Principles of Electric Machines and Power Electronicsli/slides/ch04-1Sli.pdfPrinciples of Electric Machines and Power Electronics Fig_1-1 Copyright © 2014 John Wiley & Sons, Inc

Copyright © 2014 John Wiley & Sons, Inc. All rights reserved.

Chapter 4DC Machines

Third Edition

P. C. Sen

Principles of Electric Machines

and

Power Electronics

Copyright © 2014 John Wiley & Sons, Inc. All rights reserved.Fig_1-1

Chapter 4 DC machine

•Electric machine Type: rotating machine

Applications: generator (electric source)

motor (electric load)

•Main contentsIntroduction of electric machines

DC machine basics

Armature winding

Armature voltage

Electromagnetic torque

Magnetization (saturation) curve

DC machine classifications


Convert electrical energy to mechanical energy and vice versa

Generator: prime mover rotate rotor shaft to get electric output

(voltage and current)

Motor: apply electric voltage (current) to machine to get

mechanical output at rotor shaft (torque and speed)

One machine can be used as generator or motor

Electric system: voltage and current — electric power

Mechanical system: torque and speed --- mechanical power

Electric machine


Machine types

•DC machineUse or generate DC power

Applications: Automobile(Air condition, oil pump), robots, power

tool

•AC synchronous machineUse or generate AC power

Rotational speed is the same with the electrical speed

Applications: power generator, house hold appliances, Electrical

vehicle

•Induction machine (Asynchronous machine)Use or generate AC power

Rotational speed is slower than electrical speed

Applications: house hold appliances, Electrical vehicle


• Structure

Stator (field) winding: static

Rotor (armature) winding: rotating

DC current through field winding

Commutator: rotating

Brush: stationary

• Features

Precise torque and speed control

Adjusted wide speed range

Constant mechanical torque

Rapid acceleration and deceleration

• Application

Mainly for DC motor application

DC generator is limited due to the wide use of AC power

Large dc motor: conveyors, cranes, paper mills…

Traction motors: transit cars and locomotives

Control device: tachogenerator for speed sensing; servomotor

Structure of DC machine


• Current-carrying conductor in a magnetic field is subject to

electromagnetic force

• Electromagnetic force (Lorentz force): Ԧ𝑓 = Ԧ𝑖 × 𝐵 ∙ 𝑙• Direction: right-hand (screw) rule

Physics behind motor


Operating principle of Motor


Operating principle of Motor- commuter and brush


• Conductor moving in magnetic field generates motional Electromotive

Force (EMF) ε (volt)

• 𝜺 = 𝒗 × 𝑩 ∙ 𝒍• The direction of the EMF: right-hand (screw) rule

Physics behind generator


Operating principle of generator- commuter and brush

Current reversal by commutator and brush


Multi-turns• Series connection

• Shifted physically

• Sum voltage: DC voltage with ripple

Too large ripple!

Operating principle of Generator


Four-pole DC machine--mechanical angle vs electrical angle

ed md2

p

oo md

ed

360One pole pitch 180

p

• Pole pitch: distance between centers of two adjacent poles

• One mechanical cycle, two electrical cycles are encountered


Armature windings

Turn---Coil---Winding


Lap winding of a dc armature—high current low voltage

number of parallel paths(a)=

number of machine poles (p)=

number of brushes

Current paths:

#1: 5-10-7-12;

#2: 8-3 - 6-1 ;

#3 13-2-15-4 ;

#4:16-11-14-9


Current paths:

#1:[8]10-47-36-23-12-49-38-25-14-1-40-27-16-3-42-29-18-5-44-31 [13]

#2:[8] 21-34 [20] #3:[7] 19-32 [19] #4:[7] 8-45 [20]

#5:[19] 47-6-17-30-14-4-15-28-39-2-13-26-37-50-11-24-35-48 [2]

#6:[1] 7-20 [13] #7[1]46-33[14] #8[2] 9-22 [14]

Wave winding of a dc armature


• number of parallel paths (a)= 2

• There may be two or more brushes

• Used for high voltage low current

Wave winding of a dc armature


Armature voltage in dc generator

e Blv

( ) 2 , 2t m t me B l r e B l r

,2

t m

p pB e

A rl

a t m

N N pE e

a a

, , or 2a a m a

NpE K K a p a

a

Ka machine or armature constant


Torque production in dc motor

Right hand: thumb→ current; fingertip→Lorenz

force

f Bli

( ) ( ) ac c

If B l i B l

a

( ) ,2

a a ac c c

I I pIT f r B l r T Bl r

a a a

2 cT NT

a aT K I


Armature output power and DC machine loss

a m a a mP T K I a a a a a mP E I K I

DC Generator outputs electric power DC motor outputs mechanical power


Example 4.1 (practice)


DC machine representation- two circuits

• Field flux is in d-axis direction

• Armature current flux is in q-axis direction


Magnetic circuit of two-pole dc machine

• Magnetic path: pole, air gap, rotor, yoke of the stator (laminated Ferrite)

• Field MMF: 𝐹𝑝 = 𝑁 ∗ 𝐼

• Magnetic reluctance ℛ• Magnetic flux: 𝛟 = 𝐹𝑝/ℛ


Flux-mmf (field excitation) relation in dc machine

At low values of flux, flux is determined by

reluctance of air gap;

At high values of flux, saturation occurs.


Magnetization curve: open-circuit armature voltage vs field

excitation mmf

Induced voltage is proportional to flux times speed

, , or 2a a m a

NpE K K a p a

a

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Principles of Electric Machines and Power Electronicsli/slides/ch04-1Sli.pdfPrinciples of Electric Machines and Power Electronics Fig_1-1 Copyright © 2014 John Wiley & Sons, Inc