Machines of this type are known from the state of the art, and in particular from patent application WO-A-1,169,762. In this document, the alternator-starter is a reversible alternator which can be used in a motor vehicle with a thermal engine, firstly as an electric generator, and secondly as an electric motor, in particular for the starting of the thermal engine of the vehicle.
An alternator-starter of this type is used in particular in so-called systems for stopping and re-starting of the thermal engine, hereinafter designated as the “system for stopping-re-starting of the thermal engine”, otherwise known by the term “stop and start”.
These systems make it possible to stop the thermal engine during the phases of stopping of the vehicle, and to start or re-start it when there is the occurrence of a stimulus such as the first demand by the driver, or another criterion which is aimed at generation of fuel savings.
In order to be able to function in a reversible manner, the alternator-starter is connected in rotation to the crankshaft by mean of a flexible transmission unit, for example by a drive belt.
During the use of the alternator-starter in a system for stopping-re-starting of the thermal engine, it is preferable for the response time between the stimulus and the actual re-starting of the thermal engine to be as brief as possible, for example less than 350 ms.
The alternator-starter must in particular produce a starting torque which is greater than a resistant torque of the thermal engine. The resistant torque of the thermal engine is equal to the sum of a starting torque which takes into account the inertia of the mobile elements of the thermal engine, as well as the friction forces which oppose the movement of the latter, and a moment of compression of the fluid which is contained in the cylinders of the thermal engine.
For this purpose, it is known to carry out a pre-flux step, during which the rotor of the alternator-starter is supplied with electricity, without the stator being supplied.
During this pre-flux step, the intensity of the electric current which circulates in the rotor winding increases progressively as a result of the inductance of the stator winding. In a known manner, the intensity of the magnetic rotor field is proportional to the intensity of the electric current.
When the magnetic rotor field reaches a predetermined stage corresponding to a steady state, in general for a duration of the pre-flux step of over 150 ms, the stator is supplied in a following step in order to permit the rotation of the rotor. Thus, the alternator-starter produces a very high starting torque, which makes it possible to re-start the thermal engine very quickly.
This manner of proceeding is entirely satisfactory when the thermal engine which is stopped opposes low resistant torque. This is the case in particular for thermal engines which have resistant torque up to a peak value of approximately 170 Nm. It will be noted that the aforementioned peak value is the sum of the torque caused by the gas, plus the inertia torque, plus the torque caused by the friction.
It has been found however that this method could not be applied as such to thermal engines which have greater resistant torque. In this configuration, when rotation of the rotor begins, the taut section of the drive belt between the alternator-starter and the crankshaft is subjected to sudden tension which may damage it.
In order to solve this problem, it has already been proposed to replace the standard drive belt by a stronger drive belt. However, this solution is very costly.