Single-Phase Transformers (Summary/Quiz)

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Summary

¦ All values of voltage, current, and impedance in a transformer are proportional to the turns ratio.

¦ Transformers can change values of voltage, current, and impedance but cannot change the frequency.

¦ The primary winding of a transformer is connected to the powerline.

¦ The secondary winding is connected to the load.

¦ A transformer that has a lower secondary voltage than primary voltage is a step-down transformer.

¦ A transformer that has a higher secondary voltage than primary voltage is a step-up transformer.

¦ An isolation transformer has its primary and secondary windings electrically and mechanically separated from each other.

¦ When a coil induces a voltage into itself, it’s known as self-induction.

¦ When a coil induces a voltage into another coil, it’s known as mutual induction.

¦ Transformers can have very high inrush current when first connected to the powerline because of the magnetic domains in the core material.

¦ Inductors provide an air gap in their core material that causes the magnetic domains to reset to a neutral position.

¦ Autotransformers have only one winding, which is used as both the primary and secondary.

¦ Autotransformers have a disadvantage in that they have no line isolation between the primary and secondary winding.

¦ Isolation transformers help filter voltage and current spikes between the primary and secondary side.

¦ Polarity dots are often added to schematic diagrams to indicate transformer polarity.

¦ Transformers can be connected as additive or subtractive polarity.

¦ Constant-current transformers are also known as current regulators.

¦ Constant-current transformers are generally used to provide power to series connected loads.

¦ As a general rule, transformer secondary windings should not be connected in parallel.

QUIZ:

1. What is a transformer?

2. What are common efficiencies for transformers?

3. What is an isolation transformer?

4. All values of a transformer are proportional to its … .

5. What is an autotransformer?

6. What is a disadvantage of an autotransformer?

7. Explain the difference between a step-up and a step-down transformer.

8. A transformer has a primary voltage of 240 V and a secondary voltage of 48 V. What is the turns ratio of this transformer?

9. A transformer has an output of 750 VA. The primary voltage is 120 V. What is the primary current?

10. A transformer has a turns ratio of 1:6. The primary current is 18 A. What is the secondary current?

11. What do the dots shown beside the terminal leads of a transformer represent on a schematic?

12. A transformer has a primary voltage rating of 240 V and a secondary voltage rating of 80 V. If the windings were connected subtractive, what voltage would appear across the entire connection?

13. If the windings of the transformer in Question 12 were to be connected additive, what voltage would appear across the entire winding?

14. The primary leads of a transformer are labeled 1 and 2. The secondary leads are labeled 3 and 4. If polarity dots are placed beside leads 1 and 4, which secondary lead would be connected to terminal 2 to make the connection additive?

On the job…

You are working in an industrial plant. You must install a single-phase transformer. The transformer has the following information on the nameplate:

Primary voltage-13,800 V Secondary voltage-240 V Impedance-5% 150 kVA The secondary fuse has a blow rating of 800 A and an interrupt rating of 10,000 A. Is this interrupt rating sufficient for this installation? ¦

On the job…

You have been given a transformer to install on a 277-V line. The transformer nameplate. The transformer must supply a 120-V, 20-A circuit. The transformer capacity should be not less than 115% of the rated load. Does the transformer you have been given have enough kVA capacity to supply the load? To which transformer terminals would you connect the incoming power? To which transformer terminals would you connect the load?

+++++68 Transformer nameplate.

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