Government Engineering College - Bhuj ELECTRICAL ENGINEERING DEPARTMENT 5th SEM. GROUP NO. :- 13

Elements of Electrical Design Enrollment No. :- VISHAL 130150109091 SHAHRUKH 130150109092 KETAN 130150109093 DENISH 130150109094 CHANDRESH 130150109095 RAMKUMAR 130150109096 AKSHAY 130150109097 Guided By:- Prof. H. V. Datania M.E.(Electrical)

STEPS FOR DESIGNING SMALL TRANSFORMER

Transformers having rating up to 1 kVA are called small transformers and are used for purpose like battery charger, battery eliminator and in electronic circuits for stepping down the primary voltage to the low value. Normally shell type of construction is used for the small transformers. For small transformers, following combination of stampings are used for the core:

❶ E-I Type:

❷ T-U Type:

❸ E Type:

To design a small transformer, you need to go through the following steps: Define the parameters pertaining to the small transformer design specifications Output (secondary) voltage (Vs or Vo) Output (secondary) current (Is or Io) Primary (supply) voltage (Vp) Supply frequency (f) Assume transformer efficiency, η = 80 to 92%

STEP-2: Calculation of required core size EMF equation for transformer E = 4.44 fφmN  Turns per volt,  Maximum value of flux in the core The value of Te is selected from Table A corresponding to VA (output) rating of transformer. Net area of core,

For small of core, Bm = 1 wb/m2 Gross core area, Where, Ks = stacking factor = 0.9 If we assume square cross-section for the core then depth of core = width of Limb  Gross core area, Agc = A × A  Width of central limb, A =

STEP-3: Calculation of the number of turns in primary and secondary windings No. of primary winding turns, NP = VpTe No. of secondary winding turns, NS = 1.05 VsTe 5% extra turns are taken for the secondary winding to compensate for the voltage drop.

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 current. Primary current, Area of primary winding conductor, Where,  = current density

Similarly, area of secondary winding conductor The accepted value of  for small transformer is between 2 to 2.5 A/mm2. Enamelled round conductors are used for the windings of small transformers.

STEP-5: Calculation the area of window Space factor (Sf) is defind as Usually average value of Sf = 0.8 ( )2 is used for the round conductors Where, d = diameter of bare wire d1 = diameter of insulated wire  Window area required for primary winding

Similarly, window area required for secondary winding Actual window area required will be about 20 to 25% more as to accomodate insulation between layers and formers in the window.  Gross area of window required = 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 stampings used.

DESIGN OF CHOKE COIL

CHOCK COIL A coil of copper wire wound on a laminated iron core has negligible resistance is known as choke coil. When an alternating voltage is applied to the purely inductive coil, an emf known as self-induced emf is induced in the coil due to self-inductance of the coil which opposes the applied voltage. In the case of tube-light, the sudden stop of current through the choke produce voltage of several hundred volts (approx. 900 to 1000v) across it, because of self-inductance of the coil. The high voltage starts the flow of electrons from one filament to the other through the gas filled inside the tube.

Steps For Designing Choke Coils Design of chokes is similar to the design of small transformers. In chokes the volume of copper required is half that of a corresponding transformer. Therefore a choke may be thought of a transformer having a reating of . where, Q = Volt ampere rating. Specification : Rated voltage, V Current carrying capacity, I in Amp. Supply frequency f.

STEP-1: Calculation of required core size Apparent power Q = VI The choke may be thought of a transformer having rating of . Therefore, the value of turns per volt Te is selected corresponding to volt-ampere rating of . Maximum flux in the core Net area of core, Assume maximum flux density, Bm = 1 wb/m2

Gross area of core, Where, Ks = stacking factor = 0.9 If we assume square cross-section for the core then Depth of core = width of Limb Gross core area, Agc = A × A Width of central limb, A =  

STEP-2: Design of winding Number of turns, N = V × Te Area of conductor or wire a = where,  = current density and its value is assumed between 2.2 to 2.5 A/mm2 . Enamelled round conductors are used for the winding of choke. diameter of conductor or bare wire

STEP-3: Calculate the window area required Window area required for the winding Where, Sr = space factor = 0.8 Where, d = diameter of bare wire d1= diameter of insulated wire In order to accommodate insulation between layers and former in the window, actual window area required is about 20% more. Total window area required = 1.2

STEP-4: Selection of stampings or laminations EI, TU or JJ type laminations are used for the chokes. For a Tube–light chokes, J-J laminations are usually used. The dimensions are as shown in fig.

J-J stampings :

STEP-5: Impedance of the choke coil Impedance of choke coil, The AT or MMF required for the choke coil = MMF required for iron parts + MMF required for air gaps

Usually the MMF required for the iron parts is very small as compared with MMF required for the air gaps. Neglecting MMF required for the iron parts,  Total AT = MMF required for air gaps = ngatglg Where, atg = 800000 lg = length of each air gaps ng = number of air gaps in series in the flux path 

Substituting the value of current I in equation (1), we get For EI, TU and JJ type laminations, there are two air gap in series in the magnetic flux path.

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