Chapter 6 Basic Principles of Electromechanical Energy Conversion

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6-1

Electric Drive

Electrical

Electrical

Mechanical

System

Machine

System

M o toring mode Generating mode

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Pelec

Pmech Pelec

Pmech

6-2

Basic Structure o Construction Stator Air gap Rotor

o Multi-pole structure S N

S

N

N

N

S

S N

S

2pole

4pole

Sufficient to consider a 2-pole machine © 2000 http://www.ece.umn.edu/groups/electricdrives

salient pole

6-3

Production of Magnetic Field o Radial field (H,F,B) in the air gap

is (t )

θ

H positive if away from the center

magnetic axis

o Using Ampere’s law, field

r is (t )

produced by the stator is, Hs =

µ =∞

lg Ns

N sis 2lg

mmf acting on air gap Fs = H s l g =

N s is 2

& Flux density in air gap Bs = µ o H s

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B s (t ) +

stator surface

π

0

rotor surface

−

π 2

−

N s is µo 2l g

N s isµo 2l g

θ 2π

6-4

Basic Principles of Operation o Force on a current carrying conductor subjected to an externally-established magnetic field

f em = B l i o emf induced in a conductor moving in a magnetic field

e = B lu

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6-5

Electromagnetic Force external B field B f em

l

f em

f em

i

1424 3 subtract

f em = B l i { 123 { {

[ Nm ]

[Wb / m 2 ] [ m ] [ A]

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1424 3

resultant

add

q Force Direction Higher concentration to lower concentration

6-6

Induced EMF Force on positve charges fq = q ( u × B )

+ fq+

B (into paper)

In this example a net positive charge accumulates at the top and a net negative charge accumulates at the bottom

u

fq− −

Magnitude of induced emf e = B l u { 123 { {

[V ]

[Wb / m 2 ] [m ][ m / s ]

Polarity of induced emf is given by f q and is independent of current flowing through the conductor © 2000 http://www.ece.umn.edu/groups/electricdrives

fq+ B (into paper)

u

6-7

Application of Basic Principles δ

o Assumptions u Uniform Bs , radial in direction u Rotor current of constant magnitude but polarity changes with position u counter-clockwise torque is positive

o Force acting on the conductor

ir

ωm

+ er −

stator magnetic axis

Tem

Electrical system

f em = Bs ( N r I ) l ir

and torque on coil Tem = 2 f em r = 2Bs ( N r I ) lr

torque remains constant as rotor turns o emf induced in coil er = 2econd = 2N r Bslrω mr

−I

0

er 0

180 o

360 o

δ

E −E

δ

Tem 0

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I

δ

6-8

Energy conversion ir

δ

+ er −

ir

er

ωm

Tem

stator magnetic axis

360 o

180 o

0

−I

Motoring Mode

I

E

0

δ

−E

Tem

Electrical system

0

−I ir

+ er −

Electrical system

© 2000 http://www.ece.umn.edu/groups/electricdrives

0

er ωm

δ

ir

δ

Regenerative braking Mode

δ

Tem

stator magnetic axis

0

Tem 0

I 180 o

360 o

δ

E −E

δ

δ

Power losses and Energy efficiency

6-9

o Motor losses w Conduction losses, PR w Iron losses, Phe w Friction and windage losses, Pfw w Switching losses, Psw Pin w Stray losses, Ps Ploss = PR + Phe + Psw + Pfw + Ps ηmach =

Electric Drive 64444 4744444 8

P P U

Electrical Machine

PPU losses

Motor losses

Po Po = Pin Po + Ploss

o PPU losses w Conduction losses w Switching losses ηdrive = η PPU × ηmotor

© 2000 http://www.ece.umn.edu/groups/electricdrives

(80 − 90%)

Po

6-10

Motor Ratings o Safe Operating Area Speed

Continuous Safe Operating Area Re duced torque due to lack of cooling

100%

0

100%

Torque

o Rated speed, torque and power Prated = ω rated Trated o Motor temperature rise due to losses o Expanded safe operating area during transients and for intermittent operation © 2000 http://www.ece.umn.edu/groups/electricdrives

6-11

Summary o What is the role of electric machines? What do the motoring-mode and the generating-mode of operations mean? o What are the definitions of stator and rotor? o Why do we use high permeability ferromagnetic materials for stators and rotors in electric machines? Why are these constructed by stacking laminations together, rather than as a solid structure? o What is the approximate air gap length in machines with less than 10 kW ratings? o What are multi-pole machines? Why can such machines be analyzed by considering only one pair of poles?

© 2000 http://www.ece.umn.edu/groups/electricdrives

6-12

Summary o Assuming the permeability of iron to be infinite, where is the mmf produced by machine coils "consumed"? What law is used to calculate the field quantities, such as flux density, for a given current through a coil? Why is it important to have a small air gap length? o What are the two basic principles of operation for electric machines? o What is the expression for force acting on a current-carrying conductor in an externally established B-field? What is its direction? o What is slot shielding and why can we choose to ignore it? o How do we express the induced emf in a conductor "cutting" an externally established B-field? How do we determine the polarity of the induced emf? © 2000 http://www.ece.umn.edu/groups/electricdrives

6-13

Summary o How do electrical machines convert energy from one form to another? o What are various loss mechanisms in electric machines? o How is electrical efficiency defined and what are typical values of efficiencies for the machines, the power-processing units, and the overall drives? o What is the end-result of power losses in electric machines? o What is meant by the various ratings on the name-plates of machines?

© 2000 http://www.ece.umn.edu/groups/electricdrives