors being used)
4a) Thyristor are available with a blocking voltage of many KV and conduction of many KA, more denisity can be achived by //el and series induvidiual Thyristors (at the expense of extra losses though)
4b) Thyristor's have the lowest on-state losses of the controllable silicon
4c) since they are natually commutated devices there is no switching losses (since thyristor-based converters are essentially zer-current-switching topology)
5) No extra control needed for naturally commutated loads (eg. syncronous machines)
They are used alot in steal-mills and large naval vessals that are electically driven.
6) multi-freq/phase in, multi-freq/phase out (within its limitation, 1/3 freq and no DC)



Dissadvantage
1) alot of silicon to control
2) for best performance output freq must be less then or equal to 1/3 of input freq. Any more and the quality really starts to go down
3) cannot generate freq greater then the supply freq.
4) cannot drive non-naturally commutated loads (eg. induction machines)


Voltage-source inverters (DC-link source)
Advantage over Cycloconverter
1) can force-commutate and thus can drive alot more loads (speed-control induction machines for one) with the addition of position sensing for commutation zoning
2) lot less switches to control
3) fairly easy to increase the quality of output waveform via multi-level topologies
4) since it is a voltage-source inverter its ability to provide rapid changes in current is only limited to load-inductance (as oppose to DC-choke in current-source drives)
5) only freq limitation is governed by acceptable quality and ability to force current into load (ie a high inductance load will limit yr ability to do 100kHz at 100kW at 100V if you see what I mean). Basically a very wide freq range of output waveform
5a) not limited to just sinusoidal, can do square, trapizoidal, triangle (within load limitation) if wanted.

Dissadvantages
1) Losses are in excess of 4x that for a thyristor controller (per switch)
1a)on-state losses are 2x that of an thrystor (at best) plus IF used in a forced-commuted situation then there is switching losses (equal that of conduction is a good target)
2) have bulky DC-link capacitors, for cost Electrolytic are usually used (pluss best density) but to ensure ripple current is met it means alot!!!
3) operating voltage limited to 1200V for IGBT
4) operating current limited to 100A (continous)
4a) for higher voltage need to series, increases losses
4b) for higher current need to parallel, current sharing issues
5) potential shoot-through is always a risk
6) failure modes pretty much always results in power-brick blowing up
7) only 2-quandrant (and cannot switch between the two), for 4-quadrant need some form of extra regen (either thyristor rectifier to invert back to mains or lose the extra energy in break resistor)
7) since voltage-inverter the current drawn from the utility is extreamly distorded, this is the factor that relegates voltage-source inverters to the low power section (~100kW).
7a) the Current-Thd drawn from the utility can be improved via the use of auto-trasformers to increase the rectification from 6pulse to 12pulse, 24pulse... at the expense of complex phase-shifting trasformers


On the whole the only thing that makes voltage-source inverters better then others is its ability to drive force-commutated loads


Also as I said a Cycloconverter can be viewed as the pre-desessor to a matrix-converter.
A matrix converter takes the idea of a Cycloconverter and a voltage-source ability to allow FULL multi-freq/phase in, multi-freq/phase out.
IE DC-in to 100-phase out at any freq. OR 100phase in to single phase out at any freq.

It has its limitations true (3ph to 3ph seems to provide the best operation). It has 2 GOLDEN rules.
Thou shall not open-cct the load
Thou shall not short-cct the supply

adhear to those simple rules and the horrendous gate-drive scheme & commutation and its great (although a vienna rectifier or sparce-matrix are just as good at alot less complexity)

 

[samcheetah]

that reminds me that you were planning to write a power electronics tutorial. have you written it?

by the way, i will be needing something like that because im having a power electronics course in my next semester