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Discuss Static Frequency Converter(SFC) in the Industrial Electrician Talk area at ElectriciansForums.net

S

speedtronic

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Please tell that how SFC work?
What I know is that first of aLL AC voltage is converted into DC voltage through rectifier bridge then DC is converted back to AC of desired frequency using inverters. Now what I know that converter/inverter simply change AC/DC voltage. How they change frequency without changing voltage?
 
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S

spark-doctor

  • Thread Starter Thread Starter
  • #2
To many big word there. I was lost after the first line.

Shakey would be the best one to answer this, know the ins and outs of the damm things.
 
C

Carter

  • Thread Starter Thread Starter
  • #3
Same as when you speak. Speech is a variation in frequency, how loud you speak would be a variation in amplitude. Modern micro electronics can do the same trick with an input voltage.
SFCs are employed when the demands of the machine are asynchronous with the characteristics of the supplied mains either in frequency or voltage. An example in reverse would be a wind turbine which is designed & set up to run at an optimum rpm based on average predicted wind-speed. But seeing as the wind speed is so variable so is the rotational speed of the blades and therefore the output from the genny would vary wildly with windspeed. This is no good for direct (synchronous) connection to the grid as the output frequency of such a 'directly coupled' generator would be unsuitable for connection no matter how good its physical regulation. At best all you could do would be to disconnect it every time the frequency fell out of step and so it would only be contributing power when the windspeed allowed it perfectly to match that of the mains. maybe for 2% of its operational life. That is a huge waste of the total amount of kinetic energy that hits its blades.

The easiest way to recover more energy is to take whatever the generator gives and convert it do DC.
Once you have that regular DC it's a whole lot easier to use a micro-processor controller driving switching thyristors/amplifiers/IGBTs or whatever to create pretty much any wave shape and frequency you want. Even the little micro-processors I fiddle about with run at 4Mhz so they are easily capable of reproducing a perfectly regulated 50Hz (or 400Hz for that matter) sine wave.
 
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R

rumrunner

  • Thread Starter Thread Starter
  • #4
Same as when you speak. Speech is a variation in frequency, how loud you speak would be a variation in amplitude. Modern micro electronics can do the same trick with an input voltage.
SFCs are employed when the demands of the machine are asynchronous with the characteristics of the supplied mains either in frequency or voltage. An example in reverse would be a wind turbine which is designed & set up to run at an optimum rpm based on average predicted wind-speed. But seeing as the wind speed is so variable so is the rotational speed of the blades and therefore the output from the genny would vary wildly with windspeed. This is no good for direct (synchronous) connection to the grid as the output frequency of such a 'directly coupled' generator would be unsuitable for connection no matter how good its physical regulation. At best all you could do would be to disconnect it every time the frequency fell out of step and so it would only be contributing power when the windspeed allowed it perfectly to match that of the mains. maybe for 2% of its operational life. That is a huge waste of the total amount of kinetic energy that hits its blades.

The easiest way to recover more energy is to take whatever the generator gives and convert it do DC.
Once you have that regular DC it's a whole lot easier to use a micro-processor controller driving switching thyristors/amplifiers/IGBTs or whatever to create pretty much any wave shape and frequency you want. Even the little micro-processors I fiddle about with run at 4Mhz so they are easily capable of reproducing a perfectly regulated 50Hz (or 400Hz for that matter) sine wave.
Excellent answer ,i wonder why you chose an example of a wind generator:)
 
C

Carter

  • Thread Starter Thread Starter
  • #5
Excellent answer ,i wonder why you chose an example of a wind generator:)
It illustrated the fact that they have a uses other than the usual one we are familiar with of driving a 3ph motor from a single phase source, speed control and ramping etc.
Anything that improves the **** poor efficiency of installed wind capacity has to be a good thing 'coz as a means of easing reliance on gas/nuke/coal they are signally crap and useless as on-line generators without big storage capacity to damp fluctuations, eg (batteries, pumped water storage or cracking water into hydrogen) Hasn't stopped them announcing a hugely wasteful program of infrastructure/plant installation though.
Report blows hole in wind power plan - Telegraph
In fact they misquote the cost as £100million in fact it is £100 BILLION!
A load of hot air: Why spending £100bn on windfarms to please the EU is Labour's greatest act of lunacy | Mail Online
and it still won't remove the need for conventionaly fuelled infill capacity when there is no wind....such as in Winter months....just when the demand is nighest....doh!
God spare us from these idiots, soon!
 
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M

montybaber

  • Thread Starter Thread Starter
  • #7
Vvvf drives (known as inverters) output a variable voltage and frequency (hence the name lol) to deliver speed control on AC motors, the voltage and frequency must vary proportionally so the motor can deliver its rated torque.

perhaps this is the reason you thought voltage must vary alongside frequency
 
S

Spudmiester

  • Thread Starter Thread Starter
  • #8
Basically same as invertors,AC input, change to DC , alter speed, change back to ac again for output. There is some splitting of sinewaves too somewhere (i think).
 
B

Bane

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  • #9
Bloody hell.

Not even a simple query can be prevented from having political undertones!
 
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