Constant Voltage Transformer

(CVT)Construction and

WorkingBy Er. Mirza Abdul

Construction• The CVT is simply a magnetic transformer of a special construction that has a capacitor connected across the secondary winding of the transformer

•In an ordinary transformer, the primary and secondary windings are wound near each other so that whenever there is a change of voltage across the primary there is a corresponding change in the secondary voltage depending upon the ratio of the turns on the two windings

•  In a CVT the primary and secondary windings are wound separately from each other

•To set up field in between the coils, a separate shunt path is provided between the two windings but an air gap is formed in the shunt path. A capacitor is connected across suitable tapping's of the secondary winding.

Construction

Working of CVT

Working of CVT•The portion of the magnetic core over which the secondary winding is wound is saturated, while the portion over which the primary is wound is not saturated

•A capacitor is connected across the secondary winding to tune out the output at a fre quency very close to 50 Hz

•This capacitor also makes the current in the secondary winding to increase which helps in the saturation of the secondary flux.

Working• the secondary ac flux is restricted to a

saturated value for a large range of the input voltage (170-270 V)

•So a constant voltage is available across the secondary winding.

•Air gap is placed to increase the reluctance of the core and make the impedance HIGH while the transformer is unsaturated

Output waveform•The output voltage will not be of a pure sinusoidal waveform but will be an approximate sinewave with the peaks flat tened approaching a square wave

•a compensating winding is shown connected in series to the secondary to improve the CVT performance.

The input voltage ranges 170 to 260 V and output regulation is 230 ± 2 % at no load to full-load. 

Questions•Why is the waveform Quasi- sine type?•Why is a saturating core transformer

used? How?

•The basic two-component (CVT) Ferro-resonant regulator. The inductor, L, is a linear inductor and is in series with C across the input line. The voltage across capacitor, C, would be considerably greater than the line voltage, because of the resonant condition between L and C (series resonance)

• The voltage, Vp, can be limited to a predetermined amplitude by using a self-saturating transformer, Tl, which has high impedance, until a certain level of flux density is reached.

• At that flux density, the transformer saturates and becomes a low-impedance path, which prevents further voltage buildup across the capacitor.

• This limiting action produces a voltage waveform that has a fairly flat top

• the load voltage is essentially constant and the current varies with the load. A transformer of this type is designed to operate at or just above the knee of the magnetization curve in order to ensure relatively high permeability 

Input Waveform