Government Engineering College, Bharuch Department of Electrical Engineering Elements of Electrical Design ( ) NAME:MASTER MUAATASIM EN NO:

STEPS FOR DESIGNING SMALL TRANSFORMER STEP-1 Define the parameters pertaining to the small transformer design specifications (i)Output voltage (Vs) (ii)Output current (Is) (iii)Primary Voltage (Vp) (iv)Supply frequency (f) (v)Assume transformer efficiency (ƞ)

STEP 2 Calculation of required core size  EMF equations for transformer E = 4.44fΦmN  Turns per volt Te = N/E = 1/4.44fΦmN  Maximum value of flux in the core Φm=1/4.44fΦmN  Net area of core Ac = Φm / Bm  For small transformer, Bm=1Wb/m²  Gross core area Agc= Ac / Ks Where, Ks= stacking factor = 0.9  Gross core area, Agc = A×A  Width of central limb, A = Agc

STEP 3 Calculation of the number of turns in primary and secondary winding No. of primary winding turns, Np = VpTe No. of secondary winding turns,Ns =1.05 VsTe

STEP 4 Selection of wire size for primary and secondary windings  for primary and secondary, choose a wire that does not generate too much heat in the winding at the desired currents.  Primary currents, Ip = VA rating / ƞ Vp  Area of primary winding conductor, Ap = Ip/δ Where, δ = current density.  Similarly, area of secondary winding conductor As = Is / δ  The accepted value of δ for small transformer is between 2 to 2.5 A/ mm².

STEP 5 Calculate the area of window  Space factor is defined as Sf = active area of conductor in window gross area  Usually average value of Sf = 0.8 (d/d₁)² is used for the round conductor. Where, d = diameter of bare wire d₁ = diameter of insulated wire  Window area required for primary winding = NpAp/Sfp

 Window area required for secondary winding =NsAs/Sfp  Gross area of window required  Aw = 1.2 to 1.25 (NpAp/Sfp +NsAs/Sfp) = 1.2 to 1.25 (window area required for primary and secondary)  The window area calculated by the above should be less than the window area provided by the stamping used.

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