Hints for the Transformer Lab

Thanks to Arvind for coming up with the following:

ELEC 3105

Basic EM and Power Engineering

Lab 5- The Transformer

(Please make sure you have the latest lab theory + instruction booklet)

Description of Experiments and Hints

Experiment (a)

This is a simple experiment, where we measure the coil resistances R1 and R2 of the transformer, as shown in Fig. 3 using a multimeter. Please remove all connections on the Transformer before you make the measurements.

Experiment (b)

In this experiment, we measure the turns ratio of the given transformer, assuming its ideal. Connect the Wavetech source to the primary and the measure the output voltage amplitude at the secondary and evaluate the v1/v2 ratio.

Experiment (c)

In this experiment, we compute the input impedance and its components (resistance and reactance) with the secondary short circuited. Hence, the simplified equivalent circuit Fig. 5 comes into the picture. Using the set up in Fig. 7, you will measure the current and voltage in the primary. Hence the ration v1/i1 must give the input impedance. Note that this will be complex as we have the effect of a resistance and an inductance connected in series for this set up as shown in Fig. 5. The resitance being R1+ a² R2 and inductance being 2(1-k)L1.

But first, you must figure out how to calculate a resistance and an inductance part of a load when you know the voltage and the current through them. Once you derive the relationships as given in the booklet, proceed to measure the voltage and current amplitudes and relative phase.

Experiment (d)

The purpose of this experiment is to measure the eddy current losses (Rc) and kL1. For this we open circuit the secondary and hence, Fig. 6 comes into effect. As in the previous experiment, you shall measure v1 and i1. From that you can estimate the effective series resistance and the inductance of the circuit in Fig. 6 Rs and Ls as before.

From Fig. 6 you will see that these values depend upon R1, (1-k)L1, Rc and kL1. But we already know the values of R1 (Expt (a)) and (1-k)L1 (half the value of the inductance found in Expt (c)), so to find the remaining unknowns, (Rc and kL1), you must derive the relationship (10) and then use the values of Rs, Ls, R1 and (1-k)L1 to compute Rc and kL1.

Experiment (e) & (f)

You will repeat the above experiment for increasing frequencies and plot the behavior of Rc and kL1 against frequency. Beyond a certain point, you will see a drastic change in the Reactance of the equivalent circuit. Give an explanation for this behavior.

Hint: May be some assumptions made to construct Fig.6 that you so dearly followed for all your calculations isn’t that good after all!

Experiment (g) & (h)

This is again a repetition of the previous experments but with 2 small changes:

1. you will connect a 10 ohm load to the secondary
2. you will model the resulting equivalent circuit looking “into” the source v1 as a resistance and an inductance in PARALLEL as against serial, as you have been doing till now.

Now with v1 and i1, you figure out how to measure the resistance and inductance connected in parallel. Use these relationships (13) to measure the values of Rp and Lp. Compare it with the ideal model described in (5) and speculate why your results don’t satisfy that simple explanation, especially at low frequencies. Also the currents and the voltages distort at some value of v1 as you increase it. Note that point and explain why.

Hint: What do you remember of inductors, when it comes to DC currents and very high frequency AC currents?