Influence of modern magnetic and insulation materials on dimensions and losses of large induction machines

Balavoine François; Cassoret Bertrand; Demian Cristian; Romary Raphaël; Debendere Christophe

doi:10.1515/phys-2020-0172

Article Open Access

Influence of modern magnetic and insulation materials on dimensions and losses of large induction machines

Balavoine François , Cassoret Bertrand , Demian Cristian , Romary Raphaël and Debendere Christophe

Published/Copyright: October 20, 2020

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From the journal Open Physics Volume 18 Issue 1

Abstract

The aim of this study is to present the possibilities of reducing the size of large induction motors with new electrical materials taken into account of thermal constraints. This study highlights the possibilities of reducing losses, increasing the efficiency, and decreasing the weight, by taking different grades of materials. A nonlinear multiobjective optimization is used to determine the best solution, by varying the internal and external geometric parameters.

Keywords: Induction motor; magnetic sheets; insulation; motor design; electrical materials

List of symbols

A l : specific current loading (A/m)
B ˆ : flux density (T)
B ˆ e : air gap flux density (T)
C mec : machine sizing constant (kW s/m³)
cos ϕ : power factor
D: internal diameter motor (m)
η : efficiency
H: magnetic field (A/m)
K w1 : winding factor
L: length motor (m)
n: speed revolutions per second) (rps)
p: pole pair numbers
P Cu : copper losses (W)
P mec : mechanical power (W)
P Iron : iron losses (W)
Rad i : radius of different stator parts (m)
S: apparent power (VA)
T 1 : critical temperature point (°C)

1 Introduction

A plenty of medium and large power induction machines (IMs) manufactured in the first half of the 1900s are still in operation today [1]. It is possible to improve efficiency, reduce volume, or both by replacing insulation or magnetic sheets with current, modern materials [2]. The aim of this study is to quantify the influence of the materials on the dimensions of different IMs at given rated power from 1950s to present days and to present the contribution of new materials to performance improvements. This step allows us to reduce the size of the machines. This work is based on classical methods of sizing literature.

The main dimensions of the IM are essentially related to the power density coefficient C mec , in kW s/m³ [3]. This quantity depends on the specific current loading A l in A/m, the air gap flux density B ˆ e in T, and the winding factor K w1 . Equations 1 and 2 give the mechanical power P mec in kW, which is proportional to C mec . Independently of the speed and thus the pole number, the constant C mec provides the main dimension of the machine. Figure 1 presents the evolution at different periods of this constant for pole pair numbers from two to six. The curves allow one to deduce the evolution of the inner stator diameter and length at given speed [4,5]. This evolution shows that the machines are getting smaller and smaller in size for the same power:

(1) P mec = π 2 2 ⋅ K w1 ⋅ A l ⋅ B ˆ e ⋅ D 2 ⋅ L ⋅ n ⋅ cos ϕ ⋅ η ,

(2) P mec = C mec ⋅ D 2 ⋅ L ⋅ n ,

(3) C mec = π 2 2 ⋅ K w1 ⋅ A l ⋅ B ˆ e ⋅ cos ϕ ⋅ η .

$Figure 1 Evolution of machine sizing constant C mec {C}_{\text{mec}} based on empirical knowledge and a traditional know-how, for pole pairs two to six.$

Figure 1

Evolution of machine sizing constant C mec based on empirical knowledge and a traditional know-how, for pole pairs two to six.

Section 2 of this study deals with the improvement of IM motors and give the details of the impact of electrical insulation and magnetic circuit quality. In order to check the critical temperature points, Section 3 presents a simplified thermal circuit model. Section 4 deals with the choice of a 4.5 MW IM and gives the first results. Section 5 presents the optimization and dimensions of the best case and a conclusion.

2 Improvement of the IM

2.1 Presentation of the studied machine

The study focuses on the design of a 4.5 MW, two pole pairs, a 6.6 kV voltage supply IM. This is a squirrel cage machine with 72 stator slots and 62 rotor slots. Four different configurations with the same airgap width have been studied as follows:

Old electrical insulation and old magnetic circuit with C mec = 200 kWs/m 3 (named OLD),
Recent (RE) materials, minimum value of C mec = 325 kWs/m 3 (named RE_Min),
Recent materials, maximum value of C mec = 425 kWs/m 3 (named RE_Max),
Recent materials, iron cobalt electrical steel sheets, and high value of C mec = 510 kWs/m 3 (named RE_IRCO).

The three values of C mec come from usual design, whereas the last one corresponds to the size reduction target.

2.2 Impact of electrical insulation

The electrical insulation evolution allows the increase of the dielectric strength and the acceptable temperature. This concerns the high voltage insulation and the fill factor improvement of copper in the slots [1,6].

Tables 1 and 2 give an overview about the improvement of electrical insulation. It can be seen in Table 1 that before 1960s the groundwall insulation was essentially made of micanite tube, which is an assembly of thin flakes of mica stuck together with a flexible varnish. The tube thickness depends on voltage supply. The conductor insulation was composed of various papers and cotton. A few turns around the conductors were necessary.

Table 1

Old electrical insulation

Localization	Years	Width (mm)	Composition
Flat copper	1926/1928	0.4	Cotton wrapping or tape paper
Flat copper	1955	0.3–0.4	Cotton wrapping or tape paper
Winding/groundwall	1926/1928	2.5–4	Micanite sheath/tube
Winding/groundwall	1955	2.5–3	Micanite sheath/tube

Table 2

Recent electrical insulation

Localization	Width (mm)	Composition/laminating
Flat copper^a	0.3	Mica tape with polyester film
Winding^b	0.15	Mica paper with metallic salt accelerator and zinc naphthenate
Groundwall^c	0.085	Conductor tape with polyester tape
Corona effect	0.085	Conductor tape with polyester tape
Groundwall^d	0.18	Glass and polyester and polyester felt with epoxy resin
Finish tape	0.18	Glass and polyester and polyester felt with epoxy resin
Total^e	0.415	Total insulation for the coil winding and groundwall
Winding + groundwall	0.415	Total insulation for the coil winding and groundwall

a
It is the insulation closest to the copper conductor;
b
The insulation of the winding, i.e. of all the conductors;
c
It is one of the groundwall insulations, for the corona effect;
d
It is second of the groundwall insulations (for mechanical protection);
e
It is the sum of the dimensions of the groundwall + winding insulation, for the coil insulation (without flat cooper).

The new electrical insulations presented in Table 2 are composed of several types of thin materials, each brings a specific insulation function in the IM (turn-to-turn insulation, corona effect, and mechanical protection). To the nearest of the conductor, the turn-to-turn protection is a base of mica paper with epoxy resin impregnation.

2.3 Impact of magnetic circuit quality

The electrical steel sheet quality has been improved through a better building process. Among the non-oriented steel sheet, the following three categories based on datasheet have been considered:

old iron silicon sheets (thickness 0.5 mm or 1 mm) (taken as a reference),
recent iron silicon sheets (thickness 0.5 mm), and
recent iron cobalt sheets (thickness 0.35 mm).

These materials are described in Figures 2 and 3 [7,8,9].

Figure 2

Electrical steel sheets quality comparison of B–H curves.

Figure 3

Electrical steel sheets quality comparison of specific total losses.

Among the electrical steel sheet qualities used in the paper, the lowest losses are for iron cobalt called iron and cobalt conception (IRCO). Their losses are around 1.4 W/kg at 1.5 T and 50 Hz [10], whereas those of the old sheets are between 6 and 8 W/kg.

This steel sheet is characterized by a high saturation induction at 2.3 T, whereas the others have a weaker saturation level at 1.8 T. Moreover, the difference in the bend of the curve is more important for the old characteristics [11].

3 Simplified thermal circuit model

In order to optimize the size of IM, the thermal circuit model allows one to check the temperature within the IM. The critical temperature point at the surface of the conductors in the stator slots and in the end windings is well known. Joule and magnetic losses will be the main cause of temperature increase in the machine.

The selected thermal model considers only the heat transfer by radial conduction within the machine. The use of thermal potential differences will allow us to evaluate the temperature increase at different points of the machine. The thermal model (Figures 4 and 5) will be in a steady state operating condition of the machine [12].

Figure 4

Simplified thermal circuit model.

Figure 5

Simplified stator for the thermal model.

In the thermal network, two power sources are considered in this circuit, the Joule and iron losses in the stator. The variables T i correspond to the temperature at different points in the stator. The thermal resistances of this network are calculated from the analytical equations of the heat equations at the area borders defined by the radius Rad i in Figure 5. They depend on geometrical parameters and the material thermal conductivity. The resistances are calculated at different points of the winding, insulation, stator yoke, and the frame in contact with the ambient air.

The thermal conductivities used in the model are as follows [12]:

Coil 5 W/(mK) ;
Winding insulators 0.20 W/(mK) ;
Iron and silicon magnetic circuit 25 W/(mK) ; and
Cast-iron housing 52 W/(mK) .

4 Main dimensions and results

The sizing approach is based on the usual equations of large IM design [4,5]. This is a traditional approach that enables us to obtain a fast dimensioning of the machine, but that suffers of a lack of accuracy for loss determination. However, the used method gives tendencies about the influence of modern materials on the sizing.

Partial results are presented in Table 3 that gives the main dimensions for each configuration, based on Figure 1 for C mec and considering material characteristics.

Table 3

Principal internal dimensions for 4.5 MW IM

IM	Inner diameter (m)	Outer diameter (m)	Iron length (m)
OLD	1	1.57	0.9
RE_Min	0.8105	1.19	0.90
RE_Max	0.737	1.08	0.82
RE_IRCO	0.68	1.05	0.76

For RE_IRCO, the constant C mec is chosen to be 510 kW s/m³ higher than the present day area of Figure 1 corresponding to the gray area between RE_Min and RE_Max.

The results of the design process show that for the same rated power, the air gap flux density is slightly higher for the machine with IRCO because the magnetomotive force losses in the material are lower.

Figure 6 gives the efficiency of each machine resulting from analytical calculations. The results show an improvement of efficiency between the old and the new materials. The results in Figure 7 show a decrease of 52.3% for iron weight, 15.8% for copper weight, and 0.54 point efficiency rise, between the OR_old and the best IM configuration (IRCO). However, for usual applications, the economic criteria lead to choose an iron-silicon magnetic circuit (cases RE_Min and RE_Max) instead of IRCO. Indeed, IRCO sheets can cost 5–10 times more than IRSI sheets. Concerning the efficiency, the improvement is roughly the same but the iron weight and volume are more reduced with IRCO.

Figure 6

Efficiency of each configuration.

Figure 7

Weight of magnetic circuit and copper of each configuration.

Table 4 shows the copper losses ( P Cu ) and iron losses ( P Iron ) in the stator, which can be used for the thermal model. Then, the thermal circuit model gives the temperature T 1 for each machine, which corresponds to the critical point located at the conductor surface. Table 4 also presents the temperature T 1 for each machine. This table shows a reduction of local heating at the critical point, between the old and new configurations, caused by the insulation thickness improvement. The results related to the main losses and the temperature T 1 are consistent.

Table 4

Main sources of thermal model and temperature of conductors inside the stator

IM	OLD	RE_Min	RE_Max	RE IRCO
P _Cu (kW)	28.8	24.4	22.4	27.2
P _Iron (kW)	38.4	12.85	16.6	11
T ₁ (°C)	127	82.7	75.8	83.6

5 Optimization of IRCO case

This part presents the dimensioning of the machine using a genetic algorithm. The NSGA-II code is used with MATLAB in order to determine a set of results presented in the form of Pareto curve [13,14,15]. The previous configurations stem from datasheet curves of Figure 1. The design will ensure acceptable temperature, especially, in the recommended area between RE_Min and RE_Max. The aim of this section is to calculate and control the case IRCO outside this area.

In order to obtain optimized results for iron cobalt case, a nonlinear multiobjective optimization with analytical calculations is used ref. [16,17]. This optimization makes it possible to vary the main dimensions of the machine (diameter and length) and the size of stator slots. The main objective is the smallest dimensions with equivalent efficiency to this range of motors. The constraints used in this optimization define the physical limits that can be taken into account. These limits are as follows: the saturation of the magnetic induction at 2.2 T for iron cobalt electrical steel sheet, the magnetizing current, the current density in the winding, the maximum temperature inside a conductor and one mechanical constraint, corresponding to the shaft diameter chosen at 0.25 m for this power.

The variation in the input parameters in the genetic algorithm returns a matrix with several results depending on the objectives. This matrix shows a result set in Figure 8 in the form of a Pareto curve.

Figure 8

Pareto curve for two objectives about a design motor with iron cobalt electrical sheet steel.

Based on this curve, it is possible to calculate the volume and choose a Pareto optimum. This point gives a shorter length and close diameters: the inner diameter is 0.684 m, the outer diameter 1.05 m, and the length is 0.66 m. The efficiency is 96.65%, almost the same as before (96.67%). The magnetic circuit weight is of 3.37 t a reduction of 17.1% compared to the RE_IRCO case and 0.6 t for stator and rotor copper, a reduction of 13%.

Comparing this solution with the previous RE_IRCO machine (Table 5), it can be noted that the lowest weight and dimensions are obtained with the same efficiency. The temperature increase is still acceptable. This new optimized design leads to C mec = 580 kW s/m 3 instead of 510 kW s/m³.

Table 5

Comparison between two IRCO configurations

IM	RE IRCO	Optimization IRCO
η (%)	96.67	96.65
Weight of iron (t)	4.1	3.4
Weight of copper (t)	0.69	0.6
T ₁ (°C)	83.6	90
P _Cu (kW)	27.2	31.8
P _Iron (kW)	11	11.7

6 Conclusion and discussion

This paper presents a study about the building of IM at different periods. It shows how the electrical material ameliorations have an impact on the main dimensions, the weight, and the efficiency. The results show that for the best case with the iron cobalt magnetic circuit, an important reduction of internal diameter, length, and weight, and a 0.54 point efficiency improvement can be obtained. To specify the results and validate the IM size reduction, a thermal circuit model is exploited to check the temperature at the critical point T 1 located at the surface of the stator conductors. To improve the IM design, a genetic algorithm is used to find the best solution using the analytical relations and considering the thermal constraints. The geometric parameters of IM can be changed for different objectives, as minimizing weight while keeping a good efficiency. The optimization allows us to find the best solution into the Pareto matrix. The optimal weight reduction is 60.8% for magnetic circuit and 26.8% for total copper between case OLD and optimized IRCO.

christophe.debendere@flipo-richir.com

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Received: 2020-03-13

Revised: 2020-06-10

Accepted: 2020-06-29

Published Online: 2020-10-20

This work is licensed under the Creative Commons Attribution 4.0 International License.

Articles in the same Issue

https://doi.org/10.1515/phys-2020-0172

Keywords for this article

Induction motor; magnetic sheets; insulation; motor design; electrical materials

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