1.0 Testing Line Frequency Transformers
Transformers appear in almost every electrical and electronic product
that the world produces providing the world with an enormous need for
transformers.
Testing transformers and wound components is essential before final
assembly into product. This filters out failures ahead of time, avoids
costly re working, reduces manufacturing costs and improves overall
reliability.
Transformer testing thus requires:
Fast effective quality controlled manufacturing methods.
100% testing securing zero rejects sent to the customer
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.
- Isolation.
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.
Voltech transformer testers combine virtually all tests into one box
resulting in fast test time avoiding re configuration for each test.