Electrical machine lab report 2

Electrical Machine Lab report 2

The introduction

The main aim of the paper is determining voltage and current characteristics of a transformer as well as determines its efficiency. The results will establish the core flux required. This is required at no load also, and hence it must be provided by the supply to which the primary windings are connected. In the case of an ideal transformer, the flux linking with primarily winding, core flux, and the flux lining with the secondary winding is same. In other words the two coils are having the coupling coefficient of unity.


Graph below shows that the exciting voltage increases as the applied voltage increases until level saturation is reached. This shows that the exciting voltage is enhanced by the gradual increase in applied voltage, hence the name is enhancement. If applied voltage is removed, the electric field and the excitement voltage will vanish and therefore the transformer will return back to blocking state.

electrical machine lab report 2

Transformer polarity

The graph shows the relationship between the current and voltage. From the graph it can be noted that it rises to a certain point and it begins to raise a slow rate .

electrical machine lab report 2 1

The change is the graph look is due but then saturates of conducting materials in the transformer. When the voltage increases, the current saturation rises as well, and tends to remain constant for increasing values of voltage. Consequently the drain current is enhanced by the gradual increase in the output voltage.


The winding connected to the source is known as the “primary’ winding while the other winding is known as the ‘secondary winding’. The voltage is transformed in the ratio of turns in the transformer. The secondary winding can supply electrical power at a voltage at fHz to a load connected to it. As load is connected to the secondary winding, a current I2 flows, decided by the load impedance Z1 such thatelectrical machine lab report 2 2. This I2 is due to V2 and V2 is due to the induction. According to Lenz’s Law, the induced voltage is in such a direction that it opposes its cause. In our case, V2 is induced due to a changing flux. Thus, the direction of V2 is such that a current I2 due to it tries to reduce the changing flux. But, form equation, for constant V1,electrical machine lab report 2 3 is constant, these two conditions are satisfied only when the primary winding automatically draws a proper current I1 from the supply, so that the secondary mmf (=I2N2) is neutralised. This balance of ampere turn on the two sides is another operating principle of the transformer.

So far, the discussion was related to an ideal transformer, in which the leakage fluxes, winding resistances, the core loss, and the magnetising current were ignored. As the name suggests the secondary side is on no load during this test. Since there is no secondary current, the primary current is equal to the magnetizing current. The active component of the no load current corresponds to the core loss and reactive component of this current corresponds to the magnetising (reactive-) volt amps require by the transformer. Measuring power input, current and voltage on the primary side, the active and reactive components of the no load current can be separated and thus,
electrical machine lab report 2 4 of the shunt branch of the equivalent circuit can be determined.


When a transformer is loaded, its terminal voltage changes and there are load-dependent losses in its windings, the change in terminal voltage depends on the magnitude and the power factor of the current. The variable or the load-dependent losses are the copper losses which obviously vary as the square of the winding currents. The performance of a transformer should be good, when loaded with minimal losses. These are also termed as ‘good efficiency and good regulation’ respectively, for the transformer. These can be known either after loading or from the calculations based on its equivalent circuit.