GB871460A - Control system for electric motors - Google Patents

Abstract

871,460. Synchronizing. HARLAND ENGINEERING CO. Ltd. April 22, 1959 [April 28, 1958], No. 13333/58. Class 38 (4). [Also in Group XXXVII] An independently driven frequency converter 4 is connected between the stator and rotor windings of an induction motor 10 whereby the motor is forced to run in unison with the frequency converter. A transformer has its primary windings 13 in series with the stator windings and secondary windings 14 connected to the slip-rings of the frequency converter. A choke 15 draws substantially constant current from the supply 2 and passes it to third windings 16 of the transformer so that additional excitation current is injected through the frequency converter into the rotor of the motor. This improves the power factor of the system and renders the slope of the torque/speed error characteristic more uniform. A system for two motors 10, 10a is shown, each motor driving a section of a paper-making machine. Each frequency converter is coupled through an infinitely variable ratio transmission 23 with a shaft driven by motor 25 which is supplied from main generator 26. The speed relationships between the motors 10, 10a are determined by the speeds of the associated frequency converters, whilst the overall speed of the machine is controlled by varying the frequency and voltage of the supply 2. Each induction motor is started by an auxiliary generator 27 providing an increasing frequency and voltage, the rotor circuit being shortcircuited by switches 29 across the slip-rings of the frequency converter, the switch 30 being opened and the switch 28 closed. A synchronizing device 31 comprises a pilot motor 32 for comparing the frequency of the generator 27 with the rotor frequency of the induction motor 10 transmitted through transformer 36 and the frequency converter to the rotor of the motor 32. A regulator 39 for the speed of the generator 27 is driven by the motor 32 which stops rotating when the compared frequencies are equal. A contact arm 40, attached to the shaft of the pilot motor, closes contacts 41 when the motor 10 and the frequency converter are in the correct phase relationship, a relay 42 being operated when the speed of the induction motor differs only slightly from the predetermined speed of the frequency converter. Response of the relay 42, opens the switches 28, 29 and closes the switch 30 so that the motor 10 is now supplied from the main generator. Resistors may be connected across the slip-rings of the frequency converter to prevent phase-swinging due to sudden changes of load on the induction motor. As shown, the frequency converter has an armature, with a commutator and slip-rings, rotating in an unwound laminated stator. In an alternative construction, an iron ring is shrunk on to the armature after it has been wound and replaces the stator referred to. The induction motor may have its rotor supplied from a polyphase source and its stator windings connected to the frequency converter.