A production unit of metal strips comprises various pieces of equipment, notably for rolling, dressing, pickling or other treatments.  At the exit of a given section of the unit, the strip must, generally, be reeled onto a coil, to facilitate its transportation to another section or any other location in view of its use. 
To this end, a coiler comprising at least one mandrel brought into rotation around its axis and on which the strip is reeled to form a coil, is used.  The mandrel is provided, according to well-known embodiments, with retractable expansion means, enabling the coiler to be removed upon completion of the reeling sequence, and is generally combined to appended devices such as, for instance, a belt winder facilitating the beginning of the reeling sequence, as well as means for removing the reeled coils. 
In certain improved units, especially those working in continuous flow, such as coupled rolling mills or lines, it is necessary to have outlet equipment especially suited to large production capacities. 
Particularly, as soon as a coil has reached the desired sizes, the strip must be sheared off and engaged immediately onto another coiler.  A strip accumulator permits the rolling process to continue during the replacement of the coiler, but the time necessary for this operation must obviously, remain as short as possible. 
Usually, such an installation comprises thus two independent coilers, each having a belt reeler and a carriage for removing the coils.  The passage of the product, after shearing, from one coiler to the other, is ensured by a junction system.  Such a lay-out is quite cumbersome and expensive, notably further to the infrastructure cost called for. 
In order to reduce the space requirements of the coiling unit, it has already been suggested to use, instead of the two independent coilers, a single apparatus comprising two reeling mandrels mounted on a drum-shaped rotary frame, mounted rotatably around a center axis, each mandrel being fastened in cantilever fashion on a driving shaft mounted rotatably on the drum around an axis parallel to the center axis of the latter. 
It is thus possible to index successively one mandrel or the other in a position to reel the strip, by simple rotation of the drum around its axis (DEA-3346219, JP-A-61-124478). 
Such a coiler, called "turntable coiler", calls for a single reeler and a single removing carriage for the coils, which reduces significantly the infrastructure cost.  Moreover, as the engagement geometry of the strips is constant, the junction device is not necessary any longer. 
However, to enable positioning of either of the mandrels and of the separate control unit for the rotation of the mandrel, being in reeling position, the existing installations of this type exhibit quite complex mechanisms and require numerous gears, transmission shafts, bearings, clutches, claw couplings, and the like. 
In order to simplify the mechanical layouts, it has been suggested, in JP-A-61.124478, to connect to each mandrel an electric motor, comprising a rotor turning inside a stator, the latter being fixed on the drum and the rotor turning together with the driving shaft of the mandrel. 
Thus, each driving motor of a mandrel needs simply to be connected to an electrical power supply, by a power supply circuit which can advantageously pass through the center rotation shaft of the drum, so that selective control for the rotation of one mandrel or the other is performed easily. 
Such a layout enables the withdrawal of the driving mechanisms of both mandrels while replacing them with autonomous motors.  However, the whole assembly remains rather heavy and cumbersome and overheating of the electric motors located inside the drum may be expected.