Patent GB-992538-A (GB)
Title: Electric motor starting method and means
IPC: H02P1/26
Filed: 1964-02-06
Abstract: 992,538. Control of A.C. motors. LOUIS ALLIS CO. Feb. 6, 1964 [Feb. 6, 1963], No. 5069/64. Heading H2J. In order to provide a satisfactory starting torque with a low input current, an initial flux is established in the stator winding of an induction motor, the stator current in one phase then being discontinued. The motor flux is maintained by the induced rotor current, the stator current being re-applied in phase with the flux to afford the torque. The motor may have a three-phase star-connected stator winding with two oppositely-wound coils per phase. The stator is energized from a directcurrent source in a cyclic manner the flux being established by exciting two of the stator phases, after which the first phase is disconnected and the third phase is energized to produce the motive force. The speed of the motor is sensed, by detecting the voltage induced by movement of the rotor in an unenergized portion of the stator winding, to control the sequence connections between the directcurrent source and the stator winding and to produce successive accelerating forces acting in the same direction on the motor. In one arrangement, the polarity of the second phase is reversed when the energization of the first phase is interrupted. When the motor reaches a predetermined speed, the unidirectional energization is discontinued and polyphase energization of the stator is introduced. The current from the D.C. supply may be passed into the end of one phase winding and out from another phase, or the phase windings may be centre-tapped and the unidrectional energization applied to the centre taps. As shown, to start motor 104, switch 106 is closed to pass a signal through gate 130, synchronizer 120, counter 110 to decoder 112 from rectifier 109. Decoder 112, in response to the count recorded in counter 110, operates the rectifier 109 to energize two phase windings A, B, of stator 12. After a flux has been established in the motor, time delay 108 passes a second signal through gate 130, synchronizer 120, to counter 110 which records the second input. Decoder 112 decodes the count to operate rectifier 109 so that phases A, C, of stator 12 are energized. At the same time, decoder 112 operates selector 114 to connect phase B to detector 116. Rotor 18 rotates and detector 116 supplies a third input to gate 130. This actuates counter 110 and decoder 112 so that rectifier 109 supplies power to the phases B, C. The decoder also switches selector 114 to sense the voltage in phase A. This cycle is repeated, to accelerate the motor. After the sixth input to counter 110, amplifier 132 resets the counter to zero to repeat the switching sequence. When detector 122 senses that the frequencies of detector 116 and oscillator 118 are equal, indicating that the motor has reached running speed, it passes a signal to AND gate 124 which also receives an input from decoder 112, at each sixth pulse. The gate 124 opens to operate flip-flop 126 whereby the motor is now operated by frequency converter 102. The counter 110 is then re-set to prepare for the next starting operation.