Miniaturized claw-pole generators and motors with high power density

Nowadays, energy harvesting is a topic of steadily increasing research attention. There are a number of devices which utilize a multitude of different physical principles to convert energy from the environment into electrical form. In small electronic devices such as wristwatches, thermoelectric or piezoelectric effects can be are used for supply [1]. At higher power levels however, electromagnetic generators are more suited. In the present paper, a novel topology for a claw-pole type generator is proposed which features an extremely compact design, converting kinetic energy from a rotational motion into electrical energy. The energy can either be used to power small devices directly or to charge a secondary battery. It may be used to substitute big size and heavy weight batteries in autonomous systems. The output voltage and current of the generator are transient in nature and must thus be converted to a DC signal. This requires energy converters with small volume and low weight. The proposed design allows further miniaturization, still using conventional processes and technologies for manufacturing. The principle of operation is based on the magnetic flux through the machine changing its path with rotation of the claw-pole type rotor. The different flux paths are shown in Fig. 1. As a result of time-varying flux through the cylindrical coil, an alternating voltage is induced. In the development process analytical calculations applying reluctance network techniques were first carried out in order to determine the basic geometry of the magnetic circuit. A 3D-FE model was created to perform field calculations to optimize the generator's output power. These investigations also allowed a substantial reduction of the cogging torque, which is typical for claw-pole machines. The 3D-FE model calculations are in good agreement with experimental results obtained for different protoypes, one of which is shown in Fig. 2. However, it turned out that there is a high sensitivity of the cogging torque to geometrical variations of the claws. The generator in Fig. 2 was developed for powering an autonomous door lock. When pressing down the door-handle, energy provided by the generator is first stored in a capacitor and can later be used by the unlocking mechanism. By extending the system to two electrical phases, the principle was applied to build a motor with a high torque density. In comparison with the data of commercially available motors of similar size it turns out that this new machine is more than a match to them when looking at the produced torque.