Transformer
Experiment No: 2
Open circuit and short circuit tests on single phase transformer 1 Aim
• To understand the basic working principle of a transformer. • To obtain the equivalent circuit parameters from OC and SC tests, and to estimate efficiency & regulation at various loads.
2
Theory
The physical basis of the transformer is mutual induction between two circuits linked by a common magnetic field. Transformer is required to pass electrical energy from one circuit to another, via the medium of the pulsating magnetic field, as efficiently and economically as possible. This could be achieved using either iron or steel which serves as a good permeable path for the mutual magnetic flux. An elementary linked circuit is shown in Fig.1. The principle of operation of this circuit can be explained as follows: Let an alternating voltage v1 be applied to a primary coil of N1 turns linking a suitable iron core. A current flows in the coil, establishing a flux φp in the core. This flux induces an emf e1 in the coil to counterbalance the applied voltage v1 . This e.m.f. is e1 = N1 dφp . dt Assuming sinusoidal time variation of the flux, let φp = Φm sin ωt. Then, e1 = N1 ωΦm cos ωt, The r.m.s. value of this voltage is given by: E1 = 4.44F N1 Φm Now if there is a secondary coil of N2 turns, wound on the same core, then by mutual induction an emf e2 is developed therein. The r.m.s. value of this voltage is given by: E2 = 4.44F N2 Φm where Φm is the maximum value of the (sinusoidal) flux linking the secondary coil (φs ). If it is assumed that φp = φs then the primary and secondary e.m.f.’s bear the following ratio: e1 e2
where
ω = 2πF
=
E2 E1
=
N2 N1
Note that in actual practice, φp = φs since some of the flux paths linking the primary coil do not link the secondary coil and similarly some of the flux paths linking the secondary coil do not link the primary coil. The fluxes which do not link both the coils are called the “leakage fluxes” of the primary and secondary
Open circuit and short circuit tests on single phase transformer 1 Aim
• To understand the basic working principle of a transformer. • To obtain the equivalent circuit parameters from OC and SC tests, and to estimate efficiency & regulation at various loads.
2
Theory
The physical basis of the transformer is mutual induction between two circuits linked by a common magnetic field. Transformer is required to pass electrical energy from one circuit to another, via the medium of the pulsating magnetic field, as efficiently and economically as possible. This could be achieved using either iron or steel which serves as a good permeable path for the mutual magnetic flux. An elementary linked circuit is shown in Fig.1. The principle of operation of this circuit can be explained as follows: Let an alternating voltage v1 be applied to a primary coil of N1 turns linking a suitable iron core. A current flows in the coil, establishing a flux φp in the core. This flux induces an emf e1 in the coil to counterbalance the applied voltage v1 . This e.m.f. is e1 = N1 dφp . dt Assuming sinusoidal time variation of the flux, let φp = Φm sin ωt. Then, e1 = N1 ωΦm cos ωt, The r.m.s. value of this voltage is given by: E1 = 4.44F N1 Φm Now if there is a secondary coil of N2 turns, wound on the same core, then by mutual induction an emf e2 is developed therein. The r.m.s. value of this voltage is given by: E2 = 4.44F N2 Φm where Φm is the maximum value of the (sinusoidal) flux linking the secondary coil (φs ). If it is assumed that φp = φs then the primary and secondary e.m.f.’s bear the following ratio: e1 e2
where
ω = 2πF
=
E2 E1
=
N2 N1
Note that in actual practice, φp = φs since some of the flux paths linking the primary coil do not link the secondary coil and similarly some of the flux paths linking the secondary coil do not link the primary coil. The fluxes which do not link both the coils are called the “leakage fluxes” of the primary and secondary