Abstract:
A system and a method for thermally protecting a high output vehicle alternator are provided. The high output vehicle alternator includes a duty cycle control system. The duty cycle control system includes an alternator temperature signal generator and an alternator rotor speed signal generator in communication with an alternator having a speed limit and a temperature limit. The duty cycle control system regulates the field current supplied to the alternator based on the temperature and rotor speed signals.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to automotive alternators. In particular, the present invention relates to thermal protection for high output vehicle alternators. 
     BACKGROUND OF THE INVENTION 
     A fundamental design goal for a vehicle alternator is to provide maximum power output at the lowest possible rotational speed. Additionally, smaller engine compartments in current vehicles require small alternator size combined with high efficiency. The highly efficient alternators generate high output at low speed, but generate more energy than can be consumed when the vehicle alternator operates at a higher rotational speed. In addition to generating electrical power, the alternator also generates heat. As the rotational speed of the alternator increases, the amount of heat increases, creating a potential failure of the alternator due to the elevated temperatures. 
     In order to dissipate heat, the alternator is provided with a cooling system. Liquid cooling has been used to help decrease excess heat. Liquid cooling dissipates the heat and provides a means of sealing the alternator&#39;s components for increased environmental robustness. However, liquid cooling may not be sufficient to dissipate heat at high alternator output levels. A machine designed to provide maximum output demand at low speed may exceed its thermal limit at high speed. 
     Air cooling systems also exist to help dissipate heat generated by an alternator. Similar to the liquid cooling systems, the air cooling systems cannot sufficiently cool the alternator at high output levels to prevent potential alternator failure due to excess heat generated during high speed alternator operation. 
     An additional problem exists with the temperature regulation of an alternator. The alternator itself has a large thermal capacity such that a temperature sensor may not adequately indicate increasing alternator temperature at the time the increase is actually occurring. By the time a temperature sensor indicates that the alternator has reached the thermal limit, the response time necessary to effect a decrease in temperature may be greater than the time required for sufficient cooling to prevent damage to the alternator. Temperature measurement alone is not sufficient to maintain the alternator at a thermally safe operating temperature and thereby prevent alternator failure due to excessive heating of the alternator during high speed operation. 
     Therefore, a need exists for providing a means for protecting a high output vehicle alternator from damage due to excessive heat. 
     BRIEF SUMMARY OF THE INVENTION 
     In order to alleviate one or more shortcomings of the prior art, a thermal protection system and method are provided herein. In accordance with the present invention, a thermal protection system and method are disclosed herein for thermal protection of a high output vehicle alternator. 
     According to one aspect of the present invention, there is provided a system for thermal protection of a high output vehicle alternator. The system comprises an alternator having a temperature limit and a rotor speed limit, a field current supply to the alternator, an alternator rotor speed signal generator, an alternator temperature signal generator and a duty cycle control system operably connected to the alternator and the signal generators. The duty cycle control system regulates the field current supply to the alternator based on the information from the signal generators. 
     In another aspect of the present invention, a method for thermally protecting a high output vehicle alternator is provided. The method includes the steps of determining an operating temperature for an alternator, comparing the operating temperature to a pre-determined alternator temperature limit and providing a temperature comparison signal to the duty cycle control system corresponding to the temperature comparison, determining an operating alternator rotor speed, comparing the rotor speed to a pre-determined rotor speed limit and providing a rotor speed comparison signal to the duty cycle control system indicative of the rotor speed comparison, and generating a duty cycle control signal to regulate a field current supply to the alternator based on the temperature comparison signal and the rotor speed comparison signal. 
     Advantages of the present invention will become more apparent to those skilled in the art from the following description of the preferred embodiments of the invention which have been shown and described by way of illustration. As will be realized, the invention is capable of other and different embodiments, and its details are capable of modification in various respects. Accordingly, the drawings and description are to be regarded as illustrative in nature and not as restrictive. 
    
    
     
       BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an electrical circuit for a preferred embodiment of the present invention; and 
         FIG. 2  is a logic flow diagram illustrating a preferred embodiment of a thermal protection scheme in accordance with an embodiment of the in accordance with another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An exemplary thermal protection system for use with a high output vehicle alternator that can be implemented in the present embodiment of the invention is shown in the schematic diagram of  FIG. 1 . An exemplary schematic high output vehicle alternator  10  is shown in this embodiment. The alternator  10  may be a Lundell type alternator, although any alternator commonly known in the art may be used with the present thermal protection system. As shown, the alternator  10  includes a rotor  12  inductively coupled to a stator  13  with stator windings  14 . The rotor  12  is driven by a vehicle engine  15 . The rotor  12  spins within the stator windings  14  to create three alternating currents. The three phase-alternating currents from the stator windings  14  are then rectified into direct current by a rectifier  18 , preferably consisting of six diodes as is well-known in the art. The direct current is then supplied to a positive pole  21  of a battery  22 . The battery  22  is preferably a 12 volt battery. 
     A field current is supplied through a voltage regulator  24  to the rotor  12  from the battery  22  or the rectifier  18 . As shown in  FIG. 1 , a MOSFET switch  26  controls the voltage across the regulator  24 . Any type of switch, such as a Darlington switch or a relay switch, commonly known in the art, may be used to control the voltage across the regulator  24  to the rotor  12 . The voltage regulator  24  as shown in this embodiment of the present invention illustrates the thermal protection scheme for limiting a duty cycle to limit the field current supplied to the alternator  10  when an alternator temperature limit or a rotor speed limit (described below) has been reached. 
     An AND gate  30  and an OR gate  32  form a logic circuit for generating a signal for controlling the MOSFET  26 . The reference voltage generating circuit of the AND gate  30  includes a battery reference  34 , an error amplifier  38 , a comparator  40  and a ramp generator  42 . The battery reference  34  provides a reference standard for the battery  22 , preferably in the range of 13–15 volts, most preferably in the range of 14.1–14.2 volts. The battery reference standard  34  is compared to an actual voltage level of the battery  22  in the error amplifier  38 . The error amplifier  38  connects to the comparator  40  wherein a pulse width modulator (PWM) is formed. The ramp generator  42  provides a ramp signal to the comparator  40  to provide a ramp signal of constant frequency and shape. In this embodiment, the ramp signal may be digitally generated by a 7-bit divider  44  connected to an oscillator  46 . The oscillator  46  frequency, in the preferred embodiment, is about 16 kHz. The ramp generator  42  provides a reference to the comparator  40 . The ramp generator  42  reference signal is compared to the error amplifier  38  signal at the comparator  40 . A PWM control signal from the comparator  40  that determines the duty cycle for the field current supplied to the rotor  12  through the regulator  24  is sent to the AND gate  30  and input with the signal from the OR gate  32 . From the AND gate  30 , a signal is transmitted to the MOSFET  26  to turn the MOSFET  26  on or off. 
     As described above, the AND gate  30  receives signals from the comparator  40  for reference and from the OR gate  32  for actual alternator operation. A plurality of AND gates  52 – 58  transmit signals to the OR gate  32 . A set of continuous signals is sent from timers  60  to each of the AND gates  52 – 58  as described below. The timers  60  provide a signal reflecting the duty cycle limit required based on the temperature of the alternator  10  or the speed of the rotor  12  and the temperature of the alternator  10 . Output from the 7-bit divider circuit  44  is transmitted to the timers  60 . Based on the signal sent to OR gate  32  input with the reference PWM signal from comparator  40 , the regulator  24  regulates whether to override the reference from comparator  40  that allows 100% duty cycle or whether to decrease the duty cycle based on an algorithm, described below, when the temperature or rotor speed signals indicate that a limit requiring thermal protection has been reached by the alternator. 
     A reference temperature  62  is determined for a specific alternator  10 . For each type of alternator used, a maximum operating temperature limit for the alternator  10  may be experimentally determined to provide a temperature limit above which the alternator may be subject to damage or failure due to excessive heat exposure. For example, in a liquid-cooled alternator, a maximum operating temperature limit for the alternator may be determined by running the alternator at a speed at which excess heat is generated and until the alternator is damaged due to the excess heat generated during operation. The maximum safe operating temperature may be set at a range just below the temperature range at which the alternator incurs damage from the excess heat generated during operation. The maximum operating temperature limit is typically in the range of about 140° C. to about 170° C., more preferably about 145° C. to about 155° C., most preferably about 150° C. Of course, the maximum temperature limit set for an alternator depends on the specific type of alternator used. The maximum temperature limit may be any temperature limit that protects an alternator from damage or failure. 
     An alternator temperature signal  64  is generated from a temperature sensor  20  on alternator  10 . The alternator temperature  64  may be monitored by a temperature sensor  79  placed at various points on the alternator and the signal  64  is generated while the alternator is operating. In a preferred embodiment, a temperature sensor may be placed within the voltage regulator, alternatively the temperature sensor may be placed inside the alternator on the rotor  12  or the stator windings  14 . A temperature sensor may also be placed on the exterior of the alternator for ease of placement. Although interior placement of a temperature sensor may provide a measurement of the highest temperature in the alternator, interior placement of the temperature sensor is not critical. For example, a thermistor placed on the exterior of the alternator may be used to indicate alternator temperature. Any temperature sensor, combination of sensors, and sensor placement to monitor alternator temperature commonly known in the art may be used to detect the alternator temperature. The temperature of the alternator  10  is converted to voltage to supply the alternator temperature signal  64 . The alternator temperature signal  64  is input with the alternator reference temperature  62  at a comparator  66 . A signal from the comparator  66  is input to the AND gates  52 – 58 . 
     A rotor speed signal  70  is generated from the conversion of the stator winding  14  frequency to voltage to reflect the speed of the rotor  12 . The rotor speed signal  70  is supplied to a comparator  72  and a comparator  74 . The comparator  72  compares the rotor speed signal  70  with a first rotor speed limit  76 . A signal from comparator  72  is input to the AND gates  52  and  56 . The comparator  74  compares the rotor speed signal  70  with a second rotor speed limit  78 . A signal from comparator  74  is input to the AND gates  52 ,  56 , and  58 . The rotor speed limits are described in detail in the description of the algorithm below. 
     Signals are generated in the AND gates  52 – 58  as follows. For the AND gate  52 , the timer  60  signal representing a 100% duty cycle  80  is input with the inverted signals from the comparators  66 ,  72 , and  74 . The 100% duty cycle  80  represents the maximum allowable regulator field drive duty cycle for a specific alternator. Therefore, the AND gate  52  transmits a signal to the OR gate  32  for 100% duty cycle if a) the temperature signal  64  is not greater than the reference temperature  62 ; and b) the rotor speed signal  70  is not greater than the first speed limit  76 ; and c) the rotor speed signal  70  is not greater than the second speed limit  78 . 
     For the AND gate  54 , the timer  60  signal representing a default maximum duty cycle limit  82  is input with the temperature comparator signal  66 . The default maximum allowable duty cycle is any duty cycle that is low enough for the alternator not to produce excessive temperature. For example, the duty cycle may be reduced to 20% to still provide alternator output, but not generate excess heat. Alternatively, the duty cycle may be reduced to 0% to disable voltage regulation to prevent excess heating. Of course, other duty cycle limit reductions may be used when the operating temperature of the alternator exceeds the maximum allowable temperature limit. The AND gate  54  transmits a signal to the OR gate  32  for the default maximum duty cycle limit  82  when the temperature signal  64  is greater than the reference temperature  62 . 
     For the AND gate  56 , the timer  60  signal representing a first duty cycle limit  84  is input with the comparator signal  72  and inverted signals from the comparators  66  and  74 . The duty cycle limits represent a limit on the field current supply to the alternator that is set when the alternator rotor speed reaches a certain speed. The rotor speeds and the duty cycle limits are described below in detail in the description of the algorithm for the preferred embodiment of the present invention. Therefore, the AND gate  56  transmits a signal to the OR gate  32  for the first duty cycle limit  84  if a) the temperature signal  64  is not greater that the reference temperature  62  and b) the rotor speed signal  70  is greater than the first speed limit  76  and c) the rotor speed signal  70  is not greater than the second speed limit  78 . 
     For the AND gate  58 , the timer  60  signal representing a second duty cycle limit  86  is input with the inverted signal from the comparator  66  and the signal from comparator  74 . Therefore, the AND gate  58  transmits a signal to the OR gate  32  for the second duty cycle limit  86  if a) the temperature signal  64  is not greater that the reference temperature  62 ; and b) the rotor speed signal  70  is greater than the second speed limit  78 . 
     The logic flow chart diagramed in  FIG. 2  shows a preferred implementation of the controller steps of the regulator shown in  FIG. 1  and preformed by the thermal protection algorithm  100  of the duty cycle control system. 
     The thermal protection algorithm  100  is initiated at  102 , after a vehicle engine has been turned on. The algorithm  100  continues to run through a logic loop to monitor temperature changes and duty cycle limit changes as described below. Therefore, the algorithm may signal changes to the duty cycle controller to immediately change the duty cycle in response to excess alternator temperature or excess rotor speed. 
     In the algorithm  100 , a vehicle starts at  102  and a duty cycle of 100% begins at 110. The 100% duty cycle  110  represents the maximum allowable regulator field drive duty cycle for a specific alternator. From the starting 100% duty cycle limit  110 , a temperature determination  112  for the alternator is made. As described above, any temperature sensing means known in the art may be used to determine the temperature of the alternator while the alternator is operating. From the temperature determination  112 , the algorithm continues to a determination of whether a duty cycle limit  114  has been invoked. The duty cycle limit  114  indicates that the duty cycle is less than the 100% of the duty cycle  110 . 
     The thermal protection algorithm  100  decreases the maximum allowable duty cycle from the 100% duty cycle  110  to a default maximum allowable duty cycle when the temperature determination  112  indicates that the operating temperature exceeds a maximum operating temperature limit determined for a specific alternator, described at step  116 . The default maximum allowable duty cycle is any duty cycle that is low enough for the alternator not to produce excessive temperature as described above. If the duty cycle limit  114  is not less than the 100% duty cycle limit  110  indicating that the duty cycle is operating at the maximum allowable duty cycle, the algorithm determines whether the temperature determination  112  is less than a maximum temperature limit determined for a specific alternator at step  116 . In a preferred embodiment, using a Lundell type alternator, the maximum temperature limit may be in the range of about 145–155° C., more preferably about 150° C. 
     If the temperature  112  is greater than the maximum temperature limit  116 , the algorithm continues to set a duty cycle limit reduction  118 . The duty cycle limit  118  is set to the default maximum allowable duty cycle as described above at step  114  and the algorithm returns to the temperature determination  112 . 
     If the temperature  116  is below the maximum determined temperature, an alternator rotor speed  122  is determined. The rotor speed  122  is compared to a first speed limit  124 . In a preferred embodiment, a first rotor speed limit may be set to limit generation of excess heat production by the alternator before the alternator temperature exceeds the maximum temperature limit  116 . In the present embodiment, a first rotor speed limit may be set at about 2500 rpm. Of course, other rotor speed limits are possible to set and still provide thermal protection for an alternator. If the rotor speed  122  is below the first speed limit  124 , the algorithm returns to  110  and the duty limit for the field drive duty cycle is reset to 100%. 
     If the rotor speed  122  exceeds the first speed limit  124 , the rotor speed  122  is compared to a second rotor speed limit  126 . In a preferred embodiment, a second rotor speed limit  126  may be determined for a specific alternator to be set at about 5000 rpm. When the rotor speed  122  is below the second rotor speed limit  126 , a first duty cycle limit  128  is set. The first duty cycle limit  128  may be determined experimentally by comparing the temperature of the alternator to the speed of the rotor and determining the reduction necessary in the duty cycle at the first rotor speed limit  124  that prevents the specific alternator from exceeding the maximum temperature limit. In a preferred embodiment, as described above with the maximum temperature for the specific alternator set at 150° C., the first speed limit  124  of about 2500 rpm, the first duty cycle limit  128  for the field drive duty cycle may be set at about 90%. The algorithm returns to  112  and the alternator temperature  112  is determined. 
     If the speed  122  exceeds the second rotor speed limit  126 , a second duty cycle limit  130  is set. As described for the first duty cycle limit  128 , each duty cycle limit is alternator specific. In this preferred embodiment, the second duty cycle limit  130  for the field drive duty cycle may be set at about 80%. The algorithm returns to  112  and the alternator temperature  112  is determined. 
     If the duty cycle limit  114  is operating at the maximum allowable duty cycle when the operating temperature exceeds the threshold temperature, the temperature  112  is compared to the maximum temperature, described at  116 , minus a hysteresis temperature  120  reflecting the lagging in the values of the temperature  112  resulting from the alternator&#39;s thermal capacity. In the preferred embodiment described herein with a maximum temperature for the specific alternator of about 150° C., the hysteresis temperature is preferably set at about 15° C. If the temperature  112  is below the maximum determined temperature minus the hysteresis temperature  120 , the rotor speed  122  is determined and the algorithm continues as described above. If the temperature  112  is above the maximum determined temperature minus the hysteresis temperature  120 , the algorithm returns to determine the temperature  112 . 
     Of course, a duty cycle control system may be implemented using a different number of temperature limits, rotor speed limits and duty cycle limits. Alternatively, the duty cycle control system may be implemented on a point by point temperature and rotor speed determination using a microprocessor. 
     Although the invention herein has been described in connection with a preferred embodiment thereof, it will be appreciated by those skilled in the art that additions, modifications, substitutions, and deletions not specifically described may be made without departing from the spirit and scope of the invention as defined in the appended claims. The scope of the invention is defined by the appended claims, and all devices that come within the meaning of the claims, either literally or by equivalence, are intended to be embraced therein.