throughout all the figures , same or corresponding elements are generally indicated by same reference numerals . these depicted embodiments are to be understood as illustrative of the invention and not as limiting in any way . it should also be understood that the drawings are not necessarily to scale and that the embodiments are sometimes illustrated by graphic symbols , phantom lines , diagrammatic representations and fragmentary views . in certain instances , details which are not necessary for an understanding of the present invention or which render other details difficult to perceive may have been omitted . turning now to the drawing , and in particular to fig1 , there is shown a schematic illustration of a drive device for an electrically operated vehicle . the drive apparatus possesses a motor 1 , which is designed as an alternating current motor . the motor 1 is supplied with power by an intermediate circuit 2 which is indicated by an intermediate circuit capacitor c . the intermediate circuit 2 is a direct voltage system with a negative rail 3 and a positive rail 4 . a converter 5 linked to the negative rail 3 and the positive rail 4 converts a direct current from the intermediate circuit 2 into a three - phase current or alternating current for the motor 1 . three phase lines are laid from the converter 5 to the motor 1 for this purpose : a first phase line 6 , a second phase line 7 and a third phase line 8 . a current i 1 is measured in the first phase line 6 , a current i 2 in the second phase line 7 and a current i 3 in the third phase line 8 . a voltage u 1 is present vis - à - vis the negative rail 3 at the first phase line 6 , a voltage u 2 at the second phase line 7 and a voltage u 3 at the third phase line 8 . normally the motor currents i 1 , i 2 and i 3 of the motor 1 are measured directly . the currents are at least measured in two phases , and the current in the third phase is calculated from these , as the three currents together come to zero , as long as no isolation errors are present . a method is known from the publication gb 2 440 559 b mentioned in the introduction , to which explicit reference is made here , for how using a σ - δ method ( sigma - delta method ) the output voltages u 1 , u 2 and u 3 of the converter 5 can be measured vis - à - vis the negative rail 3 of the intermediate voltage circuit . thanks to the σ - δ method all measured values are present in digital form as σ - δ data flows and can be suitably further processed in a central logic module 9 ( e . g . fpga ) ( see fig3 ). using the instantaneous values of the output voltages u 1 , u 2 and u 3 and the instantaneous values of the output currents i 1 , i 2 and i 3 the torque m emitted by the motor 1 can be calculated in the calculation unit 9 , even if the converter 5 is locked or the target voltages cannot be correctly converted for other reasons ( e . g . voltage - time integral errors as a result of dead time influences and forward voltage drops or as a result of reaching the control limit , activating minimum pulse monitoring ). in these cases no target voltages are available or the target voltages differ from the actual voltages due to non - linearities . then it is only possible to estimate the torque meaningfully on the basis of the actual voltages . fig2 shows by way of example based on a signal flow chart the calculation of the instantaneous line from the currents i 1 , i 2 and the line - to - line voltages u 13 and u 23 . in the specific example the currents i 1 and i 2 are thus measured in the first phase line 6 and the second phase line 7 . likewise the voltages u 1 , u 2 and u 3 are measured at the first phase line 6 , the second phase line 7 and the third phase line 8 and are provided as input variables . the electrical output frequency f can be determined from the currents or voltages and a corresponding angular velocity 2πf can be provided . using a first subtractor 10 a first line - to - line voltage u 13 is obtained by subtracting the third voltage u 3 from the first voltage u 1 . similarly , a second line - to - line voltage u 23 is obtained using a second subtractor 11 by subtracting the third voltage u 3 from the second voltage u 2 . the line - to - line voltage u 13 is multiplied by the current i 1 of the first phase line 6 in a multiplier 12 . likewise the line - to - line voltage u 23 is multiplied in a multiplier 13 by the current 12 in the second phase line 7 . the products of both multipliers 12 and 13 are summed in an adder 14 , and the result represents the instantaneous power that is made available by the converter . using a downstream low - pass filter 15 the electrical active power of the converter 5 is determined from the instantaneous power . the losses of the electric motor ( especially the stator losses ) are subtracted from this active power in a subtractor 16 . these losses are provided using a power loss model 17 . the output signal of the subtractor 16 thus corresponds to the actual rotating field power of the motor . using a divider 18 , with which the rotating field power is divided by the angular velocity 2πf of the converter , an estimated value { tilde over ( m )} of the actual mechanical torque m of the motor is finally obtained . the measured variables used for the estimation are thus made available independently of the control branch with which target values for the converter are provided by the control electronics . for safety reasons it should be possible to check the measured variables for plausibility . the comparison with the target torque can then be effected both in the torque monitor and in the control unit , and a two - channel switch - off into safe mode can be initiated . the voltage measurements can be checked in a switch - on sequence . in this case the converter can for example be controlled by the control electronics with a null pointer ( 000 , 111 ), so that no line - to - line voltages u 13 , u 23 are generated . in this case the null pointer 000 means for example that all phase lines 6 , 7 , 8 are short - circuited with the negative rail 3 , whereas in the case of the null pointer 111 all phase lines 6 , 7 and 8 are connected to the positive rail 4 . if no line - to - line voltages are now generated , no current will flow either , but the voltage measurement channels are controlled . it must be possible to identify this at the voltage measurement elements . it should also be possible to verify the measurement of the currents . this is simple to do , in that all three currents i 1 , i 2 and i 3 are measured . only in error - free operation is the summated current zero . otherwise a measurement or isolation error is present . the rotating field frequency ( frequency f of the rotating field ) is also simple to check . the rotating field frequency can in fact be calculated both from the measured currents i 1 , i 2 , i 3 as well as from the measured voltages u 1 , u 2 , u 3 , in that for example the temporal derivation of the angle is determined from the respective space vectors of the measured variables . an excessive deviation identified using the method outlined above between target and actual value or the overshoot of a maximum limit for the estimated torque value can be used in the calculation unit 9 for initiating further protection measures . a corresponding signal s to trigger a protection reaction can be provided by the calculation unit 9 ( see fig3 ). for example , a permanent - magnet - excited synchro - machine can be short - circuited in field weakening mode to achieve a torque - free wheel , or can be isolated from the converter . advantageously in accordance with the above principle a σ - δ output voltage measurement can thus be used to calculate the developed actual torque . the comparison to be performed very quickly in the calculation unit 9 ( e . g . logic module fpga ) with the target torque or with limit values that can be fixed can be used to respond very quickly to errors . this method works without software , resulting in a fast and reliable response , even if the software or controller exhibits a malfunction . determining speed via the electric frequency also makes it independent of the rotary transducer . in this way a very high level of safety can be achieved . while the invention has been illustrated and described in connection with currently preferred embodiments shown and described in detail , it is not intended to be limited to the details shown since various modifications and structural changes may be made without departing in any way from the spirit of the present invention . the embodiments were chosen and described in order to best explain the principles of the invention and practical application to thereby enable a person skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated .