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Z ESZY TY N A U K O W E POLITECHNIKI ŚLĄSKIEJ Seria: ELEK TR Y K A z. 176

2001 N r k o l. 1500

K a r e l C H M E L l K 1 ) , A l e S V A C H A L A 2 ), V ¿ c l a v C E C H 3 )

M A G N E T IC F IE L D IN T H E S U R R O U N D IN G S OF IN D U C T IO N MOTORS

Sum m ary: High reliability is one of the most important requirements in technical operations and can be assessed by estimation of the actual operational state. This operation is called technical diagnostics.

The magnetic induction around an induction motor is taken as a value indicating the state of its functional properties. The modelling of the magnetic field around an induction motor is performed by means of the ANSYS program in order to get information about the shape of the magnetic field and about the size of magnetic induction both outside a faultless induction motor and inside a motor with an eccentric air gap.

The results are checked by measuring the outside magnetic field of the induction motor during its operation by means of the MAGNET-PHYSIK apparatus.

Key w o rd s: AC machines, induction machines, modelling, software, diagnostics, measurement

1. IN TRO DUC TIO N

In electrical m achines e le ctric energy is transform ed into m echanical one through a magnetic field. S tator and rotor w indings are sources o f the m agnetic field in every electrical m achine. These w indings are placed eith e r in slots or on poles. Electrom agnetic transform ation e.g. in induction m otors is m ade possible by m utual influence o f the m agnetic field produced by stator windings and by the rotor w indings in w hich the e lectric current flows. There are two kinds o f m agnetic field in a m otor: the m ain and the leakage field. T he m ain field m agnetic flux is closed by a m agnetic circuit.

Since the leakage m agnetic field is closed and also outside the m agnetic circuit it can be used for the research in processes w ithin the m achine. T his enables to watch the processes taking place in the m achine and observe the state o f its functional properties during its norm al operation. This dia g n ostic requires no dism antling, is not destructive and has no harm ful effects on any part of the m achine. The shape and course o f m agnetic fields have then a direct influence on mechanical quantities, especially on the m achine torque. The torque can either positively or negatively in fluence function, reliability and service life o f the driven m echanism .

2. USE O F THE M O TO R M AG N ETIC FIELD FOR DETECTIO N OF ITS AC TU AL CONDITION

A s already m entioned, the torque o f an electrical m achine depends on electromagnetic quantities especially in an air gap. C ourses o f m agnetic fields in a m achine are very complicated and are influenced by m any factors, especially discrete distribution o f conductors in slots, non­

linear m agnetic characteristics, com plicated construction o f the m agnetic circuit, non-uniformity of the air gap, etc. Q uantitative diagnosis o f the m agnetic field in a rotating electrical machine is therefore very difficult. T his m ay be partly due to the m achine construction, non harm onic voltage or e le ctric pow er supply or any possible defects o f e lectric and m echanical parts o f the machine.

For this reason we studied and verified the possibility of obtaining inform ation about defects and failures o f Induction m otors by m easuring and analysing leakage m agnetic fields on the casing surfaces o f these m otors.

The frequency spectrum o f the leakage m agnetic field up to 100 Hz is shown in Figs.1 and 2.

The spectrum was m easured on the casing surface o f a 4 pole induction m otor by an exploring coil.

The spectrum has a lateral band w hen the m agnetic field of the stator or the air gap are asym m etrical. This w ill cause a change o f the m agnetic field during one revolution. T his frequency

11 Doc. Ing. VSB-TU Ostrava, tf. 17. listopadu 15, 708 33 Ostrava-Poruba, Czech republic tel.: (4269)6995251, facsimile num ber (4269) 6919597, e-mail: karel.chmelik@vsb.cz 2) Ing. vSB-TU Ostrava, tf. 17. listopadu 15, 708 33 Ostrava-Poruba, Czech republic

tel.: (4269)6995425, facsimile number: (4269) 6919597, e-mail: ales.vachala@vsb.cz 5) Ing. VfiB-TU Ostrava, tf. 17. listopadu 15, 708 33 Ostrava-Poruba, Czech republic

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w ill appear in the isit. It depends on the type o f the m achine and especially on the num ber o f its poles w h e th er the sp ectrum can be evaluated or not [1],

Fig.1. Magnetic field frequency spectrum of a 4-pole induction motor without a symmetric failure

Fig. 2. Magnetic field frequency spectrum of a 4-pole induction motor with a symmetric failure

The above m entioned m easured results required m ore detailed research o f the leakage m agnetic field in the surroundings o f an induction m otor. W e study this field on a m otor w ithout any electrical or m echanical defects as well as with an e ccentric air gap. It is obvious th a t the results can also be used in the field o f ele ctro m a g ne tic com patibility.

3. VE R IFIC A TIO N OF E X P E R IM E N TA LLY DETEC TED RESULTS BY M O DELLING

W e practised m odelling o f the m agnetic field in surroundings o f an induction m otor by m eans of the A N S Y S 5.5.3 program , which solves problem s by m eans o f the finite elem ent m ethod (FEM).

T he finite e lem ent m ethod is an effective m ethod fo r the solution of all boundary problem s of engineering practice described by differential equations. The m ethod was developed with the appearance o f digital com puters to solve problem s in elasticity and strength in the aircraft industry at the end of the fifties.

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Magnetic field in the surroundings o f induction motors 211

Nodes and node potentials are introduced into an area where the field is calculated. The nodes can be distributed in the area unevenly and thus they can follow the shape o f the boundary areas.

T he b igger density o f the m esh is established in places w here the acute change o f the field is expected. The system o f equations is set fo r unknown nodal values. The coefficients o f the m atrix o f the system o f equations and the right sides are not calculated by using differences substituting derivatives but as definite integrals over elem entary areas or volum es, in whose peaks the nodes are placed. T hese elem entary fo rm s are called finite elem ents. W e used areal elem ents in the shape o f quadrilaterals and trilaterals fo r the calculation o f our 2D model.

W e m odel the m agnetic field in the surroundings o f a three phase wound rotor induction m otor w ith a fram e m ade o f cast iron. T he induction m otor has the power rating o f 5,5 kW and 6 poles.

W e m ade a cutting plane o f the m oto r so that it w ent through the m otor m agnetic circuit (outside the term inal board). W e om itted the m otor fram e cast-iron bases and cooling ribs. By m eans of p re processor com m ands o f the A N S YS program we created m otor 2D m odel. The kind o f material and e le m e n t was then fitted to the m odel areas. The m odel was covered with elem ents. The current densities were m atched to the stator and rotor sheet m etal slot elem ents. W e m ade static calculation o f the 2D m odel [2],

4. THE R ES U LTS FOR AN INDUCTIO N MO TO R W ITH SYM M ETRIC AIR GAP

The shape o f the m agnetic field in the surroundings o f an induction m otor is shown in Fig. 3.

The fram e is m ade o f cast iron, the induction m otor has a sym m etric air gap and the m otor runs on no-load. 2D flux lines are shown in Fig. 4.

F or the calculation o f the external field we used the sam e m otor m odel as fo r no load, however, we determ ined the current densities into slots, in the stator and rotor fo r the load motor. The shape o f the m agnetic field in the surroundings o f the induction m otor is shown in Fig. 5.

5. R ES U LT FOR AN EC C EN TRIC A IR GAP

The m oto r m odel was adapted by m oving the rotor to the right by 0,2 m m. The calculated shape of the m agnetic field outside the induction m otor is shown in Fig. 6. The fram e is made of cast iron, m oto r has an e ccentric air gap end the m otor runs on no-load.

T he m otor m odel was adapted by m odelling the fram e cooling ribs. The calculated shape of the m agnetic field outside the induction m otor is shown in Fig. 7.

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NODAL SOLUTION STEP-2 SUB -1 TIME-2 BSUH (AVG) RSYS-0 PowerGraphica EFACET-1 AVRES-Hat SMN -.189E-04 SMX -.001183

ZV -1 DIST-.3509 Z-BUFFER

.189E-04 .148E-03 .278E-03 .407E-03 .S3 6E-03 .666E-03 .79SE-03 .924E-03 .001054 .001183

B [ T ]

Fig.3. The magnetic field in induction motor surroundings, the frame is made of cast iron, air gap is symmetric, no-load motor

STEP-2 SUB -1 TIHE-2 AZ RSYS-0 SMN --.024888 SHX -.025016

ZV -1 DIST-.1452

-.023964 -.022116 -.018419 -.016571 -.012874 -.011026 -.00733 -.005481 -.001785 .637E-04 .00376 .005609 .009305 .011154 .01485 .016698 .020395 .024092

Fig.4. 2D flux lines of induction motor, the frame is made of cast iron, air gap is symmetric, no-load motor

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Magnetic field in the surroundings o f induction motors 213

NODAL SOLUTION S T E P -2 SUB - 1 T IH E -2 BSUH (AVG) RSYS-0 P o w e r G r a p h i c s EFACET-1 AVRES-Hat SHN -. 420E-05 3MX -.162E-03

ZV - 1 D1 S T - .3 5 0 9 Z-BUTFER

. 4 2 0 E - 0 5 . 2 1 7 E - 0 4 j l l l j . 3 9 3 E - 0 4 . 5 6 8 E - 0 4 E p . 7 4 4 E - 0 4

¡ 1 ® . 9 1 9 E - 0 4 L = j . 1 0 9 E -0 3 . 1 2 7 E -0 3 . 1 4 5 E -0 3 . 1 6 2 E -0 3 B I T ]

Fig.5. The magnetic field in induction motor surroundings, the frame is made of cast iron, air gap is symmetric, load motor

NODAL SOLUTION S T E P -2 SUB - 1 T IH E -2

BSUH (AVG)

SHN - . 1 6 7 E - 0 5 SHX - . 0 0 2 9 9 6

ZV - 1 D I S T - . 3 5 0 9 XF - - . 2 0 0 E - 0 3 Z-BUFFER

. 1 6 7 E - 0 5

| . 3 3 4 E - 0 3 . 6 6 7 E -0 3 . 1 0 0 E -0 2 I——j .0 0 1 3 3 2 p H * . 0 0 1 6 6 5

| j . 0 0 1 9 9 0 . 0 0 2 3 3 .0 0 2 6 6 3 .0 0 2 9 9 6 B [ T ]

Fig.6. The magnetic field in induction motor surroundings, the frame is made of cast iron, eccentric air gap, no-load motor

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NODAL SOLUTION STEP=2 SUB =1 TIHE=2 BSUM (AVG) SMN * . B05E-06 SMX =.830E-03 ZV =1

»DIST*.37853

*XF — .001595

*YF — .004477 Z-BUFFER _ .805E-06

5 .93OE-04 . 185E-03

m

-277E-°3

-3^E-03 P=n . 462E-03 r = j .554E-03 r===j . 646E-03 . 738E-03

™ . 830E-03 B [ T ]

A W S 'V S -

Fig.7. The magnetic field in induction motor surroundings, the frame is made of cast iron, eccentric air gap, no-load motor. Cooling ribs were also modelled

6. V E R IFIC A TIO N OF TH E M AG N ETIC INDUCTIO N CALC U LA TE D VA LU ES IN SU R R O U N D IN G S O F AN IN DUCTIO N M O TO R BY M EASUREM EN TS

T he m agnetic induction value in the m odelled m otor surroundings was m easured by the M A G N E T -P H Y S IK m easuring apparatus shown in Fig. 8. The apparatus m easures m agnetic field by m eans o f the Hall probe. W e placed the probe into the plane w here the m otor m agnetic field was m odelled and we gradually m oved the probe away from the m otor fram e. W e com pared the m agnetic induction m easured values w ith those obtained by m odelling in the A N S Y S program. The calculated values o f m agnetic flux density in the induction m otor surroundings are shown in diagram no.1. T he m agnetic induction m easured values are shown in diagram no.2.

Fig.8. Measuring apparatus MAGNET-PHYSIK FH 47/3, model 9953

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Magnetic field in the surroundings o f induction motors 215

r>io*»-4i

I [ m ]

Diagram no.1. Illustration of magnetic induction calculated values in induction motor surroundings vs.

distance from the frame (no-load motor, air gap is symmetric, the frame is made o f cast iron)

I [cm ]

Diagram no.2. Illustration of magnetic induction measured values In induction motor surroundings vs.

distance from the frame (no-load motor, air gap is symmetric, the frame is made o f cast iron)

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7. C O N C LU S IO N S

C om paring the above m entioned results we cam e to follow ing conclusions:

1.The m agnetic field In the surroundings o f an induction m otor has higher values fo r no load motor because the m ag n e tic field o f the rotor interacts counter to the m agnetic field o f the stator when the m otor runs on load. T herefore the target field has low er values in surroundings of an induction m otor (influence o f anchor reaction).

2.T he shape o f the m ag n e tic field in the surroundings o f an induction m otor with an eccentric air gap is also eccentric.

3.W e can say th a t the m oto r fu lfills conditions o f EM C according to the calculated and measured values o f m ag n e tic induction in the surroundings o f an induction motor.

4 .The shapes o f m agnetic induction outside an Induction m otor obtained by modelling and m easuring correspond w ith each other. T he m agnetic induction values are o f the sam e order but th e ir values are not identical. The both processes w ere obtained in entirely different ways. The results o f m agnetic Induction obtained by m odelling represent the m agnetic induction values on the selected net nodes under the ideal conditions o f no load m otor case (there is no current in the rotor). The m ag n e tic induction values obtained from m easurem ents corespond to those for real no load m oto r case. W e also have to take into consideration that the m easurem ents were taken on a m etal d yn a m o m e te r m easuring desk in a te st room and their accuracy can therefore be influenced by the various e lectric gadgets th a t are placed there.

5 .C alculations o f the m ag n e tic field in the surroundings o f an induction m otor that are mentioned here are not final. M odelling the m agnetic field in the surroundings o f an induction m otor will be perform ed fo r fram e m ade o f alum inum and fo r other cases o f failures o f induction motors.

T his paper was w ritten w ithin the solving o f G AC R project v.6 .102/00/0192.

R EFER E N C ES

I.C e c h V.: P ractical utilization o f scattered m agnetic field m easuring fo r diagnostics o f induction m otors, proceedings o f T R A N S F E R conference, section D, BRNO, 1999, pages D7 - D8.

1 .V achala A.: M agnetic field in surroundings o f induction m otor, Diplom a work, V S B-TU OSTRAVA, 1998, pages 21 - 28.

Recenzent: Dr hab. in i. Krzysztof Kom^za Profesor Polltechniki td d zkie j

W ptyn?to do R edakcji dnia 15 lutego 2001 r.

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