Magnetizing currents in self-locking polyphase inverters

In conventional polyphase self-locking transistor-magnetic inverters, a single master inverter controls frequency, plays a key role in determining the phase-lock angles of all slave inverter units connected to it, and shares the load equally with these slaves. Implicit in this diversity of function is the need for the transistors in the master to carry magnetizing currents for a set of square-loop timing cores-one associated with each slave. The nature of these magnetizing currents is such that, in the presence of heavy loads, they tend to cause premature switching of the master inverter transistors and, hence, a loss of system synchronism. An arrangement is presented in which the load sharing and timing-locking functions of the master inverter are delegated to an in-phase slave and a submaster. The master inverter retains its key position in the operation of the system, but its residual functions, i.e., stimulation of the system and determination of system frequency, do not require great size and therefore permit simple and economic frequency regulation. The scheme has been applied to a 6-unit 3-phase twelve-step per cycle 1.5-kW inverter, with a nominal 28-volt input and a regulated operating frequency of 400 c/s.