Magnetic, Electric, and Insulation Materials for IM

Authored by: Ion Boldea

Induction Machines Handbook

Print publication date:  June  2020
Online publication date:  May  2020

Print ISBN: 9780367466121
eBook ISBN: 9781003033417
Adobe ISBN:

10.1201/9781003033417-3

 

Abstract

Main active materials – magnetic, electric, and insulation types – used in the induction machine (IM) fabrication are presented with their characteristic performance, with additional data on recently developed such materials (and pertinent source literature). Core loss basic formulae have been derived starting from Maxwell equations and tables illustrate core losses/kg of silicon steel at various flux densities and a few frequencies of interest.

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Magnetic, Electric, and Insulation Materials for IM

3.1  Introduction

Induction machines (IMs) contain magnetic circuits travelled by A.C. and travelling magnetic fields and electric circuits flowed by alternative currents. The electric circuits are insulated from the magnetic circuits (cores). The insulation system comprises the conductor insulation, slot, and interphase insulation.

Magnetic, electrical, and insulation materials are characterised by their characteristics (B–H curve, electrical resistivity, dielectric constant, and breakdown electric field (V/m)) and their losses.

At frequencies encountered in IMs (up to tens of kHz, when pulse width modulation (PWM) inverter fed), the insulation losses are neglected. Soft magnetic materials are used in IM as the magnetic field is currently produced. The flux density (B)/magnetic field (H) curve and cycle depend on the soft material composition and fabrication process. Their losses in W/kg depend on the B–H hysteresis cycle, frequency, electrical resistivity, and the A.C. (or) travelling field penetration into the soft magnetic material.

Silicon steel sheets are standard soft magnetic materials for IMs. Amorphous soft powder materials have been introduced recently with some potential for high-frequency (high-speed) IMs. The pure copper is the favourite material for the stator electric circuit (windings), while aluminium or brass is used mainly for rotor squirrel-cage windings.

Insulation materials are getting thinner and better and are ranked into a few classes: A (105°C), B (130°C), F (155°C), and H (180°C).

3.2  Soft Magnetic Materials

In free space, the flux density B and the magnetic field H are related by the permeability of free space µ0 = 4π10−7 H/m (S.I.)

3.1 B [ Wb m 2 ] = μ 0 [ H m ] H [ A m ]

Within a certain material, a different magnetisation process occurs.

3.2 B = μ H ; μ = μ 0 μ R

In (3.2), µ is termed as permeability and µR as relative permeability (non-dimensional).

Permeability is defined for homogenous (uniform quality) and isotropic (same properties in all directions) materials. In non-homogeneous or (and) non-isotropic materials, µ becomes a tensor. Most common materials are non-linear: µ varies with B.

A material is classified according to the value of its relative permeability, µR, which is related to its atomic structure.

Most non-magnetic materials are either paramagnetic – with µR slightly greater than 1.0 – or diamagnetic with µR slightly less than 1.0. Superconductors are perfect diamagnetic materials. In such materials when B → 0, µR →0.

Magnetic properties are related to the existence of permanent magnetic dipoles within the matter.

There are quite a few classes of more magnetic materials (µR ≫ 1). Amongst them, we will deal here with soft ferromagnetic materials. Soft magnetic materials include alloys made of iron, nickel, cobalt, and one rare earth element and/or soft steels with silicon.

There is also a class of magnetic materials made of powdered iron particles (or other magnetic material) suspended in an epoxy or plastic (non-ferrous) matrix. These soft powder magnetic materials are formed by compression or injection, moulding, or other techniques.

There are a number of properties of interest in a soft magnetic material such as permeability versus B, saturation flux density, H(B), temperature variation of permeability, hysteresis characteristics, electric conductivity, Curie temperature, and loss coefficients.

The graphical representation of non-linear B(H) curve (besides the pertinent table) is of high interest (Figure 3.1). Also of high interest is the hysteresis loop (Figure 3.2).

Typical B–H curve.

Figure 3.1   Typical B–H curve.

Deltamax tape-wound core 0.5 mm strip hysteresis loop.

Figure 3.2   Deltamax tape-wound core 0.5 mm strip hysteresis loop.

There are quite a few standard laboratory methods to obtain these two characteristics. The B–H curve can be obtained in two ways: the virgin (initial) B–H curve, obtained from a totally demagnetised sample; and the normal (average) B–H curve, obtained as the tips of hysteresis loops of increasing magnitude. There is only a small difference between the two methods.

The B–H curve is the result of domain changes within the magnetic material. The domains of soft magnetic materials are 10−4–10−7 m in size. When completely demagnetised, these domains have random magnetisation with zero flux in all finite samples.

When an external magnetic field H is applied, the domains aligned to H tend to grow when B is increased (region I on Figure 3.1). In region II, H is further increased and the domain walls move rapidly until each crystal of the material becomes a single domain. In region III, the domains rotate towards alignment with H. This results in magnetic saturation Bs. Beyond this condition, the small increase in B is basically due to the increase in the space occupied by the material for B = µ0Hr0.

This “free-space” flux density may be subtracted to obtain the intrinsic magnetisation curve. The non-linear character of B–H curve (Figure 3.1) leads to two different definitions of relative permeability:

  • The normal permeability µRn:
    3.3 μ Rn = B μ 0 H = tan α n μ 0
  • The differential relative permeability µRd:
    3.4 μ Rd = dB μ 0 dH = tan α d μ 0

Only in region II, µRn = µRd. In regions I and III, in general, µRn > µRd (Figure 3.3). The permeability is maximum in region II. For M19 silicon steel sheets, Bs = 2 T, Hs = 40,000 A/m, and µRmax = 10,000).

Relative permeability versus H.

Figure 3.3   Relative permeability versus H.

So the minimum relative permeability is

3.5 ( μ Rn ) B s = 2.0 T = 2.0 4 π 10 7 40 , 000 = 39.8 !

The second graphical characteristic of interest is the hysteresis loop (Figure 3.2). This is a symmetrical hysteresis loop obtained after a number of reversals of magnetic field (force) between ±Hc. The area within the loop is related to the energy required to reverse the magnetic domain walls as H is reversed. This nonreversible energy is called hysteresis loss and varies with temperature and frequency of H reversals in a given material (Figure 3.2). A typical magnetisation curve B–H for silicon steel nonoriented grain is given in Table 3.1.

Table 3.1   B–H Curve for Silicon (3.5%) Steel (0.5 mm thick) at 50 Hz

B (T)

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

H (A/m)

22.8

35

45

49

57

65

70

76

83

90

B (T)

0.55

0.6

0.65

0.7

0.75

0.8

0.85

0.9

0.95

1

H (A/m)

98

106

115

124

135

148

162

177

198

220

B (T)

1.05

1.1

1.15

1.2

1.25

1.3

1.35

1.4

1.45

1.5

H (A/m)

237

273

310

356

417

482

585

760

1050

1340

B (T)

1.55

1.6

1.65

1.7

1.75

1.8

1.85

1.9

1.95

2.0

H (A/m)

1760

2460

3460

4800

6160

8270

11,170

15,220

22,000

34,000

It has been shown experimentally that the magnetisation curve varies with frequency as shown in Table 3.2. This time the magnetic field is kept in original data (1Oe = 79.5 A/m) [1].

Table 3.2   1Οe = 79.5 A/m

Induction

Typical D.C. and Derived A.C. Magnetising Force (Oe) of As-Sheared 29-Cage M19 Fully Processed Cold Rolled Nonoriented (CRNO) at Various Frequencies

(kG)

D.C.

50 Hz

60 Hz

100 Hz

150 Hz

200 Hz

300 Hz

400 Hz

600 Hz

1000 Hz

1500 Hz

2000 Hz

1.0

0.333

0.334

0.341

0.349

0.356

0.372

0.385

0.412

0.485

0.564

0.642

2.0

0.401

0.475

0.480

0.495

0.513

0.533

0.567

0.599

0.661

0.808

0.955

1.092

4.0

0.564

0.659

0.669

0.700

0.39

0.777

0.846

0.911

1.040

1.298

1.557

1.800

7.0

0.845

0.904

0.916

0.968

1.030

1.094

1.211

1.325

1.553

2.000

2.483

2.954

10.0

1.335

1.248

1.263

1.324

1.403

1.481

1.648

1.822

2.169

2.867

3.697

4.534

12.0

2.058

1.705

1.718

1.777

1.859

1.942

2.129

2.326

2.736

3.657

4.769

5.889

13.0

2.951

2.211

2.223

2.273

2.342

2.424

2.609

2.815

3.244

4.268

5.499

14.0

5.470

3.508

3.510

3.571

3.633

3.691

3.855

4.132

15.0

13.928

8.276

8.313

8.366

8.366

8.478

8.651

9.737

15.5

22.784

13.615

13.587

13.754

13.725

13.776

14.102

16.496

16.0

35.201

21.589

21.715

21.800

21.842

21.884

16.5

50.940

32.383

32.506

32.629

32.547

32.588

17.0

70.260

46.115

46.234

46.392

46.644

46.630

18.0

122.01

19.0

201.58

20.0

393.50

21.0

1111.84

In essence for the same flux density B, the magnetic field increases with frequency. It is recommended to reduce the design flux density when the frequency increases above 200 Hz as the core losses grow markedly with frequency.

Special materials such as Hiperco 50 show saturation flux densities Bs = 2.3–2.4 T at Hs < 10,000 A/m, which allows for higher airgap flux density in IMs and, thus, lower weight designs (Tables 3.3 and 3.4).

Table 3.3   Hiperco 50 Magnetisation Curve B/H – T/(A/m)

B (T)

0.017

0.7

1.5

1.9

2

2.1

2.15

2.2

2.25

2.267

2.275

2.283

H (A/m)

0

39.75

79.5

159

318

477

715.5

1431

3975

7950

11,925

15,900

Table 3.4   Hiperco 50 Losses W/Kg

f = 60 Hz

Core loss (W/Kg)

0.8866

1.0087

1.2317

1.3946

1.5671

1.7589

1.9410

2.1614

2.4202

2.6215

2.8612

B (T)

1

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2

f = 400 Hz

Core loss (W/Kg)

8.5429

10.196

11.849

13.734

15.432

17.636

19.290

21.770

23.975

25.904

27.282

B (T)

1

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2

f = 800 Hz

Core loss (W/Kg)

23.589

27.282

31.416

35.274

40.786

45.072

51.768

58.137

65.485

72.671

76.917

B (T)

1

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2

f = 1200 Hz

Core loss (W/Kg)

40.738

47.242

58.659

65.745

77.162

84.642

92.909

104.32

113.38

128.34

135.43

B (T)

1

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2

3.3  Core (Magnetic) Losses

Energy loss in the magnetic material itself is a very significant characteristic in the energy efficiency of IMs. This loss is termed core loss or magnetic loss.

Traditionally, core loss has been divided into two components: hysteresis loss and eddy current loss. The hysteresis loss is equal to the product between the hysteresis loop area and the frequency of the magnetic field in sinusoidal systems.

3.6 P h k h f B m 2 [ W / kg ] ; B m - maximum flux density

Hysteresis losses are 10%–30% higher in travelling fields than in A.C. fields for Bm < 1.5(1.6) T. However, in a travelling field, they have a maximum, in general, between 1.5 and 1.6 T and then decrease to low values for B > 2.0 T. The computation of hysteresis losses is still an open issue due to the hysteresis cycle’s complex shape, its dependence on frequency and on the character of the magnetic field (travelling or A.C.) [2].

Preisach modelling of hysteresis cycle is very popular [3], but neural network models have proved much less computation time consuming [4].

Eddy current losses are caused by induced electric currents in the magnetic material by an external A.C. or travelling magnetic field.

3.7 P e k e f 2 B m 2 [ W / kg ]

Finite elements are used to determine the magnetic distribution with zero electrical conductivity, and then the core losses may be calculated by some analytical approximations as (3.6) and (3.7) or [5]

3.8 P core k h fB m α K ( B m ) + σ Fe 12 d 2 f γ Fe ( dB dt ) 1/f 2 dt + K ex f 1 / f ( dB dt ) 1.5

where K = 1 + 0.65 B m n Δ B i

  • Bm – maximum flux density
  • f – frequency
  • γFe – material density
  • d – lamination thickness
  • Kh – hysteresis loss constant
  • Kex – excess loss constant
  • ΔBi – change of flux density during a time step
  • n – total number of time steps.

Equation (3.8) is a generalisation of Equations (3.6) and (3.7) for nonsinusoidal time-varying magnetic fields as produced in PWM inverter IM drives. Recently, better fits of power losses formula for cold-rolled motor laminations for a wide frequency and magnetisation range have been proposed [6].

For sinusoidal systems, the eddy currents in a thin lamination may be calculated rather easily by assuming the external magnetic field H 0 e j ω 1 t acting parallel to the lamination plane (Figure 3.4).

Eddy current paths in a soft material lamination.

Figure 3.4   Eddy current paths in a soft material lamination.

Maxwell’s equations yield

3.9 H y x = J z ; H 0y = H 0 e j ω 1 t E z x = j ω 1 μ ( H 0y + H y ) ; σ Fe E z = J z

where J is current density and E is electric field.

As the lamination thickness is small in comparison with its length and width, Jx contribution is neglected. Consequently, (3.9) is reduced to

3.10 2 H y x 2 j ω 1 μ σ Fe H y = j ω 1 σ Fe B 0

B0 = µ0H0 is the initial flux density on the lamination surface.

The solution of (3.10) is

3.11 H y ( x ) = A 1 e γ x + A 2 e γ x + B 0 μ 0
3.12 γ = β ( 1 + j ) ; B = ω 1 μ σ Fe 2

The current density Jz(x) is

3.13 J z ( x ) = H y x = γ ( A 1 e γ x + A 2 e γ x )

The boundary conditions are

3.14 H y ( d 2 ) = H y ( d 2 ) = 0

Finally,

3.15 A 1 = A 2 = B 0 2 μ cosh β d 2 ( 1 + j )
3.16 J z ( x ) = β ( 1 + j ) μ B 0 sinh ( 1 + j ) β x cosh β d 2 ( 1 + j )

The eddy current loss per unit weight Pe is

3.17 P e = 2 γ Fe d σ Fe 1 2 0 d / 2 ( J z ( x ) ) 2 dx = β γ Fe d ω 1 μ B 0 2 [ sinh ( β d ) sin ( β d ) cosh ( β d ) + cos ( β d ) ] [ W kg ]

The iron permeability has been considered constant within the lamination thickness though the flux density slightly decreases.

For a good utilisation of the material, the flux density reduction along lamination thickness has to be small. In other words, βd << 1. In such conditions, the eddy current losses increase with the lamination thickness.

The electrical conductivity σFe is also influential, and silicon added to soft steel reduces σFe to (2–2.5) × 106 (Ωm)−1. This is why 0.5–0.6 mm thick laminations are used at 50(60) Hz and, in general, up to 200–300 Hz IMs.

For such laminations, eddy current losses may be approximated to

3.18 P e K w B m 2 [ W kg ] ; K w = ω 1 2 σ Fe d 2 24 γ Fe

The above loss formula derivation process is valid for A.C. magnetic field excitation. For pure travelling field, the eddy current losses are twice as much for same laminations, frequency, and peak flux density.

Given the complexity of eddy current and hysteresis losses, it is recommended that tests be run to measure them in conditions very similar to those encountered in the particular IM.

Soft magnetic material producers manufacture laminations for many purposes. They run their own tests and provide data on core losses for practical values of frequency and flux density.

Besides Epstein’s traditional method, made with rectangular lamination samples, the wound toroidal cores method has also been introduced [7] for A.C. field losses. For travelling field loss measurement, a rotational loss tester may be used [8].

Typical core loss data for M15 – 3% silicon 0.5 mm thick lamination material – used in small IMs, is given in Figure 3.5a [9].

Core losses for M15 – 3% silicon 0.5 mm thick laminations [

Figure 3.5   Core losses for M15 – 3% silicon 0.5 mm thick laminations [9], (a) and 0.1 mm thick, 3% silicon Metglass (AMM) core losses, (b).

As expected, core losses increase with frequency and flux density. A similar situation occurs, with a superior but still common material: steel M19 FP (0.4 mm) 29 gauge (Table 3.5) [1].

Table 3.5   Typical Core Loss (W/lb) As-Sheared 29 Cage M19 Fully Processed CRNO at Various Frequencies [1]

Induction (kG)

50 Hz

60 Hz

100 Hz

150 Hz

200 Hz

300 Hz

400 Hz

600 Hz

1000 Hz

1500 Hz

2000 Hz

1.0

0.008

0.009

0.017

0.029

0.042

0.074

0.112

0.205

0.465

0.900

1.451

2.0

0.031

0.039

0.072

0.0119

0.173

0.300

0.451

0.812

1.786

3.370

5.318

4.0

0.109

0.134

0.252

0.424

0.621

1.085

1.635

2.960

6.340

11.834

18.523

7.0

0.273

0.340

0.647

1.106

1.640

2.920

4.450

8.180

17.753

33.720

53.971

10.0

0.494

0.617

1.182

2.040

3.060

5.530

8.590

16.180

36.303

71.529

116.702

12.0

0.687

0.858

1.648

2.860

4.290

7.830

12.203

23.500

54.258

108.995

179.321

13.0

0.812

1.014

1.942

3.360

5.060

9.230

14.409

27.810

65.100

131.98

14.0

0.969

1.209

2.310

4.000

6.000

10.920

17.000

15.0

1.161

1.447

2.770

4.760

7.150

13.000

20.144

15.5

1.256

1.559

2.990

5.150

7.710

13.942

21.619

16.0

1.342

1.667

3.179

5.466

8.189

16.5

1.420

1.763

3.375

5.788

8.674

17.0

1.492

1.852

3.540

6.089

9.129

A rather complete up-to-date data source on soft magnetic materials’ characteristics and losses may be found in Ref. [1].

Core loss represents 25%–35% of all losses in low-power 50 (60) Hz IMs and slightly more in medium- and large-power IMs at 50(60) Hz. The development of high-speed IMs, up to more than 45,000 rpm at 20 kW [10], has caused a new momentum in the research for better magnetic materials as core losses are even larger than winding losses in such applications.

Thinner (0.35 mm or less) laminations of special materials (3.25% silicon) with special thermal treatment are used to strike a better compromise between low 60 Hz and moderate 800/1000 Hz core losses (1.2 W/kg at 60 Hz, 1 T; 28 W/kg at 800 Hz, 1 T).

6.5% silicon steel nonoriented steel laminations for low-power IMs at 60 Hz have shown capable of a 40% reduction in core losses [11]. The noise level has also been reduced this way [11]. Similar improvements have been reported with 0.35 mm thick oriented grain laminations by alternating laminations with perpendicular magnetisation orientation or crossed magnetic structure (CMS) [12].

Recent Progress on Core Loss Assessment

  • As the fundamental frequency in electric machine tends to increase (for high-speed variable speed drives) formulae as (3.8), with constant coefficients do not fit experimental results.
  • Also, high-temperature annealing core laminations have been proven to reduce the hysteresis losses but increase the eddy current losses.
  • Finally, the magnetic core properties degrade due to manufacturing process (punching, pressing, welding, packaging), and variability due to manufacturing has to be observed. To treat the above problems and thus provide reliable data on hysteresis cycle and losses and on eddy current losses, recent R & D effort concentrated on the following:
    • Proving that the variation of hysteresis cycle with frequency is due to skin effect in the lamination especially when the frequency increases [13].
    • Investigating experimentally, by a modified electromagnetic Halbach core test rig, the rotational core losses as they tend to have a maximum of around 1.5–1.6 T monotonously with flux density [14].
    • Measuring the stator core loss degradation by the manufacturing process by a stator toroidal model that uses the stator core and investigates mainly the stator back core losses [15,16].

Soft magnetic composites (SMCs) have been produced by powder metallurgy technologies. The magnetic powder particles are coated by insulation layers and a binder which are compressed such that to provide

  • Large enough magnetic permeability
  • Low-enough core losses
  • Densities above 7.1 g/cm3 (for high-enough permeability).

The eddy current loss tends to be constant with frequency, while hysteresis loss increases almost linearly with frequency (up to 1 kHz or so).

At 400–500 Hz and above, the losses in SMC become smaller than for 0.5 mm thick silicon steels. However, the relative permeability is still low: 100–200. Only for recent materials, fabricated by cold compression, the relative permeability has been increased above 500 for flux densities in the 1 T range [17,18]. On the other hand, amorphous magnetic materials with plastic fill between magnetic particles, such as Metglass (Figure 3.5b), show not only lower losses at 300–500 Hz but also allow for higher flux densities.

Added advantages such as more freedom in choosing the stator core geometry and the increase of slot-filling factor by coil in slot magnetic compression-embedded windings [19] may lead to wide use of SMCs in induction motors. The electric loading may thus be increased. The heat transmissivity also increases [17].

In the near future, better silicon 0.5 mm (0.35 mm) thick steel laminations with nonoriented grain seem to remain the basic soft magnetic materials for IM fabrication. For high speed (frequency above 300 Hz), thinner laminations are to be used. The insulation coating layer of each lamination is getting thinner and thinner to retain a good stacking factor (above 85%).

3.4  Electrical Conductors

Electrical copper conductors are used to produce the stator three (two)-phase windings. The same is true for wound-rotor windings.

The electrical copper has a high purity and is fabricated by involved electrolysis process. The purity is well above 99%. The cross section of copper conductors (wires) to be introduced in stator slots is either circular or rectangular (Figure 3.6). The electrical resistivity of magnetic wire (electric conductor) ρCo = (1.65–1.8) × 10−8 Ωm at 20°C and varies with temperature as

Stator slot with round (a) and rectangular (b) conductors.

Figure 3.6   Stator slot with round (a) and rectangular (b) conductors.

3.19 ρ Co ( T ) = ( ρ Co ) 20 ° [ 1 + ( T 20 ) / 273 ]

Round magnetic wires come into standardised gauges up to a bare copper diameter of about 2.5 mm (3 mm) (or 0.12 inch), in general (Tables 3.6 and 3.7).

Table 3.6   Round Magnetic Wire Gauges in Inches

Awg Size

Bare Wire Diameter Nominal (inches)

Film Additions (inches)

Overall Diameter (inches)

Weight at 20°C-68°F

Resistance at 20°C-68°F

Min.

Max.

Min.

Nom.

Max.

Lbs./M Ft. Nom.

Ft./Lb. Nom.

Ohms./M Ft. Nom.

Ohms./ Lb. Nom.

Wires/ ln. Nom.

Awg Size

8

0.1285

0.0016

0.0026

0.1288

0.1306

0.1324

50.20

19.92

0.6281

0.01251

7.66

8

9

0.1144

0.0016

0.0026

0.1149

0.1165

0.1181

39.81

25.12

0.7925

0.01991

8.58

9

10

0.1019

0.0015

0.0025

0.1024

0.1039

0.1054

31.59

31.66

0.9988

0.03162

9.62

10

11

0.0907

0.0015

0.0025

0.0913

0.0927

0.0941

25.04

39.94

1.26

0.05032

10.8

11

12

0.0808

0.0014

0.0024

0.0814

0.0827

0.0840

19.92

50.20

1.59

0.07982

12.1

12

13

0.0720

0.0014

0.0023

0.0727

0.0738

0.0750

15.81

63.25

2.00

0.1265

13.5

13

14

0.0641

0.0014

0.0023

0.0649

0.0659

0.0659

12.49

80.06

2.52

0.2018

15.2

14

15

0.0571

0.0013

0.0022

0.0578

0.0588

0.0599

9.948

100.5

3.18

0.3196

17.0

15

16

0.0508

0.0012

0.0021

0.0515

0.0525

0.0534

7.880

126.9

4.02

0.5101

19.0

16

17

0.0453

0.0012

0.0020

0.0460

0.0469

0.0478

6.269

159.5

5.05

0.8055

21.3

17

18

0.0403

0.0011

0.0019

0.0410

0.0418

0.0426

4.970

201.2

6.39

1.286

23.9

18

19

0.0359

0.0011

0.0019

0.0366

0.0374

0.0382

3.943

253.6

8.05

2.041

26.7

19

20

0.0320

0.0010

0.0018

0.0327

0.0334

0.0341

3.138

318.7

10.1

3.219

29.9

20

21

0.0285

0.0010

0.0018

0.0292

0.0299

0.0306

2.492

401.2

12.8

5.135

33.4

21

22

0.0253

0.0010

0.0017

0.0260

0.0267

0.0273

1.969

507.9

16.2

8.228

37.5

22

23

0.0226

0.0009

0.0016

0.0233

0.0238

0.0244

1.572

636.1

20.3

12.91

42.0

23

24

0.0201

0.0009

0.0015

0.0208

0.0213

0.0218

1.240

806.5

25.7

20.73

46.9

24

25

0.0179

0.0009

0.0014

0.0186

0.0191

0.0195

988

1012

32.4

32.79

52.4

25

26

0.0159

0.0008

0.0013

0.0165

0.0169

0.0174

779

1284

41.0

52.64

59.2

26

27

0.0142

0.0008

0.0013

0.0149

0.0153

0.0156

0.623

1605

51.4

82.50

65.4

27

28

0.0126

0.0007

0.0012

0.0132

0.0136

0.0139

0.491

2037

65.3

133.0

73.5

28

29

0.0113

0.0007

0.0012

0.0119

0.0122

0.0126

0.395

2532

81.2

205.6

82.0

29

30

0.0100

0.0006

0.0011

0.0105

0.0109

0.112

0.310

3226

104

335.5

91.7

30

31

0.0089

0.0006

0.0011

0.0094

0.0097

0.0100

0.246

4065

131

532.5

103

31

32

0.0080

0.0006

0.0010

0.0085

0.0088

0.0091

0.199

5025

162

814.1

114

32

33

0.0071

0.0005

0.0009

0.0075

0.0078

0.0081

0.157

6394

206

1317

128

33

34

0.0063

0.0005

0.0008

0.0067

0.0070

0.0072

0.123

8130

261

2122

143

34

35

0.0056

0.0004

0.0007

0.0059

0.0062

0.0064

0.0977

10,235

331

3388

161

35

36

0.0050

0.0004

0.0007

0.0053

0.0056

0.0058

0.0783

12,771

415

5300

179

36

37

0.0045

0.0003

0.0006

0.0047

0.0050

0.0052

0.0632

15,823

512

8101

200

37

38

0.0040

0.0003

0.0006

0.0042

0.0045

0.0047

0.0501

19,960

648

12,934

222

38

39

0.0035

0.0002

0.0005

0.0036

0.0039

0.0041

0.0383

26,110

847

22,115

256

39

40

0.0031

0.0002

0.0005

0.0032

0.0035

0.0037

0.0301

33,222

1080

35,880

286

40

41

0.0028

0.0002

0.0004

0.0029

0.0031

0.0033

0.0244

40,984

1320

54,099

323

41

42

0.0025

0.0002

0.0004

0.0026

0.0028

0.0030

0.0195

51,282

1660

85,128

357

42

43

0.0022

0.0002

0.0003

0.0023

0.0025

0.0026

0.0153

65,360

2140

139,870

400

43

44

0.0020

0.0001

0.0003

0.0020

0.0022

0.0024

0.0124

80,645

2590

208,870

455

44

Table 3.7   Typical Round Magnetic Wire Gauges in mm

Rated Diameter (mm)

Insulated Wire Diameter (mm)

Rated Diameter (mm)

Insulated Wire Diameter (mm)

0.3

0.327

0.75

0.7949

0.32

0.348

0.80

0.8455

0.33

0.359

0.85

0.897

0.35

0.3795

0.90

0.948

0.38

0.4105

0.95

1.0

0.40

0.4315

1.00

1.051

0.42

0.4625

1.05

1.102

0.45

0.4835

1.10

1.153

0.48

0.515

1.12

1.173

0.50

0.536

1.15

1.2035

0.53

0.567

1.18

1.2345

0.55

0.5875

1.20

1.305

0.58

0.6185

1.25

1.325

0.60

0.639

1.30

1.356

0.63

0.6705

1.32

1.3765

0.65

0.691

1.35

1.407

0.67

0.7145

1.40

1.4575

0.70

0.742

1.45

1.508

0.71

0.7525

1.50

1.559

The total cross section Acon of the coil conductor depends on the rated phase current I1n and the design current density Jcon:

3.20 A con = I 1n / J con

The design current density varies between 3.5 and 15 A/mm2 depending on the cooling system, service duty cycle, and the targeted efficiency of the IM. High-efficiency IMs are characterised by lower current density (3.5–6 A/mm2). If the Acon in (3.19) is larger than the cross section of the largest round wire gauge available, a few conductors of lower diameter are connected in parallel and wound together. Up to 6–8 elementary conductors may be connected together.

If Acon is larger than 30–40 mm2 (that is, 6–8, 2.5 mm diameter wires in parallel), rectangular conductors are recommended.

In many countries, rectangular conductor cross sections are also standardised. In some cases, small cross sections such as (0.8–2)⋅2 mm × mm or (0.8–6) × 6 mm × mm are used for rectangular conductors.

In general, the rectangular conductor height a is kept low (a < 3.55 mm) to reduce the skin effect; that is, to keep the A.C. resistance low. A large cross-section area of 3.55 × 50 mm × mm would be typical for large-power IMs.

The rotor cage is in general made of aluminium: die-casted aluminium in low-power IMs (up to 300 kW or so) or of aluminium bars attached through brazing or welding processes to end rings.

Fabricated rotor cages are made of aluminium or copper alloys and of brass (the upper cage of a double cage) for powers above 300 kW, in general. The casting process of aluminium uses the rotor lamination stack as a partial mould because the melting point of silicon steel is much higher than that of aluminium. The electrical resistivity of aluminium ρAl ≅ (2.7–3.0) × 10−8 Ωm and varies with temperature as shown in (3.19).

Though the rotor cage bars are in general uninsulated from the magnetic core, most of the current flow through the cage bars as their resistivity is more than 20–30 times smaller than that of the laminated core.

Insulated cage bars would be ideal, but this would severely limit the rotor temperature unless a special high-temperature (high-cost) insulation coating is used.

3.5  Insulation Materials

The primary purpose of stator insulation is to withstand turn-to-turn, phase-to-phase, and phase-to-ground voltage such that to direct the stator phase currents through the desired paths of stator windings.

Insulation serves a similar purpose in phase-wound rotors whose phase leads are connected to insulated copper rings and then through brushes to stationary devices (resistances or/and special power electronic converters). Insulation is required to withstand voltages associated to: brush rigging (if any), winding connections, winding leads, auxiliaries such as temperature probes, and bearings (especially for PWM inverter drives).

The stator laminations are insulated from each other by special coatings (0.013 mm thick) to reduce eddy current core losses.

In standard IMs, the rotor (slip) frequency is rather small, and thus, inter-lamination insulation may not be necessary, unless the IM is to work for prolonged intervals at large slip values.

For all wound-rotor motors, the rotor laminations are insulated from each other. The bearing sitting is insulated from the stator to reduce the bearing (shaft) voltage (current), especially for large-power IMs whose stator laminations are made of a few segments, thus allowing a notable A.C. axial flux linkage. This way, premature bearing damage may be prevented and even more so in PWM inverter-fed IMs, where additional common voltage mode superhigh frequency capacitor currents through the bearings occur.

Stator winding insulation systems may be divided into two types, related to power and voltage levels.

  • Random-wound conductor IMs with small and round conductors
  • Form-wound conductor IMs with relatively large area rectangular conductors.

Insulation systems for IMs are characterised by voltage and temperature requirements. The IM insulation has to withstand the expected operating voltages between conductors, conductors (phase) and ground, and phase to phase.

The American National Standards Institute (ANSI) specifies that the insulation test voltage shall be twice the rated voltage plus 1000 V applied to the stator winding for 1 minute.

The heat produced by the winding currents and the core losses causes hot-spot temperatures that have to be limited in accordance with the thermal capability of the organic (resin) insulation used in the machine and to its chemical stability and capability to prevent conductor-to-conductor and conductor-to-ground short-circuits during IM operation.

There is continuous, but slow deterioration of the organic (resin) insulation by internal chemical reaction, contamination, and chemical interactions. Thermal degradation develops cracks in the enamel, varnish, or resin, reducing the dielectric strength of insulation.

Insulation materials for electric machines have been organised in stable temperature classes at which they can perform satisfactorily for the expected service lifetime.

The temperature classes are (again):

class A: 105°C

class F: 155°C

class B: 130°C

class G: 180°C

The main insulation components for the random-wound coil windings are the enamel insulation on the wire, the insulation between coils and ground/slot walls, and slot liner insulation and between phases (Figure 3.7).

Random-wound coils insulation.

Figure 3.7   Random-wound coils insulation.

The connections between the coils of a phase and the leads to terminal box have to be insulated. Also, the binding cord used to tie down end windings to reduce their vibration is made of insulation materials.

Random-wound IMs are built for voltages below 1 kV. The moderate currents involved can be handled by wound conductors (eventually a few in parallel) where enamel insulation is the critical component. To apply the enamel, the wire is passed through a solution of polymerisable resin and into the high-curing temperature tower where it turns into a thin, solid, and flexible coating.

3.5.1  Random-Wound IM Insulation

Several passes are required for the desired thickness (0.025 mm thick or so). There are dedicated standards that mention the tests on enamel conductors (ASTMD-1676); American Society for Testing and Materials (ASTM) standards part 39 for electric insulation test methods: solids and solidifying liquids should be considered for the scope.

Enamel wire, stretched and scraped when the coils are introduced in slots, should survive this operation without notable damage to the enamel. Some insulation varnish is applied over the enamel wire after the stator winding is completed. The varnish provides additional enamel protection against moisture, dirt, and chemical contamination and also provides mechanical support for the windings.

Slot and phase-to-phase insulation for class A temperatures is a somewhat flexible sheet material (such as cellulose paper), 0.125–0.25 mm thick, or a polyester film. In some cases, fused resin coatings are applied to stator slot walls by electrostatic attraction of polymerisable resin powder. The stator is heated to fuse and cure the resin to a smooth coating.

For high-temperature IMs (class F, H), glass cloth mica paper or asbestos treated with special varnishes are used for slot and phase-to-phase insulation. Varnishes may interact with the enamel to reduce thermal stability. Enamels and varnishes are tested separately according to ASTM (D2307, D1973, and D3145) or International Electrotechnical Commission (IEC) standards.

Model motor insulation systems (motorettes) are tested according to Institute of Electrical and Electronics Engineers (IEEE) standards for small motors.

All these insulation-accelerated life tests involve the ageing of insulation test specimens until they fail at temperatures higher than the operating temperature of the respective motor. The logarithms of the accelerated ageing times are then graphed against their reciprocal Kelvin test temperatures (Arrhenius graph). The graph is then extrapolated to the planed (reduced) temperature to predict the actual lifetime of insulation.

3.5.2  Form-Wound Windings

Form-wound windings are employed in high-power IMs. The slots are rectangular and so are the conductors. The slot filling factor increases due to this combination.

The insulation of the coil conductors (turns) is applied before inserting the coils in slots. The coils are also vacuum impregnated outside the machine. The slot insulation is made of resin-bonded mica applied as a wrapper or tape with a fibrous sheet for support (in high-voltage IMs above 1–2 kV).

Vacuum impregnation is done with polymerisable resins which are then cured to solids by heating. During the cure, the conductors may be constraint to size to enter the slot as the epoxy-type resins are sufficiently elastic for the scope.

Voltage, through partial discharges, may cause insulation failure in higher voltage IMs. Incorporating mica in the major insulation schemes solves this problem to a large degree.

A conducting paint may be applied over the slot portion of the coils to fill the space between the insulated preformed coil and slot wall, to avoid partial discharges. Lower and medium voltage coil insulation is measured in accelerated higher temperature tests (IEEE standard 275) by using the model system called formette. Formette testing is similar to motorette testing for random-wound IMs [20].

Diagnostic non-destructive tests to check the integrity and capability of large IM insulation are also standardised [20–23].

3.6  Summary

  • The three main materials used to build IMs are of magnetic, electric, and insulation type.
  • As IM is an A.C. machine, reducing eddy current losses in its magnetic core is paramount.
  • It is shown that these losses increase with the soft magnetic sheet thickness parallel to the external A.C. field.
  • Soft magnetic materials (silicon steel) used in thin laminations (0.5 mm thick up to 200 Hz) have low hysteresis and eddy current losses (about or less 2 W/kg at 1 T and 60 Hz).
  • Besides losses, the B–H (magnetisation) curve characterises a soft magnetic material [13].
  • The magnetic permeability µ = B/H varies from (5000–8000) µ0 at 1 T to (40–60) µ0 at 2.0 T in modern silicon steel laminations. High permeability is essential to low magnetisation (no load) current and losses.
  • High-speed IMs require frequencies above 300 Hz (and up to 800 Hz and more). Thinner silicon lamination steels with special thermal treatments are required to secure core losses in the order of 30–50 W/kg at 800 Hz and 1 T.
  • % silicon steel lamination for small IMs have been proved adequate to reduce core losses by as much as 40% at 50 (60) Hz.
  • Also, interspersing oriented grain (transformer) laminations (0.35 mm thick) with orthogonal orientation laminations has been shown to produce a 30%–40% reduction in core losses at 50 (60) Hz and 1 T in comparison with 0.5 mm thick nonoriented grain silicon steel used in most IMs.
  • SMCs have been introduced and shown to produce lower losses than silicon steel laminations only above 300 Hz but at the expense of lower permeability ((100–200) µ0 in general). Cold compression methods are expected to increase slot filling factor notably and thus increase the current loading. Size reduction is obtained also due to the increase of heat transmissivity through SMCs. Amorphous magnetic materials (Metglass) have been recently introduced for lower losses and high permeability (0.1 mm thick), 400 Hz, 1.6 T: 50% of core losses of silicon steel at 0.1 mm thickness.
  • Electric conductors for stator windings and wound rotors are made of pure (electrical) copper.
  • Cast aluminium is used for rotor cage windings up to 300 kW.
  • Fabricated aluminium or copper bars and rings are used for higher power IM cage rotors. Die-cast methods for copper cage have been proposed recently [24]. Lightly ferromagnetic aluminium rotor bars have also been proposed to increase starting torque at lower starting current, while preserving rated speed performance [25].
  • The rotor cage bars are not, in general, insulated from the rotor lamination core. Interbar currents may thus occur.
  • The windings are made out of random-wound coils with round wire and form-wound coils for large IMs with rectangular wire.
  • The windings are insulated from the magnetic core through insulation materials. Also, the conductors are enamelled to insulate one conductor from another.
  • Insulation systems are classified according to temperature limits in four classes: Class A-105°C, Class B-130°C, Class F-155°C, and Class G-180°C.
  • Insulation testing is thoroughly standardised as the insulation breakdown diminishes the operation life of an IM through short-circuit.
  • Thinner and better insulation materials keep “surfacing” as they are crucial to better performance IMs fed from the power grid and PWM inverters.
  • Better finite element (FE) and analytical methods and test procedures to appraise existing and novel materials for IMs are being proposed.

References

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S. Sprague , D. Jones , Using the new lamination steels database in motor design, Proceedings of SMMA-2000 Fall Conference in Chicago, pp. 1–12.
2
M. Birkfeld , K. A. Hempel , Calculation of the magnetic behaviour of electrical steel sheet under two dimensional excitation by means of the reluctance tensor, IEEE Transactions on Magnetics, Vol. MAG-33, No. 5, 1997, pp. 3757–3759.
3
I. D. Mayergoyz , Mathematical Models for Hysteresis, Springer Verlag, New York, 1991.
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H. H. Saliah , D. A. Lowther , B. Forghani , A neural network model of magnetic hysteresis for computational magnetics,IEEE Transactions on Magnetics, Vol. MAG-33, No. 5, 1997, pp. 4146–4148.
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M. A. Mueller et al., Calculation of iron losses from time – Stepped finite element models of cage induction machines, Seventh International Conference on EMD, IEE Conference Publication No. 412, Durham, UK.
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M. Popescu , D. M. Ionel , A best-fit model of power losses in cold rolled motor lamination steel operating in a wide range of frequency and magnetisation, IEEE Transactions on Magnetics, Vol. MAG-43, No. 4, 2007, pp. 1753–1756.
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A. J. Moses , N. Tutkun , Investigation of power losses in wound toroidal cores under PWM excitation, IEEE Transactions on Magnetics, Vol. MAG-33, No. 5, 1997, pp. 3763–3765.
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M. Machizuki , S. Hibino , F. Ishibashi , Application of 6.5% silicon steel sheet to induction motor and to magnetic properties, EMPS, Vol. 22, No. 1, 1994, pp. 17–29.
12
A. Boglietti , P. Ferraris , M. Lazzari , F. Profumo , Preliminary consideration about the adoption of unconventional magnetic materials and structures for motors, IBID, Vol. 21, No. 4, 1993, pp. 427–436.
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M. Ibrahim , P. Pillay , Advanced testing and modelling of magnetic materials including a new method of core loss separation for electrical machines, IEEE Transactions on Industry Applications, Vol. IA-48, No. 5, 2012, pp. 1507–1515.
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M. Persson , P. Jansson , A. G. Jack , B. C. Mecrow , Soft magnetic materials–Use for electric machines, IEEE 7th International Conference on EMD, 1995, pp. 242–246.
19
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T. W. Dakin , Electric machine insulations, Chapter 13 in Electric Machine Handbook, Edited by S.A. Nasar , McGraw-Hill, New York, 1987.
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P. L. Cochran , Polyphase Induction Motors, CRC Press, Boca Raton, FL, 1989, Chapter 11.
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R. M. Engelmann , W. H. Middendorf (Eds.), Handbook of Electric Machines, Marcel Dekker Inc., New York, 1995.
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