# Transformers Questions and Answers – Approximate Circuit of Transformer

This set of Transformers Question Bank focuses on “Approximate Circuit of Transformer”.

1. If R is the resistance of secondary winding of an electrical transformer and K is the transformation ratio then the equivalent secondary resistance referred to primary will be _________
a) R/VK
b) R/K2
c) R2/K2
d) R2/K

Explanation: Resistances or more precisely impedances are transformed in the ratio of inverse square of the transformation ratio or turns ratio. Thus, primary to secondary resistance is equal to reciprocal of square of turns ratio.

2. The use of higher flux density in the transformer design ________________
a) reduces weight per kVA
b) reduces iron losses
c) reduces copper losses

Explanation: If a material is having higher flux density it will store and transfer maximum amount of flux from primary to secondary, which will be very helpful as less core material will be required and weight per KVA will get reduced.

3. The value of flux involved in the emf equation of a transformer is _______________
a) average value
b) rms value
c) maximum value
d) instantaneous value

Explanation: In the emf equation flux involved is maximum flux. Thus, here we can conclude that as flux increase/decrease emf at the secondary also increases/decreases. Emf varies according to the AC wave input.

4. Which winding of the transformer has less cross-sectional area?
a) Primary winding
b) Secondary winding
c) Low voltage winding
d) High voltage winding

Explanation: Winding having less cross-sectional area may be primary or secondary winding. For high voltage winding cross sectional area is less while for low voltage winding cross sectional are is more, due to inverse proportionality.

5. In constant frequency (50 Hz) power transformers, approximate form is used with ___________
a) π equivalent model
b) T equivalent model
c) π and T equivalent both
d) Another model

Explanation: In constant frequency (50 Hz) power transformers, approximate forms of the exact T-circuit equivalent of the transformer are commonly used. Thus, we lump all series parameters of the circuit on either side of T circuit.

6. Exciting current in an electrical transformer will not be affected much if primary is excited with E rather than V.
a) True
b) False

Explanation: Since winding resistances and leakage reactances are very small, V1 = E1 even under conditions of load. Therefore, the exciting current drawn by the magnetizing branch (Gi || Bm) would not be affected significantly by shifting it to the input terminals, i.e. it is now excited by V1 instead of E1.

7. In approximate equivalent circuit of the transformer _______________
a) All resistances and inductances are lumped before magnetizing branch
b) All resistances and inductances are lumped after magnetizing branch
c) Resistances and inductances aren’t changed
d) Any of the above will work

Explanation: Since Io is very small compare to like about 5-10% of full load current, voltage drop can be approximated to very large extent. These all resistances and inductances are in series, combined with each other to give approximate equivalent circuit.

8. Final approximate equivalent circuit contains _________
a) Only equivalent X series branch
b) Only equivalent Z series branch
c) Only equivalent R series branch
d) Any of the above

Explanation: We combine all series parameters of equivalent circuit together to get approximate equivalent circuit of the transformer, where we assume that equivalent resistance is zero as it is negligible, and parallel branch is removed.

Sanfoundry Global Education & Learning Series – Transformers. 