Zahnaerztliches Bohrhandstueck

1,246,753. Dental handpieces. SIEMENS A.G. 5 Feb., 1970 [5 Feb., 1969], No. 5667/70. Heading A5R. [Also in Division H2] A dental handpiece comprises an electric fractional horsepower motor and an electronic commutator which may be disposed wholly or partially within the handpiece. The motor comprises a rotor 9 constructed as a permanent magnet and a stator consisting of a plastics housing 14 on which are arranged windings 15, and which is surrounded by laminations 16. The commutator comprises voltage generators 19 (and 19a) and transistors 29 to 32 acting as control members and switching devices respectively therefor. The transistors 29 to 32 are disposed on conductor plates 25 to 28 respectively, in circular formation about a central passage of the handpiece, and carry heat dissipating members (33 to 36 respectively). The conductor plates also carry amplifiers 37 to 40 respectively. The motor and commutator are held inside a cylindrical housing 5 of the handpiece by a disc 20 which carries pins 21, 22 for supplying the motor with electric current from an external hose (7) and which has perforations to allow an inflow of compressed air into the central passage of the handpiece for cooling the motor and the components of the commutator. A control device sets and maintains constant the rotational speed of the motor by supplying a constant current to the amplifiers and voltage generators. The direction of rotation of the motor can be reversed by actuating switch 41 to reverse the control current to the voltage generators. The amplifiers and the control device may be disposed externally of the handpiece.