Efficiency of dc to ac converter problem?

Discussion in 'Homework Help' started by steven2410, Nov 1, 2014.

  1. steven2410

    Thread Starter New Member

    Jul 7, 2014
    When a dc to ac converter is used to interface dc loads or dc power sources to the ac grid its
    efficiency can be defined in a similar manner to dc converter: efficiency = Pout/Pin. However,in some special applications we may use the converter without any dc load at all, and have it behave merely as a device that draws/injects controllable amounts of reactive power from/to the grid. Describe the problem with using the above definition of efficiency in this case. As an example, consider a converter drawing 50+j2000 VAand having no power exchanged on the dc side with loads or sources. Its efficiency is clearly 0%. For such systems what would be a better normalized measure to rate the converter?
    I'm stuck on this question. TBH, I'm not getting reactive power. Is it the power dissipated by inductor/ capacitor?
  2. MrAl

    Distinguished Member

    Jun 17, 2014

    One way would be to rate it in terms of the ratio of no load power to full load power.

    Reactive 'power' is not real power, it's the power you would get with the same current and voltage if they were not really out of phase.
    For 10vac across 10 ohms we get 1 amp and 10 real watts of power, but for 10vac across an (pure) inductor that draws 1 amp we get 10 volt amps of reactive 'power'. This reactive power is not real power so it does not dissipate energy and does not create heat.
    steven2410 likes this.
  3. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    If the goal is simply to generate reactive power - say for active mode power factor compensation - then one might suggest a figure of merit indicating the real power loss (the "cost") per VAR of actual reactive power "generated".
    steven2410 likes this.