an electrically driven vehicle 1 according to fig1 , here a tram , is supplied with electric power via an overhead line 4 by means of an external electric supply system , here a 600 volt supply system , ( not illustrated in more detail ). the electric power is tapped in a conventional manner by means of a current collector 2 . referring to fig2 , the electric drive 3 in this configuration is comprised of , for example , an electric motor 9 , here an asynchronous motor , and may also , if necessary , contain a transmission as well as a power inverter 13 as a power unit 8 which supplies drive power for the electric motor 9 . a control unit 7 controls the entire electric drive 3 . power unit 8 is comprised of a power inverter 13 which is connected to an intermediate circuit capacitor 12 and to the external supply system via a main contactor 10 and a supply system reactor 11 . the grounding is carried out in this example by means of a grounding earth contact 14 . power inverter 13 may be of any conventional or desired type , such as a switched rectifier , pulse width modulation ( pwm ) drive unit having one or more stages of power transistors such as igbts or mosfets feeding the motor in common . a conventional on - board supply system comprising one or more batteries 5 , here 24 volt batteries , supplies important parts of the vehicle 1 such as the electronic control system or the basic lighting system of the vehicle 1 , etc ., with electric power independently of the external supply system . the vehicle 1 which is described above is , of course , only to be considered as an example . in particular , the individual components can be arranged at any other desired locations on the vehicle 1 , for example , instead of overhead lines 4 it would also be possible to provide lateral sliding contacts , and the individual components can of course also be provided multiply , for example , a vehicle 1 can be provided with two drives 3 . in many embodiment variants the drive 3 is also accommodated directly in the chassis of the vehicle 1 . in the normal operating mode , the electric drive 3 of the vehicle 1 is supplied with electric power by the external electric supply system . generally , the batteries 5 only supply the electronic control system and other components of the vehicle 1 here , and are recharged via the external electric supply system when necessary . if the external supply system fails , for example owing to a line break or a power failure , or if the vehicle 1 is to be operated in an area without an external supply system , for example in a depot , it has been necessary in the past , for auxiliary vehicles which are independent of a contact line to be used to move the vehicle . according to the present invention , however , as illustrated in fig2 , a circuit breaker 6 is provided which , is closed ( either manually , or automatically by control unit 7 in response to loss of external power ) to connect control unit 7 and power inverter 13 of the electric drive 3 to the on - board supply system batteries 5 . in conjunction with this , the main contactor 10 which connects the external power supply to power unit 8 , is opened by control unit 7 , and vehicle 1 is disconnected from the external supply system . both traction power and conventional on - board function power are supplied by the already present on - board supply system batteries 5 . however , as a result of the significantly lower supply voltage , here 24 v instead of 600 v , for which the drive 3 is configured , completely different electrical conditions arise . for example , as well known to those skilled in the art of designing switched rectifier motor drives , parasitic effects inherent in the semiconductor devices which comprise the motor drive unit 13 cause losses , which effectively reduce the power available to drive motor 9 . these effects are taken into account and compensated for in the design of the drive unit itself , and by the operation of control unit 7 . nevertheless , these come to bear dramatically when the traction power is supplied by on - board battery system . consider , for example the following : if the external voltage supply provides , e . g ., 600 volts , and a certain element of power inverter 13 , such as a power transistor or a diode , experiences a voltage drop of 2 volts , the reduction of the drive voltage to 598 volts will have negligible influence on the magnetic field of traction motor 9 . if traction power for the vehicle needs to be supplied by the on - board 24 volt system supply battery , a voltage drop of 2 volts is a drop of 10 % in the supply voltage , which can not be ignored anymore . the consequent reduction in motor current will have a dramatic effect on the torque produced by the motor , effectively rendering it incapable of providing traction power . according to the principles of the present invention , the lost current is compensated for by increasing the pulse width ( duty cycle ) of the pwm drive for the inverter . this permits motor 9 to be run on power provided by the onboard battery . the motor speed will , of course , be considerably lower than under normal conditions , but will be sufficient for emergency purposes , or limited travel in a depot where external power is not available . as will be understood by those skilled in the art , complete specification of the performance of the inverter 13 and controller 7 with both external and on - board battery power can , in general , be quite complex . considerable simplification could be achieved if the required compensation could be determined on the basis of transistor and diode models alone . fortunately , over a range of input voltages encountered , it has been found that these models can be used to determine the voltage drops that have to be compensated by the control unit . stated differently , a person skilled in the art will readily be able to apply conventional design practice to controller 7 such that it will be operable to adjust the duty cycle for the pwm switched power supply to provide adequate magnetization current for motor 9 when traction power is supplied by the on - board battery system on the basis of standard transistor a diode models . it will similarly be appreciated by those skilled in the art , that numerous circuit designs can be implemented which will achieve the stated objective within the scope of the invention . a person skilled in the art will also recognize that other features can also be provided . for example , a separate battery management unit , which could also be the control unit 7 , can be provided since the batteries 5 have only a restricted amount of power which should be utilized in an optimum way . for this purpose , on the one hand , a power limiting means could be provided since each battery should be loaded only with a specific maximum battery current , and in order to prevent the battery 5 becoming exhausted , and on the other hand , a maximum drive torque is to be obtained . the battery management system ensures both traction to the point where the battery 5 is exhausted and residual availability of the control voltage at the vehicle 1 . it is , of course , conceivable to use any desired number of the present on - board supply system batteries 5 to supply the electric drive 3 , or else to equip a plurality of drives 3 of the vehicle 1 in this way . in this specific exemplary embodiment , the power unit 8 has been described as comprised of a power inverter 13 which is connected to an intermediate circuit capacitor 12 and to the external supply system via a main contactor 10 and a supply system reactor 11 . in the event of a failure , the main contactor 10 is opened by means of the control - unit 7 , and the circuit breaker 6 is closed , as a result of which the power unit 8 is thus supplied by the battery 5 . the grounding is carried out in this example by means of a grounding earth contact 14 . however , basically , any desired other circuits are also possible , for example a two - pole embodiment of the circuit breaker 6 if the battery circuit is grounded differently , or the use of some other power unit 8 in which the main contactor 10 and supply system reactor 11 are not integrated in the power converter , etc . although the present invention has been described in relation to a particular embodiment thereof , many other variations and modifications and other uses will become apparent to those skilled in the art . it is intended , therefore , that the present invention is not be limited by the specific disclosure herein , but is to be given the full scope permitted by the appended claims .