Transformer Regulation

Objective:

• To study the voltage regulation of the transformer with varying loads.
• To study transformer regulation with inductive and capacitive loading.

Discussion:

The load on a large power transformer in a sub-station will vary from a very small value in the early hours of the morning to a very high value during the heavy peaks of maximum industrial and commercial activity. The transformer secondary voltage will vary somewhat with the load and, because motors and incandescent lamps and heating devices are all quite sensitive to voltage changes, transformer regulation is of considerable importance. The secondary voltage is also dependent upon whether the power factor of the load is leading, lagging or unity. Therefore, it should be known how the transformer will behave when it is loaded with a capacitive, an inductive or a resistive load.

If a transformer were perfect (ideal) its windings would have no resistance. Furthermore, it would require no reactive power (vars) to set up the magnetic field within it. Such a transformer would have perfect regulation under all load conditions and the secondary voltage would remain absolutely constant. But, practical transformers do have winding resistance and they do require reactive power to produce their magnetic fields. The primary and secondary winding possess, therefore, an overall resistance R and an overall reactance X. The equivalent circuit of a power transformer having a turn ratio of 1 to 1, can be approximated by the circuit shown in Figure-1. The actual transformer terminals are P₁ P₂ on the primary side and S₁ S₂ on the secondary.

In between these terminals we have shown the transformer as being composed of a perfect (ideal) transformer in series with impedance consisting of R and X, which represents its imperfections. It is clear that if the primary voltage is held constant, then the secondary voltage will vary with loading because of R and X.

An interesting feature arises with a capacitive load, because partial resonance is set up between the capacitance and the reactance X so that the secondary voltage E₂ may actually tend to rise as the capacitive load value increases.

Equipment Required:

• A Single Phase Transformer
• Power Supply
• Resistive, Capacitive and Inductive Load
• AC Ammeter
• AC Voltmeter
• Wires

Procedure:

CAUTION!!!

High voltages are Present In the Experiment! Do not make any connections with the power on! The power should be turned off after completing each individual measurement!!!

1. Using Single-Phase Transformer, Power Supply, Resistive Load, AC Ammeter and AC Voltmeter, connect the circuit shown in Figure-2.

2. a. Place all of the Resistive Load switches in their open position for zero load current.

b. Turn on the power supply and adjust for exactly 220 V ac as indicated by voltmeter E₁.

c. Measure and record in Table-1 the input current I₁, the output current I₂ and the output voltage E₂.

d. Adjust the load resistance Z_L to 4400 Ω. Make sure that the input voltage remains at exactly 220 V ac. Measure and record I₁, I₂ and E₂.

e. Repeat (d) for each of the listed values in Table-1.

f. Return the voltage to zero and turn off the power supply.

ZL (ohms)	I2 (mA ac)	E2 (V ac)	I1 (mA ac)
Infinitive	0	210	0
4400	0.05	215	0.06
2200	0.095	210	0.11
1467	0.14	200	0.15
1100	0.18	198	0.19
880	0.225	195	0.23

3. a. Calculate the transformer regulation using the no-load and full-load output voltage from Table-1.

b. For different value of load resistance, we can see the primary and secondary winding VA is not equal. For fixed value of E₁ is 220 V ac and for decreasing load resistance, the primary winding current I₁ increasing, the secondary winding current I₂ increasing and voltage E₂ decrease. For every value of load resistance, we can measure the primary winding VA always greater then the secondary winding VA.

4. a. Repeat procedure 2 using the Inductive Load in place of the Resistive Load.

    b. Record our measurements in Table-2.

ZL (ohms)	I2 (mA ac)	E2 (V ac)	I1 (mA ac)
Infinitive	0	210	0
4400	0.05	210	0.06
2200	0.10	200	0.11
1467	0.14	198	0.15
1100	0.17	193	0.19
880	0.22	185	0.23

5. a. Repeat procedure 2 using the Capacitive Load in Place of the Resistive Load.

    b. Record our measurements in Table-3.

ZL (ohms)	I2 (mA ac)	E2 (V ac)	I1 (mA ac)
Infinitive	0	210	0
4400	0.055	215	0.05
2200	0.11	223	0.12
1467	0.155	230	0.16
1100	0.23	235	0.21
880	0.25	240	0.25

6. Now we construct an output voltage E₂ vs output current I₂ regulation curve for each type of transformer load.

a. Plot recorded values of E₂ (at each value of I₂ listed in Table-1) on the graph of Figure-1.

b. Draw a smooth curve through plotted points. Label this curve “Resistive Load”.

c. Repeat (a) for the Inductive (Table-2) and Capacitive (Table-3) Loads. Label these curves “Inductive Load” and “Capacitive Load”.