Constant speed drives (CSD's) are typically used to convert variable speed motive power from a prime mover into constant speed motor power for driving a generator so that the generator develops constant frequency electrical power. Electrically compensated CSD's (ECCSD'S) have been proposed for use in applications where conventional hydromechanical CSD's have been found to be inadequate. Known ECCSD'S include a mechanical differential speed summer having a first input shaft coupled to the output shaft of the prime mover, a second input shaft and an output shaft at which the constant speed motor power is developed. A speed compensation link is coupled to the second input shaft of the speed summer and includes a first or compensating permanent magnet machine having a motive power shaft coupled to the second input of the differential speed summer and electrical power windings. A motive power shaft of a second or control permanent magnet machine is coupled to either the output shaft of the prime mover or the output of the differential speed summer. Electrical power is transferred between the windings of the first permanent magnet machine and electrical power windings of the second permanent magnet machine by a power converter so that the compensating permanent magnet machine develops compensating speed of a magnitude and direction which causes the output of the differential to be driven at the desired constant speed. Borger U.S. Pat. No. 4,572,961 discloses such a constant speed drive wherein the power converter comprises an AC/DC link converter coupled to the power windings of the control permanent magnet machine, a filter coupled to the output of the DC link converter which develops a DC link voltage and an inverter which converts the DC link voltage into AC power which is provided to the compensating permanent magnet machine. A control for the DC link converter is responsive to a load speed error signal and other speed related signals to produce the DC link voltage. The inverter is responsive to the current in the windings of the second permanent magnet machine to maintain a desired commutation angle.
Other ECCSD'S are disclosed in Dishner et al U.S. Pat. Nos. 4,692,671 and 4,695,776, assigned to assignee of the instant application and the disclosures of which are hereby incorporated by reference. The former patent discloses an "input differential" configuration in which the first permanent magnet machine is coupled to the output of the differential speed summer whereas the latter patent discloses an "output differential" configuration in which the first permanent magnet machine is coupled to the output shaft of the prime mover. In both patents, a power converter interconnects the electrical power windings of the first and second permanent magnet machines. The power converter includes a bidirectional AC/DC converter coupled to the electrical power windings of the first permanent magnet machine, a bidirectional DC/DC converter coupled to the first AC/DC converter and a second bidirectional AC/DC converter coupled between the DC/DC converter and electrical power windings of the second permanent magnet machine.
All of the foregoing patents suggest that the power flow through the power converter may be either bidirectional or unidirectional. However, the power converter disclosed in the '671 and '776 patents is expressly designed for bidirectional power flow. The '961 patent does disclose that the DC link converter and inverter can be used for unidirectional power flow; however, the circuit for controlling the DC link converter is not independent from the circuit for operating the inverter. Therefore, adjustment or a change in one circuit may necessitate a change in the other control circuit for proper operation of the ECCSD. Further, the control for operating the DC link converter and the inverter are overly complex due to the need to sense many operating parameters of the drive.