Inverter having self adjusting output frequency for use with other inverters in parallel

#1. Field of the Invention

#This invention relates to an inverter suitable for use with portable AC power supply units.

#2. Description of the Related Art

#Other objects, features and advantages of the present invention will become clear upon reviewing the following description of a preferred embodiment, made with reference to the accompanying drawings, in which:

#FIG. 1 is a circuit diagram showing an electrical arrangement of a portable AC power supply unit to which an inverter of an embodiment in accordance with the present invention is applied;

#FIG. 2 is a schematic block diagram showing functions of a microcomputer employed in the power supply unit;

#An embodiment in accordance with the present invention will be described with reference to FIGS. 1 to 12. The inverter of the invention is incorporated in a portable AC power supply unit in the embodiment. Referring to FIG. 1, an electrical arrangement of the portable AC power supply unit 21 generating AC power of 100 V and 50 Hz or 60 Hz, for example. The portable AC power supply unit 21 comprises a three-phase AC generator 22 driven by an engine (not shown) and a single-phase inverter unit 23 connected to a rear stage of the generator. The generator 22 includes a rotor, an armature neither of which is shown, and a stepping motor 24 controlling a flow rate of fuel (gasoline) to the engine so that a rotational speed of the engine is controlled. The armature has Y-connected primary windings 25u, 25v and 25w wound thereon and an auxiliary winding 26 wound thereon. Primary winding terminals 27u, 27v and 27w and auxiliary winding terminals 28a and 28b are connected to input terminals 29u, 29v and 29w, 30a and 30b of the inverter unit 23 respectively.

#The inverter unit 23 will be described in detail. A rectifier circuit 33 is connected between DC power supply lines 31 and 32. The rectifier circuit 33 is further connected to the input terminals 29u, 29v and 29w. A smoothing capacitor 34 and an inverter circuit 37 are also connected between the DC power supply lines 31 and 32. The inverter circuit 37 is further connected to output terminals 35 and 36 of the inverter unit 23. A filter circuit 38 is connected to the inverter circuit 37. The rectifier circuit 33 serves as a DC power supply circuit in the invention. The rectifier circuit 33 comprises thyristors 39 to 41 and diodes 42 to 44 connected into a three-phase hybrid bridge configuration. The inverter circuit 37 comprises transistors (switching elements) 45 to 48 and diodes 49 to 52 connected into a full bridge configuration.

#The filter circuit 38 comprises a reactor 55 and a capacitor 56. The reactor 55 is interposed between the output terminal 53 of the inverter circuit 37 and the output terminal 35 of the inverter unit 23. The capacitor 56 is connected between the output terminals 35 and 36 of the inverter unit 23. The inverter circuit 37 has an output terminal 54 directly connected to the output terminal 36 of the inverter unit 23. A current transformer 57 is connected across a current path between the output terminal 54 and the filter circuit 38 for detecting an output current. In a parallel operation of a plurality of the AC power supply units 21, the output terminals 35 and 36 of the inverter units 23 are connected in parallel with each other or one another, respectively.