In a manner which in itself is known, unlike an electrical machine with permanent magnets, a rotary electrical machine with excitation can produce engine torque, or supply electric energy, only when an excitation current passes through its inductor.
A common type of rotary electrical machine with excitation, which is widely used in the motor vehicle industry for the alternator and starter functions, comprises a rotary inductor and a stator with a plurality of windings.
When the machine is functioning as an alternator, the current which is generated in the stator windings by the rotating inductor is rectified such as to supply direct voltage to the vehicle battery.
This voltage depends on the speed of rotation of the inductor, the connected load and the excitation current.
For motor vehicle applications, the output voltage must be regulated such as to remain constant irrespective of the speed of rotation of the alternator and irrespective of the battery load.
For this purpose, the output voltage is measured and compared continually with a set value by a control device which controls the excitation current such as to cancel out any difference.
The company VALEO EQUIPEMENTS ELECTRIQUES MOTEUR has already proposed to carry out this control on the basis of measurements by sampling by means of digital techniques, which provide substantial advantages in comparison with the conventional analogue methods, in particular in its European patents EP 0 481 862 and EP 0 802.606.
In the design of a modern control device, the subjection of the output voltage to a set value is based on the theorisation of a proportional (P) or proportional integral (PI) control loop.
The creation of this loop by the corresponding algorithms makes it possible to design regulators with programmable functionalities which can adapt more easily to the specifications of the motor vehicle manufacturers, such as the one described in the article “An high-voltage CMOS voltage regulator for automotive alternators with programmable functionalities and full reverse polarity capability”, P. Chassard, L. Labiste, P. Tisserand et al, Design, Automation and Test in Europe Conference and Exhibition (DATE), 2010, EDAA.
In the motor vehicle industry, a plurality of characteristics of the alternator make it possible to evaluate the performance of the alternator, in particular the following characteristics:
maximum current supplied according to the speed of rotation for a given regulation voltage;
voltage regulation, i.e. the aptitude of the alternator to generate a voltage corresponding to the set value according to the loads connected;
stability of the system, i.e. the phase margin and the gain margin of the system comprising the regulator, the alternator, a battery and loads.
However the inventive body has observed a decrease in the phase and gain margin in systems comprising high-power alternators.
This decrease can become critical. In fact poor voltage regulation arises which leads to an “oscillating” voltage, or worse still, an alternator output voltage which can no longer be controlled by the regulator. This lack of control can lead for example to excess voltage; reference is then made to an unstable controlled system.
According to arbitrary criteria, the phase margin of the transfer function in an open loop (FTBO) of a controlled system must be more than 45°, and the gain margin must be more than 13 dB in order to consider the system as stable.
However, mostly, for a system which uses a very high-power alternator (which supplies a current of 300 A for example), the respective criteria of the phase margin (>45°) and gain margin (>13 dB) can no longer be complied with for a performance level of the voltage regulation which is identical to that of an alternator with lower power (which for example supplies a current of 100 A). The system is liable to be unstable.
A solution which is well-known in order to compensate for the increase in alternator gain is to create a decrease in the regulator gain.
Unfortunately, in the case of a control loop of the proportional type, the performance of the voltage regulation is affected by the change of the regulator gain.
In fact, although the criteria of stability are improved, the drop in voltage of the voltage regulation according to the current output is then increased, and can reach 600 mV, which is considered to be a deterioration of the performance of the voltage regulation in comparison with a voltage drop of approximately 200 mV for an alternator which outputs 100 A, for example.
A known solution for limiting the voltage drop of the regulation when the regulator gain is decreased is the use of an integral part in the control loop.
The voltage drop can thus be brought to a value close to 0 mV; however, it is known that the phase margin and the gain margin are slightly affected.
There is therefore a need for a solution which would make it possible to increase the phase margin and the gain margin of the transfer function in an open loop of a voltage regulation system associated with a high-power alternator, whilst maintaining the performance of the voltage regulation.