Improved rotor winding current control for an automobile alternator, starter or alternator/starter has provision for short-term operation with over-excitation

摘要

The alternator rotor winding (LEXC) is connected at one end, through a diode inactive in normal operation, to the supply positive, and, via a switching transistor (T1), to earth. A positive connection is made to the other end of the winding via another such transistor (T2), an inductance (LB) and another diode (DB). In addition, a capacitance (CB) is connected between the winding here and earth. In alternator mode, the excitation is controlled by a low side PWM regulator applied to the first transistor (T1) (the other (T2) is conducting). A further, MOSFET, transistor (TB), shunted by a diode (which is incorporated in the MOSFET), and connected on the rotor winding side of the inductance, is non-conducting at this stage. In the starter mode, operation begins at switch-on with a phase of over-excitation PWM-controlled by the MOSFET, the other transistors both conducting. The rotor voltage in this phase is e.g. 18V instead of the normal 8V. Protection against overheating is obtained e.g. by monitoring the temperature at a machine hot-spot. Normal excitation returns after a brief interval, then when the engine starts exciting voltage is reversed. Rapid demagnetization is achieved by opening all 3 switching points (T1,T2,TB), current (DMG) circulating via the MOSFET and line-connected diodes.