hb006 Constant Voltage Transformer (CVT) how does it work?
Introduction
AC stabilisation can be achieved using a simple magnetic device which has no
moving parts.
This is a process of producing a constant ac voltage from a varying ac voltage
supply and involves the use of saturable reactors. The latter may be incorporated
in a special transformer magnetic saturation being produced in a part of the
magnetic circuit.
The winding arrangement and construction of one such constant voltage transformer
is shown in the diagram:
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The core is a three-limbed shell with a magnetic leakage path dividing
the winding space. On the upper winding space is the primary and a compensating
winding while the lower winding space carries the secondary which has
a capacitor connected across it. An increasing voltage applied to the
primary produces an increasing flux in the main magnetic circuit and the
secondary voltage increases proportionally to this voltage. The increasing
flux produces an increase in the leakage reactance of the secondary and
this approaches a value which resonates with the capacitor connected across
it. As the condition of resonance is reached the secondary current rises
rapidly saturating the lower portions of the magnetic circuit. The flux
due to the primary is diverted through the magnetic shunt and further
increases in primary voltage produces little change in the secondary emf.
It increases very slowly and this is offset by the emf induced in the
compensating winding on the upper portion of the core which is connected
in series opposition to the secondary winding. |
Thus once the secondary is brought to resonance the output voltage from the
secondary and compensating windings is constant and it is under these conditions
that the transformer is used.
The advantage of this form of stabilisation is that it can be applied to the
heater supplies in addition to any HT supplies derived from it. Owing to the
non-sinusoidal waveform however readings taken with the usual rectifier-type
meters are subject to error.
The compensating winding produces a small voltage which is used to 'buck' the
output voltage.
To produce a sinusoidal waveform a further winding is added which is coupled
via a magnetic gap. This extra 'neutralising' winding can be arranged to provide
a suitable amount of 3rd and 5th harmonics which when summed with the output
'square' wave above results in a sinewave.
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A first term equivalent circuit is also shown: |
part of the AEL technical handbook return to hb000
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