26/04/2019

Constant Voltage Transformer - Construction & Working

The performance of the electrical and electronics equipment depends upon quality of power supply. The constant voltage transfer generates constant output voltage for a specific input voltage and specific load range.

Working Principle

  • The CVT is non – linear transformer and works on Ferro – resonance principle. 
  • The transformer operates in the magnetic saturation region as per ferro – resonance principle therefore if there is any change in the input voltage, magnetic flux does not change and output voltage remains constant. Simply, the ferro – resonance works as a flux limiter not voltage regulator.

Construction

  • There are three linear capacitors, non – linear inductors and series connection of core winding and zig – zag winding is shown in the figure A. 
  • The core winding is shown by A and zig – zag winding is shown by A’. 
  • The core is made of magnetic thin lamination in order to reduce eddy current loss.
construction-of-the-cvt

Working

  • When three phase supply is given at the input side, five out of six core are saturated. 
  • The inductor and capacitor makes resonance condition in this saturation condition and it will result in minimum current passes through it. 
  • The saturated core does not provide any voltage at the output and unsaturated core provides constant voltage at the output side. ( The output voltage is directly proportional to input voltage particular at low voltage but the output voltage becomes constant when core is saturated ).
  • The reactive energy requires for core is supplied by capacitors. 
  • The constant voltage at the output depends upon supply frequency, magnetic characteristic of core and number of turns on the non – saturated core. 
  • The number of turns on the zig – zag winding A’ is kept three times that of core winding A. 
  • The rating of CVT depends upon maximum voltage, current, frequency and output power. 
  • The range of CVT rating lies from 110 VA to 1000 VA, 50 Hz. The efficiency of CVT ( above 99% ) increases with increase in rated power. 
  • The power factor of the CVT close to unity, but it should be 0.95 in rated condition.

Output voltage

  • The output voltage is measured across capacitor. 
  • The CVT output voltage
        E = 4.44BNAf
Where 
B = Saturated flux density ( weber / square meter )
N = Number of turns in the output winding
F = Operating frequency
A = Cross sectional area of core ( square meter )
The transformer flux range is shown in the Figure
ΔB = BP – BQ
But ΔB ≈ 0
Therefore BP = BQ
So Eoutput = Einput ( N, A and f are constant )

b-h curve-of-the-cvt


Effect of electrical parameters on output voltage

  • There is change in output voltage in the CVT depends upon change in input voltage as well as magnetic flux of core. 
  • The regulation in the output voltage is good if the B – H curve of the core is square. 
  • The output voltage slightly reduced when the load power factor less than unity. When there is change in supply frequency, the output voltage also changes and it is shown in the graph.

variation-of-the-output-voltage-with-respect-to-input-voltage-in-the-cvt



effect-of-the-supply-frequency-on-the-output-voltage-of-the-cvt


Advantages

  • When there is ± 40% change in input voltage, the change in output voltage is in the range of 1 to 4%.
  • No need of maintenance due to absence of battery and rotating parts
  • The CVT is not effective for sudden voltage sag ( reduction ).
  • The CVT works as current limiter in some specific application and it supply constant current in spite of constant voltage.
  • When load current increases 150% that of rated current, the CVT comes out of saturation region and output voltage reduces drastically and limits the current.
  • The tank circuit of CVT is designed such that it reduces harmonics and noise.
Disadvantages
  • The efficiency of Ferro – resonance transformer is lower than conventional transformer. 
  • The efficiency of CVT is approximately 93% for its full load range and it decreases as the load decreases. 
  • The efficiency of CVT at half load is lies in the range of 75% to 85% whereas the efficiency of power transformer is approximately above 95% for that load. 
  • The design of CVT is done by considering overload or inrush current. 
  • When the size of CVT increases 2 to 3 times for rated current, its efficiency drastically reduces and it also produces humming sound.
Applications
  • Electronic equipments
  • Telecom equipments
  • Electronic type writer
  • Bio medical equipments
  • Micro / mini computer
  • EPABX
  • TV / DVD recorder and player
  • CNC system
  • X – ray machine
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