DC machines 1

DC machines

Transforms mechanical energy into electricenergy with DC voltage and current (DC generator or dynamo), or conversely (DC motor)

ball bearing

ball bearing brush

brush holder

stator (inductor)collector

rotor (armature)

29/05/2020

2

DC generators

Armature or rotor : • stack of thin magnetic sheets (some tenth of a mm)

(perpendicular to the machine axis to reduce eddycurrents) ...

• ... supporting conductors in which electromotiveforces (e.m.f.s) appear when the armature rotates(e = v ´ b) ...

• ... these e.m.f.s are time-varying and change signeach time the collector crosses a neutral line (bissector between 2 successive poles)

Inductor or stator: 2p poles withexcitation windings carrying DC current

Magnetic circuit:stator + rotor + airgap

2 poles

4 poles

Collector: copper strips isolated from each other, and connected to equidistant points of the armature winding. Fixed brushes slide on the collector and rectify (mechanically) the e.m.f.s

DC machines

3

No-load characteristic

Variation of the voltage Ev as a function of theexcitation current Ie, at constant speed and with nodelivered current

îíì

==q

=0I

constantevitesseavec)I(fE

aev

!

No-load characteristic

)I(kE evEv Fq= !

Magnetic fluxproduced by the inductor andseen by the armature winding

Nonlinear with hysteresis

(3) Increasing Ie

(2) Decreasing Ie

(1) First magnetization

DC machines

constant speed with

4

Armature reaction

Magnetic phenomena due to the currents in the armature

Armature reaction (magnetic)

)I(EE av y-=

)I(k)I( aEa DFq=y !

+ =Inducto

r field

1. Neutral line shifted(rotated) in the rotation directionÞ decrease of the e.m.f.

2. Local magnetic field reduction (entry part) and increase (exit part) not compensated due to nonlinearityÞ flux and e.m.f. reduction (+ incr. pmag)

armature fie

ld

Total field

e.m.f. with load1. and 2. armature reaction

DC machines

5

Armature reaction

aaaa IR)I()I( +y=Y

Total armature reaction

Compensatingwinding

disadvantages:• for a single value of Ia• shift direction depends on

rotation direction• shift direction depends on

operating mode (generator or motor)

Shift of the brushes w.r.t. neutral axis

Reduction of the armature reaction

DC machines

6

Exterior characteristics

Variation of delivered voltage U in terms of thedelivered current I, at constant speed and excitationcircuit

îíì =q

=invariable excitationd'circuit

constantevitesseavec)I(fU

!

Exterior characteristic of a generator

Excitation type...

seriesindependent shunt compound

inductor

armature

DC machines

withfixed excitation circuit

constantspeed

e

7

Independent excitation generator

IR)I()I(EUII

aev

a-y-=

=

Ra » 0.1 W (110V/50A machine)Compensated armature reaction

Delivered voltage quasi independentof delivered current® Voltage source

+ Y(Ie) ... max. in the magnetizationcurve corner

Excitation current Ie modification Speed modification

IR)I()I( a+y=Y

DC machines

e

8

Series excitation generator

Speed modification

Rs << since Ie = I is highcoherent: section >, ns <

Quasi-linear

U almost fixed

I almost constant

Current source

usefulzone

Inductor shunting

( )IRR)I()I( sa ++y=Y

( )IRR)I()I(EUIII

savea

+-y-===

DC machines

9

Shunt excitation generator

ed

aaaev

ea

IRUIR)I()I(EU

III

=-y-=

-=

Rd >> to reduce Joule lossesIe < Þ ns >

Picou construction

daea

212e1e

2v1vedaa

1a

eed

aev

RUIIII

UetUIetIEetEIR)I()I()pointparpointprocédure(IPour

)I(UIRconnus)I(et)I(E

-=-=

®®º+Y®Y®

=Y

IR)I()I( aaa +y=Y

no-load

short-circuit

DC machines

known

For (point by point procedure)&

& &

&

10

... the voltage varies however more than for the generator withindependent excitation

Shunt excitation generatorExterior characteristic

Operating point of the generator driving a resitance R

Delivered voltage almost independent of the delivered current® Voltage source

DC machines

11

Shunt excitation generatorSpeed modification Excitation circuit modification

Effect of hysteresis

Short-circuit current

2 branches : Ie increasing and decreasing

If the speed is too low of if Rd is too large® no operating point

DC machines

12

Compound excitation generatorMixed excitation: shunt inductor and seriesinductor wound on the same poles

(1) antagonist compound (opposite m.m.f.)(2) shunt dynamo(3) concordant compound (same direction m.m.f.)(4) hypercompound (ns >>)

fdad

sedased InInnInInIn.m.m.f =÷÷

ø

öççè

æ±=±=

÷÷ø

öççè

æ==

Y-=

ad

sfded

afv

InnIRIRU

)I()I(EU

!

÷÷ø

öççè

æ+Y= a

d

sfdafv InnIR)I()I(E !

DC machines

m.m.f.

13

Self-starting generator

Self-starting is possible thanks to the remanentmagnetization of the inductor

Condition:Rd+Ra not too large!

yesyes

no

Example: shunt generator

Operatingpoint

DC machines

14

If E decreasesÞ the generator receives energy (motor for shunt and compound machines!)

DC network connection

Conditions :E » UE and U in opposition

a

aea R

U)I,I(EI -=

After connexion (1) :

If E <<, Ia >> !

Then, increase E (2)Þ the generator produces energy

operating point

Slope should be large (to reduce the currentvariations dues to voltage perturbations)Þ compound antagonist generator OK

DC machines

15

DC motorsMain principle

aa IREU +=

Total e.m.f. (E) on brushes is equal to the integral ofthe electromotive field along the armatureconductors

... in the armature conductors as soon as they rotate,opposed to the current

Electromotive force (e.m.f.)

The armature conductors are subjected to themagnetic flux density created by the inductor

Excitation current Ie and armature current Ia

... hence to the Laplace force bjf ´=

... hence to a torque that tends to make the armaturerotate

DC machines

16

Armature reaction

aaaa IR)I()I( +y=Y

Total armature reaction

)I(EE av y-=

)I(k)I( aEa DFq=y !e.m.f. with loadarmature reaction

DC motor DC generator

aa IREU += aa IREU -=

aaaa IR)I()I( -y=Y

DC machines

17

Motor torque

aaavaa IR)I(EIREU +y-=+=

( ) 2aaaav

2aaaa IRI)I(EIRIEIU +y-=+=aI´

Electric power provided to the armature

Joule losses in the armature

Electromagneticpower

q=

q= !!

aelm IEPC

Electromagnetic torque

[ ] aaevEaaeE I)I()I(kI)I,I(kC DF-F=F=

DC machines

18

Mechanical characteristics

Motor speed in terms of the electromagnetic torque,with fixed voltage and excitation circuit

îíì =

=qinvariable excitationd'circuit

constanteUavec)C(f!

Machanical characteristic of a motor

Excitation type...

independent or shunt series compound

inductor

armature

!

DC machines

withconstant

fixed excitation circuit

19

Shunt excitation motor

[ ])I,I(fC

I)I()I(kC

ae0

aaevE=

DF-F=

1)I(

)I()I()I,I(fev

aevae £

FDF-F

=

with

C0 = torque produced by the motorif there was no armature reaction

(Ie constant)

[ ] aaaevEaa IR)I()I(kIREU +DF-Fq=+= !

[ ] )I,I(f1

)I()I(kIRU

ae0

aevE

aa q=DF-F

-=q !!

)I(kIRU

evE

aa0 F

-=q!

aevE0 I)I(kC F=

Speed almost independentof torque

DC machines

20

Shunt excitation motor

Influence of Ie

Stable and unstable zones

Small perturbation: e.g. speed increase

From P:motor torque P'' < resisting torque P'Þ speed decreases, back to P Þ stable

From Q:motor torque Q'' > resisting torque Q'Þ speed increases! Þ unstable

Limited speed range (saturation)DC machines

21

Shunt excitation motor

Influence of the voltage U

Poor efficiency!

+ power electronics...

)I(kU

evE F»q!

High dynamic torque control (since la <<)

DC machines

22

Series excitation motor

( ) asaaE

aaE

IRR)I(kUI)I(kC

++Fq=

F=!

( ) asaaME

2aME

IRRIkU

IkC

++ql»

l»!

Non saturatedmachine

Saturated machine

( ) asaSE

aSE

IRRkU

IkC

++qF»

F»!

sa

SESE RR

kUkC+

qF-F»

!

No loadÞ runaway!

Maximumcurrent !

( ) 2

2

ME2

saME

2

MEU

k1

RRk

UkCql

»++ql

l»!!

DC machines

23

Series excitation motor

Influence of the voltage source UShunting the inductor

+ power electronics...

aa'ss

's

e IIRR

RI £+

=

)I(II'I)I( aaMaMeMe F=l£l=l»F

MM'ss

's

MRR

R' l£l+

=l

Typical useElectric traction and lifts (large startup torque)

DC machines

24

Series excitation motor

BrakingaaE I)I(kC F=

Change the sign of the torque to work as a brake

Electric power changes sign(recovers energy)

DC machines

Different modes

25

Compound excitation motor

Mixed excitation: shunt and series inductor woundon the same poles

fd

ad

sed

ased

In

InnIn

InIn.m.m.f

=

÷÷ø

öççè

æ±=

±=

DC machines

m.m.f.

26

DC motor startup

>>=a

a RUI

a

E

aa R

kUREUI Fq-=

-=

!

Shunt motor Series motor

Zero speed at startup Þ zero e.m.f. E

Induced current Ia limited only by the armature resistance Ra

Startup rheostat(value progressively reduceddown to short-circuit)

Startup rheostat in series with the armature (to limit Ia) (One allows Ias = 1.5 Ian)

DC machines

27

Inverting the rotation direction

aeE I)I(kC F=

Shunt motor Series motor

Modify the direction of the current in theexcitation circuit w.r.t. the rotor

Torque changes sign

Samedirection

Oppositedirection

DC machines

28

Losses in DC machines! Mechanical losses

– friction losses in bearings (÷ v) (v = speed)– windage losses (÷ v2)– friction losses from brushes on the collector (÷ v)

! Magnetic losses– eddy current losses in armature (÷ v2, ÷ bmax

2)– hysteresis losses in armature (÷ v, ÷ bmax

1.5 ® 2)

! Electric losses– Joule losses in armature, inductor and brushes (÷ I2, function of

temperature)

! Supplementary losses– due to skin effect in the rotor and sparks at brushes/collector contact– increased magnetic losses due to the magnetic reaction

DC machines