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ELECTRICAL TRANSFORMERS
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Black_Hawk
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Joined: 15 Mar 2004
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Location: Entering The Flags...but simply an arm wielding a sword signifying power

Post ELECTRICAL TRANSFORMERS Reply with quote
Greetings,
Some wortwhile info for whomever is interested in Learning about transformers

Toroidal types:

....Norms - Our complete line of standard inductors
The simplicity of a coil of wire, the power of a Mitchell inductor, the speed of a stocked component. That's the Norm. And now it can be yours quicker than ever. ..OH REALLY?HOW COINCIDENTAL

http://www.mitchellxfmr.com/products/index.php

EXCERPT THE ADVANTAGES ."Small Size
Most toroidals are smaller than their E-1-transformer counterparts. Electrical and mechanical designers, when "painted into a corner" by a minuscule space allotment for power supplies, appreciate a toroidal's compact dimensions.

Flexible Dimensions
compounding the benefits of low weight and small size is the flexibility to vary dimensions. As long as the core cross section is held constant, the height and diameter for the toroidal may be economically varied to accommodate equipment design requirements - a great help when designing low profile, slim line equipment.

Easy to Mount
A single center bolt easily and quickly mounts the torodals, avoiding costly mechanical design and practical problems associated with traditional E-1-laminated transformers..., and three bolts are eliminated at assembly.

Low Stray Magnetic Field
Toroidals have no air gaps: primaries and secondaries are wound uniformly around the entire core. As a result, toroidals emit very low radiated magnetic fields. This makes the toroidal ideal for application in CRT displays, high quality amplifiers, and medical equipment.

Low Mechanical Hum
the core of a toroidal s formed from a single strip of grain-oriented electrical grade silicon steel tightly wound in the form of a clock spring with the ends spot-welded i place. the copper wire is wound over polyester film, forming s silent, stable unit without give or varnish coating.

Reduced No-Load Losses
Compared to traditional E-1-transformers, toroidals exhibit extremely low no-load losses. In applications where a circuit is in a "stand-by" mode for long periods, the potential cost reductions for power can be significant.
================================

REDUCED SIZE REDUCTION THROUGH DUTY CYCLE


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A significant reduction in transformer size and weight may be realized in many cases where the transformer is loaded intermittently. In such cases, the load is energized for a small portion of the period. The period is much shorter than the overall thermal time constant of the transformer. The following equation applies:



...Full Wave Bridge(FWB)


Form Factor (K) = 1.8 IL(DC)=DC current load
IAC=(K) x IDC IS(AC)=RMS current in secondary
K=Form factor associated with circuit type

FORM FACTOR (K)

IL(DC) = DC current load
Is(AC) = RMS current in secondary = K x IL(DC)
K = Form factor associated with circuit type

The "Form Factor" (K) is related to the rectifier circuit configuration and the wave form of the current in the secondary.


lAC = K x IDC

Typical form factor (K) values at capacitor input filter:

Rectifier Type Form Factor (K)
FWB 1.8
FWCT 1.3
FWCT with choke input 0.7
FWCT with dual outputs 1.8
=========================================

E1_ IRON CORE TYPES


........Losses and Iron

Early transformers used pure iron for their cores. Pure iron has a hysteresis loss of about 600 ergs per cycle per cubic centimeter, a relatively small value, and a coercive force of 0.2 Oe from 10 kG, so it is magnetically quite "soft." Commercial sheet iron has a larger, but still small, hysteresis loss. However, it is a fairly good electrical conductor, with a resistivity of 7.64 µO-cm. Therefore, it acts like a short-circuited turn, and currents are induced that circle round the magnetic field as it changes. These eddy currents dissipate large amounts of energy, making a solid core very inefficient. This can be overcome by laminating the core, and interrupting the possible current paths. Bundles of wire were used as cores for this purpose, but thin sheet laminations are now the universal practice. The eddy-current loss is proportional to the square of the thickness of the laminations. It's proportional to the squares of the frequency and the maximum flux density as well.

Using Transformers; Phasing

Three rules should always be observed in using transformers: first, a voltage greatly in excess of the rating should not be applied to any winding; second, a significant direct current should not be allowed to flow through any winding not designed for it; third, the frequency should not be significantly less than the design frequency. If you make the primary of a small 120 V transformer carry, say, 50 mA direct current, and then connect it to the 120 V line, you will have smoke and combustion! Applying 120 V to a 12 V secondary in hopes of getting 1200 V at the primary will have a similar result, accompanied by insulation failure which will add further fireworks. Merely overloading a transformer is not so serious--it will simply get hot and eventually burn up, or, more likely, a wire will fuse. There is normally little chance of running across a lower power frequency, such as 25 Hz. A 60 Hz transformer will draw too much magnetizing current on 25 Hz, and will run quite hot.

However, you can always apply less than the rated voltage to any winding, and use any winding as primary or secondary, so long as the current rating is not exceeded. A few milliamperes of direct current is no problem, either. Any frequency higher than the rated frequency is also safe.

Many transformers are made with duplicate windings, to permit flexibility in operation. Two 110 V primaries can be connected in parallel for 110 V, or in series for 220 V. Even more common are dual secondaries, which can be connected in series or parallel. In parallel, there will be a center tap as well. The windings cannot arbitrarily be connected to each other, because of the problem of phasing. The voltages across the windings will either be in phase, which is the desired state, or in antiphase. If connected in antiphase, the result is a short circuit.

Two windings on a core are shown at the right. The black dot at one terminal or the other of each winding is called a phase or polarity mark. Currents entering the marked terminals cause magnetic flux in the same direction in the core. An increasing current entering the marked terminal as shown will cause a positive voltage at the marked terminal of the other winding. If the unmarked terminal of the winding on the left is connected to the marked terminal of the winding on the right, the two windings will be in phase and their ampere-turns will add. If they are connected in the opposite sense, their ampere-turns will cancel, and there will be no inductance. Unfortunately, transformers are not provided with phase marks, and you have to figure them out for yourself. This process is called phasing.

Of course, most transformers come with numbered terminals and wiring diagrams so that phasing is not required. Still, it is a good idea to check the phasing in any case, and to know how to do it in case you have a strange transformer without instructions. An obvious method is to use an oscilloscope and a function generator, which is easy and l World info:



Hope no one found the above to be too long of a brief intro to trnasformers

Respects..Norm B
Fri Aug 06, 2004 5:20 am View user's profile Send private message
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