Design and Development of Rotor Quality Test System for Die-Cast Copper Rotors

Soby T. Varghese; K. R. Rajagopal; Bhim Singh

Abstract

t is well known in the market that the copper rotor motor can give good efficiency at par with the new era motors, and can handle higher temperatures to qualify for electric vehicle application. Being a heavy metal, die-cast copper rotor manufacturing needs absolute care for faithful rotor production. In this paper, the common faults in copper die-cast processing are identified and sufficient monitoring methods are suggested in a three-stage inspection. The final stage consists of a rotor quality test system, which detects most of the problems found in copper die-cast rotors. This rotor quality test system analysis helps to optimize the rotor manufacturing process and avoid the circumstances of fitting an inferior rotor in the motor assembly.

Korea Abstract

구리 회전자 모터가 새로운 시대의 모터와 동등한 우수한 효율을 제공할 수 있고 전기 자동차 애플리케이션에 적합하도록 더 높은 온도를 처리할 수 있다는 것은 시장에서 잘 알려져 있습니다. 중금속이기 때문에 다이캐스트 구리 로터 제조는 충실한 로터 생산을 위해 절대적인 주의가 필요합니다.

본 논문에서는 구리 다이캐스트 공정에서 흔히 발생하는 결함을 파악하고 3단계 검사를 통해 충분한 모니터링 방법을 제시합니다. 최종 단계는 구리 다이캐스트 로터에서 발견되는 대부분의 문제를 감지하는 로터 품질 테스트 시스템으로 구성됩니다.

이 로터 품질 테스트 시스템 분석은 로터 제조 공정을 최적화하고 모터 어셈블리에 열등한 로터를 장착하는 상황을 피하는 데 도움이 됩니다.

INTRODUCTION

As global interest in EV (Electric Vehicle) has been extended to a larger scale, the automotive engineers are in search of special materials for the manufacturing of the motor, which can take advantage from reduced material usage, by compacting the size and reducing the weight. Recently, the interest in die- cast copper rotor motor has increased due to concerns about the scanty availability of rare earth materials and magnet performance at elevated temperatures in permanent magnet motors. The copper rotor induction motor appears to be the viable choice for parallel hybrid electric vehicles in terms of compactness, high-power density, overall system, efficiency and durability [1].

High pressure die-casting is the most economical process for the manufacturing of die-cast rotors, and aluminium has been the material of choice since 1930. The use of copper in the rotor in place of aluminium, by taking advantage of copper's high conductivity, has proved to be a solid strategy for developing energy efficient motors for EV applications. Replacing the aluminium with the copper in die-cast rotor bars in the squirrel cage induction motor has enormous benefit in terms of reduction in rotor I2 R loss that ultimately provides improved efficiency and energy savings.

The reduction in rotor I2 R loss, leads to lower motor working temperature. Though, the copper die-cast process is identical to aluminium die-cast process, additional manufacturing challenges of increased temperature and pressures make copper die-cast rotor production difficult. The cost involved in melting the copper and the molten copper handling charges are estimated approximately three times higher than that of the aluminium.

The higher cost involved in the manufacturing of die-cast copper rotor motor is compensated by the energy savings in terms of reduction in input power consumption, reduced maintenance charges and longer life [2]-[7]. In the construction of new die-cast rotor, there are various defects that may endanger the operation of the motor. The rotor faults result in abnormal heating, in causing the presence of additional harmonics, creation of arcing, generation of vibration and noise, and speed and torque variations in the motor.

The problems in die-cast copper rotor, lead to undesirable performances in the motor, making it less trustworthy and requiring frequent servicing. The total failure of induction motor due to rotor defects, is estimated approximately to 10% [8]. The die-cast rotor manufactured with copper may pose various defects in its fabrication. Some of the problems are invisible to the naked eye to be detected, and there are unidentified problems as well.

Fig. 1 Manufacturing process flow of die-cast copper rotor
Fig. 1 Manufacturing process flow of die-cast copper rotor
Fig. 2 Three stage quality monitoring test for copper die-cast rotor
Fig. 2 Three stage quality monitoring test for copper die-cast rotor
Fig. 3 Weight test to identify the blow holes
Fig. 3 Weight test to identify the blow holes
Fig. 4 Flaw detection on end rings using ultrasonic tester
Fig. 4 Flaw detection on end rings using ultrasonic tester
Fig. 5 Conceptual design of rotor quality tester
Fig. 5 Conceptual design of rotor quality tester
Fig. 6 Hardware set up of rotor quality test system
Fig. 6 Hardware set up of rotor quality test system
Fig. 7 Parts of rotor quality test system
Fig. 7 Parts of rotor quality test system
Fig. 8 Model of electromagnet sensor arrangement with rotor
Fig. 8 Model of electromagnet sensor arrangement with rotor
Fig. 9 AC and DC flux lines and waveforms (top) in EM sensor
Fig. 9 AC and DC flux lines and waveforms (top) in EM sensor
Fig. 10 Development stages of electromagnet sensor
Fig. 10 Development stages of electromagnet sensor
Fig. 11 Aluminium sheets used for insulation in model (left) and sensor (right)
Fig. 11 Aluminium sheets used for insulation in model (left) and sensor (right)
Fig. 12 Gauss meter testing of electromagnet sensor
Fig. 12 Gauss meter testing of electromagnet sensor
Fig. 13 Electromagnet sensor with height and skew angle adjuster
Fig. 13 Electromagnet sensor with height and skew angle adjuster
Fig. 14 Digitizer board connectivity with EM sensor and speed sensor
Fig. 14 Digitizer board connectivity with EM sensor and speed sensor
Fig. 15 Rotor quality tester front panel created in NI LabVIEW software
Fig. 15 Rotor quality tester front panel created in NI LabVIEW software
Fig. 16 Rotor quality test program sequence
Fig. 16 Rotor quality test program sequence
Fig. 17 Rotor fault inspection using FFT
Fig. 17 Rotor fault inspection using FFT
Fig. 18 Prototype rotor for testing RQTS
Fig. 18 Prototype rotor for testing RQTS
Fig. 19 Prototype rotor with (a) no fault (b) broken bar (c) inter-laminar shor
Fig. 19 Prototype rotor with (a) no fault (b) broken bar (c) inter-laminar shor
Fig. 20 Waveforms obtained in RQTS for various rotor faults
Fig. 20 Waveforms obtained in RQTS for various rotor faults
Fig. 21 Inspection waveform chart of RQTS
Fig. 21 Inspection waveform chart of RQTS

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