This article describes some power transformers I wound in my youth. That is some time back, and they were wound using modern techniques for the time.
The transformer cores are 'C' cores, two halves in the shape of a U (rather than C) each of which are formed of high grade laminated steel laid up and glued into a unit with carefully ground surfaces to mate with the other half.
Fig 1 shows a power transformer conservatively rated at 800VA for continuous operation. It was intended for a 24-28V power supply with choke input filter, depending on the rectifier configuration. The transformer was tested at 1800W into an AC load for a 15min and winding temperature rise was not excessive
The C cores, in this case two pairs are held together by a stainless steel band. One side of each pair of C cores forms the central limb.
Two through bolts clamp the C cores between the mounting brackets, and some brass screws can be seen on the terminal board, but other than that, no screws are used in the transformer.
The bobbin is cut from fibreglass sheet, and the cheeks and centre parts are cut so as to interlock, no glue or adhesive tape is used to form the bobbin. Chips of fibreglass board are used to packing between the bobbin and core to secure the bobbin to the core.
The winding layers are insulated with mylar film, and windings separated with Elephant Hide.
When assembled and tested, the transformer was pre-heated and immersed in hot transformer varnish in a vacuum tank, and impregnated. Such varnish impregnation minimises the risk of winding vibration and noise.
The outcome is a transformer with mass of 7.8kg conservatively rated at 800VA with hysteresis and eddy current loss of less than 10W at 246V.
Fig 2 shows the choke intended for the same power supply project. Same technology but one size smaller C cores and again two pairs used.
The advantage of the choke input filter is better regulation from the filter, lower dissipation in a series pass regulator, lower dissipation in the rectifier (due to greater conduction angle), but at the expense of cost and weight.
The choke is a 'swinging choke', it enters saturation approaching 10A, so that its reactance decreases at higher load current. Filter capacitance requirement of the filter is lower for the choke input configuration and 10,000µF is quite sufficient for this application. The high current rating is principally to deliver a low DC voltage drop.
It weighs in at 6.1kg.
Fig 3 is a transformer intended for a battery charger, and again the same technology. This is built on one pair of C cores, and the band-it used to secure the C cores and mounting bracket can be seen. The design of the interlocking parts is obvious from this pic. The side by side winding configuration has slightly higher leakage reactance which suits a battery charger.
Again the bobbin is made from interlocking parts and packed to the core to secure it. The only screws used are the ones visible for terminals on the bobbin cheeks.
Weight is 3.4kg.
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© Copyright: Owen Duffy 1995, 2012. All rights reserved. Disclaimer.