[BC] 12 phase supply, formerly AC Voltage Drop

[Cowboy]

On Monday 29 June 2009 11:12 pm, Dave Dunsmoor wrote:
>  > Since 12-pulse (or 12-phase) transformers were expensive, ...
>  
>      Richard (or anyone else who would care to reply), I've asked this
>  question before, and now am going to have to admit to some forgetfulness:
>  how exactly does this 12-phase business work? Where do you get the
>  nomenclature "12 pulse" (or phase) from a 3 phase incoming main AC, and how
>  does combining 2 secondaries constitute something that can be considered to
>  be 12 phases?

 Here ya go.....

 Any transformer is an inductive device, so the output from any transformer
 is always phase shifted relative to the primary input.

 Three phases into a transformer produces 3 phases out, but out of phase
 with the primary. ( any transformer, and that's critical ! )
 Why ?
 Remember, a transformer is an inductive reactive device !
 The +j causes this phase shift. How much is relative to the L/R ratio.
 ( in "simple" terms )
 Consider the primaries and secondaries separately for a minute.
 The primary is a coil with a current flowing into it.
 The magnetic field stores this energy, AND because we have
 this conductor in a moving magnetic field ( remember the current
 is AC so is "moving" relatively ) a current is induced in any
 conductor in this field, which includes the primaries. If the self
 inductance of the primary is high enough, the current in that primary
 is limited to only enough to replace the field energy dissipated in
 transformer and radiation losses.
 The secondary is also a conductor in the field, so when current is
 drawn from the secondary ( seen as a real R load ) it represents a
 loss as far as the primary is concerned, so more current flows in
 the primary to make up the energy "lost" from the magnetic field.

 Now, some genius somewhere along the way decided to label the
 pulsating output of a rectifier as a "phase" for this power supply purpose,
 hence a 3 phase half-wave supply is a 3 phase supply, while a 3 phase
 full-wave rectifier is a 6 phase. Clear ?  Me neither, but let's move on. 

 The full wave unfiltered rectifier produces pulses 60 degrees apart.
 ( three phases 120 degrees rotated, plus the pulses from the other
 half of the cycle, 180 degrees shifted from the first pulses, hence
 a pulse from each of the 6 rectifiers 60 degrees shifted from the next
 rectifier ) OK, we have our "six phase" supply.

 Now, if we have two sets of secondaries, each gets that same moving field,
 inducing currents into those secondaries.
 The phase of those currents are shifted from the moving field relative to
 the L/R ratio again. Let's assume for the moment that the R is a constant.
 ( it's not, but that gets REAL complex )

 If you look at a Y connected secondary, you have coils in series, so the
 inductances of each adds to the next for that cycle, producing phase
 shift A.
 Look at a delta connected secondary, and you have each coil winding
 in parallel with the other two windings, so the inductances seen by the
 moving field are less than an individual winding, producing phase shift B.
 SO, in the Y connected, the inductances add, so there is a higher L/R ratio,
 while in the delta connection they reduce, producing a lower L/R ratio.
 This gives us separately phases A and B from the two sets of secondaries,
 each a "normal" 3 phase AC output.
 Got it ?

 Now, going back to the above, the currents output are phase shifted relative
 to the L/R ratio of that winding, hence the currents output from the Y connected
 secondaries are shifted more than the currents from the delta connection, due
 to the higher ( series ) L seen by the moving magnetic field.

 Using two sets of full wave rectification, we get 6 "phases" from each set
 of secondaries, AND since the currents in those secondaries are phase
 shifted relative to each other, the output DC pulses are slightly out of phase with
 the DC pulses from the other set of rectifiers.
 SO, when ALL of these pulses are combined on the filter input wire, we get
 12 pulses into the filter for each one full cycle of the transformer primary
 3 phase input current.

 Make sense ?

-- 
Cowboy