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Contactless Power Transfer to a Rotating Disk

J.P.C. Smeets, D.C.J. Krop, J.W. Jansen and E.A. Lomonova Electromechanics and Power Electronics Group, Eindhoven University of Technology, Netherlands Email: j.p.c.smeets@tue.nl

Abstract—This paper discusses a power transfer system from the stationary to the rotating part of a device, by means of contactless energy transfer. A rotating transformer is proposed as a replacement for wires and slip rings. A pot core geometry is used for the rotating transformer and two different winding topologies are compared. The transformer is analyzed in the electromagnetic and thermal domain. An analytic model for each domain is derived. The validity of the analytical models is confirmed with both 2D and 3D FEM simulations and mea-surements. Two prototype rotating transformers are designed for the transfer of 1 kW peak, rotating at 6000 rpm. The prototypes are manufactured using commercially available pot cores and tested in an experimental setup.

I. INTRODUCTION

In many modern mechatronic systems, the transfer of power to rotating parts plays an important role, for example, in robotics and in industrial applications where power needs to be transferred to a rotating part. Nowadays, wires and slip rings are used to transfer power to the rotating part. Disadvantages of wires are a limited rotation angle and increased stiffness Despite the significant amount of research and development of reliable and durable slip rings, contact wear as well as vibra-tion limit the lifetime, and frequent maintenance is required [1]. Furthermore, contact wear creates dust particles, which are unwanted in cleanroom and vacuum applications.

A solution to overcome the disadvantages of wires and slip rings is a contactless energy transfer (CET) system that uses a rotating transformer. The transformer converts power across an airgap, a physical separation which provides the ability to rotate the secondary side of the transformer. An extra advantage could be the freedom in winding ratio, to transform the primary voltage level to the requirements of the load.

The contactless transfer of energy by means of a rotating transformer is under investigation since the 1970’s [2]. Later,the concept of a rotating transformer is used in

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applications such as the transcutaneous energy transmission for pacemakers [3] and inductive charging [4], both cases benefit from the CET. Rotating transformers can be used for the transfer of power and data signals to the moving part simultaneously, by using an extra inductive or capacitive coupling [5].

The axial rotating and pot core transformer geometry can be used for a rotating transformer. Both are investigated by [6] in terms of total volume and efficiency. The pot core geometry, shown in Fig. 1, gives better performance indices in terms of flux density, magnetic coupling and losses. Therefore, this topology is further investigated in this paper.

This paper presents the design of a rotating transformer for a power transfer of 1 kW peak to a load, rotating at 6000 rpm. The electronics on the load require an input DC voltage of 50 V. First, the geometry of the rotating transformer is analyzed. Second, analytical models are derived for the elec-tromagnetic and thermal behavior of the transformer. Finally, two prototype transformers are designed and manufactured to verify the analytical models.

II. ENERGY TRANSFER TOPOLOGY

The working principle of a rotating transformer can be obtained from Faraday’s law and Ampere’s circuital law. Ap-plying Lenz’s law and assuming a sinusoidal excitation, yields to an equation for the induced voltage over an N-turn winding and an expression for the transferred power, independent of the number of turns

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