Abstract:
The present invention provides an apparatus for reducing the allowed output current from an electrical device such as a high output capacity vehicle alternator in response to a sensed over temperature condition. In one embodiment, a thermistor is attached to the housing of an alternator to provide temperature sensing capability. As temperature exceeds a temperature set point established by a resistor bridge, op-amps, in combination with a power FET in series with the alternator field windings, act to reduce the allowed current output from the alternator in proportion to the extent the set point temperature has been exceeded. As the alternator temperature returns to below the set point, the output current restriction is reduced such that the allowed current flow is maximized without exceeding the temperature set point.

Description:
This application claims the benefit of U.S. Provisional Application No.: 60/127,029 filed Mar. 31, 1999. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to the field of motor vehicle alternators, and more particularly to the field of temperature based controls of the output of high output capability alternators such as those found in large city busses and coaches. 
     BACKGROUND INFORMATION 
     Vehicle alternators with high output capability are used in large vehicles such as trucks, busses and passenger coaches. The alternator provides current for the vehicle which is used to charge the vehicle&#39;s battery or to run various auxiliary systems. When the alternator is operating as a generator of electricity, some amount of heat is also generated by the alternator. As the current demand on the alternator is increased, the alternator will attempt to generate more electricity, thereby increasing the heat generated. 
     Under conventional circumstances, the alternator may be cooled by circulating oil through the alternator housing and around the internal components of the alternator. In a basic system, cool oil is pumped into the alternator. The heat generated by the internal components of the alternator is then transferred to the comparatively cooler oil, thus cooling the alternator components and heating the oil. The heated oil is then conveyed out of the alternator to a heat exchanger where the oil is cooled so that it can be recirculated to the alternator for further cooling. 
     In the above system, there are two separate heat exchanges occurring. In the first heat exchange, heat is transferred from the alternator to the oil. In the second heat exchange, heat is transferred from the oil to the atmosphere. In heat exchanger systems such as this, the amount of heat transferred is highly dependent on the difference in temperature between the component or fluid from which heat is being removed and the component or fluid to which heat is being moved. For vehicle based systems, the heat transferred from the oil is ultimately transferred to the air surrounding the vehicle. Consequently, the amount of heat transferred from the oil to the air, and then from the alternator to the oil, is influenced by the temperature of the ambient air. Thus, as the ambient air temperature increases, the heat transfer capacity of the cooling system decreases. 
     A design problem that must be addressed in vehicle alternators is that as the ambient temperature increases the use of generated electricity for some components, such as air conditioners used for the comfort of passengers, also increases. In response to this increased demand for electricity, the alternator produces more electricity and necessarily generates more heat. Consequently, as the need for heat removal from the generator increases, the system&#39;s capacity for heat removal is decreased. 
     This type of system is subject to several potential failures resulting in elevated temperature of the alternator, possibly to the extent that the design temperature of the alternator is exceeded. While catastrophic failures resulting in over temperature conditions, such as a pump seizure or loss of electrical power, can occur at any time, a reduced capacity for heat removal can exacerbate otherwise nominal problems resulting in an over temperature condition. For example, the oil pump performance could become degraded, thereby limiting the amount of oil available for cooling the alternator. Additionally, the oil system could develop a leak or flow blockage restricting the amount of oil circulated through the alternator. 
     In the event that cooling oil flow is restricted or interrupted, the amount of heat conducted out of the alternator is reduced or eliminated. Consequently, the temperature of the alternator will increase. Should the reduced oil flow occur during periods of high current demand, the alternator temperature may exceed its maximum design operating temperature. Operating at a temperature in excess of design operating temperature can lead to stressing components beyond their design limits resulting in component failure. Depending on which component fails, the high temperature could result in an oil leak, reduced alternator output or even complete failure of the unit. Consequently, a vehicle may suffer catastrophic failure, resulting in passenger discomfort from loss of air conditioning or even stranding the passengers by complete shutdown of the vehicle. Additionally, restricted oil flow could result from a clogged oil filter which could otherwise be quickly and easily replaced at a minimal cost. The damage resulting from operating the alternator at high temperature, however, could necessitate costly and time consuming component replacement or repair. 
     It is readily apparent from the foregoing discussion that the severity of an alternator failure can be assessed according to two factors. The first factor is the loss of electric generating capability while the second factor is the cost of repairs. Therefore, it is desirable to provide a control system for high output capacity vehicle alternators, which minimizes operation at elevated temperatures, while avoiding complete loss of electricity generating capability and damage to the equipment. 
     Various devices have been used in other arts to avoid the extreme damage caused by operating electric equipment at elevated temperatures. One such device is disclosed in U.S. Pat. No. 5,546,262 issued to Baurand et al. Baurand et al. discloses a device which uses a thermistor to monitor the operating temperature of a load. A thermistor is simply a resistor which changes resistance as its temperature changes. The resulting voltage drop across the resistor is then used, typically by comparing the voltage to a reference voltage, to activate other devices. A device according to Baurand et al., in response to a high temperature condition of a load monitored by a thermistor, can interrupt power to the load thereby avoiding the catastrophic damage which could be caused by operating the load at elevated temperature. 
     Although the device of Baurand et al. is useful in many applications, it is of limited benefit when used to protect high capacity output alternators. As noted above, an essential factor in assessing the severity of an alternator failure is the loss of generating capability. While Baurand et al. does ameliorate the potential for damage to a piece of equipment, it does so by shutting the equipment off. This device would disable the vehicle in an over temperature condition, stranding the passengers. This is a severe shortcoming of Baurand et al. if used in conjunction with a vehicle alternator. 
     Baurand et al. also discloses the use of a bimetallic strip as a means for protecting an electrical load. A bimetallic strip is merely two smaller strips of metal which are joined into a single strip. A bimetallic strip operates under the principle that as the amount of current passing through the strip increases, the bimetallic strip heats up, in the same manner as any other resistor. The difference, however, is that the two metals used in a bimetallic strip expand at different rates as they are heated. Thus, the strip begins to curl as more current passes through it. At a designed current/temperature level, the strip will curl such that the electrical circuit is broken and current is no longer supplied to the load. After some amount of time, the strip cools down and returns to its original shape, thus closing the electrical circuit and current can once more be passed to the load. 
     The use of a bimetallic strip does mitigate damage due to operating a piece of equipment when too much current is being demanded, however, like the thermistor device of Baurand et al., a bi-metallic strip is not appropriate for use in a high capacity output alternator for a vehicle. The use of bi-metallic strips would sacrifice the operation of the alternator as a consequence of protecting the alternator from damage. 
     There is a significant need, therefore, to provide a control device which protects an electrical piece of equipment, such as an alternator, from over temperature conditions. Preferably, the device should not totally de-energize the electrical equipment, rather it should gradually decrease the current available to the equipment. Upon easing of the over temperature condition, the device should allow resumption of full capacity operation. The device should not require penetrations be made through the alternator housing to minimize the potential for oil leaks. The device should be easy to install on new equipment, it must also be easy to retro-fit onto existing equipment, the device should be inexpensive, comprise a minimum number of components, be relatively small, not require long leads, be compatible with other protective devices, not be subject to failure in extreme operating environments and be of simple construction. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides a control device which protects an electrical piece of equipment, such as an alternator, from over temperature conditions. Advantageously, the present invention gradually lowers the allowed current output from an alternator in providing over-temperature protection so that there is not a total interruption of current output. Upon easing of the over-temperature condition, the invention allows resumption of full capacity current production. The invention does not require penetrations be made through the alternator housing in retrofitting or installation with new equipment, and thus, minimizes the potential for oil leaks. The invention is easy to install on new equipment, as well as easy to retro-fit onto existing equipment. Further, the invention is inexpensive, comprises a minimum number of components, is relatively small, does not require long leads, and is compatible with other protective devices, and is not subject to failure in extreme operating environments while being of simple construction. 
     In accordance with the present invention, a sensing means monitors the temperature of the alternator and produces an output signal representative of the sensed temperature. The output signal is passed to a variable current control device which is used to control the allowed output current from the alternator. A temperature sensor is mounted on the alternator housing to sense the temperature of the alternator. In response to an over temperature condition, a power field effect transistor is alternately energized and de-energized thus interrupting the current flow through the field winding of an alternator, thus reducing the current generated by an alternator and reducing the heat generated by the alternator. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a conventional prior art remotely regulated alternator. 
     FIG. 2 is a block diagram of a thermal protection device for a remotely regulated vehicle alternator in accordance with the present invention. 
     FIG. 3 is a schematic circuit diagram of an embodiment of the thermal protection device of the present invention. 
     FIG. 4 is an end view of an alternator showing the mounting of an embodiment of a thermal protection device according to the present invention. 
     FIG. 5 is a graph of test results for an embodiment of a thermal protection device mounted to an alternator in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, a simplified block diagram of a conventional prior art alternator is shown. Alternator  4  generates electrical current on output conductor  5  connected to load  2 . Conductor  3  provides a path to ground from field winding L 1  of alternator  4 . A voltage regulator  6  may be connected to conductor  1  to control the current in the field winding L 1  of alternator  4  to control the voltage of the output of alternator  4  to protect any voltage sensitive components of load  2 . A cooling system  8  may optionally be provided to cool alternator  4  as alternator  4  creates internal heat as a result of generating electrical current. Cooling system  8  may comprise an oil cooling system that pumps cool oil into heat exchanger  10  through input conduit  11  and heated oil out through output conduit  12 . Other alternators may be air cooled without an oil cooling system. 
     Referring now to FIG. 2, a simplified block diagram of thermal protection device  20  for a remotely regulated alternator in accordance with the present invention is shown with the alternator of FIG.  1 . Operating power for thermal protection device  20  is supplied through conductor  21  connected between conductor  1  and power conditioner  24 . Power conditioner  24  is operably connected to temperature sensing circuit  26  by conductor  23 , and current regulator  28  is operably connected to temperature sensing circuit  26  by conductor  25 . Temperature sensor  29 , mounted to alternator  4 , is operably connected by conductor  27  to temperature sensing circuit  26 , to provide a signal indicative of the temperature of alternator  4 . Current from alternator  4  to load  2  is regulated by field current passing through current regulator  28  under the control of temperature sensing circuit  26 . As the sensed temperature of alternator  4  exceeds a predetermined threshold, the amount of current allowed to pass through current regulator  28  is decreased, thus lessening the field current which reduces the generated current, and therefore the heat generated by alternator  4  is reduced. 
     Temperature sensor  29 ′ can be placed at the output of heat exchanger  10  to sense the temperature of the heated oil thereby measuring temperature representative of the internal temperature of alternator  4  depending on the flow rate and the input temperature of the oil in alternators having an oil cooling system. It is also possible to place temperature sensor  29 ″ inside alternator  4  on, for example, the stator laminated stack or other component of alternator  4 , to measure the temperature of the stator laminated stack or other component. It is also possible to place the temperature sensor on the exterior of the alternator at a location where the temperature will be a direct representation of the internal temperature. The placement of the sensor will depend on a number of variables. For example, in the case of certain alternators, the laminated stack temperature is the limiting temperature, but the laminated stack is imbedded within the alternator housing. In that case, it may be desired to place the sensor on the exterior of the alternator housing at a temperature representative location. In alternators wherein the limiting component is not directly contacting the alternator housing, it may be desired to measure the oil outlet temperature. These variations and others are within the scope of the present invention. 
     Referring now to FIG. 2, the general operation of an embodiment of the present invention is described. In operation, alternator  4  is providing current to load  2  through output conductor  5 . The output current of alternator  4  is governed by voltage regulator  6  in the conventional manner. Heat generated by alternator  4  in supplying current to load  2  is removed by cooling system  8  through heat exchanger  10 . Under normal conditions, the heat removal capacity of cooling system  8  is such that when supplying maximum output current, the temperature of alternator  4  is within its maximum temperature limit. 
     The temperature of alternator  4  is sensed by temperature sensor  29  which conveys a corresponding signal to temperature sensing circuit  26  through conductor  27 . Temperature sensing circuit  26  compares the sensed temperature to a predetermined threshold temperature. If the sensed temperature exceeds the predetermined threshold temperature, current regulator  28  is controlled by temperature sensing circuit  26  to limit the field current and thus the output current allowed to be supplied by alternator  4  to load  2  to an amount less than the maximum output current. Should the temperature of alternator  4  continue to increase, temperature sensing circuit  26  will sense this condition and current regulator  28  will further limit the allowed output current of alternator  4 . 
     As the current output from alternator  4  is decreased, the heat generated by alternator  4  will also decrease. Therefore, at a current level less than the maximum output level, the temperature of alternator  4  will return to its maximum temperature limit as the heat generated by alternator  4  is removed by cooling system  8 . Thus, current regulator  28  effectively establishes a first restricted current output based upon the sensed temperature of alternator  4 . 
     At some point, either the current demanded by load  2  will drop below the first restricted current output level established by current regulator  28  or the heat removal capacity of cooling system  8  will increase. In either event, the temperature of alternator  4  will drop below the maximum temperature limit as the heat removed by cooling system  8  exceeds the heat generated by alternator  4 . This drop in temperature is sensed by temperature sensing circuit  26  and current regulator  28  is controlled by temperature sensing circuit  26  to allow more current through the winding L 1  thereby allowing more current to be supplied to load  2 . If the drop in alternator temperature was due to increased cooling capacity, and load  2  has a demand greater than the first restricted current level, more current will be produced by alternator  4  and supplied to load  2 . The additional current being generated produces more heat, driving the temperature of alternator  4  back toward its temperature limit. Thus, a new equilibrium will be established based upon the increased cooling capacity. The current level at this new equilibrium may be at the maximum current level or at a level intermediate the maximum current level and the first restricted current level. 
     Alternatively, if the drop in temperature is the result of a lessened current demand from load  2 , the cooling capacity of cooling system  8  will exceed the heat generated by alternator  4  and temperature will return to a level below the maximum temperature limit. The lower temperature will be sensed by temperature sensing circuit  26  and current regulator  28  will return the allowed output current of alternator  4  to its maximum level. 
     A more detailed circuit diagram of a thermal protection device  300  according to the present invention is shown in FIG.  3 . Power for thermal protection device  300  is supplied by voltage regulator  6  as it supplies voltage to the field windings L 1  of alternator  305  (shown in dotted lines). Connected across field outputs  410  and  415  is diode  308  which protects thermal protection device  300  from inductive surges, produced as power field effect transistor (FET)  356  is cycled, by providing an alternate current path. 
     Alternator field output  415  is connected via conductor  309  to the series combination of resistor  311  and diode  312 . Capacitor  310  is connected between the output of diode  312  and ground. Diode  312  prevents discharge of capacitor  310  when the field current is interrupted as will be discussed below. Capacitor  310  filters out the switching transients of the voltage and provides a stable power supply through resistor  313  to the other components of thermal protection device  300  at connection  314 . 
     Capacitor  380  is connected between connection  314  and ground to provide a filter for high frequencies. Zener diode  385  is located between connection  314  and ground in parallel with other components of thermal protection device  300  to provide over voltage protection for the components powered from connection  314 . 
     Because thermal protection device  300  in this embodiment is powered by voltage across field windings L 1  of alternator  305 , the size of capacitor  310  is critical. The current through field windings L 1  may be intermittently perturbed by normal function of the voltage regulator as it intermittently interrupts the field voltage, however, certain components of thermal protection device  300  depend upon constant power for proper operation. Consequently, capacitor  310  must be sized to provide adequate power to thermal protection device  300  whenever current through windings L 1  is perturbed. 
     In this embodiment, the series of thermistor  315  and resistor  320  is in parallel with the series of resistors  321  and  322  to form a bridge circuit between connection  314  and ground. The resistance of resistors  320 ,  321  and  322  in conjunction with thermistor  315  may be selected so as to determine the sensed temperature at which temperature op-amp  330  output will be altered as is well known in the art, thus serving as a means for establishing a temperature set point. As the resistance of thermistor  315  changes, the relative voltage at points  318  and  316  change due to the changes in resistance of thermistor  315 . 
     Thermistor  315 , may be of the type negative thermal coefficient thermistor commercially available from Keystone Electronics Corp. of Astoria, N.Y., or other comparable thermistors which exhibits a lowered resistance as its temperature increases. Thermistor  315  is operatively positioned relative to alternator  305  such that the temperature of alternator  305  influences the temperature of thermistor  315  in a manner proportional to the internal temperature of alternator  305 . As described with respect to FIG. 2, thermistor  315  may be placed in a variety of positions relative alternator  305  or cooling system  8 , as long as temperature of alternator  305  can be sensed directly or indirectly. Therefore, as temperature of alternator  305  increases, temperature of thermistor  315  increases and resistance of thermistor  315  decreases. Thus, voltage at point  318  decreases. Those of skill in the art will recognize that alternate circuits may be used. By way of example, but not of limitation, thermistor  315  may be replaced with a resistor and resistor  320  may be replaced with a positive temperature coefficient thermistor. This and other variations being within the scope of the present invention. 
     Point  318  is electrically connected through resistor  319  to inverting input  325  of op-amp  330 . Op-amp  330  is connected as a one quadrant differential voltage to current converter and may be of the type M33172 commercially available from Motorola, Inc. of Austin, Tex. Point  316  is connected to non-inverting input  335  of op-amp  330  through resistor  317  to provide a first voltage threshold at non-inverting input  335  which is representative of the temperature set point. When the voltage at inverting input  325  drops below the voltage at non-inverting input  335 , the output of op-amp  330  which is initially at a low value increases to a higher value, effectively turning op-amp  330  “on.” Thermistor  315  thus provides a means for sensing temperature of alternator  305  so that when a predetermined temperature is reached, the voltage at point  318  will cause the output of op-amp  330  to increase. 
     The output of op-amp  330  is connected to inverting input  325  through connector  399  and resistor  326  and to non-inverting input  335  through connector  399  and resistor  371  in series with resistor  328 . Thus, when the output of op-amp  330  is high, the voltage difference between point  318  and point  316  is impressed across resistor  371  and current flows from op-amp  330  through connector  399  and resistor  371  to point  329 . Point  329  is connected to inverting input  340  of op-amp  345  via the parallel combination of diode  372  and resistor  373 . Capacitor  352  is connected between non-inverting input  340  and ground. Thus, when the output of op-amp  330  increases to a higher value and current flows through resistor  371 , capacitor  352  is charged and the voltage at inverting input  340  increases. 
     Non-inverting input  355  of op-amp  345  is connected through resistor  382  to connection  314  and through resistor  384  and connector  400  to the output of op-amp  345 . Thus, when the output of op-amp  345  is high, resistor  382  and resistor  384  are effectively connected in parallel and serve as the upper leg of a voltage divider with resistor  386  providing the lower leg. The voltage divider thus provides a predetermined first reference voltage at non-inverting input  355  when the output of op-amp  345  is high. Op-amp  345  is, in this embodiment, connected as a very high hysteresis comparator, and is of the type MC33172. 
     The output of op-amp  345  is connected to base gate  354  of power (FET)  356  through connector  400  and resistor  353 . Power FET  356  which may be of the type MTP3055E available from Motorola, Inc. of Austin, Tex., requires voltage to be present at base  354  in order to allow current to flow between field output  410  and ground through power FET  356 . Thus, when the output of op-amp  345  is high, voltage is present at base  354  through a voltage divider between op-amp  345  output and ground consisting of resistor  353  and resistor  392 . Consequently, thermal protection device  300  in this condition is “on” and does not restrict the flow of current through field windings L 1  of alternator  305 . In the event thermal protection device  300  is off or has been de-energized such that capacitor  310  is not charged sufficiently for proper operation of thermal protection device  300 , normal alternator operation is allowed since voltage may be provided at base  354  from conductor  309  through resistor  390  and resistor  353  when thermal protection device  300  is inactive. The operation of alternator  305  may also be controlled by means well known in the art such as with a voltage regulator (not shown in FIG.  3 ). 
     The output of op-amp  345  is also connected to point  316  through diode  394 . Diode  394  is oriented such that when the output of op-amp  345  is low, the voltage at connection  316 , and thus the voltage at non-inverting input  335  of op-amp  330 , is forced to be low. This forces the output of op-amp  330  to be low. Diode  394  thus operates as a means to turn off the current output of op-amp  330  when the output of op-amp  345  goes low or “off” by establishing a second voltage threshold. 
     Detailed operational description of thermal protection device  300  is made in reference to FIG.  3 . When the temperature of alternator  305  is below the set point established by thermistor  315 , resistor  320 , resistor  322 , and resistor  321 , the resistance of thermistor  315  forces the voltage at point  318  to be higher than the voltage at point  316 . Consequently, the voltage at inverting input  325  of op-amp  330  will be higher than the voltage at non-inverting input  335  of op-amp  330 . Thus, the output of op-amp  330  will be low and capacitor  352  will not be charged. 
     The voltage at inverting input  340  of op-amp  345  is therefore lower than the voltage at non-inverting input  355  of op-amp  345 , which, because the output of op-amp  345  is high, is determined by the voltage divider of resistor  386  and parallel resistors  382  and  384 . The voltage divider of resistor  386  and parallel resistors  382  and  384  thus acts as a means for providing a first reference voltage for op-amp  345 . Sufficient voltage is therefore present at base  354  of power FET  356  to cause power FET  356  to be turned on, allowing full field current and thus full output current flow, as determined by, for example, a voltage regulator or the demand on alternator  305 . 
     As the temperature of alternator  305  increases, the resistance of thermistor  315  decreases and the voltage at point  318  decreases. When the voltage at point  318  is lower than the voltage at point  316 , op-amp  330  output goes high and the voltage difference between point  316  and point  318  is impressed across resistor  371 . Consequently, current flows from the out put of op-amp  330  through resistor  371 . The current passes through diode  372  and charges capacitor  352 . Capacitor  352  continues to charge until the voltage at inverting input  340  of op-amp  345  exceeds the voltage at non-inverting input  355  of op-amp  345  which is set by the voltage divider consisting of resistor  386  and the parallel resistors  382  and  384 . When the voltage at inverting input  340  of op-amp  345  exceeds the voltage at non-inverting input  355  of op-amp  345 , the output of op-amp  345  goes low, and voltage at base  354  of power FET  356  is forced low, turning power FET  356  off and interrupting the flow of current through winding L 1  of alternator  305 . The polarity of winding L 1  is thus reversed, and diode  308  provides a current path as is well known in the art. 
     The low output of op-amp  345 , due to the polarity of diode  394 , also causes the voltage at point  316  to go low. Consequently, the voltage at point  318  is higher than the voltage at point  316 , and op-amp  330  output is flipped back to low. This allows capacitor  352  to begin discharging through resistor  373  and resistor  371  through the output of op-amp  330  which is low. The series resistors  373  and  371  thus provide a discharge path for capacitor  352  and an RC time constant for a constant discharge rate. 
     Additionally, the low output of op-amp  345  acts to “reconfigure” the voltage divider initially provided by resistor  386  and the parallel resistors  382  and  384 . Effectively, when the output of op-amp  345  is low, resistors  386  and  384  are in parallel and resistor  382  becomes the upper leg of a voltage divider. This causes the voltage at non-inverting input to drop, in this embodiment, to about one fourth of its previous value. Consequently, the output of op-amp  345  is held low until capacitor  352  discharges to a voltage one fourth of the voltage which initially caused the output of op-amp  255  to go low. Once capacitor  352  discharges sufficiently, the voltage at non-inverting input  355  will exceed the voltage at inverting input  340  and the output of op-amp  345  will once again go high. Thus, base  354  is biased so that current is once again allowed to flow through power FET  356  and, if the high temperature condition still exists, capacitor  352  begins to charge as described above. Under these conditions, the voltage divider comprising parallel resistors  386  and  384  and resistor  382  thus acts as a means for providing a second reference voltage for op-amp  345 . 
     Thermal protection device  300  thus provides for temporary interruption of the current flow through alternator  305 , the period of which is a function of the RC discharge rate of capacitor  352 . Additionally, the period of interruption in this embodiment is also a function of the extent to which the temperature of alternator  305  exceeds the desired temperature. Specifically, the voltage at point  318  is a function of the sensed alternator temperature by way of thermistor  315 . As the voltage at point  318  decreases in response to increasing temperature of alternator  305 , the voltage difference impressed across resistor  371  increases and the flow of current through resistor to capacitor  352  likewise increases according to the process above described. Thus, as alternator temperature increases, op-amp  330  produces a variable current output that allows more rapid charging of capacitor  352 , and thus a more rapid return to a condition of low output from op-amp  345 . As the temperature of alternator  305  increases above the established temperature limit, the on time of power FET  356  is proportionally decreased, resulting in less allowed current flow through power FET  356  as temperature increases. 
     Those of skill in the art will recognize that in accordance with the above described embodiment, the discharge time of capacitor  352  is relatively constant. Thus, the time during which current is not allowed to flow through power FET  356  is relatively constant. The time required to re-charge capacitor  352 , however, decreases as the sensed temperature increases. Consequently, the time that power FET  356  allows load current to flow is, in this embodiment, inversely related to the temperature of the over-temperature condition. 
     The above described embodiment further provides a means for limiting the allowed current reduction so as to maintain a minimum level of output current. As noted above, the charging time of capacitor  352  in this embodiment determines the time that current is allowed to flow through power FET  356 . Consequently, establishing a minimum time to charge capacitor  352  effectively establishes a minimum output current level. Once the minimum current level is reached, further increase in temperature of alternator  305  will not result in further current limitation by thermal protection device  300 . The minimum charge time in this embodiment is provided by the voltage at connection  314  which is limited by Zener diode  385 . This voltage limits the maximum voltage input, voltage VCC, to op-amp  335  which in turn controls the maximum voltage which may be provided at the output of op-amp  335  and impressed across resistor  371 . Consequently, the voltage at connection  314  limits the current which may flow through resistor  371  to charge capacitor  352 . 
     The effect of the above described embodiment is that of a pulse width modulating system. When power FET  356  is off and current demand exceeds the allowed generator output, battery voltage will drop below the voltage regulator set point. The voltage regulator responds by attempting to provide maximum field current to the alternator. Consequently, actual allowed current output is fully controlled by thermal protection device  300 . Thus, the cycling of power FET  356  serves as a means for variably controlling current output of alternator  305  by variably controlling the pulse width of the field current. 
     The determination of a temperature threshold for the initiation of current limiting activity as described above will depend on the particular alternator and its application. However, all alternators will have a limiting component which defines a maximum allowed temperature. For the purpose of example, alternator model 50DN commercially available from Delco Remy America, of Anderson, Ind. is briefly described. This alternator is manufactured, in part, by pressure fitting the stator laminated stack into the alternator housing. For considerations not herein discussed, the stator laminated stack comprises steel while the housing comprises aluminum. During normal operations this difference in composition is of no import. During high temperature operations however, the two metals expand at different rates, with the aluminum expanding more rapidly than the steel. Consequently, the fit of the stack in the housing becomes loose. The critical temperature for the Delco Remy 50DN, at which temperature the fit becomes unacceptably loose, has been determined through testing to be 140 degrees Celsius (C.). 
     Once the critical temperature for a particular alternator is understood, the determination of a desired temperature threshold may be accomplished according to practices commonly followed in the art. Additionally, the placement of the temperature sensing device may be determined. For example, the Delco Remy 50DN alternator is of a type which is oil cooled. Thus, directly sensing the stator laminated stack temperature would normally result in the difficulty and expense of creating penetrations through the housing of the alternator which would then result in increased susceptibility to oil leakage. The stator laminated stack temperature in the Delco Remy 50DN alternator, however, is directly related to the alternator housing temperature. The relationship between a critical component and a readily accessible location for monitoring temperature for a given alternator is readily determined by experimentation is well known to those of skill in the art. Once the relationship of the stator and housing temperature is established, stator temperature can be indirectly sensed by sensing the housing temperature, for example, via thermistor  315 . 
     Referring to FIG. 4, an embodiment of a thermal protection device  300  of the type illustrated in FIG. 3, used to indirectly monitor stator laminated stack temperature is shown. Thermal protection device  300  is electrically connected to the rectifier end of alternator  305  on housing  401  at field coil output terminals  410  and  415  by mounting conductors  430  and  436 . Housing bolts  420  and  425  physically attach mounting conductors  432  and  434  of device  300  to the housing  401  of alternator  305 . This configuration allows the voltage at field coil terminals  410  and  415  to be used to power thermal protection device  300  in the manner described with reference to FIG.  3 . Additionally, this mounting configuration allows for easy assembly during initial production or as a retrofit. No modifications or penetrations of housing  401  of alternator  305  are required to mount device  300 , and the need for long external leads, for either the thermistor or the power supply, is eliminated. 
     Mounting conductors  430 ,  432 ,  434  and  436  of thermal protection device  300  in this embodiment are made of tin plated brass. Those of skill in the art will recognize that the salient properties of the material for fashioning terminals  430 ,  432 ,  434  and  436  are those of thermal and electrical conduction. Consequently, other materials comprising those properties are within the scope of the present invention. Electrical components in the circuit board are capable of high temperature operation (125 degrees C.) and are soldered to an FR4 circuit board (or ceramic substrate). The device is potted in an appropriate material, such as epoxy, once built to provide environmental protection and resistance to mechanical vibration. 
     Because thermal protection device  300  generates some heat when in operation, it is important to thermally isolate the heat generating components from the thermistor to reduce the risk of an unrepresentative temperature indication by thermistor  315 . Also, power FET  356  is mounted in series to conductor  432 , which acts as a heat sink for thermal protection device  300 . Thermistor  315  is mounted in series with conductor  434  to housing bolt  425  of housing  401 . Housing  401  is electrically grounded at housing bolt  425 . As the temperature of housing  401  increases, conductor  434  conducts heat to thermistor  315  thereby varying its temperature. Thus, thermistor  315  indirectly measures the internal temperature of alternator  305  by sensing the temperature of housing  401  through conductor  434 . 
     EXAMPLE 
     A test of an embodiment of the present invention as described in reference to FIG.  3  and FIG. 4 was conducted using a Delco Remy 50DN alternator. As noted above, a ground strap  434  was mounted on housing bolt  425 , and due to the thermal properties of grounding strap  434 , the temperature being sensed by thermal protection device is reflected in the temperature of grounding strap  434 . A thermocouple was therefore connected to the ground strap to provide an indirect indication of the laminated stack temperature. Thermocouples were also attached to the alternator and cooling system to monitor the actual alternator stator lead temperature, laminated stack temperature and oil outlet temperature. Additionally, oil flow rate, and field voltage were monitored. As noted above, when the thermal protection device is controlling output current, field voltage will drop below the set point of the voltage regulator, thus providing an indication of alternator output current being controlled by the thermal protection device. 
     The alternator was loaded to run at full field at 2100 RPM with an output voltage of 27.5 volts. The coolant oil was initially heated to 93 degrees C. to facilitate establishment of steady state operating parameters and a flow rate of 1.4 GPM into the alternator was initiated. An overheating condition was created by decreasing oil flow rate to 0.3 GPM. The results of this test are shown in FIG.  5 . At time “A”, the oil flow rate is decreased from 1.4 GPM to 0.3 GPM. In this test, the predetermined temperature of the stator laminated stack for initiation of output limiting operation was established at 136 degrees C. This was determined to correlate to a housing bolt temperature of approximately 97 degrees C. Shortly after stator laminated stack temperature reached 136 degrees C., at time “B”, the thermal protection device, sensing the over temperature condition through ground strap temperature, effected the modulation of the power FET, limiting the field current. 
     As a result of the temperature lag between the laminated stack temperature and the ground strap conductor  434  temperature, some rise in temperature is observed in laminated stack temperature after initiation of current limiting operation (see time “B” to time “C”). At time “C”, the heat generated by the alternator has dropped below the heat removal capacity of the coolant oil which is most clearly seen in the stator lead temperature which drops rapidly. This is further reflected in the temperature of the laminated stack which drops below the temperature limit at time “D”. 
     The thermal protection device in this test was designed to sense temperature indirectly and a temperature lag exists between the laminated stack temperature and the ground strap, where the temperature is sensed. The resultant lag in sensing the removal of the over temperature condition results in the output current being initially limited slightly more than necessary for a given over temperature condition. Consequently, the field current is dropped lower than its steady state value for a given oil flow rate. Once the temperatures within the alternator fall below the critical temperature, indicating that the alternator is once more operating within design temperatures, the thermal protection device gradually allows field current to increase to a new steady state value. This is shown at time “D” where the voltage begins to increase, resulting in alternator temperatures also increasing. 
     At time “E”, before the system returned to a new reduced output steady state condition, the oil flow was further reduced to 0.2 GPM. As seen in FIG. 5, the thermal protection device further restricted output current at time “F”. Full oil flow was resumed at time “G” and all limitation on field current was quickly removed as reflected in field voltage being returned to its set point. 
     A final test was initiated at time “H” where oil flow was sharply reduced to 0.2 GPM. The resulting over temperature condition was sensed at time “I”, and the outlet field current quickly reduced as evidenced by the drop in field voltage. Even this catastrophic reduction in heat removal capacity resulted in only a brief period of operation above the desired maximum temperature, and the laminated stack exceeded the predetermined limit by only a couple of degrees. 
     Those of skill in the art will realize that while a particular embodiment of the present invention has been described herein, other embodiments are possible in practicing the present invention. By way of example, but not limitation, various means of determining the temperature of the alternator exist including bi-metallic strips and thermocouples. It is also possible to determine alternator temperature from the outlet temperature of the coolant oil. From this information, the temperature of the alternator may be determined. Alternatively, a temperature sensing device could be placed in physical contact with the stator laminated stack or other limiting component to sense temperature. By way of further example, the means for variably controlling current may comprise a device which regulates the output current in discreet steps, such as a stepping circuit or a switching circuit, possibly in conjunction with a sequential shutdown or patterned operation of load subsystems. Also, load subsystem shutdown and/or operation could be logic based using a microprocessor control. Additionally, the op-amp circuit of FIG. 3 could be replaced by an appropriately programmed microprocessor capable of controlling a power FET or semi-conductor as a result of temperature based input signals. Also, any type of power transistor or semiconductor device in addition to power FETs can be used to control the output current of the alternator so long as they are capable of handling the current load and can be modulated by an input signal indicative of alternator temperature. 
     Those of skill in the art will further realize that the invention as described herein may be easily modified so as to provide additional benefit. By way of example, but not of limitation, an alarm or indicator light may be connected across power FET  356  such that when current is not allowed to flow due to an over temperature condition as described above, the alarm or indicator light provides an indication to the operator that the alternator is in an over temperature condition. The operator, being thus alerted, may take corrective action to correct the over temperature condition. Additionally, a device may be connected to point  318  to provide an indication of the alternator temperature to the operator. These and other variations are within the scope of the present invention. 
     The invention herein described provides a significant advantage over the prior art systems. The present invention is capable of protecting an alternator from over temperature conditions while allowing the useful output of the alternator, the invention varies the degree of current limitation based upon the actual temperature of the alternator, in one embodiment, no penetrations into a fluid filled system are required, the device is easily installed on existing or newly manufactured systems, it is reliable, of simple construction, and compatible with other subsystems used in controlling operation of an alternator. Those of skill in the art will recognize that these significant benefits and others are provided by the present invention.