totally otally enclosed permanent magnet synchronous motor

9090909090 QR of RTRI, Vol. 46, No. 2, June. 2005

PAPERPAPERPAPERPAPERPAPER

TTTTTotally Enclosed Permanent Magnet Synchronous Motorotally Enclosed Permanent Magnet Synchronous Motorotally Enclosed Permanent Magnet Synchronous Motorotally Enclosed Permanent Magnet Synchronous Motorotally Enclosed Permanent Magnet Synchronous Motorfor Commuter Tfor Commuter Tfor Commuter Tfor Commuter Tfor Commuter Trainsrainsrainsrainsrains

1. Introduction1. Introduction1. Introduction1. Introduction1. Introduction

Ventilation systems are fitted to railway vehicle trac-tion motors to save weight and boost motor power out-puts. In particular, self-ventilated motors that have aventilation fan connected directly to the motor axis (Fig.1(a)) are commonly used as traction motors on local trainsin Japan. The self-ventilated cooling system is simpleand effective. However, the cooling fan becomes a majornoise source when rotating at high speed. Another prob-lem with ventilation cooling systems is the contamina-tion inside the motor caused by the dust carried in thecooling air. To avoid the problems associated with con-tamination, railway-companies have to clean inside themotor periodically. If the motor has a totally enclosedstructure (Fig.1 (b)), the noise is shielded and dust doesnot penetrate inside the motor. However, simply adopt-ing the totally enclosed structure for conventional trac-tion motors would cause overheating. Thus, we proposeto use a high-efficiency, totally enclosed permanent mag-net synchronous motor. Generally, permanent magnetsynchronous motors are highly efficient because no lossis generated in its rotor, unlike the induction motors thatare generally used in conventional motors. This meansthat less heat generation requires less effort to cool themcompared with conventional induction motors. There-fore, by combining a permanent magnet synchronousmotor and a totally enclosed structure, we developed aquiet, maintenance-free and energy-saving traction mo-tor that has practically the same size, weight and powercapacity as conventional self-ventilated induction mo-tors.

This paper describes the design of the prototypemotor that has been developed. In addition, the resultsof temperature rise tests are shown to verify the powercapacity along with the results of noise measurementand energy consumption calculations. The test resultsshow that temperature rises are within its limits andthe noise level decreased 10 dB when compared with con-ventional motors. The energy consumption of both pro-totype motor and conventional motors are calculated

based on the performance test results on the assump-tion that they are used for commuter trains running intypical urban areas. The calculation results show thatthe use of the prototype motor reduces energy consump-tion by about 10% compared with trains using conven-tional motors.

Minoru KONDOMinoru KONDOMinoru KONDOMinoru KONDOMinoru KONDOAssistant Senior Researcher,

YYYYYasuhiro SHIMIZUasuhiro SHIMIZUasuhiro SHIMIZUasuhiro SHIMIZUasuhiro SHIMIZUAssistant Senior Researcher,

Jun-ya KAJun-ya KAJun-ya KAJun-ya KAJun-ya KAWWWWWAMURAAMURAAMURAAMURAAMURAResearcher,

Drive Systems Laboratory, Vehicle Control Technology Division.

Ventilated-type induction motors are widely used as traction motors on railway ve-hicles. However, they require overhauls for internal cleaning and are a major noise source.To solve these problems, we propose to use a totally enclosed permanent magnet synchro-nous motor as the traction motor, which has the same power-to-weight ratio as that ofconventional ventilated type traction motors. This paper reports the results of tempera-ture rise tests, noise measurement and energy consumption calculations. The results showthat the noise level fell 10dB and energy consumption decreased by 10%.

KeywordsKeywordsKeywordsKeywordsKeywords: traction motor, totally-enclosed type, permanent magnet synchronous motor,energy consumption, noise reduction

Fig. 1 Sectional views of traction motorsFig. 1 Sectional views of traction motorsFig. 1 Sectional views of traction motorsFig. 1 Sectional views of traction motorsFig. 1 Sectional views of traction motors

(a) Conventional self-ventilated induction motor

(b) Totally enclosed permanent magnet synchronous motor

9191919191QR of RTRI, Vol. 46, No. 2, June. 2005

2. Motor design2. Motor design2. Motor design2. Motor design2. Motor design

2.1 Specifications2.1 Specifications2.1 Specifications2.1 Specifications2.1 Specifications

The prototype motor was manufactured to verify theeffectiveness of the cooling structures described later andto confirm the reduced noise level. The specificationsfor the motor were set to match those of a conventionalmotor. Tables 1 and 2 show the specifications of the cor-responding trains and traction motors, respectively.

As shown in Table 2, the prototype motor is lighterin weight than the conventional motor though their sizesare almost identical. This is because induction motorsare fitted with rotor end rings, which permanent mag-net synchronous motors do not have. The measured ef-ficiency of the prototype motor is 5% higher than that ofthe conventional motor, as had been expected.

TTTTTable 1 able 1 able 1 able 1 able 1 Assumed train specificationAssumed train specificationAssumed train specificationAssumed train specificationAssumed train specification

TTTTTable able able able able 22222 TTTTTraction motor specificationsraction motor specificationsraction motor specificationsraction motor specificationsraction motor specifications

Train set 2 motor cars and 4 trailer carsWheel diameter Maximum：860mm Minimum：774mm

Wheel gauge Narrow gauge (1067mm)Gear ratio 99/14=7.07

Weight 295t (under maximum load)Maximum speed 120km/h

Maximum acceleration 2.5km/h/sLine voltage DC1500V

Prototype motor Conventional motor

Motor type Permanent magnetsynchronous motor

Induction motor

Cooling system Totally enclosed Self-ventilatedRating Continuous 1 hour 1 hour

Rated output 140kW 200kW 200kWEfficiency 97% 97% 92%

Weight 570kg 595kg

2.2 Electrical design2.2 Electrical design2.2 Electrical design2.2 Electrical design2.2 Electrical design

We used an interior permanent magnet motor inwhich permanent magnets are buried inside the rotoriron core. One reason for adopting this type of motor isthat there is no risk of breaking the fragile magnetsduring maintenance, as they are protected by the ironcore. Another reason is that it can generate sufficienttorque using reluctance torque under limited inverteroutput current and limited open-circuit voltage. Theopen-circuit voltage produced by the permanent magnetflux should be limited in a railway vehicle traction mo-tor. The peak value of the open-circuit voltage shouldbe limited under the withstand voltage of the tractioninverter to protect it. In addition, it is desirable to limitthe open-circuit voltage's peak value to less than the DCline voltage. The reason is that undesirable regenera-tive braking force will be generated when coasting if theopen-circuit voltage's peak value is higher than the DCline voltage. This problem can be solved by controlling

the electric current to suppress the permanent magnetflux while coasting. However, it is still preferable to keepthe open-circuit voltage as low as possible from the view-point of loss reduction when coasting.

Figure 2 shows a sectional view of the prototypemotor's rotor. The magnets are placed in a V-shape toincrease magnetic saliency needed to generate reluctancetorque. The flux barrier is also placed to increase sa-liency as well as to reduce the centrifugal stress on thebridge by reducing the mass supported by the bridge.

2.3 Thermal management2.3 Thermal management2.3 Thermal management2.3 Thermal management2.3 Thermal management

2.3.1 Thermal problems in totally enclosed motorsThe major technical problem in totally enclosed mo-

tors is temperature rise, especially those of bearings. Thetemperature rise limit of grease-lubricated bearings isfar lower than that of the stator coil. In addition, tem-perature rise distribution tends to be uniform in totallyenclosed motors. Therefore, it is difficult to keep bear-ing temperature rise within its limit in a totally enclosedmotor.

Another thermal problem is magnet temperaturerise. The magnet temperature rise limit (130K aboveambient temperature) is set to protect the magnetagainst demagnetization and is also lower than that ofthe stator coil. In addition, the rotor in which the mag-nets are buried is surrounded by stator coils. These alsomake it difficult to keep the magnet temperature risewithin limits.

2.3.2 Cooling structure around the bearingsIt is important to separate the bearings thermally

from the air inside the motor that is heated by the hightemperature coils, in order to keep bearing temperaturerise within its limit. Therefore, we placed a circular cool-ing space around the bearing and a heat insulator insidethe frame on the counter fan-side as shown in Fig. 4.

Fig. 2 Sectional view of prototype motorFig. 2 Sectional view of prototype motorFig. 2 Sectional view of prototype motorFig. 2 Sectional view of prototype motorFig. 2 Sectional view of prototype motor'''''s rotors rotors rotors rotors rotor

Permanent magnetFlux barrier

Vent hole Axis

Bridge

Permanent magnetFlux barrier

Vent holeAxis

Bridge


Fig. 3 Prototype motorFig. 3 Prototype motorFig. 3 Prototype motorFig. 3 Prototype motorFig. 3 Prototype motor

Fig. 4 Structures around the bearing (non-gear side)Fig. 4 Structures around the bearing (non-gear side)Fig. 4 Structures around the bearing (non-gear side)Fig. 4 Structures around the bearing (non-gear side)Fig. 4 Structures around the bearing (non-gear side)

A mini-fan, which a small disk with grooves attacheddirectly on the axis that sends air to the surface of thebearing bracket when rotating, is placed outside themotor on the fan side (Fig. 5). On the other hand, thespace between the bearing bracket and the circulationfan functions as a heat insulator. The space and the airfrom the mini-fan keep the temperature of the bearingbracket low and bearing temperature rise within its limit.

2.3.3 Inner air circulationBecause the rotor is framed inside the stator, the only

practical way to cool the magnet is by inner air. There-fore, we adopted inner air circulation. A circulation fan,which is connected directly to the axis in the same wayas a conventional motor's self-ventilated fan, forces in-ner air through a circulation duct placed on the upperside of the motor. The circulation duct is composed offinned tubes that serve to increase the surface area toimprove heat release (Fig.6) and is designed to functionas a heat exchanger, so that air is cooled when it flowsthrough it.

Fig. 5 Structures around the bearing (gear side)Fig. 5 Structures around the bearing (gear side)Fig. 5 Structures around the bearing (gear side)Fig. 5 Structures around the bearing (gear side)Fig. 5 Structures around the bearing (gear side)

Bearing

Stator coil

Circulation

fanMini-fan

Fig. 6 Circulation duct using finned tubesFig. 6 Circulation duct using finned tubesFig. 6 Circulation duct using finned tubesFig. 6 Circulation duct using finned tubesFig. 6 Circulation duct using finned tubes

Stator coilHeat insulator

Rotational

position sensor

Bearing

Cooling space


2.3.4 Magnet splittingIdeally, there should be no loss in the rotor of a per-

manent magnet synchronous motor. In reality, however,there is eddy current loss in the magnets due to the fluc-tuations of the magnetic flux. This loss increases thetemperature rise of the magnets. To reduce eddy cur-rent loss, the magnets were split and insulated beforethey were buried in the rotor. Electromagnetic analysishad been carried out in advance to decide the optimalnumber of splits to reduce heat generation. In responseto the calculation results, it was decided to split themagnet into 14 pieces along its axis.

2.3.5 Utilization of airflow during runningGenerally, totally enclosed motors are cooled only by

natural convection from their surfaces. However, theairflow around a motor is available when the train isrunning because the motor is exposed to open air in thecase of railway vehicle traction motor. To make gooduse of this airflow, many fins are provided on the sur-face facing the direction of travel. The fins on the down-side of the motor are particularly important, becausemeasurements carried out in advance showed that themotor's upstream wind speed is faster at lower points.

3. T3. T3. T3. T3. Testsestsestsestsests

3.1 T3.1 T3.1 T3.1 T3.1 Temperature rise testemperature rise testemperature rise testemperature rise testemperature rise test

3.1.1 Purpose and methodTemperature rise tests conforming to JIS/JEC Stan-

dard were performed to verify the effectiveness of theproposed structures. Continuous rating and one-hourrating tests were carried out with a sinusoidal sourceand an inverter source. During the tests, cooling airwas blown to simulate the airflow around the motor whiletrains are in motion. The speed of the cooling air wascontrolled to reproduce the upstream airflow of the mo-tor on an operational train in service. The wind speeddistribution in the train was measured in advance. Con-sequently, the wind speed at the height of the motor axisat rated speed was about 2m/s.

Temperature measurements were taken as follows:

*Stator winding: direct-current resistance method*Permanent magnet: thermocouple (via slip ring)*Bearing: thermocouple (on the motor surface)

3.1.2 Results and discussionThe test results are shown in Fig.7. The tempera-

ture rise of each part was well within its limits. Thus, itcould be concluded that this prototype motor achievedits targeted output power. In addition, the temperaturerise was so small that there is a margin for increasingthe rated output power for this motor. Alternatively, thestructure could be simplified by removing or simplifyingsome of the proposed structures. The effectiveness ofthe each proposed structure was verified by conductingtemperature rise tests for structures with and withoutthe proposed structure prior to these tests.

Fig. 7 TFig. 7 TFig. 7 TFig. 7 TFig. 7 Temperature rise test resultsemperature rise test resultsemperature rise test resultsemperature rise test resultsemperature rise test results

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3.2 Acoustic noise measurement3.2 Acoustic noise measurement3.2 Acoustic noise measurement3.2 Acoustic noise measurement3.2 Acoustic noise measurement

3.2.1 Purpose and methodAcoustic noise measurement conforming to JIS stan-

dards was carried out to confirm the noise reduction ef-fect of a totally enclosed structure. The motor was ro-tated with no load and the rotational speed controlled toremain constant throughout the measurement process.Noise level were measured with sound level metersplaced at a distance of 1m from the motor surface in fourdirections horizontally (parallel with and perpendicularto the motor axis) and one direction above the motor.The measurements were done not only at the rated speed(2550/min) but also at high speed (5000/min).

3.2.2 Results and discussionFigure 8 shows the measurement results, with re-

sults of conventional motor test shown for comparison.The noise level of the prototype motor in each directionwas 10 dB lower than that of the conventional motor athigh speed (5000/min). Thus, the comparison confirmsthe noise-reducing effect of the prototype motor.

A conventional motor has a self-ventilated fan thatproduces high noise level at high speed. The prototypemotor also has a ventilation fan to circulate air, whichproduces noise in the same way as the conventional mo-tor. However, the fan noise is shielded inside the framein the case of a totally enclosed structure. In addition,the rotors of permanent magnet motors have smoothsurfaces without copper bars that are in inductionmotor's rotors and produce wind noise at high speed.Therefore, prototype motor noise levels show a signifi-cant improvement on those of conventional motors.


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Fig. 8 Noise measurement resultsFig. 8 Noise measurement resultsFig. 8 Noise measurement resultsFig. 8 Noise measurement resultsFig. 8 Noise measurement results

4. Energy Consumption Calculation4. Energy Consumption Calculation4. Energy Consumption Calculation4. Energy Consumption Calculation4. Energy Consumption Calculation

4.1 Purpose and method4.1 Purpose and method4.1 Purpose and method4.1 Purpose and method4.1 Purpose and method

Energy consumption was calculated to estimate theenergy-saving effect of installing a permanent magnetsynchronous motor. Trains and routes were assumed(Table 3) based on those being operated. As shown inTable 3, two types of service (local train and rapid train)were assumed, because the loss generation characteris-tics of the traction motors depend on the speed, which inturn depends on the type of operation. The calculationswere carried out for the prototype motor and the conven-tional motor, their loss generation characteristics beingmodeled based on bench test results. In the energy con-sumption calculations, only the loss from the tractionmotors and traction gears was calculated and the loss fromother devices such as traction converter is ignored. There-fore, the energy consumption components considered inthis calculation are running resistance, pneumatic brak-ing, motor loss and gear loss. Regenerative braking was

TTTTTable able able able able 33333 Calculation assumptionsCalculation assumptionsCalculation assumptionsCalculation assumptionsCalculation assumptions

Local train Rapid train

Train set 3 motor cars and4 trailer cars

3 motor cars and5 trailer cars

Load 100% load 100% loadCar weight 270t 327tGear ratio 7.07 6.53

Wheel diameter 820mmAcceleration 2.5km/h/sDeceleration 3.0km/h/s

Maximum speed 110km/h 130km/hDistance traveled 100km 200km

Stations 45 21Traveling time 1.54h 2.10h

assumed effective at all times. It was assumed that themotors were operated at a current vector that minimizesthe total loss in the motor as long as the voltage was lowerthan the maximum value.

4.2 Results and discussion4.2 Results and discussion4.2 Results and discussion4.2 Results and discussion4.2 Results and discussion

The calculation results of loss generation character-istics in the conventional and prototype motors are shownin Figs. 9 and 10 respectively. The stator copper lossand the stator iron loss were almost the same in thesemotors. The conventional motor's mechanical loss wasgreater than that of the prototype motor, because venti-lation requires more power than air circulation in a to-tally enclosed motor. Rotor copper loss appears only inthe conventional induction motor. Therefore, the total

Fig. 9 Loss generation characteristics of conventionalFig. 9 Loss generation characteristics of conventionalFig. 9 Loss generation characteristics of conventionalFig. 9 Loss generation characteristics of conventionalFig. 9 Loss generation characteristics of conventionalmotormotormotormotormotor

Fig. 10 Loss generation characteristics of prototype motorFig. 10 Loss generation characteristics of prototype motorFig. 10 Loss generation characteristics of prototype motorFig. 10 Loss generation characteristics of prototype motorFig. 10 Loss generation characteristics of prototype motor

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loss in the prototype motor while powering was less thanthat in a conventional motor. In addition, the total losswhile coasting was also less in the prototype than in theconventional motor, though stator iron loss is generatedonly in the prototype motor under such conditions. Thisis because the mechanical loss is so small in the proto-type motor that the sum of mechanical loss and the sta-tor iron loss is less than the mechanical loss in the con-ventional motor. From these results, we can concludethat the prototype motor is more likely to save energythan the conventional motor regardless of the servicesfor which it is used.

The calculation results of accumulated losses in thetraction motors are shown in Figs. 11 and 12. Again, ascan be seen in these Figures, a significant differencebetween the two motors is that there is no copper loss inthe prototype motor's rotor because it uses a permanentmagnet synchronous motor. Another difference is thatthe mechanical loss is greater in the conventional motorthan in the prototype. In particular, the difference inmechanical loss is more significant in rapid trains becauseit increases in approximate proportion to the third powerof the rotational speed. In permanent magnet synchro-nous motors, iron loss is generated not only when power-ing but also when coasting due to the magnetic flux gen-erated by the permanent magnet. Therefore, accumulatediron loss tends to be larger in permanent magnet syn-chronous motors, although the iron loss while coasting isnot significant as can be seen in the calculation results.Thus, total accumulated loss generation in the prototypemotor is about a half that of the conventional motor. Thisloss reduction results in lowering total energy consump-tion by about 10%, as shown in Figs. 13 and 14. In actualservice, the regenerative braking is not always availabledue to the lack of sufficient loads (other powering trains),and there are other losses ignored in this calculation.Thus, in actual service the percentage reduction will beless than that indicated by these calculation results.

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Fig. 1Fig. 1Fig. 1Fig. 1Fig. 11 Calculation results of losses in traction motors1 Calculation results of losses in traction motors1 Calculation results of losses in traction motors1 Calculation results of losses in traction motors1 Calculation results of losses in traction motors(local train)(local train)(local train)(local train)(local train)

Fig. 12 Calculation results of losses in traction motorsFig. 12 Calculation results of losses in traction motorsFig. 12 Calculation results of losses in traction motorsFig. 12 Calculation results of losses in traction motorsFig. 12 Calculation results of losses in traction motors(rapid train)(rapid train)(rapid train)(rapid train)(rapid train)

Fig. 13 Calculation results of energy consumption (localFig. 13 Calculation results of energy consumption (localFig. 13 Calculation results of energy consumption (localFig. 13 Calculation results of energy consumption (localFig. 13 Calculation results of energy consumption (localtrain)train)train)train)train)

Fig. 14 Calculation results of energy consumption (rapidFig. 14 Calculation results of energy consumption (rapidFig. 14 Calculation results of energy consumption (rapidFig. 14 Calculation results of energy consumption (rapidFig. 14 Calculation results of energy consumption (rapidtrain)train)train)train)train)

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5. Conclusions5. Conclusions5. Conclusions5. Conclusions5. Conclusions

A novel, totally enclosed permanent magnet synchro-nous motor is presented in this paper. A prototype motorwas manufactured and compared with a conventional self-ventilated induction motor through tests and calculations.

It was proved that the prototype motor has the sameoutput at the same size and weight as those of the con-ventional motor according to the results of temperaturerise tests. Through noise measurement, it was confirmedthat the acoustic noise of the prototype motor at highspeed was reduced by 10 dB when compared with theconventional motor. In addition, the total loss gener-ated in the prototype motor while in service is calcu-lated to be about half that of the conventional motor.

From these results, we conclude that the totally en-closed permanent magnet synchronous motor is a main-tenance-free, low-noise and energy-saving traction mo-tor with the same size and weight as those of a conven-tional motor.

ReferencesReferencesReferencesReferencesReferences

1) Kondo, M. et al.: " Development of Totally enclosedType Traction Motor Using Permanent Magnet Syn-chronous Motor," RTRI REPORT, Vol.17, No.4, pp.17-22, 2003.

2) Kawamura, J. et al.: "Comparison of Energy Con-sumption of Traction Motors," RTRI REPORT, Vol.18,No.5, pp.23-28, 2004.

3) Matsuoka, K. et al.: "Totally enclosed Type TractionMotor Using Permanent Magnet Synchronous Mo-tor," IEEJ Trans. IA, Vol.124, No.2, 2004.

4) Miyata, K. et al.:" 3-D Magnetic Field Analysis ofPermanent Magnet Motor Considering Magnetizing,Demagnetizing and Eddy Current Loss," IEEJ Trans.IA, Vol.123, No.4, 2003.

totally otally enclosed permanent magnet synchronous motor

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