Lab Information
TRANSFORMERS AND AC MACHINES
EXPERIMENT NO. 1
TRANSFORMER VOLTAGE, CURRENT AND IMPEDANCE RATIOS
PURPOSE:
To study the relationship between voltage and current in a transformer.
DISCUSSION:
When a load is connected to the secondary of a transformer, the current which flows sets up an
MMF which obeys Lenz's Law, i.e., it opposes the flux produced by the applied primary voltage,
when that flux is increasing in magnitude. As a result, the primary flux and the primary counter
EMF (primary induced voltage) are reduced. The primary current increases because the applied
primary voltage has less opposition from the induced primary voltage. This increase in primary
current supplies the energy required by the load. Note that the primary ampere-turns increase the
flux while the secondary ampere-turns decrease the flux. The net effect is that the flux remains
practically unchanged for various conditions of load.
Neglecting the small exciting current and other transformer losses, the primary and secondary
ampere-turns are equal:
IpNp = ISNS where: Ip = primary amperes
IS = secondary amperes
Np = primary turns
NS = secondary turns
Thus, in this experiment the relationship between voltage, currents, and number of turns is as
follows:
(transformation ratio)
The impedance, or total opposition to current flow under load is:
It can be shown that:
APPARATUS REQUIRED:
1. One Hampden Transformer (Model T-100-3A)
2. One Hampden RLC-100 Resistance-Reactance Load
3. One Hampden Resistance Load
4. Two Hampden AC ammeters
5. One Hampden AC voltmeter
6. One Hampden 120 volt fixed AC power supply
7. One portable AC voltmeter (rated at least 75 volts)
8. One portable AC ammeter (rated at least 1 amp)
PROCEDURE:
1. Make the connections shown in figure 1. Apply approximately 0.4 amperes resistive load
(333Ω) to the secondary of the transformer using the Hampden Resistive Load. Record the
primary and secondary voltages and amperes.
2. Calculate the voltage, current, and impedance ratios and record your results.
3. Make the connections shown in figure 2. This is most easily done by first placing an AC
ammeter in series with the resistive load of figure 1. Then before connecting the reactance load,
apply power to the primary of the transformer. Observe and record the current in the secondary.
Divide the secondary current by a power factor of 0.8. Now connect the resistance-reactance load
as shown in figure 2. After reapplying power, turn the reactance knob counter-clockwise until the
total secondary current equals the value calculated above. This gives a 0.8 lagging power factor
load. Record the primary volts and amps. Now apply a 0.8 leading power factor load to the
secondary. Record the primary and secondary volts and amps.
4. Calculate the voltage, current, and impedance ratios for the lagging power factor load and then
for the leading power factor load. Record the results.
5. Determine the power dissipation, PL of the loads in Steps 1 and 3.
QUESTIONS:
1. What load conditions must be met so that a load in the primary side "sees" an impedance
match in the secondary?
2. What commercial use is made of impedance transformation?
3. What apparent effect did power factor have on the voltage, current, and impedance ratios?
4. Explain the reason for the difference between the secondary voltage obtained for the inductive
and capacitive loads? Why are the current values equal?
REPORT:
Prepare a report containing:
1. Diagrams of each circuit.
2. All calculations and required data.
3. An error analysis.
4. Answers to questions.