ΦAbstract -- This paper consists a study of the influence of specific design parameters on the double-cage induction motor behavior. The study is carried out using FEM analysis. Three double rotor bar induction motors have been simulated and their electromagnetic characteristics are compared to each other. The differences in the three motors' design are: the first one has a single rotor cage and the other two have double rotor cage with different materials for the upper and inner rotor bars. The simulations performed, offer an insight and comparison between the three motors electromagnetic variables and behavior under the same speed operation, as well as under the same applied mechanical load.

Index Terms--Bar material, Design, Double-cage, FEM analysis, Induction motors.

I. INTRODUCTION he induction motors in which each rotor slot has two conducting bars are characterized by higher starting torque, lower starting current and normal efficiency at

nominal speed, compared to the standard NEMA's class A induction motors [1]. The upper rotor bar contributes during the starting of the motor because the magnetic flux does not penetrate deep into the rotor core, due to increased leakage flux, and also because of the skin effect. Upper and inner rotor bars contribute at nominal speed both, because of the low slip frequency and also because of the strongly magnetized rotor body. Due to the above characteristics, common double cage induction motors applications are: conveyors, crushers, stirrers, compressors, loaded pumps, etc [2], [3]. Despite their higher manufacturing cost, these motors give solution in applications, where the motor needs to start loaded and continue to carry the load at nominal speed. This is an advantage compared to the standard NEMA class A induction motor, which is characterized by difficult starting due to the increased starting stator current [2]. Double bar induction motors can be divided into two large categories, depending on the construction of the rotor cage. If both, the upper and the deep bar, are from the same material, they are short-circuited to the same end-ring and the rotor has one cage, similar to the standard class A induction motor. On the other hand, if the two bars are from different materials, then the upper bars are short-circuited independently from the deep bars and the rotor is manufactured with two cages. Usually, when two different conducting materials are implemented into the rotor, the material of the greatest resistivity, forms the outer cage, in order to improve the motor's starting behavior. The middle area of the rotor slot between the upper and the deep rotor bars can be of iron or dielectric material and it is a subject of interest and research [4].

ΦThis work was supported by the research program: "K. Karatheodoris

2010", of the Research Committee of the University of Patras, Greece.

Several works have been published in the literature concerning double cage induction motors. In [5] the authors have presented a numerical method for the estimation of double cage induction motor parameters from standard manufacturer data. In the same work, it is also indicated that the leakage flux of the inner cage is always greater than the one of the upper cage. Furthermore, through the appropriate design of the rotor slots, the skin effect can be used to benefit for high starting torque or high breakdown torque in double cage induction motors [6]. On the other hand, it has been shown in previous publications that the outer cage of double cage induction motors is vulnerable to failure due to its structure and applications [3], [7], [8]. Also, because the manufacturing cost is increased with the double cage structure [1], special care should be given through the design process of these motors concerning not only their electromagnetic characteristics but also their general behavior (thermal, vibrations, etc) and reliability. This can be achieved, as mentioned in [9], with oversized rotor bars and good quality magnetic plates. In this paper, the dependence of the electromagnetic characteristics of double cage inductions motors, on the specific manufacturer design options is studied. For this purpose, three different induction motor models have been created and studied with FEM analysis. The stator of the three models has been kept intact, as well as the number and shape of rotors bars. In the first model, both the upper and inner rotor bars are from aluminum, whereas in the other two the upper bar is from aluminum and the inner bar from copper. Moreover, what differs in the last two models is the middle slot area, between aluminum and cooper, which is considered as iron for the first and dielectric for the second. In sections III-V, the three models will be examined under FEM AC time-harmonic analysis, whereas in section VI they will be examined under FEM transient analysis which takes the rotor movement into account, in order to extract both their spatial and time-dependent electromagnetic characteristics. In all simulations, FEM analysis will take into account the non-linear magnetic B-H characteristic of the rotor and stator iron core, which was extracted from the manufacturers data.

II. THE MODELS' DESIGN In Fig. 1, the three models are presented. One may observe in Fig. 1-a that the rotor bar is consisted by a single material, which is in this case aluminum. In Fig. 1-b and Fig. 1-c the other two models are presented. In these models the upper and inner rotor bars are independent and from different materials. The upper bar is from aluminum whereas the inner from copper. Moreover, the middle bar area between upper and inner bars is considered to be air for the case presented in Fig. 1-b model and iron for the case of Fig. 1-c model. The number of rotor slots for all models is 28 and the rotors are considered un-skewed.

Study of Double Cage Induction Motors with Different Rotor Bar Materials

K. N. Gyftakis, D. Athanasopoulos, J. Kappatou

T

978-1-4673-0142-8/12/$26.00 ©2012 IEEE 1450

Furthermore, the stator is designed accopole single-cage induction motor and hsame for all models with 36 stator slots. form a delta wound and the phase resistain the laboratory through DC current injeare fed by symmetrical sinusoidal 3-phassystem.

a)

b)

c)

Fig. 1. The three simulated motors. a) Single aluUpper bar from aluminum, inner bar from copper anUpper bar from aluminum, inner bar from copper an

The three models, for simplicity reason

to as: model A, B and C from now, corresponds to Fig. 1-a, model B to Figmodel C to Fig. 1-c.

III. THE MOTORS STARTING BE

In Fig. 2, the magnetic flux distribumodels at starting, is presented. It is magnetic flux lines in models B and C pthe rotor core, compared to model A.

Furthermore, in Fig. 3 the current d

ording to a real 4-has been kept the The stator circuits

ance was measured ection. The models se 380V and 50Hz

uminum rotor bars. b) nd air between them. c) nd iron between them.

ns, will be referred where model A

g. 1-b and finally

EHAVIOR ution for the three

obvious that the penetrate less into

density amplitude

along the depth of the rotor bamodels. The skin effect is presignificant current displacemetheir surface. At the same timflowing through the middle contribute to the torque producsince it is not made from condu

a

b

c

Fig. 2. The magnetic flux distribution B and c) model C.

ars is presented for the three esent in all cases, provoking ent inside the bars towards me, there is not any current

bar area, which does not ction in the models B and C, ucting material.

a)

b)

c) at starting for a) model A, b) model

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a)

b)

c)

Fig. 3. The current density amplitude along the depstarting for: a) model A, b) model B and c) model C

In TABLE I, one can see the values of

current amplitude for the three simulated The stator current is practically the samodels. Also, the electromagnetic torquereduced in models B and C. The modelstarting torque than the model A, whereaModels B and C present lower total bamodel A. As, a consequence their electromstarting was expected to be significantly A. This does not happen and the differstarting torque of the three models is difference presented in the motors startinfrom the fact that, although in models B aresistance is lower due to the copper, thgreater compared to model A as seen befo

TABLE I

TORQUE AND STATOR CURRENT AMPLITUDE FO

Model Torque (Nm) StaA 79.26 B 75.05 C 76.69

pth of the rotor bars at .

f torque and stator models at starting.

ame for the three e has been slightly l B has 5.3% less as model C 3.2%. ar resistance, than magnetic torque at less than model's

rence between the small. This small

ng behavior occurs and C the total bars he leakage flux is ore in Fig. 2.

OR THE 3 MODELS ator current (A)

56.1 56.3 55.6

IV. TORQUE AND CURCHARACTE

The electromagnetic torque

versus speed for the three motoin Fig. 4-a and Fig. 4-b. Modgreatest starting and pull-out tostability area. On the other h1350rpm, one may observe significantly lower electromagtwo models, as well as lower st

The starting behavior of significant difference than thetime, for the same speed valuimportantly greater output powgreat value if one considers thapumps, the motors are compvalues (eg. 1450rpm and 2950r

Furthermore, in Fig.4-c,d,efficiency, input and outprespectively. Model A is chafactor and greater efficiency fopower is greater than the o1200rpm and the same stands fless than 1300rpm. Models B ahave both greater input pow1200rpm and greater output p1300rpm.

a)

b)

RRENT VERSUS SPEED ERISTICS

and stator current amplitude ors are presented respectively del A is characterized by the orque as well as the greatest

hand, for speed greater than that the model A presents

gnetic torque than the other tator current. models B and C, has no

e model A and at the same ues they are characterized by wer. The present remark is of at for applications such as the pared under specific speed rpm). e,f, the motors' power factor,

put power are presented racterized by greater power

or every speed. Also, its input others for speed less than for its output power for speed and C, compared to model A, wer for speed greater than power for speed greater than

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c)

d)

e)

f)

Fig. 4. Characteristics versus speed: a) Electromagnetic torque, b) Stator current amplitude, c) Power factor, d) Efficiency, e) Input power and f)

Output power for the three motors.

V. ELECTROMAGNETIC CHARACTERISTICS AT 1470RPM In the following TABLE II, the analysis results of the three models at the same speed 1470rpm, are presented. The electromagnetic torque as well as the mechanical output power of model B is 40% greater than the model A, whereas for the model C it is 35.7%. At the same time, model B draws 21.4% more stator current and model C, 20% respectively than the model A. The power factor has increased for both models B and C, but on the other hand their efficiency has decreased, compared to the model A. The results indicate that for fixed speed, double-cage induction motors present much greater output power than double bar single cage motors, at the cost of lower efficiency and increased stator current. In order to be more precise and accurate in order to compare the three models, it is important to make one more step and examine the motors behavior under the same load operation.

TABLE II ANALYSIS RESULTS FOR THE 3 MODELS AT 1470RPM

models A B C Speed (rpm) 1470 1470 1470 Torque (Nm) 27.38 38.36 37.15

Stator Current (A) 8.84 10.73 10.61 Output Power (W) 4213 5902 5717

Cosφ 0.63 0.74 0.72 Efficiency 0.94 0.92 0.92

In Fig. 5, the spatial harmonic content of the radial

component of the magnetic flux density, in the middle of the air-gap and for speed 1470rpm, is presented for the three simulated models. The relative position of the rotor and stator are the same and as a consequence the even rank harmonics do not present any difference between the three models. The even rank harmonics are indicative of the air-gap asymmetry in space, for a specific relative position between rotor and stator. Furthermore, harmonic rank numbers indicative of the saturation, such as 3 and 9, have slightly increased in models B and C compared to model A, but this is easily explained if one thinks that the stator current is increased in models B and C (Table II). Moreover, the stator MMF harmonics such as 5, 7 etc as well as the rotor MMF harmonics such as 13, 15 etc have increased about 4dB in the models B and C compared to model A. In order to compare the spatial harmonic index of the three models, their THD has been calculated as an amplitude ratio. The model A presents THD equal to 44.1%, whereas for model B it is 48.7% and for model C it is 48.3%.

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a)

b)

c)

Fig. 5. The spatial harmonic index of the radial comflux density for the: a) Model A, b) Model B and c)

VI. SIMULATION UNDER THE SAM

In this paragraph, the three models wil

compared while operating under thmechanical load. The analysis, also for into account the non-linear B-H magnetithe stator and rotor iron core. Two cases a

mponent of the magnetic Model C.

ME LOAD

ll be simulated and he same applied

these cases, takes ic characteristic of are examined. The

applied mechanical load has 30Nm and secondly 60Nm. Thextracted after the models reach

In Fig. 6, the electromagnpresented for all models forelectromagnetic torque oscillatmodels because the load torque

a)

b)

c) Fig. 6. The electromagnetic torque fomodel C, for applied mechanical load 3

Also, in Fig. 7, the electrom

presented for all models for apand B present similar torque presents about 6% greater elect

a)

b)

been chosen to be: firstly e presented results have been hed steady-state. netic torque versus time is r applied load 30Nm. The tions are similar for the three e is low.

)

or a) model A, b) model B and c) 30Nm.

magnetic torque versus time is pplied load 60Nm. Model A

oscillation while Model C tromagnetic torque pulsation.

)

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c) Fig. 7. The electromagnetic torque for a) model Amodel C, for applied mechanical load 60Nm.

Moreover, in Fig. 8 the speed versus

for the two cases examined. In Fig. 8-atorque is 30Nm, whereas in Fig. 8-b, it is

a)

b) Fig. 8. The speed versus time for the three models torque equal to 30Nm and b) load torque equal to 60

It is clear that the model B operates atthe same applied mechanical load comptwo models. Furthermore, one may obserthe model C operates at a middle speed other two models. But, as the mechanicthe model C tends to operate at speed simB (Fig. 8-a).

In Table III and IV, the analysis resuapplied load torque 30Nm and 60Nrespectively.

In Table III, for applied load equal tomodels B and C operate at greater speedstator current than the model A. Moreovermodel C is much greater than the modeOn the other hand, model's B power facthe model A about 3.4%. The difference bfactor and the efficiency of models Bexplained. Model C is characterized byrotor flux because of the iron area betweinner rotor bars. As a consequence, its poless than model B, which leads to improve

In Table IV, it is obvious that the efficiB is greater than the other two mode

A, b) model B and c)

time is presented a, the applied load 60 Nm.

for the case of: a) load 0Nm.

t greater speed for pared to the other rve in Fig.8-b, that value between the

cal load decreases, milar to the model

ults for the case of Nm are presented

o 30Nm, both the d and draw greater r, the efficiency of l A (about 6.1%). tor is greater than between the power B and C can be y greater leakage een the upper and

ower factor will be ed efficiency. iency of the model els. Same as the

previous case (30Nm), the staand C is greater than the mod26% greater stator current thanthe model A is characterized which is 15% greater than the m

TABLE

ANALYSIS RESULTS FOR APPLIED MEMODE

models A Speed (rpm) 1470.4Torque (Nm) 30

Stator Current (A) 5.66 Output Power (W) 4617

Cosφ 0.836Efficiency (%) 85.6

TABLEANALYSIS RESULTS FOR APPLIED ME

MODE

models A Speed (rpm) 1423.6Torque (Nm) 60

Stator Current (A) 11.5 Output Power (W) 8940

Cosφ 0.9 Efficiency (%) 75.6 Finally, in Fig. 9, the spat

radial component of the air-gpresented for the three modetorque is 30Nm. The 3rd harsaturation, has the greatest valupresents THD equal to 43% whto be 49.1% and for model C it

a)

ator current of the models B del A. The model C presents n the model A. Furthermore, by the greater power factor,

model C.

E III ECHANICAL LOAD 30NM FOR THE 3 ELS

B C 4 1480 1478

30 30 5.76 6 4646 4641

6 0.87 0.74 81.3 91.7

E IV ECHANICAL LOAD 60NM FOR THE 3 ELS

B C 6 1446.5 1428.3

60 60 11.6 14.5 9084 8970 0.89 0.75 77.5 72.3

tial harmonic content of the gap magnetic flux density is els, when the applied load rmonic, which indicates the ue in model A. The model A hereas for model B it proved is 43.3%.

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b)

c)

Fig. 9. The spatial harmonic index of the radial component of the magnetic flux density for the: a) Model A, b) Model B and c) Model C, when 30Nm mechanical load is applied.

VII. CONCLUSION In this paper, 3 different models of double cage

induction motors have been studied using FEM analysis. Since the models B and C have two different rotor cages, their manufacturing cost will be greater than model A. On the other hand, the comparison between their characteristics reveals that the use of different materials for the upper and inner rotor bars presents several advantages compared to a single aluminum double bar induction motor. The comparison of the 3 motors for the same speed operation showed that: the models B and C present much greater output power as well as greater power factor and almost the same efficiency, than the model A. The simulation results under the same mechanical load for the three motors indicated that: for high load operation model B presented 2% greater efficiency than the model A, while for low load operation the model C was characterized by 6.1% greater efficiency than the model A. For both models B and C, the advantage of greater efficiency came at the cost of lower power factor and slightly greater stator current. Finally, depending on the specific application criteria, the optimization of the design of models B and C could lead to even better characteristics, and this is considered as promising future work.

VIII. REFERENCES [1] J. Park, B. Kim, J. Yang, S.B. Lee, E.J. Wiedenbrug, M. Teska and

S. Han, “Evaluation of the Detectability of Broken Rotor Bars for Double Squirrel Cage Rotor Induction Motors”, 2010 IEEE Energy Conversion Congress and Exposition, ECCE 2010 - Proceedings , art. no. 5617950, pp. 2493-2500, 2010.

[2] Motors and generators, NEMA standards pub. MG 1-2006, 2006. [3] H.A. Toliyat, G.B. Kliman, Handbook of electric motors, 2nd

edition, Marcel Dekker, 2004. [4] I. Boldea and S.A. Nasar, "The Induction Machines design

handbook", 2nd Edition 2010 by Taylor and Francis Group, LLC, pp. 23-24.

[5] J. Pedra and F. Corcoles, “Estimation of Induction Motor Double-Cage Model Parameters From Manufacturer Data”, IEEE Trans. on Energy Conversion, Vol. 19, No. 2, pp 310-317, June 2004.

[6] S. Williamson and C.I. McClay, “Optimization of the Geometry of Closed Rotor Slots for Cage Induction Motors”, IEEE Trans. on Industry Applications, Vol. 32, No. 3, May/June 1996

[7] T.A. Lipo, "Introduction to AC machine design", Wisconsin Power Electronics Research Center, University of Wisconsin, 2004.

[8] ] M.G. Say, "The performance and design of alternating current machines", Sir Isaac Pitman and Sons LTD., London, 1955.

[9] M.A. Saidel, M.C.E.S. Ramos and S.S. Alves, “Assessment and Optimization of Induction Electric Motors Aiming Energy Efficiency in Industrial Applications”, 19th International Conference on Electrical Machines, ICEM 2010, Rome, Italy, 6-8 Sep. 2010.

IX. BIOGRAPHIES

Konstantinos N. Gyftakis was born in Patras, Greece, in May 1984. He received the diploma in Electrical and Computer Engineering from the University of Patras, Patras, Greece in 2010. He is a PhD candidate in the Department of Electrical and Computer Engineering, University of Patras. His research activities are in FEM design, fault diagnosis and optimization of electrical machines. He is an IEEE member, member of IEEE PES and Magnetics Society, member of the HELIEV (Hellenic Institute of Electric Vehicles) and finally member of the Technical Chamber of Greece. (E-mail: kosgyftak@upatras.gr)

Dimitrios Athanasopoulos was born in Patras, Greece in November 1989. He is a senior undergraduate student at the Department of Electrical and Computer Engineering, University of Patras, Greece. Currently, he pursues his diploma thesis focusing on FEM design and electromagnetic analysis of double-cage induction motors. (E-mail: athanas_3@yahoo.gr)

Joya C. Kappatou was born in Argostoli, Greece. She received the diploma in Electrical Engineering from the University of Patras, Patras, Greece and the PhD from the same University in 1991 in the field of Electrical machines and Power Electronics. She is Assistant Professor in the Electrical and Computer Engineering Department of the University of Patras. Her teaching and research activities are in electrical machines, power electronics, modeling and design using FEM, faults diagnosis in electrical machines. (University of Patras, Electrical and Computer Engineering Department, 26500 Rion-Patras, Greece, Tel: +30 2610/996413, Fax: +30 2610/997362, E-mail: joya@ece.upatras.gr)

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