EXPERIMENT 14

14.1Object: Back-to-Back Test on Two Identical Transformers (Sumpner Test)

14.2Experimental Setup

In this test power is required to overcome the total losses in two identical transformers. The power loss is divided equally in the two transformers. The primary windings of 60th transformers are connected in parallel and a supply of 50 Hz frequency is given. See the circuit of Figure 14.1.

The wattmeter W_i reads total iron loss of the two transformers. The wattmeter W₀ reads the copper loss of the two transformers. I₂ is the secondary current and V₂ is the secondary voltage.

To create a short-circuit we connect the secondary of the two transformers in opposition, so that there will be no voltage and no secondary current. The polarity is checked by the voltmeter. The voltmeters will read 0 V when the polarities are in opposition.

The voltage of the primary transformer varies till the rated secondary current flows.

Through the secondary winding circuit, the readings of the wattmeters are noted down for secondary currents. The constants of the current transformers and wattmeters are noted. A table for experimental results follows. The V₁ and I₁ designate the primary voltage and current, respectively. The V₂ and I₂ designate the secondary voltage and current, and W_i and W_c designate the iron loss and copper loss, respectively. Temperature θ₁ and θ₂ can be tabulated with time t in hours, at full load. The object is to find the losses and the parameters of the transformers.

Figure 14.1

EXPERIMENTAL RESULTS

A rated open-circuit voltage, a no-load current, and power are noted to find the equivalent circuit at no load. The rated short-circuit current, voltage input, and power are noted to find an equivalent circuit at the short-circuit.

14.3Discussion

Efficiency can be found when the losses are measured according to the open-circuit and short-circuit tests described earlier. Refer to the O.C. and S.C. tests described (Experiment 12) previously. The efficiency can be determined with considerable accuracy from the O.C. and S.C. tests. The full-load test is made to determine the temperature rise. A small transformer can be tested on full load by using an artificial load. A water resistance may be an artificial load.

The large units require the Sumpner test or regenerative test due to the difficulty of absorbing a large load. The energy cost will be high for a test. In the back-to-back test we can load the two transformers on full-load condition for several hours to find the temperature rise. The only energy required in the back-to-back test will be to meet the losses.

14.4Questions and Answers on the Experiment

Q1. Why do we use this method when it is very complicated?

Ans. It is used to save the energy loss and because large loads are not needed. Large capacity transformers will require the large load and the back-to-back method will be required. With wasting energy two transformers are tested at full load.

Q2. How are the open-circuit and short-circuit conditions recognized?

Ans. The windings have full-load current to give short-circuit conditions. The rated voltage is applied to give the core loss. Hence the working conditions are recognized.