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Transformers – Part 2

Old Smokey - 70 VA transformer.

Old Smokey – 70 VA transformer.

Tonight’s post is relatively short compared to last night’s post. Last night I wanted you to really understand how the load on the secondary is reflected back to the primary through the magnetic coupling. It was primarily about concepts. Tonight’s post will talk about one other function transformers are used to do. Then I will talk some about “old smokey” and some what I consider to be shoddy workmanship done any more. (A little bit of a rant.)

Last night we talked about turns ratio or N1/N2 where N1 was the number of windings (turns) in the primary coil and N2 was the number in the secondary coil.

The important ratios are  V1/V2 = N1/N2 and the ratio of currents is:  I1/I2 = N2/N1.

Lets say the transformer has a 2:1 turns ratio and that the primary voltage is 100 Vac rms.   Then using our voltage ratio equation the secondary voltage is 50 Vac rms.   To make everything nice, we will say we have a 50 ohm resistor connected to the secondary winding so the current through that winding would be 50 V/ 50 Ω = 1 A rms.  Using our current ratio formula the primary would be seeing 0.5 A rms load.  This is equivalent to  100 V / 0.5 A or  200 Ω load on the primary.   This example shows another equation and another reason to use transformers.
Z1/Z2 = N12 / N22.   In words: The ratio of the impedance seen by the primary to the impedance at the secondary is equal to the square of the turns ratio.   This was and probably still is very useful in audio circuits to match the impedance required by the final amplifier to the very low impedance of the loudspeakers.  (I say probably, because I am not really up-to-date on the current circuits and there may be solid state circuits now able to directly drive the loud speakers.)

Schematic representation of an iron core transformer with phasing marks.

Schematic representation of an iron core transformer with phasing marks.

The diagram for a a transformer is show in the picture to the right. The lines shown between the two windings represent the iron core.  The transformer may have the turns ratio as shown but usually has the input voltage and output voltage.   There are other combinations used on transformers.  Often the secondary is center tapped.  This means the secondary can provide a large voltage between the outer leads and a smaller voltage between the center lead and either of the two outer leads.   This is exactly the case for the power transformers feeding most houses in the USA.   120 V is provided from the center tap to each outer lead and 240 V is available between the two outer leads.

Other combinations are available also where there are two sets of identical windings on both the primary and secondary windings and depending if these are wired in series or parallel different voltage combinations are available.  Each of these combinations has different current ratings.   Also sometimes one winding has various taps available to compensate for low or high voltage at the power connection.   Both the taps and combination of windings is most often done on industrial equipment and is very seldom done on devices used in residential wiring.

The final  thing to point out about the drawing is the dots shown on the two coils.  A small positive DC voltage is applied to the primary winding and the direction of the magnetic field in the core is determined.   The lead with the primary positive voltage is marked with the dot.   The secondary winding is then marked the same way when the positive voltage produces a magnetic field in the same direction.   This can become very important if there are multiple windings on the primary or secondary.

A weld on the core laminations.

A weld on the core laminations.

I had intended to have the postmortem done on old smokey.  My goal was to remove the windings to show that the center leg of the E in the core is twice as wide as the outer legs.  The problem is it will have to grind some welds off of the laminations of the core to disassemble it.  This weld bothered me because it effectively shorts out all the laminations and provides a path for the eddy currents I talked about in a previous post.  I did some researching on the net about this, because every thing I have read describes the purpose of the laminations exactly as I did.   Some low cost transformers have started doing this welding to produce a cheaper product.   It saves some on labor and some on purchasing the extra hardware for the screws.  It also causes the transformer to run a little more inefficient and hotter because of the eddy currents.  I had never seen this before and I have torn apart many transformers in my younger days.

old smokey's windingsIf you look at the picture to the right, the laminations are provided with holes for the bolts to hold the laminations together.All they are saving by doing the weld is some on labor and the hardware. The hardware if done right would include some fiber bushings to insulate the bolts from the laminations. By doing the quick and easy they are doing three things:

  1. Cutting someone out of employment.
  2. Making it impossible to repair the transformer, or reuse the laminations.
  3. Wasting energy throughout the life of the transformer.

All three of these are examples of waste currently being done only to make a cheaper product.  That in general ticks me off…. (rant over.)   Please note:  Old Smokey is not the transformer with the Jameco name.   I have not removed the cover of that one and I have no idea how it is constructed, but considering the metal cover over it I doubt very much that is has the problems associated with Old Smokey.  Old Smokey was removed from some industrial equipment.




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