Patent GB-729313-A (GB)
Title: Improvements in or relating to electric film driving apparatus
IPC: H02P1/56
Filed: 1952-02-15
Abstract: 729,313. Frequency changers; converting. KLANGFILM GES. Feb. 15, 1952, No. 4140/52. Classes 35 and 38 (2). [Also in Group XXXVII] A number of synchronous motors which drive separate parts of film-driving mechanism are operated at equal speeds during acceleration, running at a set speed, and deceleration by means of a frequency converter which supplies the motors in common and is adapted to have a direct-current output so that the synchronous motors may be locked in the same relative positions when they are stationary. The con verter may take the form of a commutator machine driven by a synchronous motor and having an unwound stator and a rotor winding connected both to the commutator and to the supply-source through slip-rings, Fig. 1 (not shown). The converter has a three-phase output of substantially constant voltage and of the same frequency as the supply when the converter is stationary. As the speed of the converter increases the output frequency drops until, when the converter-driving motor is at synchronous speed, the output of the converter becomes direct-current. Temperature-responsive resistors, such as incandescent lamps, protect the synchronous motors against excessive current at low frequencies. Thus, for starting the synchronous motors, the converter is first driven at maximum speed, the motors are connected to the converter, and then the speed of the converter is reduced to standstill, with the aid of a brake if necessary. In Fig. 2, the converter has fixed transformer windings W1, W2, W3, the commutator K also being fixed and connected to tappings in the windings through resistors R. Brushes B1 B2, B3 are mounted on slip-rings S1, S2, S3, and are driven by motor 5. The converter operates in a manner similar to that described, and its output may be selectively applied to motors 9, 91 or 9<SP>11</SP> by means of change-over switch U, so that the groups of motors may be started in sequence. When each group of synchronous motors is operating at the supply frequency it is connected directly to the source through switch U1 U2, U3. The frequency converting assembly may comprise two machines both driven by a synchronous motor, Fig. 3 (not shown). The first converting machine is of the commutator type supplying: a constant voltage independent of frequency. The second converting machine is constructed as an induction motor having its rotor connected to the supply through sliprings and its separate stator windings connected in series with the brushes of the first converting machine and to the working motors. The second machine supplies a three-phase output which is equal in frequency to that of the first converter, but at a voltage which increases from zero with an increase in frequency. The resultant voltage fed to the working motors thus increases from a low finite value to a preset value as the frequency increases, Fig. 4 (not shown), and the use of current-limiting resistors in the motor circuit is avoided. The frequency converting assembly may be reversed to provide an output of double the mains frequency, so doubling the speed of the working motors. The working motors themselves may be reversed. Mechanical, or electrical -interlocking, Fig. 5 (not shown), may be used to prevent the connection of the working motors to the converter assembly until the assembly has reached its full operating speed. The frequency converting assembly may have its constant voltage and its variable voltage output components derived from a single machine of the three-phase shunt commutator type, Fig. 6 (not shown), having its rotor connected to the mains through slip-rings, its separate stator windings providing the variable voltage component and connected on the one hand to the brushes of the machine and on the other to the synchronizing network. The relative phase positions of the two component voltages may be altered, by brush-shifting and so arranged that when the converter is at'standstill and its commutator is short-circuited for protective purposes, so removing the constant component, no excessive change occurs in the resultant output voltage.