Добавил:
Опубликованный материал нарушает ваши авторские права? Сообщите нам.
Вуз: Предмет: Файл:
Jack H.Dynamic system modeling and control.2004.pdf
Скачиваний:
73
Добавлен:
23.08.2013
Размер:
5.61 Mб
Скачать

electromagnetics - 23.1

23. ELECTROMECHANICAL SYSTEMS

Topics:

Objectives:

23.1 INTRODUCTION

• Magnetic fields and forces are extremely useful. The fields can allow energy storage, or transmit forces.

23.2MATHEMATICAL PROPERTIES

Magnetic fields have direction. As a result we must pay special attention to directions, and vector calculations.

23.2.1Induction

Magnetic fields pass through space.

electromagnetics - 23.2

resistivity of materials decreases with temperature

Amperes Circuit Law

I = °∫Hdl

where,

I = current flowing along a line (A)

H = magnetic field intensity (A/m)

l = A perpendicular path around the current flow (m)

For example, at a fixed radius (r) around a wire,

 

I = 2

π

Hrdθ

 

0

 

 

 

I = 2π

Hr

 

H =

 

I

--------

 

 

2

π r

• Flux density can be calculated for low H values. As the value climbs the relationship becomes non-linear.

B = µ H = µ rµ 0H

where,

B= Flux density (Wb/m*2 or T)

µ= permeability of material

µ

0

=

permeability of free space = 4π × 10

–7 Henry

---------------

 

 

 

 

m

µ

r

=

relative permeability

 

• Permeability,

electromagnetics - 23.3

µB

=---

H

µ 0

=

4π

10

–7 H

---

 

 

 

 

m

µ

= µ

rµ

0

 

where,

H = Magnetic field intensity A--- m

µ

µ

0

r

=permeability of free space

=relative permeability of a material

µ= permeability of a material

Permeability is approximately linear for smaller electric fields, but with larger magnetic fields the materials saturate and the value of B reaches a maximum value.

B

 

1.5

 

linear

saturation

H

 

4000

 

Figure 23.1 Saturation for a mild steel (approximately)

 

electromagnetics - 23.4

 

1.5

 

 

 

 

silicon sheet steel

 

 

cast steel

1.0

 

 

B (T)

 

 

0.5

 

 

 

 

cast iron

0

 

 

0

500

1000

 

H (At/m)

 

Figure 23.2 Magnetization curves (Sen, 1989)

 

• Flux density about a wire

 

electromagnetics - 23.5

B =

 

I

For an infinitely long straight conductor

--------

 

2

π r

 

 

 

where,

 

 

Wb

 

 

B =

Flux density

 

------- or

Tesla

 

 

 

 

m2

 

 

I

=

Current in the conductor (A)

r= radial distance from the conductor

Flux and flux density,

Φ= BdA

where,

Φ = Flux density ( Wb)

A= Cross section area perpendicular to flux

When a material is used out of the saturation region the permeabilities may be written as reluctances,

R =

L

------

 

µ A

where,

R = reluctance of a magnetic path

L = length of a magnetic path

A = cross section area of a magnetic path

electromagnetics - 23.6

• Electric circuit analogy

 

 

Φ

 

 

 

 

 

 

 

Φ

 

Ni

F

 

 

 

 

 

 

 

 

 

 

F

 

 

 

= ------------ =

--

 

 

 

 

 

l

R

 

 

 

 

 

 

 

 

 

 

 

 

------

 

 

 

 

 

 

 

µ A

 

 

 

 

 

 

 

R

• Example,

I

 

Rc

=

lc

 

-----------

 

 

 

 

µ cAc

 

 

lg

=

lg

 

 

Rg

-----------

 

 

 

 

µ gAg

 

lc

Φ

 

Ni

 

 

=

-----------------

 

 

Rc + Rg

• Faraday’s law,

electromagnetics - 23.7

e = N

d

Φ

For a coil

----

 

dt

 

 

where,

 

 

 

e = the potential voltage across the coil

N= the number of turns in the coil

Field energy,

W =

B2

V =

B2

Al

2------

µ

2------

µ

 

 

 

• Force can be derived from the energy,

F =

B2

------A

 

2µ

• The basic property of induction is that it will (in the presence of a magnetic field) convert a changing current flowing in a conductor to a force or convert a force to a current flow from a change in the current or the path.

electromagnetics - 23.8

F

=

( I ×

B) L

B

F

=

( L ×

B) I

 

I, L

F

where,

L = conductor length

F = force (N)

The FBD/schematic equivalent is,

I

M

 

 

F

 

 

 

 

 

 

 

 

 

 

 

 

 

 

NOTE: As with all cross products we can use the right hand rule here.

Figure 23.3 The current and force relationship

• We will also experience an induced current caused by a conductor moving in a magnetic field. This is also called emf (Electro-Motive Force)