Description

Figure 1       Transformer equivalent circuit

The secondary winding resistance and leakage inductance components RSS and XSS can be referred to the primary side using the transformer turns-ratio, where they are denoted by RSS’ and XSS’ respectively as shown in Figure 2.

Figure 2       Transformer equivalent circuit secondary components referred to primary

Using the assumption that the magnitudes of the impedances RSP, XSP, RSS’, XSS’ are very much less than RM, XM, the elements RM and XM can be moved so that they connect across the transformer input terminals. Consequently, RSS’ and XSS’ can be lumped together with the primary-side impedances RSP and XSP to give the equivalent components RS=RSP+RSS’ and XS=XSP+XSS’, as shown by the equivalent circuit in Figure 3,

Figure 3       Transformer equivalent circuit with lumped primary side components RS and XS.

The Feedback laboratory equipment will be used to measure and calculate the equivalent circuit parameters shown in Figure 3. Tests will be carried out on a 230V/230V, 50Hz, 100VA single-phase transformer.

1. Transformer Equivalent circuit

The first part of the experiment will determine the parameters for the equivalent circuit shown in Figure 4.

Make sure the ‘3 phase circuit breaker’ is in the off position and the ‘variable output voltage’ is set to 0%, and then connect the Feedback equipment and Voltech PM1000 power analyser as shown in Figure 4.

Figure 4       Schematic connection of the Feedback and Voltech equipment

The input Voltech PM1000 power analyser is connected so that the following parameters can be measured:

V = transformer primary voltage

I = transformer primary current

W=input power (W)

p.f. = input power factor

The load Voltech PM1000 power analyser is connected so that the following parameters can be measured:

V = transformer secondary voltage

I = transformer secondary current

W= load power (W)

p.f. = load power factor

2. Transformer Open-Circuit Test

The first part of this test will establish the turns ratio NS/NP, the transformer magnetising current characteristic and the magnetising components RM and XM.

The switches on the three-phase resistive load 67-142, should be in the off position so that the transformer secondary is open-circuit. On the Universal Power Supply ensure the ‘variable output voltage’ control is set to 0%.

·       Set the ‘3 phase circuit breaker’ to the on position and then rotate the ‘variable output voltage’ in approximately 10% steps up to 100%. Record the primary and secondary voltages and also the primary current using the PM1000 power analysers, for each step.

Table 1 – Open circuit characteristics

Plot a graph of the transformer secondary voltage against the primary voltage and determine the turns-ratio of the transformer from the graph.

Compare and comment on the measured transformer secondary voltage considering the rating of the transformer.

·       With the primary voltage set to 230V, record the primary voltage and secondary voltage, and using the power analyser measure the primary current and primary active power.

Table 2 – Open circuit test results at nominal primary voltage.

From these measurements calculate the transformer equivalent core-loss resistance RM and the magnetising reactance XM. Mark these values on an equivalent circuit of the transformer. Hint - use the equation , where P is the active power, V and I are the primary voltage and current and fM is the phase angle between the voltage and current. Separate the primary current into resistive and inductive components and hence calculate RM and XM.

3. Transformer  Short-Circuit Test

This test will be used to determine the primary-side winding resistance RSP and leakage inductance XSP. For this test the secondary of the transformer is to be connected as a short circuit. Make sure the ‘3 phase circuit breaker’ is in the off position and the ‘variable output voltage’ is set to 0%, and connect the short-circuit link as shown in Figure 5.