AC Electric Circuits

Step-up, Step-down, and Isolation Transformers

34 questions By Tony R. Kuphaldt

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Question 19 of 34

Explain how this special transformer is able to control power to the light bulb:

What advantages might there be to using a transformer to control AC power, as opposed to a variable resistor?

Note: a similar type of device is called a Variac, and it enjoys the same advantages of AC power control as the variable transformer shown in the question.

Reveal answer

This transformer controls power to the light bulb by providing a variable voltage ratio between source and load.

Notes:
It may help to give some numerical examples of voltage step-down ratio for the transformer in this circuit, for students to better understand how this device controls light bulb power. Remind your students that modern transformers are very efficient devices, with full-load efficiency ratings typically in excess of 95%.

If students ask about the Variac, you may want to show them this diagram:

Of course, the Variac is a type of autotransformer, and as such does not provide the electrical isolation of a regular transformer. In some instances, this may be important!
Question 20 of 34

In this variable-voltage transformer circuit, the input voltage (120 VAC) is switched to different “taps” on the transformer’s primary winding to create different step-down ratios.

While it is possible to “tap” the secondary winding of the transformer to achieve different output voltages instead of the primary, there is a good reason for locating the switch in the primary side of the circuit. Identify this practical reason.

Reveal answer

To minimize the amount of current the switch contacts have to handle.

Notes:
It is important to always keep in mind practical limitations of components such as switch contacts when designing circuits. Sure, there may be many alternative ways of building a working circuit, but some ways will be more practical than others.

In some cases, it might be better to locate the switch (and winding taps) on the secondary side of a step-down transformer rather than the primary. Imagine if the primary winding voltage was 100 kVAC instead of 120 VAC. Pose this scenario to your students and ask them what practical switch limitations might force re-location to the secondary winding of the transformer.
Question 21 of 34

Suppose a power system were delivering AC power to a resistive load drawing 150 amps:

Calculate the load voltage, load power dissipation, the power dissipated by the wire resistance (R_wire), and the overall power efficiency (η = P_load/P_source).

E_load =

P_load =

P_lines =

η =

Now, suppose we were to use a pair of perfectly efficient 10:1 transformers to step the voltage up for transmission, and back down again for use at the load. Re-calculate the load voltage, load power, wasted power, and overall efficiency of this system:

E_load =

P_load =

P_lines =

η =

Reveal answer

Simple system (no transformers):

E_load = 210 volts

P_load = 31.5 kW

P_lines = 4.5 kW

η = 87.5 %

Complex system (with transformers):

E_load = 239.7 volts

P_load = 35.96 kW

P_lines = 45 W

η = 99.88 %

Follow-up question: can you think of any disadvantages of the circuit using 10:1 transformers compared to the original (transformerless) power system?

Notes:
An example like this usually clarifies the benefits of using AC instead of DC for transmission of large amounts of electrical power over substantial distances, better than simply telling students why transformers are used in power systems. Even with modest power losses in the transformers (say, 3% loss in each), the overall efficiency is still much greater in this system than without using transformers at all.

In discussing the follow-up question, be sure to bring up safety as a consideration if none of your students do.