1. G H PATEL COLLEGE OF ENGG. AND TECH.
ELEMEMNTS OF ELECTRICAL DESIGN
TOPIC : DESIGN OF SMALL TRANSFORMER
PRESENTATION BY :
parth trivedi ( )
valiya lalji ( )

2. INTRODUCTION
Practical transformer design requires knowledge of electrical principles, materials, and economics.
For a transformer using a sine or square wave, one needs to know the incoming line voltage, the operating frequency, the secondary voltage(s), the secondary current(s), the permissible temperature rise, the target efficiency etc.
Specifications for transformer design varies with transformer application, tolerance level and cost limitations.

3. CONTINUED
Transformers having rating upto 1 kVA are called small transformers and are used for purpose like battery charger, battery eliminator and in electronics circuits for steeping down the primary voltage to the low value(i.e. 6 V , 12 V , 24 V etc.)
Normally shell type of construction is used.
For small transformers, following combination of stampings are used for the core :
> EI
> TU
> E in pairs

4. STEPS FOR DESIGNING SMALL TRANSFORMER
STEP 1- DEFINE THE PARAMETERS
Output (Secondary) Voltage ( Vs OR V0)
Output (Secondary) Current (Is OR Io)
Primary (Supply) Voltage(Vp)
Supply Frequency(f)
Assume transformer Efficiency, η = 80 to 92%

5. STEP 2: CALCULATIONS OF REQUIRED CORE SIZE
Emf equation for transformer E = 4.44 fΦmN
Turns per volt(Te) = N/E = 1/4.44 fΦm
Maximum value of flux in the core(Φm) = 1/4.44 fTe
The value of Te is selected from table shown in previous slide.
Net area of core , Ac = Φm/Bm
For small transformer, Bm = 1 Wb/m2
Gross core Area , Agc = Ac / Ks

6. STEP 2: CALCULATIONS OF REQUIRED CORE SIZE
Ks is staking factor = 0.9
If we assume square cross sectional for the core then depth of core = Width of limb
Gross core area , Agc = A × A
Width of central limb, A = √Agc A A

7. STEP :- 3
Calculation of the number of turns in primary and secondary windings :-
No. of primary winding turns, Np=Vp.Te
No. of secondary winding turns, Ns=1.05Vs.Te
5% Extra are taken for the secondary winding to compensate for the voltage drop.

8. STEP:-4 Selection of wire size for primary and secondary windings :-
We choose wire that does not generate too much heat.
Ip = VA rating/µVp
now area of primary winding conductor
Ap = Ip/δ

10. STEP:-5

12. The window area calculated by the above should be less than the window area provided by the stamping used.
i.e window area provided by the selected stampings > window area calculated .
Now, one Example for better understanding……