BACKGROUND OF THE INVENTION

FIELD OF THE INVENTION This invention relates to marine propulsion systems and in particular to propulsion systems utilizing multiphase induction motors.

DESCRIPTION OF THE BACKGROUND ART One conventional marine drive application is the propulsion drive of torpedoes and the like. In such application, it is desirable to provide quiet propulsion at high power densities. It has further been conven¬ tional in electric propulsion systems to use direct cur¬ rent motors as the means for driving the propellers.

Because of increasing power requirements, it has been found desirable to utilize alternating current motors in lieu of the limited capacity direct current motors previously used. One proposed application of such alternating current motor drives utilizes a three-phase squirrel—cage induction motor excited by a three-phase half-bridge inverter connected to the battery power sup¬ ply. It has been further proposed that such motors uti¬ lize a conventional three-phase winding configuration and that the inverters be arranged to provide three-phase variable frequency, variable amplitude alternating cur- rent power to the motors. It is further proposed that amplitude control may be achieved by means of pulse width modulators.

In the proposed controls for use with such three-phase drive motors, power transistors are utilized as switching devices to reduce the weight of the propul¬ sion system. However, such available transistor devices are limited in the power switching capabilities and have

been found to be inadequate to provide the desired pro¬ pulsion system power levels at the present time. As a result, in the proposed systems, paralleling of transis¬ tors is necessitated to provide sufficient power rating of the inverters. Such paralleling of power transistors causes a number of serious technical problems. Because of the exacting weight and space requirements in such marine devices, such as torpedoes, the use of additional equipment to prevent unequal current sharing among the parallel transistor devices has been a serious problem. Another problem encountered in the three-phase motor drives utilized heretofore has been the production of substantial harmonic currents which cause pulsating torque components in the motor rotor which, in the prior art devices, may have amplitudes which are a substantial fraction of the average torque output of the system. Such harmonic currents result from the inverter means delivering nonsinusoidal excitation waveforms to the motor. Thus, the three-phase half-bridge inverters con- ventionally employed deliver quasi-square wave (referred to hereinafter as "six-step") voltage waveforms under the full speed and full power conditions. The pulsating torque developed by the harmonic currents is convention¬ ally delivered directly to the speed reducing gear means in the torpedo, which not only adversely affects the gear life but has the serious undesirable effect of increasing propulsion system noise as a result of gear chatter and the like.

The excitation harmonic currents further con- tribute to a substantial increase in the rotor losses in the drive motor, reducing the capability of the drive systems and presenting heat removal problems from the rotor assembly, which is desirably designed to be as com¬ pact as possible.

'BUREA

OA.PJ

Thus, there has been a need for an improved drive system providing improved reliability, efficiency, smooth torque production, -and increased power capabili¬ ties in such alternating current drive systems.

SUMMARY OF THE INVENTION

The present invention comprehends an improved marine drive system for driving the propulsion propeller from a direct current battery power supply.

In the illustrated embodiment, the drive system utilizes a multiphase alternating current drive motor driving the propeller, the number of phases defined by the drive motor stator windings being at least four. The motor is designed so that the magnetic axes of the stator phase windings are spaced at equal intervals along the inner periphery of the motor stator. Current is deliv¬ ered to the drive motor from a corresponding plurality of inverters connected one each to the motor windings from the power supply batteries for exciting the windings suc¬ cessively at 360° time intervals wherein N equals the N number of phases.

In the illustrated embodiment, the drive motor further defines a stator portion having high leakage reactance for effectively suppressing harmonic currents in the windings. In the illustrated embodiment, the number of windings comprises a whole multiple of three-, and more specifically, in the illustrated embodiment, the stator comprises three sets of three-phase windings arranged in wye connections. The inverters are arranged as three-phase half-bridge inverters providing a six-step waveform to the drive motor under full speed, full power conditions.

Thus, the marine drive system of the present invention is extremely simple and economical of construc¬ tion while yet providing improved reliability, effi¬ ciency, torque production smoothness, and power capabili- ties.

BRIEF DESCRIPTION OF THE DRAWING

Other features and advantages of the invention will be apparent from the following description taken in connection with the accompanying drawing wherein: FIGURE 1 is a fragmentary elevation of a marine device having a drive system embodying the invention?

FIGURE 2 is a schematic diagram illustrating a preferred form of the invention utilizing three-phase half-bridge inverters in a drive system wherein the num- ber of phases of the alternating current drive motor is a multiple of three;

FIGURE 3 is a waveform diagram illustrating the line-to-line motor excitation voltage waveforms for the multiphase system illustrated in Figure 2; FIGURE 4 is a wiring diagram illustrating one embodiment of the invention utilizing a symmetrically wound, nine-phase induction motor excited by three-phase half-bridge inverters;

FIGURE 5 is a diagram illustrating the line- to-line motor excitation voltage waveforms for the system configuration of Figure 4; and

FIGURE 6 is a schematic wiring diagram illus¬ trating a conventional circuit arrangement for providing the inverter control of the motor phase windings as broadly illustrated in Figure 4.

"BU ^RE A

OMPI

DESCRIPTION OF THE PREFERRED EMBODIMENT

In the illustrative embodiment of the invention as disclosed in the drawing, a marine drive system gener¬ ally designated 10 is shown for use in the propulsion system of a torpedo 11. The present drive system is advantageously adapted for use in such an application in providing a high power, low noise drive.

The drive system drives the propeller 12 from a direct current power supply defined by batteries 13. The direct current power of the batteries 13 is converted by an inverter device 14 to provide improved operation of a multiphase alternating current motor 15 connected through a gear box 16 and propeller shaft 17 to the propeller 12. As indicated above, the invention comprehends the provision of drive motor 15 as a multiphase alter¬ nating current drive motor. In the illustrated embodi¬ ment, the drive motor defines at least four stator wind¬ ings and inverter 14 defines a corresponding plurality of inverters connected one each to the windings for exciting the windings successively. In the illustrated embodi¬ ment, the windings are spaced apart 360 electrical degrees divided by the number of phases.

In one form of the invention illustrated in Figure 2, the number of phases is caused to be a multiple of three and the inverters are arranged so as to define three-phase half-bridge inverters. Thus, each of the phase winding sets comprises a three-phase winding con¬ nected in a wye. In broad form, the number of such three-phase windings may be any number as desired, and in Figure 2 of the drawing, the number of such three-phase windings is designated m.

The arrangement of the inverters is generally illustrated in Figure 4, wherein the wiring diagram is illustrated specifically for a nine-phase motor arrange¬ ment. As shown in Figure 4, inverters 14a, 14b and 14c connected to three-phase motor windings 18a, 18b and 18c, respectively, each comprise three half-bridge inverters 19. Each inverter 19 includes a pair of transistor switches 20 and 21 connected respectively to the opposite power supply leads I_ι and L2 from battery 13. The con- nection 22 between the switches 20 and 21 is connected to one winding 23 of the three-phase windings.

Each of the other half-bridge inverter circuits 19 of the inverters 14a, 14b and 14c is similarly con¬ nected so that each of the nine windings of the three three-phase windings 18a, 18b and 18c is fed from the half-bridge connection 22 of an associated inverter 19, as illustrated in Figure 4.

As further illustrated in Figure 4, in the nine-phase arrangement, each of the three sets of three- phase windings is offset from the others by 40 electrical degrees.

The line-to-line motor excitation voltage wave¬ forms for the nine-phase system illustrated in Figure 4 are shown in Figure 5 as an illustrative example of the waveforms broadly shown in Figure 3.

Switches 20 and 21 illustrated schematically in Figure 4 may comprise, as illustrated in Figure 6, a parallel arrangement of a controlled electrical switch 24 and a freewheeling diode 25. In the embodiment of figure 6, the electronic switch 24 comprises an NPN bipolar power transistor having its base connected to a base con¬ trol module 26 of conventional construction, which, in turn, is controlled by conventional synchronization cir¬ cuit means 27, also of well known conventional construc-

UREX

OMPI

tion. As further shown in Figure 6, the collector of transistor 24 is connected to power supply lead L_ and the emitter is connected to connection 22. As shown in switch 21, the collector of the electrical switch 24 is connected to connection 22. The collector of the tran¬ sistor 24 of switch 21 is connected to connection 22 and the collector thereof is connected to power supply lead I_2, whereby the transistor switches 20 and 21 are con¬ nected in series between the power supply leads, with the connection therebetween being connected to the motor winding 23.

As shown, each of the nine motor windings of Figure 4 is connected to the connection between such pairs of electronic switches so controlled by the syn- chronization circuitry through associated pairs of base drive modules 26.

As is well known to those skilled in the art, the transistors 24 function as controlled switches as a result of the selective feeding of positive current to the transistor base, which turns the transistor on while removing of the base current turns the transistor to the high impedance "off" state in the conventional manner. The freewheeling diodes are provided to circulate induc¬ tive currents caused by the motor windings, thereby pre- venting transient voltages from developing during switch turnoff. The transistor switch module configurations, the base drive module circuitry for controlling base cur¬ rent to such transistor, and synchronization ^' circuitry for controlling the base drive modules in effecting the timed switching operation are well known to those skilled in the art and require no further description herein. However, details on the concept of providing synchroniza¬ tion signals for controlling base drive modules may be found in conventional reference sources, such as "Adjust-

able Speed AC Drive Systems," by B. K. Bose, IEEE Press, 1981. Similarly, circuitry of conventional base drive modules is illustrated in "The Power Transistor in Its Environment," a handbook of Thomson-CSF, Semiconductor Division.

As will further be obvious to those skilled in the art, the use of power transistors as the switching means in the inverters is exemplary only. Thus, the switches may comprise conventional SCR gated switches, or GTO gate-turn-off thyristor switches. Suitable control circuitry for timing the operation of such gated switches is again well known to those skilled in the art and requires no further illustration herein.

As will still further be obvious to those skilled in the art, the use of the supra-tertiary phase operation may be effected by means of a current source power supply in lieu of the voltage source power supply illustrated in Figure 4. Such a current source itself limits the maximum current in the motor phase windings, thereby simplifying considerably the task of minimizing the motor harmonic current amplitudes associated with the switching operations. In addition, the harmonic currents in such an excitation waveform react against a low imped¬ ance which provides a desirable limiting of the voltage transient amplitudes during the switching steps.

Still further, as will be obvious to those skilled in the art, while the invention has been de¬ scribed in connection with multiphase induction motors, the control is advantageously adapted as well for use with reluctance motors, synchronous motors, etc.

It will be apparent to those skilled in this art that the performance characteristics of a symmetri¬ cally wound multiphase motor having N phases can be essentially duplicated in a motor having only phases,

where N is an even number. This results from the fact that, when N is even, a symmetrically wound motor (phase spacing of 360 electrically degrees) includes only . distinct winding magnetic axes. In such a motor, each of these axes is shared by two phase windings which are excited with inverted excitation waveforms (waveforms displaced by 180 electrical degrees) . By combining each of these pairs of windings into a single winding along each magnetic axis, none of the motor performance charac- teristics are changed even though there are half as many motor windings. This technique is valuable for securing the performance advantages of a multiphase motor with N (even) phases using only half of the number of windings, switches, and other hardware elements normally neces- sary. For example, the characteristics of a symmetri¬ cally wound twelve (12) phase motor can be obtained with a six (6) phase motor when the six phases are separated by 360 - 3Q electrical degrees. In this case, the six phase excitation waveforms will likewise be spaced at 30 electrical degree time intervals.

The invention comprehends constructing the multiphase motors to have increased inductive impedance so as to reduce the amplitude of the harmonic current components produced by any nonsinusoidal voltage excita- tion waveforms. By eliminating the need for external magnetics in controlling the harmonic components, the weight and volume of the propulsion system is greatly improved. Thus, illustratively, the improved circuitry of the present invention permits the use of induction motors in torpedo-propelling application wherein the power requirements exceed 200 hp, while yet using tran¬ sistor switches in nonparallel configurations.

The foregoing disclosure of specific embodi¬ ments is illustrative of the broad inventive concepts comprehended by the invention .

-BUREA

OMPI