various embodiments will be hereinafter explained with reference to the drawings . throughout the embodiments , the same structures are attached with the same reference numerals , and redundant explanations thereabout are not repeated . each figure is a schematic view illustrating the embodiments for helping the understanding thereof . in each figure , some of shapes , sizes , ratios , and the like may be different from those in an actual apparatus . as necessary , these may be changed in design in view of the following explanation and known techniques . one example embodiment of a drive system for vehicles is shown in fig1 . this example embodiment includes an engine 1 , alternators 2 , converter 3 , voltage sensor 4 , filter capacitor 5 , inverter 6 , electric motor 7 , control section 10 , voltage detector 11 , comparing element 12 , and engine rotation number commanding part 13 . in the example embodiment , engine 1 is connected with the rotor of alternator 2 . the stator winding of alternator 2 is connected with converter 3 . converter 3 is connected with inverter 6 via filter capacitor 5 . inverter 6 is connected with electric motor 7 . in certain example embodiments converter 3 is a pulse - width modulation ( pwm ) controlled converter . in certain example embodiments inverter 6 is a pwm controlled inverter . voltage sensor 4 is connected in parallel with filter 5 . control section 10 is connected with voltage sensor 4 . control section 10 has voltage detector 11 , comparing element 12 , and engine speed controller 13 . voltage detector 11 connects with voltage sensor 4 and comparing element 12 within control section 10 . comparing element 12 connects with voltage detector 11 and engine speed controller 13 . engine speed controller 13 is connected with comparing element 12 and engine 1 . engine 1 is engaged when the vehicle is on . engine 1 generates torque , which is transmitted to the rotor of alternator 2 , rotating the rotor of alternator 2 . alternator 2 is rotated and electric power is generated and sent to converter 3 . filter capacitor 5 is charged by the electric power from converter 3 . once filter capacitor 5 is charged , converter 3 supplies direct - current electric power to inverter 6 . inverter 6 , in turn , converts the direct - current electric power supplied from converter 3 into alternating current electric power . electric motor 3 is driven with the alternating current electric power from inverter 6 . with the drive of electric motor 3 , torque is transmitted via coupling ( not illustrated ) to an axle to move the vehicle . to use an alternator 2 as a dynamo , it is necessary for converter 3 to maintain the voltage across filter capacitor 5 at or above a predetermined value . in order to maintain the voltage across filter capacitor 5 above a predetermined value , the magnetic flux of the rotor of alternator 2 is used . by rotating the rotor of alternator 2 voltage in induced in the stator of alternator 2 . filter capacitor 5 is therefore charged by the output of by converter 3 from alternator 2 . by using the residual magnetic flux of a rotor and generating induction voltage , of the voltage across filter capacitor 5 is maintained above a default value , allowing alternator 2 to be used as a dynamo . in this case , filter capacitor 5 can be charged without a separate circuit by using residual magnetic flux . when alternator 2 is used as a dynamo as mentioned above , the voltage of filter capacitor 5 is controlled by control section 10 . the voltage value detected by voltage sensor 4 is relayed to voltage detector 11 . the voltage value inputted into voltage detector 11 is inputted into comparing element 12 from voltage detector 11 as voltage value ( i ). voltage value ( i ) inputted into comparing element 12 is compared with a command value ( a ) previously set by comparing element 12 . the command value ( a ) may be set to the voltage value of the filter capacitor that will allow the drive system to operate . the comparison result of comparing element 12 is inputted into engine speed controller 13 . when the comparison result is “ voltage value ( i )& gt ; command value ( a )” ( i . e ., when the measured voltage is above the command voltage value ), the voltage of filter capacitor 5 is assumed to be sufficient to operate the drive system , and engine speed controller 13 causes the engine rotation speed to decreases . when the comparison result is “ voltage value ( i )& lt ; command value ( a )” ( i . e ., when the measured voltage value is less than the command voltage value ), the engine speed controller 13 causes the engine rotation speed to increases . thus , the minimum voltage of filter capacitor 5 is maintained by the electric power generated with alternator 2 is supplied to electric motor 7 , and it enables it to run vehicles . according to embodiments of the vehicle drive system described above , it may be possible to provide the drive system for vehicles which can run in a location without available power by using engine 1 and converting its output with alternator 2 . a second example embodiment of a drive system for vehicles is shown in fig2 . in this example embodiment , a dc - to - dc converter 21 is connected with a backup power supply 22 . as shown in fig2 , between filter capacitor 5 and inverter 6 , a backup power supply 22 is connected by dc to dc converter 21 . when the magnetic flux of alternator 2 is insufficient to charge filter capacitor 5 , backup power supply 22 charges filter capacitor 5 by the dc - to - dc converter 21 . alternator 2 can be used as a dynamo once filter capacitor 5 is charged . thus , the drive system of the second example embodiment can extend a period of maintenance work by using the backup power source less frequency than an example system that relied more on the backup power supply 22 . a second example embodiment of a drive system for vehicles is shown in fig3 a third example embodiment is shown in fig3 . the third example embodiment differs from the second example embodiment in that it has a first gear 31 , a dynamo 32 , and a rectifier 33 . as shown in fig3 , the first gear 31 is connected to engine 1 . the other end of the first gear 31 is connected to , alternator 2 and dynamo 32 . dynamo 3 is connected with rectifier 33 . rectifier 33 is connected between filter capacitor 5 and inverter 6 . engine 1 is engaged when control section 10 determines that the residual magnetic flux of alternator 2 is insufficient to charge filter capacitor 5 . when engine 1 is engaged , torque is transmitted to dynamo 32 by the first gear 31 . dynamo 32 rotates by the torque transmitted from the first gear . dynamo 32 &# 39 ; s rotation generates alternating current electric power occurs . alternating current electric power generated from dynamo 32 is rectified by rectifier 33 and is then supplied to filter capacitor 5 . once filter capacitor 5 is fully charged , the vehicle drive system will operate . in certain example embodiments , a transformer may be inserted between dynamo 32 and rectifier 33 to adjust the output voltage of dynamo 32 . in certain embodiments from the output from alternator 32 is an alternating current . in other example embodiments a direct current generator is used . according to embodiments of the vehicle drive system described above , it may be possible to provide the drive system for electric - powered vehicles that will allow the vehicle to operate in a location that does not have external power . a fourth example embodiment of the drive system is show in fig4 . in this example embodiment , first gear 41 and second gear 42 are connected . the first gear is connected between engine 1 and alternator 2 . electric motor 7 is mechanically connected with second gear 42 . the first gear 41 and second gear 42 are connected mechanically . engine 1 is engaged when control section 10 determines the residual magnetic flux of alternator 2 is insufficient to charge filter capacitor 5 . when engine 1 is engaged , the first gear 41 is rotated . torque is transmitted to second gear 42 . the rotation of the second gear 42 will , in turn , cause torque and rotation of electric motor 7 . the rotation of electric motor 7 , in turn , generates alternating current electric . in this example embodiment , electric power generated by electric motor 7 is then supplied to pwm inverter 6 . the alternating current electric power of electric motor 7 is transformed into direct - current electric power by pwm inverter 6 . this direct - current electric power charges filter capacitor 5 . once filter capacitor 5 is fully charged , the drive system is set to operate . in this embodiment the filter capacitor 5 can be fully charged even when the residual magnetic flux of alternator 2 is insufficient to charge filter capacitor 2 . according to embodiments of the vehicle drive system described above , it may be possible to provide the drive system for electric - powered vehicles that will allow the vehicle to operate in a location that does not have external power . it may also be possible to avoid the maintenance of having to charge a power supply in the drive system . while certain embodiments of a vehicle drive system have been described , these embodiments have been presented by way of example only , and are not intended to limit the scope of the disclosure . indeed , the novel systems described herein may be embodied in a variety of other forms ; furthermore , various omissions , substitutions and changes in the form of the systems described herein may be made without departing from the spirit of the disclosure . the accompanying claims and their equivalent are intended to cover such forms or modifications as would fall within the scope and spirit of the disclosure .