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G8MNY  > TECH     28.11.23 08:55z 361 Lines 17095 Bytes #20 (0) @ WW
BID : 61226_GB7CIP
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Subj: Understanding Transformers
Path: SP7YDD<SR8BBS<SR6DWH<SR5DDD<SR4BBX<DB0RES<ON0AR<OZ5BBS<CX2SA<PE1RRR<
      KA1VSC<GB7CIP
Sent: 231128/0757Z @:GB7CIP.#32.GBR.EURO #:61226 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To  : TECH@WW

By G8MNY                                       (Update Aug 15)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

Although the principles have been known since Faraday, here are some practical
pointers. Transformers have the ability to change AC voltages (& hence
impedance match), isolate several different windings, saturate the core to
protect against large current spikes.

THE IDEAL TRANSFORMER
All the copper must be wrapped around all the iron, & all the iron wrapped
around all the copper, as this is not possible the most efficient shape for a
transformer would be a two piece chain, where one end of each link is thinned
down to passes through the hole in the other link.

     __      _                          旼컴컴컴컴컴
   /'  ~~=-~~ '\                         F  ____   
     .-~                Practical      E   旼좔컨커
    (                   Option used    R  /        \
     ~-_                in Line & EHT  R 납 WINDINGS 
   \._..-=-.._./          Transformers   I  \        /
   Copper   Iron                         T   읕쩡컫켸
   MOST EFFICIENT                        E  ~~~~    
                                        읕컴컴컴컴컴

Maximum efficiency happens when the copper losses equals the iron losses.

Typical power efficiency is about 95% for a modest transformer, with smaller
power ones (plug top) down to about 70% & a large substation sized 3 phase ones
up to 98%.

SYMBOL
    컴    旼                        컴    旼
     o )||( o                            )::(
       )||(      the DOT o               )::(
       )||(      indicates               )::(
    컴    읕   same phase           컴    읕
    IRON CORE                     IRON DUST/FERRITE
 The windings can               The core can be tuned
  have tapings                   with a movable part

EQUIVALENT CIRCUIT
As transformers are in the real world they have short comings, this equivalent
circuit demonstrates most of them. (for primary only!)

       Copper                                Interwinding C
     Resistance                 Stray C       旼컴캑쳐컴
  컴컫컴컴R컴컴(((()컴컴컴쩡컴컴컴컴쩡컴컫컴컨컴    旼좔
               =====       )||     ===        o )||( o  Ideal
    ===       Leakage       )||   旼캑   Riron    )||(   Transformer
     쿎      Inductance     )||  __===          )||(
  컴컨컴컴컴컴컴컴컴컴컴컴컨컴컴컴컴컨컴컴좔컴쩡켸    읕쩡
   winding               Primary              읕컴캑쳐컴
 capacitance            Inductance

The iron losses are often indicated with "Riron" like this, but it represents
both eddy current losses & BH curve losses, where the value is not linear with
voltage but a curved relationship with the magnetising current.

           Flux       __.
   IRON         _.-~~/~               A MODERN       ..-컴
   LAMINATE  ./'  ./'                 FERRITE      //
           ./   ./'                              //
    컴컴컴/컴탠/컴컴컴 +H              컴컴컴컴//컴컴컴 +H
       ./'   /'    Amp Turns                 // 
     ./' _./'                              //   
  _.:..-"    -B                        --''    -B
 ~~
With ferrite the eddy current & BH losses are very small but have a sudden
limit, & the BH curve magnetic cycle area which determine the power loss per
cycle is also much smaller than for iron. (difficult to draw here).

The high frequency possibilities of ferrite means much smaller lighter
transformers. For most ferrite cores the power handling is proportional to
frequency up to about 250kHz. So a core handling 50W @ 25kHz can handle 500W @
250kHz, but can the copper & insulation!

CONVENTIONAL MAINS TYPES
These are made of layers on iron sheet with a central bobbin for the windings.
to reduce eddy currents..

  Solid iron           Laminated        Welded Laminated       Iron Dust
      __
 /|\     |             볶                 菉                  같
  |      |             볶                 볶                  같
  |  __ \|/            볶                 볶                  같
      <--

High circulating    Little eddy          Still little        Very little
eddy currents       current in           eddy currents       eddy currents
flow around sides   each sheet as        as only welded      as only tiny
of large cross      little area &        on one side.        area per
section like a      Insulated (oxide)                        particle.
shorted turn.       Surface to next
                    Laminate.
   旼컴
  _   _          The weight of the transformer determines the energy
 /  I  \         through put for 1 cycle, so a 60Hz transformer is 20%
   R           smaller & lighter than a 50Hz one of the same rating.
   O           Ratings also depend on maximum safe enamelled wire
 \_ N _/Bobbin   temperature & the dissipation. E.g. a fan cooled one
                 in a projector would catch fire without the fan!
   읕컴
The laminates are either E I or U T shaped to form the core, with a copper wire
wound bobbin placed on the middle leg. Note the size of the centre limb is
about 2x the outer part as it has to cope with 2x the magnetic flux. These
transformers are normally run close to the core saturation point, so increases
& voltages or a drop on supply frequency soon cause problems.
E.g. a 2x 110V primary 60Hz designed transformer run at 242V across the winding
on series & at 50Hz, would have the magnetic flux increased by 10% from the
voltage, & a further 20% from the frequency change, This could take the iron
beyond super-saturation & cause the inductance to drop many times, producing a
heavy magnetising load current, high harmonic currents & associated heavy
heating of the core & copper windings!

A heavily saturated cores cause high 3rd harmonic line currents & increased no
load powers in the transformer. But properly utilised & choke fed, a saturated
tuned transformer makes the heart of a good voltage regulator/line conditioner.

                        旼컴컴컴커旼       旼컴컴컴컴컴컴컴
                         旼컴컴켸        읕컴컴   旼컴컴
The EI format is often                  旼         旼
made with 2 x I shapes   읕컴컴커                   
cut out of 2 Es, so it          납                   
is efficient in          旼컴컴켸            읕컴    
material usage.                          읕컴컴컴컴컴 
                         읕컴컴커        읕컴컴컴컴컴컴컴
                        읕컴컴컴켸읕          Not Popular

Each lamination is oxidised on one side to produce a poor conductor to the next
lamination to reduce eddy currents. Also to minimise magnetic losses further
the laminations are normally interlaced so that the joining gaps are covered in
the adjacent layers.

To hold the transformer together you can get away with welds across the
laminates provided no electric loop is formed.
                               ___________         ______________
                              쿽         o           ______    
More conventional methods       旼컴컴            쿞mall    
are to use bolt holes in the    쿗arge         읕 쿟rans- 旼
corners or centre of the                      旼 쿯ormer 읕
laminates sides, but the      쿟ransformer          ______   
hole area must not degrade      읕컴컴         ______________
the magnetic circuit.         쿽         o
                               ~~~~~~~~~~~
The bolts hold down 2 clamping plates to squeeze the laminations together, as
when magnetised the laminations will oppose each other & try to push the
laminations apart. The end plates can be fully enclosed to provide full
electrical & some magnetic shielding.
    _____
  _//~~~\\_ Outer       Further magnetic shielding can be
 |~~~~~~~|Copper      proved by a copper strip around
 |_______|Shorted     the whole bobbin former, but
  ~\\___//~ Turn        outside the laminations.
    ~~~~~
Transformer acoustic hum is made from the steel & copper movement due to the
magnetic forces, both at 2x the frequency (6x for 3 phase), but also from
magneto-striction where the steel parts actually shrink & expand as their
magnetic domains in each crystal of iron rotate, so clamping does not reduce 
this noise!

Quite often the whole lot can be varnished dipped/impregnated to provide the
best low noise transformers & corrosion & dampness resistance. But repair is
normally impossible afterwards! 

End plates or crimped folded steel tabs casing, provide mounting holes either
around frames or just 2 holes on Tabs.

            _n_n_n_  
       ____/_______\___
 Side >0  0       0  0          _  /~~~~~~~\  _           旼컴컴컴
 frame ~~~~~~낢~~~~~~     Tab  ~~~~~~~~~~~         旼좔컴컴컴좔
 holes       낢          holes0쿑olded Case0           P C B   
  as   ______낢______         _____________        읕쩡컴컴컴쩡
 well >0  0       0  0             \_______/              잃켠켠켠
        ~~~\~~~~~~~/~~~
            ~u~u~u~

PCB mounts can be by soldering the connection pins moulded on to the plastic
bobbin former.

The bobbin can be either over wound with an insulating layer separating primary
& secondary, or a single bobbin with 2 wind areas, so the insulation integrity
is maintained even in a burnt out state (meets double insulation safety
standard).
   ____________
  /  Secondary \        ~~~~~~~~~~~~~~~~~~~~~~~~~
 /  __________  \           _____       _____    
  /          \            佶袴鳩袴袴鳩袴    
   ________                Primary       
                                       
    CORE              妄袴袴袴袴袴袴    
  ________              Secondary      
   Primary                               
  \__________/            鳩袴佶袴袴佶袴    
 \ Insulation^  /           ~~~~~  CORE ~~~~~~   
  \____________/        _________________________
   LAYERED BOBIN               SPLIT BOBBIN

Another safety feature often included nowadays is a thermal fuse fitted inside
the transformer primary layer. These protect from fires & should not be
bypassed, replacements are available.
The enamelled copper wire windings must be wound fairly tight as like the
laminations there is forces between wires & any movement will eventually rub
through the enamelling & cause a shorted turn & an instant failure.

Connections
These can be wired remotely or tags mounted on the bobbin etc.

TURNS PER VOLT (from Tom GM4PRO)
A blanket approximation of 7 turns per volt, even for the fairly small
transformer.

       E = 4.44*f*B*A  Volts

Where "E" is the e.m.f. per turn (NOT turns per volt),
      "f" is the supply frequency (i.e. 50 Hz.),
      "B" is the peak flux density of the core material (typically 1.5 Tesla
          for cold rolled grain orientated silicon steel, as used in most
          modern transformers),
      "A" is the cross sectional area of the core in square metres.

For a typical small mains transformer, this can be reduced to:

                    Turns per volt = 4.65/A

Where "A" is the core sectional area in square inches.

TURNS RAIIO & MATCHING
For AF transformers in audio stages, an output transformers tapings sets the
load line seen by the output valves. Impedance ratio is just the winding ratio
squared. So a 20:1 transformer turns or voltage ratio gives a Z ratio 400:1
e.g. used on a 15 LS, the Anode sees a 6k load.
Or to match a 17 ribbon mic to 600 AF preamp input, that is a Z ratio of 36:1
or 6:1 turns ratio, so an AF transformer with 1mV input that gives 6mV output
may do.

UNCONVENTIONAL TYPES
In some applications other designs have reduced fields or offer low profile
etc.

L Shaped Core
These are magnetically inferior depending whether the primary & secondary
overlap on common bobbins or are on different legs.
 旼컴컴컴컴
    ___   
憫컴좆 憫컴좆    The core can be made from flat laminations or
             from U curved sections where each lamination
             is a different shape! In that case the magnetic
             flux leakage is much reduced.
읓컴쬔 읓컴쬔
    ~~~   
 읕컴컴컴컴

Toroidal
These are very popular nowadays & offer a much lighter transformer as less
steel is used & they have quite magnetic low flux radiation.

The main problem is in winding them, as this has to be done once the core is
put together. The core is just very long a strip of laminated steel with an
oxide layer on one face to reduce eddy currents, wound on itself in spiral
fashion with an insulating layer on the outside.

  .-~~~~-.     The primary is wound first, as it is normally the thinner wire &
 /  .--.  \    more able to bend around the square cross section of the core.
(  (    )  )   Very thick wire for a high current secondary can be difficult &
\  '--'  /   either several thinner wires used in parrellel, or flat strip
 \'-,__.-'/    used, as the whole secondary has to be on a winding machine's
  '-.__.-'     bobbin shuttle & has to pass through the centre of the toriod.

The cross section looks the same as a conventional layerd bobbin transformer.
Primary to secondary insulation failure is always a possibility with a
toroidal. So using an earth on kit using them is recommended!

Due to the reduced amount of steel & efficient use of copper they also suffer
"randomly" from very high fluxing up turn on currents, as a full half cycle at
turn on looks like "DC" & will keep the core saturated over several cycles,
greatly reducing the inductance. E.g. a 230W transformer, will take about 1A on
full load, including a very low 50mA magnetising current, but up to 13A surge
on turn on. So fusing MUST be a slow blow type.

Mounting them is a problem, as too high a clamping force can damage the
windings. & too little pressure will allow for transit movement & winding
damage "CATCH 22"! So a bolt clamp & plate with rubber mats are normally used.

Circular     Shaped metal plate
rubber mat ======      ======= _   W A R N I N G
           /~~~~\ \_[~]_/ /~~~~\ |\   Low AC Voltage @
          퀃oroid      퀃oroid     VERY HIGH CURRENT
Circular                         available over 1 turn
rubber mat \____/        \____/ |/_  DO NOT SHORT
-----------=====================-----------
     Chassis        賽Bolt   

OTHER TRANSFORMER USES
In audio work they are still used where... good isolation, balancing, phantom
powering, impedance matching are needed, well as provide good RFI protection.

                Phantom supply
                            1:10     10mV     \
Mic  컴컴컴컴컴컴컴컴컴컴)컴  旼컴컴컴컴컴컴캑 >컴
with      1V of RF & hum    )||(      Z=10K > /
Preamp     pickup on a    __)||(
100      long screened      )||( Magnetically    AF MIXER
nominal   balanced cable     )||( Screened        With no RFI
1mV  컴컴컴컴컴컴컴컴컴컴컴컴冒  Case            & no hum
     -------------------------쳐좔컴컴컴컴컴컴
     Dirty Earth return      __ RF Ground

There are costs of course, as it is difficult to proved a wide frequency range
needed for really super audio, but 40Hz - 15kHz @-1dB is quite achievable, with
>60dB common mode hum & RFI rejection.

RF TRANSFORMERS
These iron dust cores or shaped ferrite cores. Shapes can be EE or UU, pot
core, toroidal, or even just a rod.

旼컴컫컴컴    _________       .--~~~~--.
 旼컨컴                  /          \        _.--._
 _____      旼컴      旼컴컴컴컴컴커      /      \       _______
                      납~~~  ~~~납       /~~\       ()      )
  ____     쳐캑   쳐캑    척   쳐캑   척                 ~~~~~~~
                     납___  ___납       \__/        rod type
 읕컫컴      읕컴      읕컴컴컴컴컴켸      \._  _./      (radiating!)
읕컴컨컴컴   _________    section through       ~~
     EE            UU          a pot core        Toroidal

For switch mode applications at 10-100kHz the power capability can be enormous
compared to conventional mains frequency type of the same size.

Several stacked ferrite rings can be used to provide useful broadband HF power
output transformer.

              .=====.                       O/P 1__    ____
   O/P 1콘複//複 複\\複複횽/P 2                   )::(
          ( | | ) ( | | )                   +12V___)::( Aerial
          ( | | ) ( | | ) -6x rings                )::( Secondary
          ( | | ) ( | | )                   O/P 2__)::(____
  +12V賽賽賽\\賽賽//賽賽              2x 1/2 turn    5 turns
Primary is    '=====\\ Secondary
brass tubes inside   \\ turns inside
rings connected          the tubes
to end pates.

For higher frequencies & low power, bead types (miniature toroid), or double
hole versions give extended frequencies up to 1GHz. The 2 hole one offers
complex RF hybrid possibilities used in mixers etc.
 __    _ __
((o)  ( (oo)
 ~~    ~ ~~

See my Tech bul on special uses like in "Microwave Ovens" & "Fluorescent Lamp
Circuits".
 

Why don't U send an interesting bul?

73 De John, G8MNY @ GB7CIP


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