GB1347191A - Starting arrangements for electric motors - Google Patents

Abstract

1347191 Control of A.C. motors SIEMENS AG 2 Feb 1972 [3 Feb 1971] 4861/72 Heading H2J A single phase asynchronous motor (not shown) is supplied via a terminal 3 and changeover switch U which is connected to an A.C. mains 7 during motor starting and to a mainsfed inverter 4 controlled by a pulse generator 5 during subsequent running, the change-over being effected after the starting period is completed and there is phase equality between the output voltage of the inverter and that of the mains. The latter condition is detected by a phase comparator 12 which controls a device 11 actuating the switch U, a switch D also being controlled by comparator 12 to connect a mainsfed synchronizing device 13 to the pulse generator 5 to maintain phase equality once it has been detected. In a simple arrangement, the inverter is constantly synchronized with the mains and change-over of the switch U is determined by a time delay element in the device 11, which element is started when the motor is connected to terminal 3 and causes operation of switch U after an adjustable time corresponding to the starting period. It is preferred, however, to use a threshold value circuit 14 sensing motor current and starting the delay element when the current exceeds a predetermined value. If the current falls below this value, for example when the motor is switched off, the switch U automatically changes over to reconnect the motor to the mains. The circuit 14 can have two threshold values to permit the delay element to be omitted, since the fall of current below the upper threshold value can be used to cause device 11 to change over the switch U. A similar operation can be achieved by using a single response threshold circuit. To avoid constant synchronization of the inverter with the mains, this may be effected only during starting by using a two-threshold value circuit which actuates the comparator 12 when the motor is switched on. When synchronization is effected, the comparator 12 puts the device 11 into a state ready to actuate switch U, this operation being effected when the current falls below the upper threshold value. Simultaneously, a time delay element is operated to block the synchronizing pulses when changeover has been completed. An AND gate may be included to ensure that mal-operation is not caused by the motor reaching its normal running speed before synchronization is effected. In a final embodiment, the threshold value circuit has one response value which, when exceeded, actuates the comparator 12 whose closure of switch D also starts a time delay element in the device 11 to cause the latter to subsequently change-over the switch U, synchronization being discontinued after a further delay following the change-over. Modification of the circuit may be made to start a three phase motor with a shortcircuited secondary winding.