Tightening system and a method of tightening

Claim 1

A tightening system comprising: a three phase high frequency AC induction motor; fastening means driven by said motor; an inverter to supply pulse width modulated wave forms to drive said motor in one of two modes being: a high speed low torque mode, or a slow speed high torque mode, or to cause said motor to stop, said inverter comprising switching means; control means to cause said inverter to vary the pulse width of said waveforms, said control means comprising a sinewave generator, a triangular wave generator, and a comparator fed by said sinewave generator and said triangular wave; said switching means being fed by said comparator, said switching means comprising three pairs of switching devices arranged as a ridge of six fed by three comparators, one of reach phase, each comparator controlling a pair of switching devices so that a selected one of a said pair of switching devices is switched on while the other switching devices is switched on while the other switching device of said pair is switched off depending upon whether the output from said comparator is high or low, the output of said switches driving said motor; and an inverter protection circuit for switching predetermined period if said inverter becomes overloaded, said inverter protection circuit comprising a first and second voltage reference means each providing a reference voltage near DC bus rails for each pair of switching devices, a pair of further comparators each having applied to the input thereof the output voltage of a pair of switching devices and the reference voltage of one of said two reference voltages, said further comparators having the outputs thereof applied to an OR gate the output of said OR gate being high except for overload conditions or when the pulse width modulated output changes state.

Claim 8

A method of tightening comprising: providing pulse width modulated waveforms from an inverter; driving a three phase high frequency AC induction motor by said waveforms in one of two modes comprising (a) a high speed low torque mode, and (b) a low speed high torque mode, or causing said motor to stop; controlling said inverter to vary the pulse width of said waveforms by generating a sinewave, generating a triangular wave, comparing said sinewave and triangular wave in a comparator, and utilizing crossovers of said sine-wave and said triangular wave to switch between a high and a low output of said comparator; driving a fastening device by said AC induction motor; providing a three phase system and including the steps of providing a comparator for each phase, associating with each comparator a pair of switching devices arranged as a bridge of six, and switching "on" or "off" a selected one of said switching devices depending upon whether said comparator output is high or low and driving said motor from the output of said switching devices; and protecting said inverter by switching said switching devices off if said inverter becomes overloaded, wherein protecting said inverter comprises providing first and second reference voltages near the DC bus rails for each pair of switching devices, comparing in pairs the output voltage of said pair of switching devices and one of said reference voltages, and applying the pair of outputs thus gained to an OR gate so that the output of said OR gate is high except during an overload condition or as the pulse width modulated output changes state.

Claim 9

A method of tightening comprising: providing pulse width modulated waveforms from an inverter; driving a three phase high frequency AC induction motor by said waveforms in one of two modes comprising (a) a high speed low torque mode, and (b) a low speed high torque mode, or causing said motor to stop; controlling said inverter to vary the pulse width of said waveforms by generating a sinewave, generating a triangular wave, comparing said sinewave and triangular wave in a comparator, and utilizing crossovers of said sine-wave and said triangular wave to switch between a high and a low output of said comparator; driving a fastening device by said AC induction motor; providing a three phase system and including the steps of providing a comparator for each phase, associating with each comparator a pair of switching devices arranged as a bridge of six, and switching "on" or "off" a selected one of said switching devices depending upon whether said comparator output is high or low and driving said motor from the output of said switching devices; and protecting said inverter by switching said switching devices off if said inverter becomes overloaded, wherein protecting said inverter comprises providing first and second reference voltages near the DC bus rails for each pair of switching devices, comparing in pairs the output voltage of said pair of switching devices and one of said reference voltages, and applying the pair of outputs thus gained to an OR gate so that the output of said OR gate is high except during an overload condition or as the pulse width modulated output changes state.