Controller for starting and stopping electric motors

An apparatus for controlling the torque of an alternating current electric motor comprising:

• a switch means, coupling the electric motor to a source of alternating electricity, said switch means being of a type which is conductive from the moment of application of a trigger pulse until the magnitude of alternating current therethrough goes to zero;
• voltage sensing means for providing a first indication when the voltage of the source makes a zero voltage crossing;
• detecting means for sensing when said switch means becomes non-conductive;
• means, responsive to said voltage sensing the means and to said detecting means, for measuring a first interval .phi. between when the voltage makes a zero crossing and when said switch means becomes non-conductive;
• means for defining a reference value .theta..sub.REF and for altering the reference value in a predefined manner;
• means for calculating a control value .theta. according to the equation .theta.=.phi.+K.gamma., where K is a positive number and where .gamma. has a value that corresponds to the length of a previous time interval during which said switch means was non-conductive;
• means for producing an error signal representing the difference between the control value .theta. and the reference value .theta..sub.REF ;
• means for applying a trigger pulse to said switch means at a time determined in response to the error signal;
• and
• means for redefining the value of .gamma. in response to a second interval between when said switch means became non-conductive and when the trigger pulse is applied to said switch means.

A method for controlling the torque of an electric motor that is coupled by a switch means to a source of alternating voltage and current, the switch means being of a type which is conductive from when a trigger pulse is applied until the magnitude of alternating current therethrough goes to zero, said method comprising repeatedly executing the steps of:

• (a) measuring a phase difference .theta. between the alternating voltage and current from the source;
• (b) defining a reference value .theta..sub.REF ;
• (c) calculating a control value .theta. according to the equation .theta.=.phi.+K.gamma., where K is a positive number and where .gamma. corresponds to the length of a previous interval during which the switch means was non-conductive;
• (d) producing an error signal representing the difference between the control value .theta. and the reference value .theta..sub.R.sub.REF ;
• (e) altering the value of .gamma. in response to the error signal;
• and
• (f) applying a trigger pulse to the switch means at the end of an interval substantially equal to the altered value of .gamma. from when the alternating current from the source has a zero magnitude.