System and method for controlling load dump voltage of a synchronous machine

Abstract

In an automobile electrical system having an alternator drivingly connected to a vehicle engine the alternator has a polyphase output winding producing a polyphase output in the form of a polyphase alternator voltage for each phase. A load dump apparatus and method for varying the output of the alternator employs a bridge coupled to the polyphase output winding and having a pair of direct voltage output terminals. The bridge has lower and upper switchable elements for each corresponding phase winding, the switchable elements are forward biased and connected in series across the output of the bridge and have a common or neutral input note connected to the corresponding phase winding. Each switchable element is controllable in accordance with a respective conduction signal applied thereto. A controller produces a plurality of conduction signals, which are applied to the switchable elements for controlling the conduction thereof for controlling the conduction timing for each of the lower switchable elements to render them conductive when the output voltage of the corresponding phase winding is above a selected over-voltage limit.

1 . For use in an automobile electrical system having an alternator drivingly connected to a vehicle engine, the alternator having a polyphase output winding producing a polyphase output in the form of a polyphase alternator voltage for each phase, a load dump apparatus for varying the output of the alternator comprising: 
 a control bridge adapted to be coupled to said polyphase output winding and having a pair of direct voltage output terminals, said control bridge including a first switchable element and a second switchable element for each corresponding phase winding, said first switchable element comprising at least one of a rectifier, a silicon control rectifier, a field effect transistor and a TRIAC, and said first and second switchable elements being connected in series across the output of the bridge and having a common input note adapted to be connected to the corresponding phase winding, each controllable in accordance with a respective conduction signal applied thereto,    a controller for generating a plurality of conduction signals, said conduction signals being applied to said first and second switchable elements for controlling the conduction thereof under normal operating condition and for controlling the switchable elements under an over voltage condition; and wherein the normal operating condition occurs when the output voltage of each phase winding is up to a selected limit and the over-voltage condition occurs when the output voltage of any phase winding exceeds 14 volts, the conduction of the first switching element for a phase being on for a time when the corresponding phase voltage is above 14 volts.    
   
   
       2 . An apparatus according to  claim 1 , wherein the first switchable elements comprise at least one of a rectifier, a silicon control rectifier, a field effect transistor and a TRIAC.  
   
   
       3 . An apparatus according to  claim 1 , further including an over-voltage sensing device for sensing an alternating voltage in each phase winding of the polyphase output winding and producing a voltage level signal therefrom.  
   
   
       4 . An apparatus according to  claim 3  including means comparing the phase voltage in each phase winding with a selected voltage limit to generate an over voltage signal, said controller responsive to the signal for adjusting the conduction timing of the conduction signals so as to reduce the phase voltage to a desired level.  
   
   
       5 . An apparatus according to  claim 1 , wherein the first and second switchable elements include rectifiers.  
   
   
       6 . The system according to  claim 5  when the rectifiers comprise diodes being forward biased relative to the output coupled between the neutral point and ground.  
   
   
       7 . The system of  claim 1 , further including at least one of a zener diode and varistor coupled to the bridge output.  
   
   
       8 . The system of  claim 1 , wherein the stator winding output is a three phase stator winding.  
   
   
       9 . The system of  claim 1 , wherein conduction control signals comprise gate pulses for respective switchable elements.  
   
   
       10 . The system of  claim 1  wherein the over-voltage condition occurs when at least one of the phase voltages exceeds 14 volts.  
   
   
       11 . The system of  claim 1 , wherein the computer includes a comparator for comparing the output voltage with a reference indicative of an over-voltage condition.  
   
   
       12 . The system of  claim 1  wherein the switchable elements for each phase comprise first and second switchable elements comprise forward biased diodes.  
   
   
       13 . The system of  claim 12 , wherein the first switchable elements comprise a rectifier and a parallel coupled normally open switch.  
   
   
       14 . The system of  claim 1 , wherein the alternator comprises at least one of a synchronous alternator and a permanent magnet alternator.  
   
   
       15 . In an electrical system having an alternator having a field winding, a polyphase output winding for producing a polyphase output in the form of a polyphase alternating voltage and a bridge having a direct current output, including a first and second series connected switchable elements for each phase coupled across the output and coupled to each corresponding phase winding at a neutral point therebetween, a method for varying the output of the alternator comprising the steps of: 
 generating a plurality of conduction signals for each corresponding phase of the polyphase alternating voltage;    sensing the voltage in each phase winding of the polyphase output winding;    applying the conduction signals to the switchable elements of the bridge;    comparing the output voltage with a selected voltage limit; and    adjusting the conduction timing of the conduction signals for only the first switchable elements and the current in the field winding so as to reduce the voltage for the phase below the voltage limit.    
   
   
       16 . The method of  claim 15 , wherein the alternator has a field winding and further comprising adjusting the current in the field winding to regulate the output voltage of the alternator.  
   
   
       17 . The method of  claim 15  wherein the adjusting step further comprises limiting the stator current to control the minimum output voltage of the alternator to a percentage of the maximum allowable output voltage during an over-voltage condition.  
   
   
       18 . The method of  claim 15  wherein limiting the minimum output voltage comprises maintaining a minimum voltage sufficient to prevent system voltage collapse.