Abstract—An attempt has been made to analyze the methods to improve the electromagnetic properties of a solid-rotor high speed induction motor. The slitting patterns of the solid rotor are examined. It is shown how the slitting parameters affect the produced torque. The paper considered the effect of variation of slit depth and slit width for two materials with different electrical properties, on the performance of 180 kW, 170 Hz 3-phase induction motor using Flux-2D Maxwell software package from ANSYS.It is found that deeper rotor slit gives better and higher starting torqueas well as maximum torque.Also a comparative study with a double cage rotor slot is presented which leads to further improvement of running performance of the high speed induction motor.

Keywords— ANSYS, Double Cage Rotor slot, FEM, Induction motor, Solid rotor, Slit rotor slot,.

I. INTRODUCTION T is due the remarkable development in field of frequency converters thatit’s become feasible to apply variable speed

technology of AC motors to a wider range of applications. The need ofdirect drives system is on an increase. Since direct drives do not require different gear system which is important part of conventional electric drives therefore its economical in energy and space consumption, also direct drives are easy to install and maintain. High speed induction motors require such drives as they run on frequencies in the range of 100 to 200 Hz. The applications of high speed induction motors are also vast and growing. Area were they are currently being used are domestic, commercial, and light industrial air conditioning, refrigeration, or heat pump, blowers spindle machines and large gas compression turbines[3]. High speed induction motor with solid rotor can be applied in power range of few kilowatts to tens of megawatts. Solid rotor is preferred over laminated rotor because of mechanical point of view as laminated rotor is not rigid enough[5]. The solid rotor construction also a has several advantages such as high mechanical rigidity and durability, higher thermal durability, high reliability, simple construction, low level of noise and vibrations since the surface is smooth and also it’s cheap to manufacture. On the flipside there are certain

Ranjay Singh is with the Department of Electrical Engineering, Motilal

Nehru National Institute of Technology, Allahabad-211004, India (e-mail: ranjay1988@gmail.com).

SarikaKalra andVineetaAgrawal are with the Department of Electrical Engineering, Motilal Nehru National Institute of Technology, Allahabad-211004, India (e-mail: sarika@mnnit.ac.in, vineeta@mnnit.ac.in).

__________________________________________________________ 978-1-4799-4939-7/14/$31.00 ©2014 IEEE

disadvantages of solid rotor such as lower output power, efficiency and power factor caused due to large inductive impedance of rotor therefore its material should be carefully examined and selected[2]. For the consideration of this paper solid rotor motor is used and its constraints are reduced by different rotor slot design. The tool used for analysis of the machine is Flux-2D Maxwell software package from ANSYS.

II. FEM MODELOFINDUCTION MOTOR Finite Elements Method (FEM) has been used extensively, in the analysis of electrical machines. In this paper, two-dimensional time-stepping Finite- element method is performed for modeling and analyzing of a solid slitted rotor induction motor.The non-linearity of rotor material and the movement of the rotor require the use of the time stepping method to accurately solve the magnetic field. The calculation is based on the numerical solution of the magnetic vector potential A. In the two dimensionally diffusion equation [4] × υ × )A J∇ ( ∇ = (1)

Where J= current density in z-direction, A= vector in z-direction; Current density, J as having three parts: one due to the applied source, another due to the induced electrical field produced by time-varying magnetic flux, and the third due to motion-induced or speed voltage Therefore,

σ σ σbV AJl t

∂= − + υ× Α∂

(2)

Where υ= velocity of the conductor with respect to A, Vb = voltage applied to the finite element region; σ = electrical conductivity l = length of region in z-direction (in 2-D analysis). The time-dependent magnetic diffusion equation is now become:

× υ × ) σ σ σbV AAl t

∂∇ ( ∇ = − + υ× Α∂

(3)

By applying the a reference frame theory the relative velocity become zero and the diffusion equation simplifies to

× υ × ) σ σbV AAl t

∂∇ ( ∇ = −∂

(4)

Analysis of Slit Rotor Slot of high speed SRIM and Comparison with Double Cage Rotor Slot

Ranjay Singh, Sarika Kalra and Vineeta Agrawal

I

In order to couple the circuit and the fielnecessary to calculate the total current conductor of the induction motor. The total current in the conductor is founabove equation over the cross section of the c

σ σbconductor

V AI dxdyl t

∂⎛ ⎞= −⎜ ⎟∂⎝ ⎠∫∫

Solid-rotor induction motors are built with a

a solid single piece of ferromagneticelectromagnetic properties of such a rotor arpoor, as, e.g., the slip of the rotor tends to be larmodifications of the solid rotor is required. improvement in a solid rotor is achieved bysection of the rotor in such a way that a betteinto the rotor will be enabled. Fig. 1 shows the of a solid-rotorslitted induction motor. The stsame but the rotor geometry that is, its slots is the main analysis of this paper.

Fig.1. Cross-section of solid-rotor induction motor

III. TEST MACHINE CONSTRUCT

The test machine parameters are given in depth, width and type are varied to oboperating conditions.The stator and rotor aresteel and the conductor material is copper. Thmaterial used also has immense effects on ththe machine.

Table: I

Machine Details

Number of poles Number of phases

Rated output power [kW] Rated frequency [Hz]

Output diameter of the stator [mm] Inner diameter of the stator [mm]

Core length [mm] Number of stator slots

Outer diameter of the rotor[mm] Number of rotor slots

Width of rotor slots [mm] Depth of rotor slots [mm]

There are distinctly 4 different topologies for slots of the induction machine mainly [7]:

ld equations, it is flowing in each

nd by integration conductor [5]:

(5)

rotor core made of c material. The re, however, quite rge, and thus some The performance

y slitting the cross er flux penetration cross-section view tator geometry is are varied which

TION Table I. The slot

btain to different e simulated using he variation of the he performance of

2 3

180 170 400 200 280 48

195 34 3 40

the solid rotor

1. Solid rotor with smooth sur2. Solid rotor Coated with cop3. Solid rotor with slitted rotor4. Lastly solid rotor with slitt

conductors in the slits. The mechanical integrity and the

achieved is maximum for type 1 down to type 4 but on the contrarstarting current is higher as shownpenetration is maximum in type4 anless eddy current losses and heating reducing effect of skin effect [7,8].

Fig.2. Comparison Of slot types Slitting the rotordecreases the low fproviding more torque at very lowbetter torque at rated running of tincreases the high frequenctherebydecreasing eddy current loinverter current ripples [7]. Hence fopaper slitted rotor is considereconstruction parameters is studied.

IV. SLOT DEPTH VARIA

It’s already been establishedthat properties and efficiency of a slitteslip than a smooth non slitted rotorthe slitted rotor the depth of the selected as well as their boundary defined. The solid rotor is verymechanical properties. It is due to linto the rotor core as there are nofrequency the skin effect becomes mslit slots are provided so that theconductors can be placed closer to tpenetration of flux. It is known that the surface less is the skin effect[1skin effect depends are:

Where represents penetration de

permeability, represents conductivfrequency. Since these are inherent mchoice of material also has a significperformance of SRIM.

0

20

40

60

80

100

Type 1

Type 2

Type 3

Type 4

rface and no slots. pper conductor. r slots. ted rotor slots with copper

highest speed that can be and reduces as we move ry the starting torque and n in Fig 2. Also the flux d least in type 1 leading to of the machine hence also

frequency impedance thus w slip. Therefore we get he motor as well. It also y surface impedance sses due to slotting and or the consideration of this d and variation in its

ATION EFFECTS the power factor, torque

ed rotor is higher at lower r [3]. But while designing slits have to be carefully conditions should be well

y robust but it lacks in lack of penetration of flux o slots and also at higher more prominent. Therefore e current carrying copper the core thereby aiding the more the penetration from

10]. The factors on which

(6)

epth of flux, represents ity and f represents

material property therefore cant effect on the

Maximum Speed (×1000) rpm

Starting Torque(Nm)

To show the effect two materials are consi

analysis as shown in table II:

TableII Important Material Properties

Property Steel 1010 Ir

grRelative Permeability

2000 50

Bulk Conductivity (simens/m)

20,000,000 13

Mass Density (Kg/m3)

7874 7

The depth of the slit haskey effect on th

high speed induction motor as shown in Fig 3

Fig.3. Torque vs. Speed characteristics with Slit d1010)

Fig.4. Torque vs. Speed characteristics with Slit depth

The depth of the rotor slit is varied fromwith an increment of 5mm and torque speare obtained. We can manifestly observmaterial as the depth of the slit is incrtorque increases from 125Nm to 603Nm atorque also increases from 670 Nm to 1519be observed that the maximum torque is aslip leading to better characteristics. Similwas observed for iron rotor material but deffect the improvement was less in comrotor. The current variation is obtained fvariation as shown in Fig. 5 and Fig 6.

25mm

Increasing

Increasing

25mm

idered for

ron(Industrial rade) 000

3,000,000

870

he performance of 3 and Fig 4.

depth variation (Steel

h variation (Iron)

m 5mm to 25 mm eed characteristics ve that for steel rease the starting and the maximum 98Nm. It can also

achieved at higher lar variation trend due to higher skin mparison to steel for the slit depth

Fig.5.Current vs. Speed characteristics w1010)

Fig.6. Current vs. Speed characteristics wi

As shown in Fig.5 the input currtremendously as the depth increasgetting better torque characterises current requirement impedes us to compromise between the torque anddepth selection. An optimised valconsidered. Obviously for iron rotor of the skin effect the area reduces anreluctance and thereby reducing cuhas been preferred for the further aproblem related with iron rotor asperiphery. The field formation for sshown in Fig. 7.

Fig.7. Field formation for a Slit depth of 1

5mm

5mm

Increasing

Increasing

with Slit depth variation (Steel

ith Slit depth variation (Iron)

rent requirement increases ses. Even though we are for lower depth the high go for 5mm depth and a

d current is considered for lue of 15mm depth was the current is less because

nd therefore increasing the urrent. But still steel rotor analysis as there is heating s major flux flows in the teel rotor is simulated and

5mm (Steel rotor)

25mm

5mm

5mm

25mm

There was no saturation observed for 15mm a purely sinusoidal flux formation was observcan be observed in Fig.7.

V. SLOT WIDTH VARIATION EFF

Another parameter considered is the variation. Though the variation in width haseffect on the performance but it helps in optislit design. Torque speed characteristics are oin Fig.8 for variation of width of slit from 1an increment of 1mm.

Fig.8. Torque vs. Speed characteristics for Slit width var

It was observed that with increase in slitimprove the starting torque from 215Nm tmaximum torque from 605Nm to 1487 Nm, in slip at maximum torque. This is obtainewidth increases flux has more area to penesolid rotor core. There was no significant diin the characteristics for iron rotor. But a sihigher current requirement is observed forwidth as shown in Fig.9.

Fig.9. Current vs. Speed characteristics for Slit width var Since the current requirement increases witwidth a tradeoff between torque and currentis performed and an optimized value 3mm is

Increasing

1mm

6mm

Decreasing

1mm 6mm

depth considered ved in the rotor as

FECTS rotor slit width

s a relatively less imization of rotor obtainedas shown mm to 6mm with

riation

t width we could to 355Nmand the with no variation

ed because as the etrate through the ifference obtained imilar problem of r increase in slit

riation

th increase in slit t is characteristics considered.

VI. DOUBLE CAGE ROTOR SLOT ANROTOR SLO

The double cage rotor slot is starting torque and lower starting with normal efficiency [6] in compslot. During the starting of the contributes as the magnetic flux is into the rotor core because of skin efflux. As the rotor slip frequency delower cage contribute at nominalmagnetized rotor body as now the fmore than at lower speed [9]. Duedouble cage rotor slot the performanfurther be improved. Even thougincrease in cost of the machineperformance of the machine especstarting. Therefore the analysis of dbecomes necessary for the scope machine as used in previous analysicage rotor slot. The slot design and Figure 10.

Fig.10.Double cage rotor slot of induction mo

In order to find the most optimiupper and lower cage the depth of thtorque-speed characteristic is obtadepicts the result obtained. The devaried from 5mm to 25mm with inand lower cages have a diameter ooptimum width as found out before.

m

Increasing

25mm

ND COMPARISON WITH SLIT OT

characterized by higher current at nominal speed arison to normal slit rotor

motor the upper rotor not able to penetrate deep ffect and increased leakage ecrease both the upper and l speed due to strongly flux lines can penetrate far e to this characteristics of nce of the ac machine can

gh these slot lead to an e but they enhance the cially in case of on load ouble cage rotor slots also of this paper. The same

is is used here with double configuration is shown in

otor

ized distance between the he lower cage is varied and ained for each. Figure 9 epth of the lower cage is ncrement 5mm. the upper

of 3mm which is the most

5mm

Fig.11. Torque vs. Speed characteristics with variation(Steel 1010)

Fig.12.Torque vs. Speed characteristics with variation (Iron)

Careful observation of Fig. 11 shows tha

lower cage is increased starting torque improto 725Nm and the maximum torque is 1711Nm to 1775Nm. Though there is ndifference with the variation of depth of lowdepth of 20mm gives us the best result as starting torque and maximum breakdowndepicts the same analysis performed for iexpected an inferior result was obtained in corotor. Also comparing Figure 11 and Figure starting torque for the most optimized of depof double cage rotor slot than in case of slit romaximum torque that can be achieved with sthan in case of cage rotor. On analysis of tdouble cage rotor slot with depth of 20mmthat there was a significant improvement. Taresult of the machine running performandouble cage slot and slit rotor slot SRcomparison of torque speed characteristics oslots is also shown in Figure 13 emphasizinimprovement.

Table III Performance of Different Rotor Slot T

Parameter Double cage rotor

slot Breakdown Torque(Nm)

1760

Locked Rotor Torque(Nm)

670

Breakdown Slip 0.18 Efficiency 70

As can be seen from the result the starting

the maximum torque is increased also thmaximum torque is obtained is more in casrotor. This signifies that the maximum toearlier in case of double cage rotor which is a

Increasing

5

25mm

n on depth lower cage

on depth lower cage

at as the depth of oves from 626Nm

improved from not a significant

wer cage but still a it has maximum

n torque. Fig 12 ron rotor and as

omparison to steel 3 reveals that the th is more in case otor slot. Also the slit slot is far less the machine with

m it was observed able III shows the nce of optimized RIM; a graphical

f both the type of ng the significant

Type

Slit rotor slot

1157

308

0.09 67

g torque as well as he slip at which se of double cage orque is achieved a special

advantage in application where highoverall efficiency is also improved in

Fig.13. Torque vs. Speed comparison of SlRotor Slot SRIM (Steel 1010)

VII. CONCLUS

It is concluded in this paper that

have grave importance on the perforthat how the slitting parameters afAnd it is observed that deeper slit gias maximum torque, but it also inrefined optimization of the slot imachine configuration considered hand width of 3mm has been focomparison of slit slot with double further improvement in torque perfwith the later but at additional cost.made to show the effect of rotoperformance as the material propertieffect problem.

VIII. REFEREN

[1] K.N. Gyftakis, D. Athanasopoulos, andCage Induction Motors with Different Programme K. Kartheodoris 2010 university of Patras, Greece.

[2] Vicente Aucejo Galindo, Xose M.LopA.Paulo Coimbra, “Parametric Study Induction Motor”, in press.

[3] Wen L. Soong, Greald B.Kliman, and RSpeed Induction Motor for a CommeIEEE Transaction on Industry Applicati

[4] Juha Pyrhönen, Janne Nerg, Panu KurSpeed High-Output Solid Rotor InducCompressor,” IEEE Transaction On pp.272-280, 2010.

[5] Blagoja Arpinoski, Mirka Popnikolovand Milan Cindev, “Modelling of ThreeThree- Phase Induction Motor with Dojournal, vol 2, pp.80-82, 2013.

[6] Hao Zhou, and Fengxiang Wang, “CoInduction” Proceeding of InternatioMachines and Systems, Seoul, Korea, pp

[7] Sarac Vaasilija, and Stefanov Goce, “fields In Electrical Machines usinInternational Journal of Engineering 2011.

[8] T. Aho, J. Nerg, J. Sopanen, J. Huppunthe effect of rotor slit depth on electric mrotor induction motor”, IREE vol. 1, pp.

[9] P.S. Bhimbra, Electrical Machines. Indi[10] G.K. Dubey, Electrical Drives. India 20

Do

Slit Slo

mm

inertia load is driven. The n double cage rotor.

lit Rotor Slot and Double Cage

SION

t slit rotor slot dimensions rmance of SRIM. It is seen ffect the produced torque. ives higher starting as well crease the current so, the is presented and for the here a slit depth of 15mm found. Another study of cage rotor slot shows that

formance can be achieved An attempt has also been

or material on the motor ies strongly affect the skin

NCES d J. Kappatou, “Study od Double

Rotor Bar Materials,” Research of the Reserch Committee of

pez-DFdez, J.A.Dias Pinto, and of Rotor Slot Shape on Cage

Roger N. Johnson, “Novel High-ercial Centrifugal Compressor,” ions, vol. 36, pp.706-713, 2001. rronen, and Uwe Lauber, “High-ction Motor Technology for Gas

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a Radevska, Vesna Ceselkoska, e Dimensional Magnetic Field in

ouble Cage Squirrel Cage,” TEM

omparative Study on High speed onal Conference on Electrical p.1009,1012, 2007. “Calculation of Electromagnetic ng Finite Element Method,” and Industries, vol 2, pp.53-63

nen, and J. Pyohonen, “Analyzing mechanical performance of solid-. 516-525, 2006. ia 1995. 012.

ouble Cage Slot

ot