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1

Electric Motors

• Classification / types– DC Motors– AC Motors– Stepper Motors– Linear motors

• Function– Power conversion - electrical into mechanical– Positional actuation – electrical signal to position

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DC Motors

– DC Motors• Fundamental characteristics

– Basic function

• Types and applications– Series– Shunt– Combination– Torque characteristics

• Modeling

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Fundamental characteristics of DC Motors

N

S

StatorCoils

N

SS

N

Rotor

Stator

S

N

S

N

N

S

End viewTime 0

N

S

StatorCoils

N

S NRotor

Stator

S

N

S

N

N

S

S

End viewTime 0+

Shifting magnetic field in rotor causes rotor to be forced to turn

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Nature of commutation

• Power is applied to armature windings– From V+– Through the +brush– Through the commutator

contacts– Through the armature (rotor)

winding– Through the – brush– To V-

• Rotation of the armature moves the commutator, switching the armature winding connections

• Stator may be permanent or electromagnet

Rotor

V-

V+Brush

Assembly

S

S

N

N

N

Stator

Stator

Comutator

V-

V+

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DC motor wiring topologiesP

erc

en

t o

f ra

ted

Sp

ee

d

Percent of Rated Torque

120

100

80

60

40

20

0

400300200100

0

Sh

un

t F

ield

Series Field

Sh

un

t F

ield

Series Field

Shunt

Series

Compound

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Series Wound DC motors

• Armature and field connected in a series circuit.

• Apply for high torque loads that do not require precise speed regulation. Useful for high breakaway torque loads.

– locomotives, hoists, cranes, automobile starters

• Starting torque – 300% to as high as 800% of full load torque.

• Load increase results in both armature and field current increase– Therefore torque increases by the square of a current increase.

• Speed regulation– Less precise than in shunt motors

» Diminished load reduces current in both armature and field resulting in a greater increase in speed than in shunt motors.

– No load results in a very high speed which may destroy the motor.» Small series motors usually have enough internal friction to prevent

high-speed breakdown, but larger motors require external safety apparatus.

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Shunt wound DC motors

• Field coil in parallel (shunt) with the armature.– Current through field coil is independant of the armature.

» Result = excellent speed control.

• Apply where starting loads are low– fans, blowers, centrifugal pumps, machine tools

• Starting torque– 125% to 200% full load torque (300 for short periods).

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Compound wound DC motors

• Performance is roughly between series-wound and shunt-wound

• Moderately high starting torque

• Moderate speed control

• Inherently controlled no-load speed– safer than a series motor where load may be disconnected

» e.g. cranes

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Permanent magnet DC motors

Permanentmagnetpoles

Pe

rce

nt

of

rate

d S

pe

ed

Percent of Rated Torque

40

20

02001000

120

100

80

60

400300

PermanentMagnet

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Permanent Magnet DC Motors

– Have permanent magnets rather than field windings but with conventional armatures. Power only to armature.

– Short response time– Linear Torque/Speed characteristics similar to shunt wound

motors. Field magnetic flux is constant• Current varies linearly with torque.

– Self-braking upon disconnection of electrical power• Need to short + to – supply, May need resistance to dissipate heat.

– Magnets lose strength over time and are sensitive to heating.• Lower than rated torque.

• Not suitable for continuous duty

• May have windings built into field magnets to re-magnetize.

– Best applications for high torque at low speed intermittent duty.• Servos, power seats, windows, and windshield wipers.

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Modeling DC motors

• A linear speed/torque curve can be used to model DC motors. This works well for PM and compound designs and can be used for control models for narrow ranges for the other configurations

• Model will assume!– Linearity

– Constant thermal characteristics

– No armature inductance

– No friction in motor

Pe

rce

nt

of

rate

d S

pe

ed

Percent of Rated Torque

120

100

80

60

40

20

0

4003002001000

nNo load speed

Stalled rotortorque

s

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DC Motor modeling

+

V R

Armature

E (back emf)

I

T,

]/[ ANmK t

bEIRV

eb KE

IKT t

Motor equations

From the circuit

et

KRK

TV

RKK

T

K

V

tet

Substituting the above:

R

VKT es

tn K

V

For stalled rotor torque

And no-load speed

TKK

R

ten

In terms of no-load speed torque/speed equation is:

2TKK

RTTP

ten

Power is:

Max power is:

R

VP

4

2

max

]/[ radVsKe Units:

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Application

• Use motor voltage and no-load speed to calculate Kt

• Kt = Ke in SI units

• Use stalled rotor torque, V, and Ke to find R– Note, R varies with speed and cannot be measured at rest

• See web download for explanation of Kt, Ke:

http://biosystems.okstate.edu/home/mstone/4353/downloads/

Development of Electromotive Force.pdf

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DC motor control – H-bridge

• Switches control direction– “A” switches closed for

clockwize– “B” switches for counter-

clockwise

• PWM for speed control– “A’s” duty cycle for clockwise

speed– “B’s” duty cycle for counter-

clockwise speed

• Can be configured to brake– Bottom “B” and “A” to brake

M

12V

A

A

B

B

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H-Bridge implementation

• Elements in box are available as single IC

InputLogic

PWM

Direction

Brake

Vsupply

M

GroundH-Bridge Circuit

DC Motor

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Brushless designs

• Commutation is done electronically– Encoder activated switching

– Hall effect activated switching

– Back EMF driven switching

• PM armature• Wound/switched fields• Application

– Few wearing parts (bearings)

– Capable of high speed

– Fractional HP• Servos• Low EMC

+V

Optical Encoder

Armature

+V

Field

Encoder activated switching

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AC Motors

– AC Motors• Fundamental characteristics

• Types– Fractional

» Shaded Pole» Capacitor Start Induction Run

– Integral» Service Factor» Insulation class

– Stepper

• Modeling

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AC motor model

Rs Ls Lr

Lm (Magnetizing Inductance) Rr

mm Lf

EI

2

E Iw

f

E wIkT

E - Magnetizing voltageIm - Magnetizing currentf - FrequencyT - TorqueIw - rotor current - Magnetic Flux, rotor

22wms III

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AC Motors

• Relationship between number of poles and motor speed

P

fN s

120

Poles Speed

(RPM)

2 3600

4 1800

6 1200

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Squirrel Cage Rotor

Seimens AG

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Torque/speed curve

% o

f F

ull-

Lo

ad

To

rqu

e

% of Synchronous Speed

Slip (Full load)

100806040200

250

200

150

100

50

0

Breakdown

Torque

Full-Load Torque

Pull-up Torque

Locked rotor

torque

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Typical starting current

% o

f F

ull-

Lo

ad

Cu

rre

nt

Time

500

400

300

200

100

0

Full-Load CurrentLocked Rotor

(Starting Current)

700

600

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NEMA Motor Characteristics

Design Locked Rotor

Torque

% FL

Pull-up Torque

% FL

Breakdown Torque

% FL

Locked Rotor

Current

% FL

Slip

%

Efficiency

A 70-275 65-190 175-300 NA 0.5-5 Med-High

B 70-275 65-190 175-300 600-700 0.5-5 Med-High

C 200-285 140-195 190-225 600-700 1-5 Med

D 275 NA 275 600-700 5-8 Low

E 74-190 60-140 160-200 800-1000 0.5-3 High

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PWM Variable Frequency Drives

L1

L3L2

Co

ntr

ol L

og

ic

M

Rectifier Filter Inverter

480V

650 V