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.


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.


Transformer Theory

• Induction

• Turns ratio principle

• Transformer power

• Transformer losses

• Impedance

• Phase relationship

• Taps


Transformer Theory (Cont.)

• Primary and secondary windings


Transformer Theory (Cont.)

• Core- and shell-type construction


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


Induction (Cont.)

• Mutual induction– No physical motion in

transformer– Rising and collapsing

magnetic field produces motion


Induction (Cont.)

• Self induction– Separate fields combine to form an overall magnetic

field

– EMF produced in direction opposite to force generating magnetic field


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


Turns Ratio Principle (Cont.)

• Primary of a transformer connects to the source

• Secondary of a transformer connects to the load



• Relationship between primary and secondary voltages and current values



• Voltage ratio

• Current ratio

S

P

S

P

N

N

E

E

ratio Voltage

S

P

P

S

N

N

I

I

ratioCurrent



• Step-up transformers• Step-down transformers


Transformer Power

• Power on primary side equals power on secondary side

• PP = PS• Expressed in VA (volt-amps) or kVA

(kilovolt-amps)


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


Transformer Power (Cont.)

• Power companies raise voltage to supply cities with electricity

• Wires of least resistance capable of carrying strong current


Transformer Losses

• Copper losses

• Eddy current losses

• Hysteresis losses


Impedance

• Combination of conductor resistance and the self induction

• Total resistance value


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


Taps

• In secondary winding

• Allow a transformer to provide different voltages


Review

What is a transformer?

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


Review

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

Induction


Review

How is voltage induced on a conductor?

With motion and a magnetic field


Review

What produces an emf?

A coil, motion, and a magnetic field


Review

If two coils are coupled together by a rising and collapsing field, what kind of induction are they using?

Mutual induction


Review

What does increasing or decreasing the number of turns on the secondary winding affect?

It increases or decreases the voltage


Review

What is the current ratio?

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


Review

What unit is used to express power in a transformer?

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


Review

What are three types of transformer losses?

Copper losses, eddy current losses, and hysteresis losses


Review

What is impedance?

Total opposition to current in an ac circuit, a combination of the conductor resistance and self induction


Review

What is a tap?

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


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


Typical Distribution Systems (Cont.)

• High voltage results in lower current values

• Less current means less power lost


Delta and Wye, Three-Phase Power Systems

• Similar to generators• Windings connected in series are delta connected• Windings connected in parallel are wye connected


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


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


Troubleshooting Transformers

• Megohmmeter

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


Review

How are delta and wye connections made in transformers?

Delta connections are in series, and wye connections are in parallel


Review

What is the function of a ground?

Provides a safe path for fault current


Review

What is a megohmmeter?

An ohmmeter that uses higher than normal voltages and accurately reads resistance into the megohm values


Special Transformer Applications

• Autotransformers

• Induction circuit breakers

• Lighting ballasts

• Coupling transformers

• Isolation transformers

• Automobile ignition coils


Autotransformers

• Isolation for transformers

• One winding without isolation

• Used in low-voltage applications


Induction Circuit Breakers

• Inductor triggers a switch to shut off current

• Strength of magnetic field breaks circuit

• Used to protect equipment or prevent fires


Lighting Ballasts

• Start lamp by producing voltage

• Limit current through lamp

• Use capacitors to reduce power loss


Coupling Transformers

• Separate sections of an amplifier system

• Impedance matching• Make sections

compatible with one another


Isolation Transformers

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

• Transient voltages


Automobile Ignition Coils

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


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


Review

What does isolation of a transfer refer to?

The electrical separation between the primary and secondary windings


Review

How does an induction circuit breaker function?

An inductor triggers a switch to shut off current


Review

Which transformer starts a lamp by producing a voltage?

Lighting ballast


Review

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

Coupling transformer


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.

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

Documents

educational use

motion slide

va slide

load slide

magnetic field slide

transformer power power

secondary winding slide

strong current slide