Concept Design and Performance Analysis of HTS Synchronous Motor for Ship Propulsion. Jin Zou, Di Hu, Mark Ainslie

Transcription

1 Concept Design and Performance Analysis of HTS Synchronous Motor for Ship Propulsion Jin Zou, Di Hu, Mark Ainslie Bulk Superconductivity Group, Engineering Department, University of Cambridge, CB2 1PZ, UK address: Abstract: With the technology development of High Temperature Superconductors (HTS), advanced synchronous rotating machines, called SuperMachines [1], have been constructed and tested. These machines utilize the features of superconductivity, such as higher power density and current density. Therefore the superconducting machine has advantages over the conventional one, such as lower power losses, compact volume, higher output power and output torque. A 2.5 MW class synchronous ship propulsion motor has been designed based on 2-dimension finite element method. The design procedure and HTS motor performance will be discussed in the paper. Key words: 2.5 MW class, Load Angle, High Temperature Superconducting Motor (HTS), Ship Propulsion Motor, Finite Element Method (FEM) I. Introduction: Normal HTS superconducting machine has only half or one-third volume, compared to the conventional motor with the same rating power. When the rating is over 1000hp, HTS motor offers the opportunity to reduce losses by 50% [2]. As for military ships, the lower weight of the ship body ensures higher speed and more loaded weapons, which in turns guarantees more powerful compact force. Employed with the HTS superconducting motor, the commercial ship has more spaces to carry more passengers and goods, therefore the economic aspect is improved. Thus, the HTS superconducting motor is the ideal machine for next-generation ship propulsion application. In this paper 2.5 MW class synchronous motor is designed for the ship propulsion, which requires a low rotational speed and high thrust force. The motor performance is analysed by Finite Element Method (FEM). The paper is organised as following: The second section of the paper introduces the HTS motor structure specification, and the rationale of it; the third section studies the determination of the operation condition. II. HTS Motor Structure Specification HTS motor configuration is shown in the Fig.1, which consists of stator yoke, armature winding, rotor iron core, HTS field winding and field winding supportive structure. The conventional copper winding is utilized in the stator design with some modification to maximize the air gap field. HTS tape in racetrack coil is used as the field winding.

2 Fig.1 HTS machine with copper armature winding and HTS field winding A. Rotor Topologies: Although the advantages of air core structure, such as less eddy currents, lower moment inertia, and less noise are obvious, a more robust winding support structure must be designed to transmit the toque of the motor. And more Ampere turns will be required in the design. Therefore the magnetic core is inserted back into the rotor as part of the machine s magnetic circuit to reduce the Ampere turns required for a definite flux density at the air gap. This can reduce the amount of HTS tape required, which enable the economic comparativeness against conventional motor. B. Stator Topology: The stator design is similar to the conventional one, a magnetic stator yoke is employed in the motor to guide the magnetic flux with nonmagnetic teeth and high Ampere turn loading (See Fig 2). A short-pitch winding employed in the two-layer winding is utilized in this HTS motor design. Therefore the voltage of the phase winding and the copper consumption are reduced. Another advantage of employing the short-pitched winding is that it can generate a more sinusoidal current linkage distribution than a full-pitched winding.

3 Fig 2. Toothless stator design with copper Litz winding Considering the limitation of flux density level by the saturation of the teeth, the conventional iron teeth configuration is replaced by the nonmagnetic teeth [3]. As a consequence the efficiency will be decreased a little, but a higher flux density and higher voltage can be achieved, therefore higher power density and more compact volume. In addition, this 2.5 MW HTS still has higher efficiency (over 97%) compared to the conventional counterpart (93%). Those features are more attractive for high torque machines for maritime propulsion. G10 FRP (Fibre Reinforced Plastic) which has the same permeability with air [4], is utilized in the simulation model to support the armature winding. Those nonmagnetic teeth with smaller width can benefit the omission of slot harmonics, but increase the transversal flux components. Therefore smaller diameter copper stands (Litz) are utilized in this design to decrease the eddy current. Also due to the smaller width in the teeth, more Ampere turn loading is achieved in this HTS 2.5MW motor design. C. End winding inductance calculation Although the MagNet FEM software is good at modelling rotational machine, there are still some limitation. For the copper armature winding in a motor with a larger diameter compared with the effective length, the end winding inductance cannot be ignored. The end winding leakage inductance of armature winding can be calculated by the empirically determined factors [5]. They are added back into this model (see Fig 3). The end winding inductance of copper winding is H per phase. The end effect of HTS coil can be calculated by 3D model.

4 III. Operation Specification Fig 3. End winding inductance of armature winding A. Determination of HTS operation point The 2G-YBCO tape is chosen as the conductor in the field winding. The specification at 77K is shown in table I. Table I HTS specification Minimum Ic Critical current Width Total wire Thickness 80A 2MA/CM 2 4mm 0.1mm At 77K, the critical current is 80A without any self-field, but under different background field and difference angle between the field and the HTS surface, the critical current will decrease. Under the perpendicular field, the critical current has the largest decline, and over this critical current the HTS tape will loss superconductivity. The current characteristics can be found in the Fig 4.

5 Fig 4. The dependence of the critical current on magnetic fields for various temperature with the field oriented perpendicular to the ab-plane [6]. The air gap flux density requirement for this 2.5 MW ship propulsion motor is 1T. According to my MagNet model, the maximum flux density around HTS coil is1.6 T. Those flux density profile are shown in Fig 5, and Fig 6. Fig 5 The flux density profile of air gap

6 Fig 6 The flux density profile of HTS Motor According to Fig 4-6, if the temperature is high as 77K, the critical current will be too low under perpendicular field, so that much more HTS tapes will be required. However cooling system will be cheaper. If we run the motor at 30K, according to Fig 4 with perpendicular magnetic field 1.6T, the critical current density decreases to 1.8 times of Ic at 77K 0T. Multiplied by the cross section of HTS tape, the critical current at 1.6 T comes to be 144A. Considering the Ic degradation is large after superconducting coil manufacture, with the proper margin current the designed motor will run at 30K, with 75 A excitation current. The specification of motor operation point can be found in the table II. Table II Specification of HTS synchronous motor Pole number 6 Rating Speed 480 rpm Maximum Field Coil Flux Density 1.6T HTS Tape Requirement 2.2 km Stator Turn Number Per Phase 630 Stator Slot Number 54 Rotor Turn Number Per Pole 1200 Effective Length 600 mm Stator Outer Diameter 1000 mm Stator Inner Diameter 500 mm Air Gap 3.5 mm Field Coil Exciting Current 75A Stator Terminal Voltage 6000V Nominal Output 2.5MW Exciting voltage 5805 V

7 B. Determination of load angle The determination of load angle depends on the maximum torque, obtained around 90 electrical degree. If the load angle is over 90, the motor will loss synchronous. The load angle characteristic of this 2.5 MW HTS motor is shown in Fig 7~9. Fig 7. Output Power & Current with 44 degree load angle Fig 8. Nominal Torque and Maximum Torque with 44 degree load angle

8 Fig 9 Efficiency of HTS motor over load angle compared with conventional machine When the load angle is 90, the output torque is 72 knm and the output power is 3.6 MW. Considering the load angle margin, the nominal torque is set as 2/3 of the maximum torque. The output torque and power are 50kNm and 2.5 MW, with the corresponding load angle around 44 degree. When the HTS motor working at 44 degree load angle, the efficiency (cooling system loss is not included) is around 97.6% at the 0.9 power factor. In the figure 9, compared with traditional induction and synchronous machine, the HTS machine can keep high efficiency over small load angle. And the overall efficiency is much higher, compared with the traditional machines. When the load angle is set as 44 degree, the magnetic field profile is shown in table III. Table III Flux Density around the HTS Field Coil Perpendicular Flux Density Parallel Flux Density Absolute Flux Density 0.78T 1.2 T 1.4 T

9 III. Conclusion A concept design of a 2.5 MW synchronous HTS motor to obtain 50 knm torque under 480 round per minute (rpm) is described in the paper. The specific structure of this 2.5 MW HTS motor is discussed. Iron core is inserted back into the rotor, in order to generate higher flux with relative lower Ampere turn. This configuration helps to decrease the HTS tapes used in the design, which increases the economic availability against the conventional motor. Determination method of the HTS field work point and load angle is provided. Although the HTS motor with maximum efficiency and compactness cannot be achieved, due to the utilization of the stator, the efficiency is still higher than the conventional counterpart. For a conventional 2.5 MW class motor, the weight would be 9.5 t with 93% efficiency, which is much heavier compared to 7.2t 2.5 MW HTS motor with efficiency over 97 % [7]. For the high torque and low speed maritime propulsion ship, the maximum compactness is more important.

10 References: [1] S. S. Kalsi "Development status of superconducting rotating machines", IEEE Power Engineering Society Winter Meeting, 2002 [2] R. Schiferl, A. Flory, S. Umans and W. Livoti "High temperature superconducting synchronous motors: Economic issues for industrial applications", Conf. Rec. IEEE Ind. Appl. Soc. 53rd Annu. Petroleum Chem. Ind. Conf., pp [3] Klaus G, M Wilke, J Frauenhofer, W Nick and H-W Neumüller 2007 "Design Challenges and Benefits of HTS Synchronous Machines" Proc IEEE PES General Meeting (Tampa, FL, USA, June) [4] Seung-Kyu Baik, Myung-Hwan Sohn, et al, " Performance Evaluation of HTS Synchronous Motor Using Finite Element Method," IEEE Trans. Appl.Supercond., vol. 14, no.2, pp ,2004,6 [5]J. Pyrhnen, T. Jokinen and V. Hrabovcov Design of Rotating Electrical Machines, 2008 :Wiley [6] S. Fleshlera, D. Buczeka, Scale-up of 2G wire manufacturing at American Superconductor Corporation (2009) Physica C: Superconductivity and its Applications, 469 (15-20), pp [7] Maki N, Takao T, Fuchino S, Hiwasa H, Hirakawa M, Okumura K, Asada M and Takahashi R 2005 Study of practical applications of HTS synchronous machines IEEE Trans. Appl. Supercond