2. Chapter 12
Transformers

3. Objectives Explain the operation of a transformer.
Discuss the relationship between mutual induction and transformers.
Describe the effect of self induction.
Calculate the various values of currents and voltages in transformer circuits.
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4. Objectives List three types of transformer losses.
Identify delta and wye transformer connections.
Discuss grounding an electrical circuit.
Explain troubleshooting procedures for transformers.
Describe several special transformer applications.
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5. Transformer Theory Induction Turns ratio principle Transformer power
Transformer losses
Impedance
Phase relationship
Taps
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6. Transformer Theory (Cont.)
Primary and secondary windings
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7. Transformer Theory (Cont.)
Core- and shell-type construction
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8. Induction
Voltage can be induced on a conductor with motion and a magnetic field
With a coil, motion, and a magnetic field, an emf is produced
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9. Induction (Cont.) Mutual induction No physical motion in transformer
Rising and collapsing magnetic field produces motion
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10. Induction (Cont.) Self induction
Separate fields combine to form an overall magnetic field
EMF produced in direction opposite to force generating magnetic field
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11. Turns Ratio Principle
To increase or decrease a voltage, increase or decrease number of turns on the secondary winding
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12. Turns Ratio Principle (Cont.)
Primary of a transformer connects to the source
Secondary of a transformer connects to the load
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13. Turns Ratio Principle (Cont.)
Relationship between primary and secondary voltages and current values
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14. Turns Ratio Principle (Cont.)
Voltage ratio
Current ratio
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15. Turns Ratio Principle (Cont.)
Step-up transformers
Step-down transformers
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16. Transformer Power Power on primary side equals power on secondary side
PP = PS
Expressed in VA (volt-amps) or kVA (kilovolt-amps)
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17. Transformer Power (Cont.)
Find power in primary
Find power in secondary
PS = IS × ES
IS × ES = IP × EP = 900 VA
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18. Transformer Power (Cont.)
Power companies raise voltage to supply cities with electricity
Wires of least resistance capable of carrying strong current
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19. Transformer Losses Copper losses Eddy current losses Hysteresis losses
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20. Impedance Combination of conductor resistance and the self induction
Total resistance value
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21. Phase Relationship in Transformers
Output voltage should be in phase or 180 degrees out of phase with primary voltage
Polarity of induced voltage is opposite to voltage producing it
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22. Taps In secondary winding
Allow a transformer to provide different voltages
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23. Review What is a transformer?
Device used to transfer energy from one circuit to another using electromagnetic induction
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24. Review
What is the ability to produce electrical energy in a conductor without making physical contact with the conductor?
Induction
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25. Review How is voltage induced on a conductor?
With motion and a magnetic field
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26. Review What produces an emf? A coil, motion, and a magnetic field
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27. Review
If two coils are coupled together by a rising and collapsing field, what kind of induction are they using?
Mutual induction
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28. Review
What does increasing or decreasing the number of turns on the secondary winding affect?
It increases or decreases the voltage
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29. Review What is the current ratio?
Ratio between currents in secondary and primary, inversely proportional to turns ratio
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30. Review What unit is used to express power in a transformer?
Volt-amps (VA) or kilovolt-amps (kVA)
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31. Review What are three types of transformer losses?
Copper losses, eddy current losses, and hysteresis losses
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32. Review What is impedance?
Total opposition to current in an ac circuit, a combination of the conductor resistance and self induction
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33. Review
What is a tap?
Fixed electrical connection made to a winding at a point other than its terminals
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34. Typical Distribution Systems
Power generated at plant, voltage stepped up
Energy is routed onto transformers, voltage stepped down
Residential areas have lower voltage distribution systems
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35. Typical Distribution Systems (Cont.)
High voltage results in lower current values
Less current means less power lost
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36. Delta and Wye, Three-Phase Power Systems
Similar to generators
Windings connected in series are delta connected
Windings connected in parallel are wye connected
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37. Delta and Wye, Three-Phase Power Systems (Cont.)
Only two lines of delta connection are equal to 120 V, the other is equal to 200 V
High leg
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38. Grounding Provides safe path for fault current
If one conductor is not connected to earth, a conductor-to-ground voltage reading will behave similarly to an open circuit
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39. Troubleshooting Transformers
Megohmmeter
Disconnect load, insulate secondary leads, fuse primary with small value fuse
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40. Review How are delta and wye connections made in transformers?
Delta connections are in series, and wye connections are in parallel
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41. Review What is the function of a ground?
Provides a safe path for fault current
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42. Review What is a megohmmeter?
An ohmmeter that uses higher than normal voltages and accurately reads resistance into the megohm values
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43. Special Transformer Applications
Autotransformers
Induction circuit breakers
Lighting ballasts
Coupling transformers
Isolation transformers
Automobile ignition coils
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44. Autotransformers Isolation for transformers
One winding without isolation
Used in low-voltage applications
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45. Induction Circuit Breakers
Inductor triggers a switch to shut off current
Strength of magnetic field breaks circuit
Used to protect equipment or prevent fires
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46. Lighting Ballasts Start lamp by producing voltage
Limit current through lamp
Use capacitors to reduce power loss
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47. Coupling Transformers
Separate sections of an amplifier system
Impedance matching
Make sections compatible with one another
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48. Isolation Transformers
Used to separate commercial power systems from sensitive equipment power supply systems
Transient voltages
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49. Automobile Ignition Coils
Type of autotransformer
High turns ratio develops 30,000 V or more
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50. Electric Resistance Welding
When electrical current passes through metal, friction caused by resistance heats the metal
Includes spot welding, projection welding, seam welding, and upset welding
Welding current can be ac or dc, but resistance spot welding typically uses ac
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51. Review What does isolation of a transfer refer to?
The electrical separation between the primary and secondary windings
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52. Review How does an induction circuit breaker function?
An inductor triggers a switch to shut off current
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53. Review Which transformer starts a lamp by producing a voltage?
Lighting ballast
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54. Review
What kind of transformer is used to separate the sections of an amplifier system?
Coupling transformer
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55. For Discussion
Discuss why power companies use high voltage in power lines.
Explain the differences between step-up transformers and step-down transformers.
Discuss how electricity might be distributed in the future.
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56. Glossary Copper loss Current ratio
Heat loss in motors, generators, and transformers as result of resistance of wire. Sometimes called the I2R loss.
Current ratio
The ratio between the currents in the primary and secondary windings in a transformer.
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57. Glossary Eddy current loss High leg
Heat loss resulting from eddy current flowing through resistance of a core.
High leg
The hot conductor in a delta-connected transformer bank that is not common to the neutral. The high leg exhibits a higher voltage to ground than the other hot conductors.
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58. Glossary Hysteresis loss Impedance
Energy loss in a substance as molecules or domains move through the cycle of magnetization. Loss is due to molecular friction.
Impedance
Total resistance to the flow of an alternating current as a result of resistance and reactance.
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59. Glossary Induction Isolation
The ability to produce electrical energy in a conductor without making physical contact with the conductor.
Isolation
Electrical separation between two locations.
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60. Glossary Megohmmeter Mutual induction
An ohmmeter that uses higher than normal voltages and accurately reads resistance into megohm values.
Mutual induction
Two coils located so the magnetic flux of one coil can link with turns of the other coil. The change in flux of one coil will cause an emf in the other.
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61. Glossary Primary Secondary Self induction
The input winding in a transformer.
Secondary
The output winding in a simple transformer.
Self induction
EMF is self-induced when it is induced in a current-carrying conductor.
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62. Glossary Step-down transformer Step-up transformer Tap
Used to lower a voltage.
Step-up transformer
Used to raise a voltage.
Tap
Connection made to a coil at a point other than its terminals.
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63. Glossary Transformer Turns ratio
Device that transfers energy from one circuit to another by electromagnetic induction.
Turns ratio
The number of turns of the primary winding of a transformer compared to the number of turns of the secondary winding.
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64. Glossary Voltage ratio
The ratio between the voltages of the primary and secondary windings.
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