Most transformer problems are nothing more than ratios, but some students find ratios difficult to handle. Questions such as this are great for having students come up to the board in the front of the classroom and demonstrating how they obtained the results. Note to your students how the distinction between a step-up and a step-down transformer is simply a matter of usage. It is possible to use a transformer either way! Question 17 In a typical step-up or step-down transformer, the higher-voltage winding usually uses finer gauge wire than the lower-voltage winding. Explain why this is. Hide answer The higher-voltage winding handles less current than the lower-voltage winding. Notes: If you happen to have a transformer that has been cut in half (right through the core), it will make an excellent demonstration piece for discussion. The difference between windings will be immediately apparent to the students when they see one. Question 18 A mechanic goes to school and takes a course in AC electric circuits. Upon learning about step- up and step-down transformers, he makes the remark that “Transformers act like electrical versions of gears, with different ratios.” What does the mechanic mean by this statement? What exactly is a “gear ratio,” and is this an accurate analogy for a transformer? Hide answer Just as meshing gears with different tooth counts transform mechanical power between different levels of speed and torque, electrical transformers transform power between different levels of voltage and current. Notes: Not only is this a sound analogy, but one that many mechanically-minded people relate with easily! If you happen to have some mechanics in your classroom, provide them with the opportunity to explain the concept of gear ratios to those students who are unaware of gear system mathematics. Question 19 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. Hide 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!
- Spring '14
- Alternating Current, secondary winding, Vsecondary