Transformer for Drop Machine 55v + 55v Test Transformer Wire Gauges |
Electrical Transformers in Theory and PracticeDuring the early part of my working life I was a student apprentice at the firm of Partridge Wilson & Co Ltd., whose trade name was Davenset. During my apprenticeship I learned about various transformer devices... From 10 mm cube to items that weighed half a ton. |
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Basic design and construction
A transformer generally consists of one or more coils (winding) of conducting wire, wound on a former (bobbin) that surrounds the centre limb (sometimes all limbs) of a circuit of magnetic material (core). The winding wires are insulated and the core is made from thin sheet steel plates known as laminations (this reduces 'eddy current' losses). The assembly is held together by metal cheeks known as clamps, these clamps are held in place by long screws that are insulated from the rest of the structure (again to limit eddy currents). The winding wires are either made off to terminals mounted on the clamps or the wire may leave the coil by 'flying leads'. A typical transformer... |
Laminations
| Common types of lamination are shown at left, they are known by the shape of alphabetic letter that they form. The most common type is the 'E' & 'I' form, the 'T' & 'U' is still used, but was more common in days past. The laminations are often oxidised to form a surface film of oxide that has a higher resistance than plain steel, thus isolating each layer to a certain extent and reducing eddy currents that may occur perpendicular to the plane of lamination. Sometimes one or both sides of a lamination are sprayed with lacquer for insulation purposes. |
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Other forms are possible and the 'F' & 'F' type shown
right is one of these.
Laminations are mostly used at power distribution frequencies of 50 Hz or 60 Hz and audio frequencies, if higher frequencies (up to a hundred or so kHz) are envisaged then ferrite or other sintered iron oxide compounds are used to make solid split cores. Commonly used versions are RM Cores and ETD Cores The linked pages also give some rudimentary design details. |
The magnetic flux runs around the two side limbs and combines in the centre limb which is twice the cross sectional area of a single side limb, thus keeping flux density constant. The flux lines that are indicated in red in the diagram at right would follow a slightly tighter path near the corner fixing holes, than my graphic skills can reproduce. It should be noticed that there is practically zero flux in the centre of the long side and fixing holes or notches are common at this point. |
Coil formers or bobbins
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These are of two types known as 'plain' or 'split', the one shown at
left is plain and the other at right is the split version.
Bobbins these days are mainly injection moulded in plastic, but larger ones often have paxolin or bakalised paper board cheeks. Occasionally, transformers are constructed with two or more independent bobbins, each having it's own winding(s). This method of construction is rare, but has it's uses at very high voltages (above 3,000 volts). |
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Winding Coils
| Two coils are shown, one crimson and one green, they indicate primary and secondary coils. An alternating current flowing in the primary coil will cause an alternating flux in the core which in turn couples with the secondary coil inducing an alternating voltage in it. If this alternating voltage is applied to a load then an alternating current will result. The ratio of turns between the primary and the secondary is proportionate (minus losses) to the voltages on primary and secondary. The number of turns per volt is a function of the cross sectional area of the magnetic circuit, the duty cycle and the allowable temperature rise. Eddy current losses are constant and the fraction of the primary current that is due to this cause is known as the 'magnetising current'. The resistive losses in the windings, due to the current flowing is generally known as 'copper loss' and is proportionate to the percentage of full load that the device is run at. |
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After the core and wound coils are assembled
| the transformer will
look some thing like this. All that remains is to add a pair of
clamps and connect the wires to terminals that are mounted on one
of the clamps. Testing is then performed to ensure that the
specification is correct and that the finished transformer will
operate safely for the rest of it's life.
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Insulating stepped washers
This small flanged fibre washer provides a break in any possible eddy current paths that would occur due to the fixing bolts passing perpendicular to the plane of lamination. |
Terminals
These are made in strips of 10 or 12 and sub divided (using a hacksaw) to provide the correct number of ways to suit the windings and tappings. |
Waste Free Laminations
There is a way of punching 'E' & 'I' laminations that produces a pair of 'I's from the window spaces of two 'E's. This method utilises all of a sheet of rectangular material, hence the name... The principal is illustrated at right. |
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Written... Summer 2001, Revised... 09 October 2001, Revised & spellchecked... 31 Mar 2002, Additions... 01 May 2002, New Domain... 21 April 2004, |