Rating for ac & dc

Replies

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  Anand Tamariya

  What has more voltage rating got to do with running machinery?
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  Vishal1485

  Even i don't understand, My teacher asked me this question and hence posted the same words here.
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  lal

  AC can be stepped up and stepped down very easily using a transformer which is the reason AC is available at many kilo Volts. But, at the consumer end, it is almost always 230V and 415V, unless there are special needs.
  
  And who said companies prefer DC voltage over AC? DC is mostly used for machines which need immense precision. Then those are mostly low power machines. Even that kind of accuracy is achieved by AC machines these days. Almost all companies prefer AC machines for high power applications. Besides, the grid power is in AC.
• 

  Anand Tamariya

  Vishal1485

  Even i don't understand, My teacher asked me this question and hence posted the same words here.

  I'm just stimulating you to think. Think what electrical components or systems - resistor, capacitors, diodes, motors etc. - need. Think about losses in transmission in AC vs DC. That will give you an idea where to use AC and where to use DC.
• 

  yogi.bharadwaj

  Vishal1485

  AC has more voltage rating than DC, but still companies prefer DC voltage for running machinery. Why?

  i think you are talking about the small instruemts like laptops mobiles pcs etc these are required dc because of two reasons the first one is we uses processors or mermories storage which are designed to work on a specific voltage like +5v or+9v etc and in ac stepped down supply it flactuates from +5 to -5 and+9 to-9 so it is not constant so we uses dc for continuous supply the other thing is that we require battery backup which can stores dc not ac can be stored. i think u understand and if i am wrong pls correct me who knows the real reason.
• 

  vamsi kancharla

  Vishal1485

  AC has more voltage rating than DC, but still companies prefer DC voltage for running machinery. Why?

  will you please mention the machinary so that proper answer can be given and more over now a days dc machines are replaced by the ac machines in most of the applications
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  Jayapriya babu

  in the transmission side India is having a HVDC transmission line in north side... in my training days I found most of the dc motors are replaced by ac motors..
• 

  yogi.bharadwaj

  Jayapriya babu

  in the transmission side India is having a HVDC transmission line in north side... in my training days I found most of the dc motors are replaced by ac motors..

  i think its very old from old days we uses ac motors over dc motor correct me if i am wrong,
• 

  lal

  In the past DC machines used to rule over AC machines. That was because squirrel cage motors made its entry quite late than the DC machines! But right now, it is the squirrel cage motor that dominates. A squirrel cage motor has a few amazing advantages like it is very rugged, seldom needs a repair and long lasting operation. Even if it has to be repaired, it is much easier to do.
  
  There is another reason for the DC machines to have dominated the industrial drives even after the squirrel cage made its entry for some time. Controlling the speed of DC machines is as easy as clapping the hands. And it has amazing accuracy. Simply controlling the voltage to its field winding, one can achive variable torque! But, then, it had to be maintained a lot. It couldn't be used roughly. It was a machine which operated well only if it was cared for!
  
  Squirrel cage re-wrote these disadvantages of dc machines. Then the 'power electronics' made its entry, here giving yet another advantage for ac machines - 'variable frequency drives'. Controlling the frequency of the AC supply smoothly adjusts the speed of a squirrel cage. And maintaining the frequency steady, gives it accuracy. This was made possible by power electronics.
  
  Now things got simpler, an AC supply to drive an AC machine and a six inch cube (a power electronics drive like the cyclo-converter) to control it. Again, which will last for years and years, and doesn't have to be looked back or cared about!
  
  Thus, AC machines now dominate!
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  Jayapriya babu

  I think I m going out of our topic...
  everyone is saying that AC is suitable for transmitting as its voltage can be stepped up or down using transformers...
  but I think HVDC is suitable for transmission and not for distribution...
  so only our country is having HVDC transmission line from Rihand and Dadri of about 814km and transmitting voltage is 500kv.. it was constructed in 1990 by ABB after a huge damage caused by loose connection of grading capacitor..
  
  if anything wrong correct me...
• 

  lal

  With the invention and implementation of newer technologies in power electronics, HVDC links will definitely become more feasible in near future. One big limiting factor for HVDC transmission is the converter station. High initial cost and comparitively frequent maintenance are the most hated things about high power AC - DC converters. But, the huge advantages of high voltage DC 'transmission' over AC cannot be left un-acknowledged.
  
  I think we'll have to wait a little more to find HVDC transmission links being implemented extensively.
• 

  Jeffrey Arulraj

  HVDC Is setting up intensive project. But efficiency wise HVDC is cheaper than HVAC transmission. When it comes to distribution, DC loses all it's plus cause of high power loss in the transmission cables.
  
  Skin effect is a boon and a bane in all cases
• 

  lal

  @Jeffry, In fact the transmission losses for HVDC are lower than equivalent HVAC transmission. The only un-attractive part about HVDC transmission are the converter stations whihc convert DC to AC and vice versa. There are limitations on the amount of power the semiconductor devics in the converter stations can handle and higher maintenance. Not to mention very high installation and maintenance cost!