Chapter 12 Transformers © Goodheart-Willcox Co., Inc.Permission granted to reproduce for educational use only. Objectives Explain the operation of a.

Chapter 12

Transformers

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.

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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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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Transformer Theory

• Induction

• Turns ratio principle

• Transformer power

• Transformer losses

• Impedance

• Phase relationship

• Taps

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Transformer Theory (Cont.)

• Primary and secondary windings

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Transformer Theory (Cont.)

• Core- and shell-type construction

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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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Induction (Cont.)

• Mutual induction– No physical motion in

transformer– Rising and collapsing

magnetic field produces motion

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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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Turns Ratio Principle

• To increase or decrease a voltage, increase or decrease number of turns on the secondary winding

S

P

N

N ratio Turns

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Turns Ratio Principle (Cont.)

• Primary of a transformer connects to the source

• Secondary of a transformer connects to the load

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Turns Ratio Principle (Cont.)

• Relationship between primary and secondary voltages and current values

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Turns Ratio Principle (Cont.)

• Voltage ratio

• Current ratio

S

P

S

P

N

N

E

E

ratio Voltage

S

P

P

S

N

N

I

I

ratioCurrent

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Turns Ratio Principle (Cont.)

• Step-up transformers• Step-down transformers

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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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Transformer Power (Cont.)

• Find power in primary

• Find power in secondary– PS = IS × ES– IS × ES = IP × EP = 900 VA

A 3 100

V 300

R

EI S

A 9 V 100

VA 900

P

PP E

PI

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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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Transformer Losses

• Copper losses

• Eddy current losses

• Hysteresis losses

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Impedance

• Combination of conductor resistance and the self induction

• Total resistance value

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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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Taps

• In secondary winding

• Allow a transformer to provide different voltages

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Review

What is a transformer?

Device used to transfer energy from one circuit to another using electromagnetic induction

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Review

What is the ability to produce electrical energy in a conductor without making physical contact with the conductor?

Induction

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Review

How is voltage induced on a conductor?

With motion and a magnetic field

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Review

What produces an emf?

A coil, motion, and a magnetic field

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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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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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Review

What is the current ratio?

Ratio between currents in secondary and primary, inversely proportional to turns ratio

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Review

What unit is used to express power in a transformer?

Volt-amps (VA) or kilovolt-amps (kVA)

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Review

What are three types of transformer losses?

Copper losses, eddy current losses, and hysteresis losses

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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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Review

What is a tap?

Fixed electrical connection made to a winding at a point other than its terminals

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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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Typical Distribution Systems (Cont.)

• High voltage results in lower current values

• Less current means less power lost

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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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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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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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Troubleshooting Transformers

• Megohmmeter

• Disconnect load, insulate secondary leads, fuse primary with small value fuse

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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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Review

What is the function of a ground?

Provides a safe path for fault current

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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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Special Transformer Applications

• Autotransformers

• Induction circuit breakers

• Lighting ballasts

• Coupling transformers

• Isolation transformers

• Automobile ignition coils

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Autotransformers

• Isolation for transformers

• One winding without isolation

• Used in low-voltage applications

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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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Lighting Ballasts

• Start lamp by producing voltage

• Limit current through lamp

• Use capacitors to reduce power loss

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Coupling Transformers

• Separate sections of an amplifier system

• Impedance matching• Make sections

compatible with one another

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Isolation Transformers

• Used to separate commercial power systems from sensitive equipment power supply systems

• Transient voltages

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Automobile Ignition Coils

• Type of autotransformer• High turns ratio develops 30,000 V or more

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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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Review

What does isolation of a transfer refer to?

The electrical separation between the primary and secondary windings

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Review

How does an induction circuit breaker function?

An inductor triggers a switch to shut off current

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Review

Which transformer starts a lamp by producing a voltage?

Lighting ballast

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Review

What kind of transformer is used to separate the sections of an amplifier system?

Coupling transformer

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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.