[ 0026 ] fig3 shows a control schematic of one embodiment of the present invention . the inner control loop of the prior art remains in place , so that as the power train moves into braking , pid controller 23 increases the motoring duty of the electrical machine 12 in order to keep the bus voltage at or about vs 1 . the engine , or its mechanical loads , absorbs power , with its speed rising as required in order to achieve power balance . a speed limiter 30 is connected to monitor the engine speed and to provide an output to the controller 23 to cause the generator 12 to clamp its mechanical input power to the engine 10 at a safe limit . when the engine speed reaches the maximum safe operating speed , the limit function 30 prevents the controller 23 from demanding more torque from the machine 12 acting as the motor , and a second pid controller 26 ′ is enabled in an outer control loop . this second controller 26 ′ is responsive to the outputs from both the limit function 30 and the comparator 24 . as the braking power increases further and the voltage bus tries to rise , the controller 26 ′ progressively brings in the dump 21 while maintaining the bus at or around vs 1 . in this embodiment , the control of the power switch 27 is by pid control using pwm based on feedback of the bus voltage , according to embodiments of the invention , allowing the duty cycle to be gradually increased in line with rising braking power . other control algorithms optionally are used for any of the controllers referred to as will be appreciated by the skilled person . note that the control loop using vs 2 and comparator 25 is not strictly necessary , since the voltage of the bus is now held at vs 1 throughout the braking cycle . however it is a contingency measure , so that if there were a failure in the motor controller 23 or the emu 22 , the consequent rise in bus voltage would be held at the higher level of vs 2 . it would also come into play if , for example , there were a sudden removal of load while the power train was motoring and the emu could not shut the engine down fast enough . the technique illustrated in fig3 is a significant improvement over the prior art . in this embodiment the strategy is to saturate or limit the inner closed loop control around the machine 12 when the speed of the engine 10 becomes excessive and to enable the outer control loop . this outer loop is responsive to both engine speed and bus voltage and dissipates power accordingly through the dump resistor 21 . however , it sometimes requires considerable care to arrange the parameters of the controllers 23 and 26 ′ to achieve a smooth transfer of control when the action of limiter 30 causes the controller 23 to saturate . this is particularly the case if integral terms are used in the controllers . a second embodiment of the invention is shown schematically in fig4 . in this arrangement , the inner control loop of the controller 23 and the comparator 24 , based on control of the output of the motoring machine 12 , remains active throughout the entire braking cycle . as the braking power being fed onto the bus 13 increases , this loop increases the demand on the electrical machine 12 , which in turn drives up the speed of the engine to achieve power balance in the system . as the engine speed reaches its limit , an outer control loop , comprising pid controller 54 and comparator 52 , becomes active and sets the duty of the power switch 27 to dump the power which cannot be absorbed by the engine . it will be noted that there is now no handover of control from one loop to another : the inner loop remains active . as before , the safety control loop comprising controller 26 ′ and 25 is optionally left in place to operate the power switch 27 if there is a failure in another part of the system . in this embodiment the enabled outer control loop actuates the switch 27 to dump power from the voltage bus , allowing the inner control loop to react to the new voltage accordingly . if the bus voltage reduces below that causing maximum speed of the engine 10 , the inner controller simply carries on while the outer control loop reduces the duty of power switch 27 to zero . this system controls the voltage bus 13 around the same set point vs 1 regardless of the amount of braking power being produced by the traction motors 14 . this system now exhibits several further advantages : the auxiliary loads are supplied at a stable voltage , generally eliminating any voltage transients ; any mechanical loads on the engine are supplied with power from the traction motors whenever it is available , thus economizing on fuel ; the braking effort of the engine is used to its maximum , thus making use of the existing cooling system ( s ) on the engine , and the duty of the energy dump is minimized , thus reducing the cooling requirements and saving in cost and space . in practice , the implementation of the controllers 23 , 26 ′ and 54 and associated functions , such as the comparators 24 , 25 and 52 , is conveniently implemented in a microprocessor with analog interfaces to the transducers for speed and voltage , as will be well understood by those skilled in the art . the control algorithms used by the controllers can be any of the appropriately known control laws . the skilled person will appreciate that variation of the disclosed arrangements is possible without departing from the invention . for example , embodiments of the invention are described in the context of vehicle traction . it will be readily appreciated by those of ordinary skill in the art that the load driven variously by the electrical motors fed by the generator and the prime mover could equally well be some other driven load of a moving or static nature . accordingly , the above description of is made by way of example and not for the purposes of limitation . it will be clear to the skilled person that minor modifications can be made to the arrangements without significant changes to the operation described above .