Totally Enclosed Permanent Magnet Synchronous Motor Otally

PAPER

Totally Enclosed Permanent Magnet Synchronous Motor for Commuter Trains

Minoru KONDO Yasuhiro SHIMIZU Jun-ya KAWAMURA Assistant Senior Researcher, Assistant Senior Researcher, Researcher, 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 of conventional ventilated type traction motors. This paper reports the results of tempera- ture rise tests, noise measurement and energy consumption calculations. The results show that the noise level fell 10dB and energy consumption decreased by 10%.

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

1. 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 a ventilation fan connected directly to the motor axis (Fig.1 (a)) are commonly used as traction motors on local trains in Japan. The self-ventilated cooling system is simple and effective. However, the cooling fan becomes a major noise 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 the cooling air. To avoid the problems associated with con- tamination, railway-companies have to clean inside the (a) Conventional self-ventilated induction motor motor periodically. If the motor has a totally enclosed structure (Fig.1 (b)), the noise is shielded and dust does not penetrate inside the motor. However, simply adopt- ing the totally enclosed structure for conventional trac- tion motors would cause overheating. Thus, we propose to use a high-efficiency, totally enclosed permanent mag- net synchronous motor. Generally, permanent magnet synchronous motors are highly efficient because no loss is generated in its rotor, unlike the induction motors that are generally used in conventional motors. This means that less heat generation requires less effort to cool them compared with conventional induction motors. There- fore, by combining a permanent magnet synchronous motor and a totally enclosed structure, we developed a (b) Totally enclosed permanent magnet synchronous motor quiet, maintenance-free and energy-saving traction mo- tor that has practically the same size, weight and power Fig. 1 Sectional views of traction motors capacity as conventional self-ventilated induction mo- tors. This paper describes the design of the prototype based on the performance test results on the assump- motor that has been developed. In addition, the results tion that they are used for commuter trains running in of temperature rise tests are shown to verify the power typical urban areas. The calculation results show that capacity along with the results of noise measurement the use of the prototype motor reduces energy consump- and energy consumption calculations. The test results tion by about 10% compared with trains using conven- show that temperature rises are within its limits and tional motors. the noise level decreased 10 dB when compared with con- ventional motors. The energy consumption of both pro- totype motor and conventional motors are calculated

90 QR of RTRI, Vol. 46, No. 2, June. 2005 2. Motor design Permanent magnet Flux barrier Permanent magnet 2.1 Specifications Flux barrier

The prototype motor was manufactured to verify the effectiveness of the cooling structures described later and to confirm the reduced noise level. The specifications Bridge for the motor were set to match those of a conventional motor. 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 lighter in weight than the conventional motor though their sizes are almost identical. This is because induction motors are 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 of the conventional motor, as had been expected.

Table 1 Assumed train specification Train set 2 motor cars and 4 trailer cars Vent hole Axis Wheel diameter Maximum：860mm Minimum：774mm Vent hole Wheel gauge Narrow gauge (1067mm) Fig. 2 Sectional view of prototype motor''' s rotor Gear ratio 99/14=7.07 Weight 295t (under maximum load) the electric current to suppress the permanent magnet Maximum speed 120km/h flux while coasting. However, it is still preferable to keep Maximum acceleration 2.5km/h/s the open-circuit voltage as low as possible from the view- point of loss reduction when coasting. Line voltage DC1500V Figure 2 shows a sectional view of the prototype motor's rotor. The magnets are placed in a V-shape to Table 2 Traction motor specifications increase magnetic saliency needed to generate reluctance Prototype motor Conventional motor torque. The flux barrier is also placed to increase sa- liency as well as to reduce the centrifugal stress on the Motor type Permanent magnet Induction motor bridge by reducing the mass supported by the bridge. synchronous motor Cooling system Totally enclosed Self-ventilated 2.3 Thermal management Rating Continuous 1 hour 1 hour Rated output 140kW 200kW 200kW 2.3.1 Thermal problems in totally enclosed motors Efficiency 97% 97% 92% The major technical problem in totally enclosed mo- tors is temperature rise, especially those of bearings. The Weight 570kg 595kg temperature rise limit of grease-lubricated bearings is far lower than that of the stator coil. In addition, tem- 2.2 Electrical design perature rise distribution tends to be uniform in totally enclosed motors. Therefore, it is difficult to keep bear- We used an interior permanent magnet motor in ing temperature rise within its limit in a totally enclosed which permanent magnets are buried inside the rotor motor. iron core. One reason for adopting this type of motor is Another thermal problem is magnet temperature that there is no risk of breaking the fragile magnets rise. The magnet temperature rise limit (130K above during maintenance, as they are protected by the iron ambient temperature) is set to protect the magnet core. Another reason is that it can generate sufficient against demagnetization and is also lower than that of torque using reluctance torque under limited inverter the stator coil. In addition, the rotor in which the mag- output current and limited open-circuit voltage. The nets are buried is surrounded by stator coils. These also open-circuit voltage produced by the permanent magnet make it difficult to keep the magnet temperature rise flux should be limited in a railway vehicle traction mo- within limits. tor. The peak value of the open-circuit voltage should be limited under the withstand voltage of the traction 2.3.2 Cooling structure around the bearings inverter to protect it. In addition, it is desirable to limit It is important to separate the bearings thermally the open-circuit voltage's peak value to less than the DC from the air inside the motor that is heated by the high line voltage. The reason is that undesirable regenera- temperature coils, in order to keep bearing temperature tive braking force will be generated when coasting if the rise within its limit. Therefore, we placed a circular cool- open-circuit voltage's peak value is higher than the DC ing space around the bearing and a heat insulator inside line voltage. This problem can be solved by controlling the frame on the counter fan-side as shown in Fig. 4.

QR of RTRI, Vol. 46, No. 2, June. 2005 91 Stator coil

Circulation fan Mini-fan

Bearing

Fig. 5 Structures around the bearing (gear side) Fig. 3 Prototype motor 2.3.3 Inner air circulation Because the rotor is framed inside the stator, the only Heat insulator practical way to cool the magnet is by inner air. There- Stator coil fore, we adopted inner air circulation. A circulation fan, which is connected directly to the axis in the same way as a conventional motor's self-ventilated fan, forces in- ner air through a circulation duct placed on the upper side of the motor. The circulation duct is composed of Rotational finned tubes that serve to increase the surface area to position sensor improve heat release (Fig.6) and is designed to function as a heat exchanger, so that air is cooled when it flows Cooling space through it.

Bearing

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

A mini-fan, which a small disk with grooves attached directly on the axis that sends air to the surface of the bearing bracket when rotating, is placed outside the motor on the fan side (Fig. 5). On the other hand, the space between the bearing bracket and the circulation fan functions as a heat insulator. The space and the air from the mini-fan keep the temperature of the bearing bracket low and bearing temperature rise within its limit. Fig. 6 Circulation duct using finned tubes

92 QR of RTRI, Vol. 46, No. 2, June. 2005 2.3.4 Magnet splitting 120 Sinusoidals source/ Ideally, there should be no loss in the rotor of a per- continuous rating manent magnet synchronous motor. In reality, however, Invert source/ there is eddy current loss in the magnets due to the fluc- 100 continuous rating tuations of the magnetic flux. This loss increases the temperature rise of the magnets. To reduce eddy cur- Sinusoidal source/ one-hour rating rent loss, the magnets were split and insulated before 80 they were buried in the rotor. Electromagnetic analysis Inverter source/ had been carried out in advance to decide the optimal one-hour rating number of splits to reduce heat generation. In response 60 to the calculation results, it was decided to split the magnet into 14 pieces along its axis. 40

2.3.5 Utilization of airflow during running Temperature rise (K) Generally, totally enclosed motors are cooled only by natural convection from their surfaces. However, the 20 airflow around a motor is available when the train is running because the motor is exposed to open air in the case of railway vehicle traction motor. To make good 0 use of this airflow, many fins are provided on the sur- Stator Permanent Bearing Bearing face facing the direction of travel. The fins on the down- winding magnet (gear side) (non-gear side of the motor are particularly important, because side) measurements carried out in advance showed that the Fig. 7 Temperature rise test results motor's upstream wind speed is faster at lower points. 3.2 Acoustic noise measurement

3. Tests 3.2.1 Purpose and method Acoustic noise measurement conforming to JIS stan- 3.1 Temperature rise test dards was carried out to confirm the noise reduction ef- fect of a totally enclosed structure. The motor was ro- 3.1.1 Purpose and method tated with no load and the rotational speed controlled to Temperature rise tests conforming to JIS/JEC Stan- remain constant throughout the measurement process. dard were performed to verify the effectiveness of the Noise level were measured with sound level meters proposed structures. Continuous rating and one-hour placed at a distance of 1m from the motor surface in four rating tests were carried out with a sinusoidal source directions horizontally (parallel with and perpendicular and an inverter source. During the tests, cooling air to the motor axis) and one direction above the motor. was blown to simulate the airflow around the motor while The measurements were done not only at the rated speed trains are in motion. The speed of the cooling air was (2550/min) but also at high speed (5000/min). controlled to reproduce the upstream airflow of the mo- tor on an operational train in service. The wind speed 3.2.2 Results and discussion distribution in the train was measured in advance. Con- Figure 8 shows the measurement results, with re- sequently, the wind speed at the height of the motor axis sults of conventional motor test shown for comparison. at rated speed was about 2m/s. The noise level of the prototype motor in each direction Temperature measurements were taken as follows: was 10 dB lower than that of the conventional motor at high speed (5000/min). Thus, the comparison confirms *Stator winding: direct-current resistance method the noise-reducing effect of the prototype motor. *Permanent magnet: thermocouple (via slip ring) A conventional motor has a self-ventilated fan that *Bearing: thermocouple (on the motor surface) produces high noise level at high speed. The prototype motor also has a ventilation fan to circulate air, which 3.1.2 Results and discussion produces noise in the same way as the conventional mo- The test results are shown in Fig.7. The tempera- tor. However, the fan noise is shielded inside the frame ture rise of each part was well within its limits. Thus, it in the case of a totally enclosed structure. In addition, could be concluded that this prototype motor achieved the rotors of permanent magnet motors have smooth its targeted output power. In addition, the temperature surfaces without copper bars that are in induction rise was so small that there is a margin for increasing motor's rotors and produce wind noise at high speed. the rated output power for this motor. Alternatively, the Therefore, prototype motor noise levels show a signifi- structure could be simplified by removing or simplifying cant improvement on those of conventional motors. some of the proposed structures. The effectiveness of the each proposed structure was verified by conducting temperature rise tests for structures with and without the proposed structure prior to these tests.

QR of RTRI, Vol. 46, No. 2, June. 2005 93 assumed effective at all times. It was assumed that the Prototype motor 100 motors were operated at a current vector that minimizes Conventional motor the total loss in the motor as long as the voltage was lower than the maximum value. 95 4.2 Results and discussion 90 The calculation results of loss generation character- istics in the conventional and prototype motors are shown 85 in Figs. 9 and 10 respectively. The stator copper loss and the stator iron loss were almost the same in these 80 motors. The conventional motor's mechanical loss was greater than that of the prototype motor, because venti- 75 lation requires more power than air circulation in a to- tally enclosed motor. Rotor copper loss appears only in A-weighted sound pressure level (dB) the conventional induction motor. Therefore, the total 70 Gear side Bogie axis Non- gear Non- Upper side side bogie axis side side 25 Total loss Stator copper loss Fig. 8 Noise measurement results Rotor copper loss Stator iron loss Mechanical loss 4. Energy Consumption Calculation 20

4.1 Purpose and method 15 Energy consumption was calculated to estimate the energy-saving effect of installing a permanent magnet synchronous motor. Trains and routes were assumed Loss (kW) 10 (Table 3) based on those being operated. As shown in Table 3, two types of service (local train and rapid train) were assumed, because the loss generation characteris- 5 tics of the traction motors depend on the speed, which in turn depends on the type of operation. The calculations were carried out for the prototype motor and the conven- tional motor, their loss generation characteristics being 0 0 20 40 60 80 100 120 modeled based on bench test results. In the energy con- sumption calculations, only the loss from the traction Speed (km/h) motors and traction gears was calculated and the loss from Fig. 9 Loss generation characteristics of conventional other devices such as traction converter is ignored. There- motor fore, the energy consumption components considered in this calculation are running resistance, pneumatic brak- 25 Total loss Stator copper loss ing, motor loss and gear loss. Regenerative braking was Stator iron loss Mechanical loss

20 Table 3 Calculation assumptions Local train Rapid train Train set 3 motor cars and 3 motor cars and 15 4 trailer cars 5 trailer cars Load 100% load 100% load

Car weight 270t 327t Loss (kW) 10 Gear ratio 7.07 6.53 Wheel diameter 820mm 5 Acceleration 2.5km/h/s Deceleration 3.0km/h/s Maximum speed 110km/h 130km/h 0 Distance traveled 100km 200km 0 20 40 60 80 100 120 Stations 45 21 Speed (km/h) Traveling time 1.54h 2.10h Fig. 10 Loss generation characteristics of prototype motor

94 QR of RTRI, Vol. 46, No. 2, June. 2005 loss in the prototype motor while powering was less than that in a conventional motor. In addition, the total loss 30 Rotor copper loss while coasting was also less in the prototype than in the conventional motor, though stator iron loss is generated Stator iron loss only in the prototype motor under such conditions. This 25 Stator copper loss is because the mechanical loss is so small in the proto- type motor that the sum of mechanical loss and the sta- Mechanical loss tor iron loss is less than the mechanical loss in the con- 20 ventional motor. From these results, we can conclude that the prototype motor is more likely to save energy 15 than the conventional motor regardless of the services for which it is used. The calculation results of accumulated losses in the 10 traction motors are shown in Figs. 11 and 12. Again, as Loss in traction motor (kJ) motor traction in Loss can be seen in these Figures, a significant difference 5 between the two motors is that there is no copper loss in the prototype motor's rotor because it uses a permanent magnet synchronous motor. Another difference is that 0 the mechanical loss is greater in the conventional motor Conventional motor Prototype motor than in the prototype. In particular, the difference in Fig. 12 Calculation results of losses in traction motors mechanical loss is more significant in rapid trains because (rapid train) it increases in approximate proportion to the third power of the rotational speed. In permanent magnet synchro- nous motors, iron loss is generated not only when power- 60 ing but also when coasting due to the magnetic flux gen- erated by the permanent magnet. Therefore, accumulated 50 iron loss tends to be larger in permanent magnet syn- chronous motors, although the iron loss while coasting is not significant as can be seen in the calculation results. 40 Thus, total accumulated loss generation in the prototype motor is about a half that of the conventional motor. This 30 loss reduction results in lowering total energy consump- tion by about 10%, as shown in Figs. 13 and 14. In actual service, the regenerative braking is not always available 20 due to the lack of sufficient loads (other powering trains), and there are other losses ignored in this calculation. (kJ) consumption Energy 10 Thus, in actual service the percentage reduction will be less than that indicated by these calculation results. 0 Conventional motor Prototype motor 16 Rotor copper loss Fig. 13 Calculation results of energy consumption (local Stator iron loss train) 14 Stator copper loss 160 12 Mechanical loss 140

10 120

8 100

6 80 60 4 Loss in traction motor (kJ) motor traction in Loss 40 2 Energy consumption (kJ) 20

0 0 Conventional motor Prototype motor Conventional motor Prototype motor Fig. 11 Calculation results of losses in traction motors Fig. 14 Calculation results of energy consumption (rapid (local train) train)

QR of RTRI, Vol. 46, No. 2, June. 2005 95 5. Conclusions References

A novel, totally enclosed permanent magnet synchro- 1) Kondo, M. et al.: " Development of Totally enclosed nous motor is presented in this paper. A prototype motor Type Traction Motor Using Permanent Magnet Syn- was manufactured and compared with a conventional self- chronous Motor," RTRI REPORT, Vol.17, No.4, pp.17- ventilated induction motor through tests and calculations. 22, 2003. It was proved that the prototype motor has the same 2) Kawamura, J. et al.: "Comparison of Energy Con- output at the same size and weight as those of the con- sumption of Traction Motors," RTRI REPORT, Vol.18, ventional motor according to the results of temperature No.5, pp.23-28, 2004. rise tests. Through noise measurement, it was confirmed 3) Matsuoka, K. et al.: "Totally enclosed Type Traction that the acoustic noise of the prototype motor at high Motor Using Permanent Magnet Synchronous Mo- speed was reduced by 10 dB when compared with the tor," IEEJ Trans. IA, Vol.124, No.2, 2004. conventional motor. In addition, the total loss gener- 4) Miyata, K. et al.:" 3-D Magnetic Field Analysis of ated in the prototype motor while in service is calcu- Permanent Magnet Motor Considering Magnetizing, lated to be about half that of the conventional motor. Demagnetizing and Eddy Current Loss," IEEJ Trans. From these results, we conclude that the totally en- IA, Vol.123, No.4, 2003. 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.

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