PURPOSE:To assure that a constant number of rotations can be obtained always by providing a resonance means which resonants in an alternating field of specific frequency and switching a gear ratio of a speed change feature using an action at time of resonance. CONSTITUTION:A speed change feature 3 is composed of the first and second pinions 14, 15 fixed in two stages to a rotary shaft 8 jutting out upward through the top plate 4a of a housing 4, the first speed reduction gear 16 arranged in such position that a shaft 8 is intermeshed with the pinion 14 at a descending location and with the pinion 15 at an ascending location, the second speed reduction gear 18 which is intermeshed with the third pinion 17 installed on the same shaft as the first gear 16, a plural-gear array which is intermeshed with the fourth pinion 19 installed on the same shaft as the second gear 18, etc. At the last stage of this gear array, is fixed a last output shaft. Under this constitution, the speed change feature is switched over automatically depending on power supply frequency, so that a constant number of rotations can be obtained always.
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