the present invention discloses a drive system being able to do bi - directional rotation of positive or reverse rotation by electric motor for driving input end of the constant directional output transmission device and further through the output end of the constant directional output transmission device to provide constant directional rotation output of different speed change ratio in different directions of driving inputs via the constant directional output transmission device of different speed change ratio for driving the loading wheel trains , wherein the present invention can be applied in electric motor driven carriers such as electric bicycles , electric motorcycles , electric vehicles , or electric driven industry machinery . fig1 is a basic block schematic view of the electric motor drive system of the present invention with bi - directional input and constant directional output . fig2 is a block schematic view showing that the drive system in fig1 is additionally installed with a transmission device ( 104 ) between the electric motor ( 101 ) and the constant directional output transmission device ( 102 ). fig3 is a block schematic view showing that the drive system in fig1 is additionally installed with a transmission device ( 104 ) between the constant directional output transmission device ( 102 ) and the driven loading wheel train ( 103 ). fig4 is a block schematic view showing that the drive system in fig1 is additionally installed with a transmission device ( 104 ) between the electric motor ( 101 ) and the constant directional output transmission device ( 102 ) as well as between the constant directional output transmission device ( 102 ) and the driven loading wheel train ( 103 ). as shown in the fig1 ˜ 4 , for the electric motor drive system with bi - directional input and constant directional output , the electric power from the power supply device ( 100 ) is subject to the operative signal or commands of the input device ( 111 ) via the electric motor operative control device ( 110 ) to drive the electric motor ( 101 ) in the first driven rotational direction or in the second driven rotational direction of contrary rotational direction , wherein the rotational kinetic energy output from the output end of the electric motor ( 101 ) is directly or through the transmission device ( 104 ) provided to the input end of the constant directional output transmission device ( 102 ), thereby by means of the constant directional output transmission device of different speed change ratio to output the rotational kinetic energy in constant rotational direction of different speed change ratio in different directions of driving inputs , and further to drive the loading wheel train ( 103 ) as shown in fig1 ˜ fig . 4 , wherein : the electric motor ( 101 ): it is comprised of ac or dc , brushless or brushed , synchronous or asynchronous operated electric power driven electric motor , and can be operated in both positive direction and reverse direction ; electric motor operative control device ( 110 ): it is constituted by dynamo - electric components or devices , or solid state electronic components or device , or the control circuits having microprocessor and associate operative software for inputting ac or dc power from the utility power source or the generator , or inputting electric power from the power supply device ( 100 ) such as fuel battery , primary battery , or charge / discharge secondary battery , thereby to operatively control the on / off , rotational direction , rotational speed , torque to the electric motor ( 101 ), or provide over - voltage or over - currents protection to the electric motor ( 101 ); input device ( 111 ): it issues signals or commands to the electric motor operative control device ( 110 ) for operatively controlling or setting the on / off , rotational direction , rotational speed , torque to the electric motor ( 101 ); the input kinetic energy of the first driven rotational direction and the second driven rotational direction from the electric motor ( 101 ) is for driving the input end of the constant directional output transmission device ( 102 ); the first driven rotational direction is contrary to the second driven rotational direction ; the constant directional output transmission device ( 102 ): the constant directional output transmission device ( 102 ) directly receives or is through the transmission device ( 104 ) to receive kinetic energy in different rotational direction from the electric motor ( 101 ) so as to provide kinetic energy output in constant rotational direction ; wherein the internal transmission components of the constant directional output transmission device ( 102 ) are constituted by one or more than one transmission components of the 1 ) gear train ; or 2 ) friction wheel train ; or 3 ) chain and chain sprocket train ; or 4 ) belt and belt wheel train ; or 5 ) transmission crankshaft and wheel train ; or 6 ) fluid transmission device ; or 7 ) electromagnetic transmission device , etc ., wherein if the constant directional output transmission device ( 102 ) is driven by the inputs of the first driven rotational direction and the second driven rotational direction in different rotational directions , the speed ratio of the constant directional rotation between the one at the input end and the one at the output end is the same or different or a variable speed ratio ; the loading wheel train ( 103 ): it is the wheel train type load being directly driven by the constant directional output transmission device ( 102 ) or being driven in the single constant directional rotation via the transmission device ( 104 ). the electric motor drive system with bi - directional input and constant directional output of the present invention can be further installed with a transmission device between the constant directional output transmission device ( 102 ) and the loading wheel train ( 103 ), or between the electric motor ( 101 ) and the constant directional output transmission device ( 102 ), or between the electric motor ( 101 ) and the constant directional output transmission device ( 102 ) as well as between the constant directional output transmission device ( 102 ) and the loading wheel train ( 103 ); fig2 is a block schematic view showing that the drive system in fig1 is additionally installed with a transmission device ( 104 ) between the electric motor ( 101 ) and the constant directional output transmission device ( 102 ). fig3 is a block schematic view showing that the drive system in fig1 is additionally installed with a transmission device ( 104 ) between the constant directional output transmission device ( 102 ) and the driven loading wheel train ( 103 ). fig4 is a block schematic view showing that the drive system in fig1 is additionally installed with a transmission device ( 104 ) between the electric motor ( 101 ) and the constant directional output transmission device ( 102 ) as well as between the constant directional output transmission device ( 102 ) and the driven loading wheel train ( 103 ). the transmission device ( 104 ): the transmission device ( 104 ) is constituted by one or more than one transmission devices of the following : 1 ) various gear - type , belt - type , friction - type fixed speed ratio rotating type transmission functioning structure , or 2 ) a stepped or stepless variable speed device being operated manually , or by the mechanical force , or fluid force , or centrifugal force , or rotating torque or counter - rotating torque , or 3 ) the transmission device of the capability to operatively control the switching relationship of the relative rotating directions at the input end and output end , or 4 ) the clutch device or the single way clutch being operated manually , or by the mechanical force , or electromagnetic force , or fluid force , or centrifugal force , or rotating torque , or counter - rotating torque for connecting transmission or interrupting transmission clutch functioning structure . for the electric motor drive system with bi - directional input and constant directional output of the present invention , the input device ( 111 ) is manually operated to through the electric motor operative control device ( 110 ) operatively control the rotational direction of the electric motor ( 101 ). in addition , the electric motor drive system with bi - directional input and constant directional output of the present invention can be additional installed with an electric motor loading currents detection device ( 109 ), thereby to automatically switch the output speed ratio according to the loading currents ; fig5 is a basic block schematic view of the electric motor drive system with bi - directional input and constant directional output of the present invention being installed with an electric motor loading currents detection device . fig6 is a block schematic view showing that the drive system in fig5 is additionally installed with a transmission device ( 104 ) between the electric motor ( 101 ) and the constant directional output transmission device ( 102 ). fig7 is a block schematic view showing that the drive system in fig5 is additionally installed with a transmission device ( 104 ) between the constant directional output transmission device ( 102 ) and the driven loading wheel train ( 103 ). fig8 is a block schematic view showing that the drive system in fig5 is additionally installed with a transmission device ( 104 ) between the electric motor ( 101 ) and the constant directional output transmission device ( 102 ) as well as between the constant directional output transmission device ( 102 ) and the driven loading wheel train ( 103 ). as shown in aforementioned fig5 ˜ 8 , the electric motor drive system with bi - directional input and constant directional output of the present invention is further installed with an electric motor loading currents detection device ( 109 ), wherein the electric motor loading currents detection device ( 109 ) can be comprised of various buck / step down type current detection device , or electromagnetic effect induction type current detection device , or magnetic detecting type current detection device , or heat accumulated type current detection device for detecting loading currents of the electric motor ( 101 ), thereby feedback to the electric motor operative control device ( 110 ), wherein when the loading currents of the electric motor exceeds the predetermined value and the status exceeds the preset time , the signals from the electric motor loading currents detection device ( 109 ) is transmitted to the input device ( 111 ) or the electric motor operative control device ( 110 ) so as to change the rotational direction of the electric motor ( 101 ) and further change the speed ratio of the constant directional output transmission device ( 102 ) to a relatively enlarged deduction ratio to increase the output in the same rotational direction of the output rotational torque to further drive the load . when the loading currents of the electric motor ( 101 ) return and drop below the predetermined value , the following two methods can be operated to make the electric motor back to the normal status , including : 1 ) by manually control the input device ( 111 ) to operatively control the electric motor operative control device ( 110 ) to enable the electric motor ( 101 ) returning to the original rotational direction , thereby driving the load at original speed ratio ; or 2 ) when the electric motor loading currents detection device ( 109 ) detects currents dropping to the predetermined value , by means of the electric motor operative control device ( 110 ) to automatically select the rotational speed which is not only capable of driving the load at that time , but also capable of outputting corresponding power volume according to the variation of the speed ratio so as to drive the load fluently without pauses or unexpected accelerations , thereby to drive the electric motor ( 101 ) being fluently operated in the original rotational direction . if the electric motor drive system with bi - directional input and constant directional output of the present invention further requires to perform constant directional output in different driving directions , it can be achieved by one of following methods : 1 ) by means of the electric motor operative control device ( 110 ) to operatively control the initial driving direction of the electric motor ( 101 ); or 2 ) a motor direction switch is additionally installed to manually switch the initial rotational direction of the motor ; or 3 ) one of the transmission devices ( 104 ) is constituted by the transmission device with functions of mechanically switching rotational directions of the output and changing the speed ratio for switching the rotational direction of output .