Lab Information
EXPERIMENT NO. 7
THE AUTOTRANSFORMER
PURPOSE:
To observe the operating principles of the autotransformer, to show the conventional (isolation)
transformer being used as an autotransformer.
DISCUSSION:
The autotransformer is distinguished by the fact that the primary and secondary share a common
portion of its single winding, which is called the common winding. The remaining portion of the
single winding of the autotransformer is called the series winding; this portion is unique to the
primary in the case of a step-up autotransformer. Transformer action takes place between the
series and common portions of the autotransformer winding rather than between the "full"
primary and "full" secondary. For example in figure 13, V1 * I1 = VL * I2. Ideally this is called
the transformed V-A. The remainder of the V-A to the load VL*I1, is called the conducted V-A.
The total V-A to the load is then VL* (I1+I2) = VL*IL.
The advantage of the autotransformer are that it is smaller, lighter and more efficient than a
conventional transformer of comparable rating. Its major disadvantage is the absence of
electrical isolation between the primary and secondary side. As a result d.c. current will be
conducted by the autotransformer as well as overvoltages resulting from transients in a system.
The conventional transformer can be used at a much higher V-A rating when wired as an
autotransformer if the voltage and current ratings of the primary and secondary coils are
observed.
APPARATUS REQUIRED:
1. One Hampden transformer (Model T-100-3A)
2. One Hampden RL-100 resistance load
3. Two Hampden AC ammeters
4. One Hampden AC voltmeter
5. One Hampden variable AC power supply
6. One portable AC ammeter
PROCEDURE:
1. Make the connections shown in figure 11. Measure and record the voltages between X1 and
X2, X3, X4, X6 and X7.
2. Connect the transformer as shown in figure 12. Adjust the load current to approximately 2
Amps. Record all meter readings. NOTE: DO NOT APPLY MORE THAN 2 AMPS.
3. Connect the transformer as shown in figure 13. Adjust the load current to approximately 1.5
Amps. NOTE: DO NOT APPLY MORE THAN 1.5 AMPS.
REPORT:
Prepare a written report to include the following:
1. All circuits and meter readings
2. Calculation of VA in and VA out for each connection
3. Illustrate coil voltage and current ratios for steps 2 or 3
4. Answer to all questions
QUESTIONS:
1. What would happen to the secondary voltage if the connection between X1 and the ammeter
were open circuited in figure 12? DO NOT ACTUALLY PERFORM THIS.
2. How would you change the connections of figure 13 to develop approximately 180 volts
across the secondary, keeping the applied voltage at 120 volts? Draw a schematic to answer this
question.
3. Continuing question #2, would the secondary current be more or less than the secondary
current measured in step #3 if the load is the same. Support your answer.
A SINGLE MULTI-TAPPED WINDING AUTOTRANSFORMER
EXPERIMENT NO. 7
CONVENTIONAL TRANSFORMER USED AS AN AUTOTRANSFORMER