Laminate Transformer Testing
Laminate transformers are mostly used as line frequency, low frequency and low/high voltage step-up, step-down transformers. Two coils are wound over a core such that they are magnetically coupled. The two coils are known as the primary and the secondary. The core material tends to be constructed from thin sheets of a soft magnetic material (approx. 0.35mm thick), usually made of 4% silicon steel, called laminations, these are insulated from each other by varnish. These thin sheets reduce eddy currents by increasing the resistance to the flow of such currents. Eddy currents are one of the elements associated with overall core losses. Core loss is the sum of hysteresis and eddy current loss in a magnetic core. Hysteresis is the energy used up by changing the magnetic state of the core during each cycle and eddy currents are currents induced in the core by time varying fluxes. The core is partially assembled prior to the windings being inserted and once inserted the remaining laminate sheets are then interleaved to avoid all of the joints coming into one place, the joints are then staggered similar to laying bricks. Laminate transformers are used in most low frequency applications usually between 50Hz and 400Hz. The primary tends to have a high inductance this allows low frequency use with minimal core losses.
Laminate transformers provide the following: -
- High voltage step-up.
- Low voltage step-down.
- High current output.
For the purpose of this document we will concentrate on a step-down laminate transformers. By designing the number of turns in the primary and secondary windings, any desired step-up or step-down transformer can be realised. The coupling between the primary and secondary must be 'tight' in a power transformer in order to reduce the leakage reactance, otherwise the drop in reactance will be considerable and will vary with secondary voltage and current. Therefore laminate transformers are wound with concentric windings (the primary and secondary are wound with half the turns onto the core limb, one o ver the other (to give a close coupling) with intervening insulation.