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ECET 211 Electric Machines & Controls

Lecture 5-1 Electric Motors(1 of 4)

Text Book: Chapter 5 Electric Motors, Electric Motors and Control

Systems, by Frank D. Petruzella, published by McGraw Hill,

2015.

Paul I-Hai Lin, Professor of Electrical and Computer Engr. Tech.

P.E. States of Indiana & California

Dept. of Computer, Electrical and Information Technology

Purdue University Fort Wayne Campus

Prof. Paul Lin

Lecture 5-1 Electric Motors

Chapter 5. Electric Motors

• Part 1. Motor Principles

• Part 2. Direct Current Motors

• Part 3. Three-Phase Alternating Current Motors

• Part 4. Single-Phase Alternating Current Motors

• Part 5. Alternating Current Motor Drives

• Part 6. Motor Selection

• Part 7. Motor Installation

• Part 8. Motor Maintenance and Troubleshooting

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Part 1 Motor Principle

Magnetism

Electromagnetism

Generators

Motor Rotation

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Part 1. Motor Principle - Magnetism Magnetism

• In physics, magnetism is a force that can

attract (pull closer) or repel (push away)

things that have a magnetic material like iron

inside them; magnetic lines of forces of a bar

magnet shown by iron filling on paper;

https://simple.wikipedia.org/wiki/Magnetism

• Magnetism is the force that creates rotation

for a motor to operate.

• Permanent Magnet

Inherent magnetic force (field)

Attract and hold magnetic materials such

as iron and steel

Figure 5-1 Magnetic field of a permanent

bar magnet: Lines of flux travel from the

N pole to the S pole

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Part 1. Motor Principle - Magnetism Magnetism

• Demonstration of an electric current in a

wire, by Christian Oersted, a Danish

scientist, 1821; source - http://www-

istp.gsfc.nasa.gov/Education/Imagnet.html

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Part 1. Motor Principle - Electromagnetism

Reference: Chapter 6 Introduction to

Motors and Generators of Electrical

Power and Controls, by Timothy L.

Skvarenina and William E. DeWitt, 2004,

Prentice Hall

Right-hand rule (current flow from

+ to -)

• Thumb – current direction

• Rest of four fingers – Flux lines

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Magnetic field produced

around a current carrying

conductor.

Figure 5-2 Magnetic field

around a straight current

carrying conductor

Left-hand rule (electron

flow, from – to +)

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Part 1. Motor Principle - ElectromagnetismReference: The AC’s and DC’s of

Electric Motor, Motor Mastery

University, Regal Beloit America, Inc,

https://www.centuryelectricmotor.com/

Motor-Mastery-University.aspx

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Magnetic Fields

• Magnetic flux (Φ), SI unit –

Webers, abbreviated Wb

DC Flux – DC circuits

AC Flux – AC circuits

• Flux density B = Φ/A , SI

unit Wb/m2, or Telsas

Figure 5-3 Magnetic field

produced by a current-carrying

conductor

Part 1. Motor Principle - Generator

An electric generator:

• uses magnetism to convert

mechanical energy into electrical

energy

• A mechanical torque is applied to turn

conductors through a magnetic field

and generate electric current.

• A voltage is induced in a conductor

whenever the conductor is moved

through a magnetic field so as to cut

lines of force.

• Faraday’s law of induction: Induced

voltage: e = N dΦ/ dt ≈ N /t (volts)

Figure 5-4 Simplified AC generator

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Part 1. Motor Principle - Generator

DC Generator and Motor

Generating DC is basically the same as generating AC.

The only difference is the manner in which the generated

voltage is supplied to the output terminal – Commutator

Any DC machine can act as a generator or as a motor.

Figure 5-5 Simplified DC generator

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Part 1. Motor Principle – Motor Rotation

An electric motor rotates as the results of the interaction of two

magnetic fields.

Figure 5-6 Motor principle

• Fixed stator N-S magnet

• Moving armature (electromagnet)

• Rotation force is produced as a result of

Like poles repel ( N N; S S)

Unlike poles repel (N – S)

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Part 1. Motor Principle – Motor Rotation

The interaction between

• Magnetic field produced by

the current Permanent N-

S field

• A force being experienced by

the conductor

Figure 5-7 A current-carrying

conductor, placed in a magnetic

field

Figure 5-8 Right-hand motor

rule

• Fixed stator N-S magnet

• Moving armature

(electromagnet)

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Part 1. Motor Principle – Motor Rotation

Developing Motor Torque

(Rotational force)

T = F*r

Figure 59 Developing motor

torque

• By a current-carrying coil or

loop of wire placed in a

magnetic field

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Part 2. Direct Current Motors

Figure 5-11 Typical DC industrial motor

application

For applications with high torque and

variable speed; Examples: Mine hoist,

steel rolling mills, ship propulsion,

cranes, conveyors, and elevators

Ohio Electric Motors,

http://www.ohioelectricmotors.com/dc-

motors-used-in-electric-hoists-reels-and-

winches-1597

Figure 5-12 Major components of a DC

motor

More complicated and expensive than

that of an AC motor

Major components: Commutator,

brushes, and armature windings

Cost for maintenance of the

brush/commutator assembly Prof. Paul Lin 13

Part 2. Direct Current Motors

DC Motor Parameters

Speed: Motor shaft rotational speed, in RPM

Torque:

• Turning force supplied by the motor’s shaft

• T = F * r (force acting on a radius)

• Units:

lb-in (pound-inches)

lb-ft (pound-ft)

Horsepower: the rate at which work is done.

• 1 hp ≈ lifting 33,000 pounds of object to a height of 1 foot in 1

minute

• 1 hp = 746 watts of electric power

DC Current Motors and Drives (1/50 to 3000 hp),

http://www.baldor.com/mvc/DownloadCenter/Files/BR600

Baldor Shekby Plant - DC Manufacturing Plant, 2:35 min, video,

https://www.youtube.com/watch?v=I7WXETxQm30Prof. Paul Lin 14

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Part 2. Direct Current Motors

Permanent-Magnet DC Motors

Use permanent magnets to

supply the main field flux

and electromagnets to

provide the armature flux.

Movement of the magnetic

field of the armature is

achieved by switching current

between coils within the

motor – called commutation

Figure 5-13 Permanent-

magnet DC motor operation

Youtube Video

DC Motor, How it works,

Learn Engineering, 4:49 min

video,

https://www.youtube.com/wat

ch?v=LAtPHANEfQo Prof. Paul Lin 15

Part 2. Direct Current Motors

Permanent-Magnet DC Motors

Figure 5-14 Permanent-

magnet DC motor

Figure 5-15 Armature

commutation of switching

effect

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Part 2. Direct Current Motors

Permanent-Magnet DC Motors

Figure 5-16 Reversing the direction of rotation of a PM motor

Forward/Reverse Control

• Current direction through the armature

Speed control:

• Vary the voltage value apply to the armature

• Higher voltage => Higher speed

• Lower voltage => Lower speed

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Part 2. Direct Current Motors

Series DC DC Motors

Figure 5-17 Series-type DC motor

Series filed: low resistance field (Rs) and low resistance armature

(Ra)

At starting, when DC supply voltage E is applied to the motor, the

starting current is high I = E/(Rs + Ra)

High current => Strong magnetic fields inside the motor =>

produce high torque (Torque & I2)

Idea for starting very heavy mechanical loads

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Part 2. Direct Current Motors

Series DC Motors

Figure 5-18 Speed-torque characteristics

curves for a DC series

Speed Curve

Speed varies widely between no load

and rated load => cannot be used where

a constant speed I required

Run faster with a light load (low current);

Run slower as the load increases

Ability to start very heavy loads

Application areas: cranes, hoists and

elevators

Caution: Never operate a series motor

without a load

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Part 2. Direct Current Motors

Shunt DC Motors

Figure 5-19 Shunt-type DC motor

Shunt field (F1, F2) – higher

resistance

Armature and Shunt field –

connect in parallel

Figure 5-20 Speed-torque

characteristic for a shunt DC

motor

The current through the shunt

field wining: constant, does not

vary with the motor speed

The torque will vary only with

the current through the

armature

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Part 2. Direct Current Motors

Shunt DC Motors

Figure 5-21 Separately excited motor (variable speed drive)

Independent control of the field and armature

• DC controller => Armature voltage

• DC controller => Shunt field voltage

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Part 2. Direct Current Motors

Compound DC Motors

Figure 5-22 Compound-type DC

motor

Shunt field (F1, F2) – higher

resistance

Armature and Series field –

connect in series

Cumulative-compound

connection

• Under load the series field

flux and shunt field flux act

in the same direction to

strengthen the total field

flux

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Part 2. Direct Current Motors

Compound DC Motors

Figure 5-23 DC cumulative-compound motor connections and speed-

torque characteristics

Long-shunt compound

Short-shunt compound

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Part 2. Direct Current Motors

Direction of Rotation

Figure 5-24 DC series motor reversing

motor starter

Figure 5-25 DC shunt and compound

motor starting

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Part 2. Direct Current Motors

Motor Counter Electromotive Force

(CEMF)

Figure 5-26 Motor CEMF

CEMF (back EMF) – a form of

resistance that opposes and limits

the flow of armature current

IA = (VMTR – CEMF)/RA

where

IA = armature current

VMTR = motor terminal voltage

RA = armature circuit resistance

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Part 2. Direct Current Motors

Motor Counter Electromotive Force

(CEMF)

Example 5-1 The armature of a 250V

DC motor draws 15A when operating at

full load and has a resistance of 2Ω.

Determine the counter EMF (back EMF)

produced by the armature when

operating at full load.

Solution:

IA = (VMTR – CEMF)/RA

CEMF = VMTR –( IA * RA)

= 250 V – (15 * 2) = 230V

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Part 2. Direct Current Motors

Speed Regulation

A measure of motor’s ability to maintain its speed from no load to full

load without a change is the applied voltage to the armature or fields

Percent speed regulation = ((No-load speed – Full-load speed)/Full-

load speed) * 100

Example 5-2 A DC shunt motor is running with a measured no-load

speed of 1775 rpm. When full load is applied, the speed drops slightly

to 1725 rpm. Find the percentage speed regulation.

Solution:

Percent speed regulation = ((1775 – 1725)/1725) * 100 = 2.9%

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Part 2. Direct Current Motors

Varying DC Motor Speed

Figure 5-29 DC motor speed

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Figure 5-30 Armature-controlled

DC motor

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Part 2. Direct Current Motors

Varying DC Motor Speed

Figure 5-30 Armature

controlled DC motor

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Part 2. Direct Current Motors

DC Motor Drives

Figure 5-32 The block diagram for a typical

DC motor drive

Figure 5-32 Typical DC motor drive unit

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Summary & Conclusion

Questions?Contact Prof. Lin through:

Email: lin@ipfw.edu

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