Abstract:
A diagnostic tool for an auxiliary power unit comprising an alternating current generator having an exciter winding and an output winding and an automatic voltage regulator for providing control over the excitation winding of the generator includes a harness with plug connectors allowing the diagnostic tool to be directly introduced between the generator and automatic voltage regulator replacing the conventional connections without affecting operation of the auxiliary power unit. The diagnostic tool has a plurality of multimeters between the automatic voltage regulator and the alternating current generator providing voltage readings across and resistance readings for the generator&#39;s stator exciter winding and main and auxiliary output windings. A protocol relates voltage and resistance readings taken with the multimeters when the auxiliary power unit is on and off to possible faults in the generator or with the automatic voltage regulator.

Description:
BACKGROUND 
     1. Technical Field 
     The technical field relates to diagnostic testing of motor vehicle electrical systems and more particularly to electrical testing of an alternating current generator and an associated automatic voltage regulator in an auxiliary power unit. 
     2. Description of the Technical Field 
     The electrical system of a motor vehicle can be difficult to troubleshoot, particularly where components of a system interact operationally. An example of such a system is a self-excited alternating current generator which uses an automatic voltage regulator to maintain output voltage of the generator by controlling the energization of field coils used for excitation of the generator. When generator performance varies from its nominal values, it may reflect problems in any one of several locations within the generator or with the automatic voltage regulator. Repair of the system can involve repair of the generator, including replacement of parts within the generator. It is current practice to try to repair a malfunctioning generator, but to replace a defective automatic voltage regulator. 
     The electrical connections between the generator and the automatic voltage regulator include some high voltage connections. Diagnostic testing of the equipment can expose technicians to high voltages. 
     SUMMARY 
     A diagnostic tool for an auxiliary power unit comprising an alternating current generator and an automatic voltage regulator is described. In the auxiliary power unit the automatic voltage regulator provides control over an excitation winding of the generator, derives its energization from an auxiliary winding in the generator and senses the voltage of the main output winding as a feedback signal relating to control of energization of the excitation winding. The diagnostic tool includes wiring allowing simultaneous connection of each of a plurality of multimeters into the connections between the automatic voltage regulator and the alternating current generator. The plurality of multimeters provides for reading the voltage applied across the direct current excitation winding and the voltage sensed across the output coil when the generator is running and for measuring the resistance of the both windings when the generator is off. The automatic voltage regulator further provides connection to an auxiliary output winding. A multimeter in this connection provides an auxiliary output voltage reading from across the auxiliary output voltage winding and resistance of the winding. A diagnostic protocol relates voltage and resistance readings taken with the multimeters when the auxiliary power unit is on and off to possible faults in the generator or with the automatic voltage regulator. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Reference to the following detailed description is aided by reference to the accompanying drawings, wherein: 
         FIG. 1  is a side elevation of a truck with an auxiliary power unit. 
         FIG. 2  is high level block diagram of the electrical system for the truck of  FIG. 1  including an auxiliary power unit. 
         FIG. 3  is a circuit schematic for a generator in the auxiliary power unit. 
         FIG. 4  is a schematic of a diagnostic tool for the auxiliary power unit of  FIG. 2  illustrating connection of the tool between the generator and an automatic voltage regulator of the auxiliary power unit. 
         FIG. 5  is a perspective view of the diagnostic tool of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION 
     The accompanying drawings illustrate the embodiment described here, however, the claims which follow are not to be construed as limited to the illustrations and described embodiment. Like numbers refer to like elements throughout. 
       FIG. 1  illustrates a truck tractor  11  that has a forward passenger compartment or cab  13  and a rear sleeper or living compartment  15 . A driver is situated in cab  13  while driving truck  11 , and uses the sleeper  15  for storage, a living area, an entertainment center, and sleeping at other times. Truck tractor  11  has an engine compartment  17  in front of or below cab  13 . A housing  19  may be mounted to a side frame of truck  11 , below cab  13 , to enclose most components of an auxiliary power unit (APU)  21  ( FIG. 2 ). The APU  21  and its housing  19  can alternately be located elsewhere on truck  11 , such as behind fuel tank  20 , behind sleeper  19 , or other locations on the vehicle. 
     Referring to  FIG. 2 , truck tractor  11  has a main or primary diesel engine  25  housed in engine compartment  17 . A primary fan  35  is driven by primary engine  25  for drawing air through radiator  30 . Primary engine  25  drives a conventional direct current (DC) alternator  41  that charges batteries  43  and supplies electrical power to the vehicle when the engine is running. Batteries  43  are typically located under the cab  13  along one side of truck  11 . Primary engine  25  operates under the control of an engine controller  27 , which may be an element in a more extensive vehicle control network incorporating transmission controllers, antilock brake controllers, other components and a data link enabling communication among the controllers and components. 
     Truck  11  is equipped with an auxiliary power unit  21  generally enclosed in compartment  19 . The auxiliary power unit  21  has an engine  47 , which may be a four-stroke diesel engine that runs on fuel from the same source as the engine of truck tractor  11  but which is of smaller displacement allowing its efficient operation at lower power output levels than the main engine  25 . Fuel line connections can be readily made with minor modifications to a fuel pickup area (not shown) of the vehicle fuel tank  20 . Engine controller  27  can be adapted for the control of auxiliary engine  47  or an auxiliary engine controller  127  can be provided to control operation of the auxiliary engine  47 . Where truck  11  is equipped with a vehicle control network such as a controller area network the auxiliary engine controller  127  may be connected to the network for communication with other controllers. 
     Auxiliary engine  47  drives an auxiliary generator  67  by a mechanical linkage  51 , typically directly from the auxiliary engine fly wheel. Mechanical linkage  51  may also be a drive belt, timing gear or timing belt. Because generator  67  is operated as a synchronous machine slippage in mechanical linkage  51  between auxiliary engine  47  and generator  67  is minimized. Auxiliary generator  67  is configured to supply single phase alternating electrical current (AC) at 110-120 volt and 60 cycles to sleeper  15 . This allows the use of conventional household appliances (AC load  59 ) in the sleeper  15 . This power may also be supplied to components on truck  11  which usually use line current, such as a block heater  45 . The output voltage and frequency of generator  67  are readily changed to accommodate different regional standards by changing the main stator winding, typically to provide 220 volt AC. Frequency may be changed by changing the operating speed of the auxiliary engine  47 , or changing gearing in mechanical linkage  51 . Output voltage is maintained under loads by maintaining the DC voltage across the stator exciter winding  81 . Automatic voltage regulator  73  provides this control, using a set of sense and field lines  23  connected between itself and generator  67 . 
     Three conductors are provided from APU  21  to cab  15  for supporting AC load  59 : a hot line  77 ; a neutral line  63 ; and a ground line  61 . The output frequency is controlled by controlling the operating speed of the auxiliary engine  47 . Engine speed may be monitored by equipping auxiliary engine  47  with a tachometer connected to report engine speed to engine controller/computer  127 , or by providing appropriate electronic circuitry to monitor the line frequency appearing on output lines  15  and  63  and providing this data to the engine controller  127 . Engine speed is maintained by the auxiliary engine controller  127 . Engine controller  127  increases fuel flow in response to an increase in the electrical load, which tends to pull output frequency down, or by decreasing fuel flow should the load lighten, with a consequent increase in output frequency. 
     Direct current for truck  11  components such as the engine controller  27  or for charging battery  43  may be supplied from a combination step down transformer/rectifier  75  which is connected to receive the output of generator  67 . Step down transformer/rectifier  75  is considered to be part of the APU  21 , however, the device itself is not usually located in housing  19 , but rather in sleeper  15  in order to physically protect the device. An automatic starter system (not shown) for auxiliary engine  47  may include a starter motor supplied with power from battery  43 . 
     Referring to  FIG. 3 , the major components within generator  67  and the relationship of the sense and field lines  23  to those components is detailed. There are three pairs of wires within the set of sense and field lines  23 : an AC output voltage sense pair  23 C; an AC automatic voltage regulator power pair  23 A; and a DC exciter or field voltage pair  23 B. The pairs  23 A, B and C are color coded. For example, both wires of pair  23 A may be made blue, both wires of pair  23 C may be made yellow, and the positive wire of pair  23 B red with the negative or ground wire black. Color coding assists the technician is quickly connecting diagnostic tool  100 . 
     Generally, generator  67  includes repairable components, while automatic voltage regulator (AVR)  73  is replaced if it fails. Diagnosis of faults in the combined sub-system isolates faults within generator  67 . The locations of faults within AVR  73  are not isolated beyond determining that the AVR is not operating properly. If generator  67  is operating properly, its output voltage is maintained by controlling the current through an exciter stator coil  81  connected into wire pair  23 B. The exciter current is set in response to the sensed AC voltage appearing at the output terminals of generator  67  (i.e. across the terminals of a main stator coil  89 ). This voltage appears on wire pair  23 C. Finally automatic voltage regulator  73  draws power to provide the DC excitation current from the AC AVR power pair  23 A, which are connected to the terminals of an auxiliary stator coil  91 . Coil  91  is excited from the main rotor coil  87 . The terminals of generator  67  have associated pairs of wires from the sense and field lines  23 . 
     AC generator  67  includes a rotor  84  between the exciter stator (input) coil  81  and the main and auxiliary stator (output) coils  89 ,  91 . Rotor  84  is mechanically coupled to the auxiliary engine  47  for rotation. Rotor  84  carries an exciter rotor coil  83  which is flux linked to the exciter stator coil  81  and a main rotor (field) coil  87  which is magnetically flux linked to the main stator coil  89  and auxiliary stator coil  91 . Exciter coil  83  is Wye-connected and produces three phase electrical power which is full wave rectified by rectifier  88  to produce a DC current though main rotor coil  87 . Rotation of the main rotor coil  87  past the main and auxiliary stator coils  89 ,  91  results in the generation of AC electricity appearing on the output terminals of the stator coils. The frequency is controlled by controlling rotor  84  rotational speed and the voltage is maintained by control of the DC exciter voltage, which allows the stator exciter  81  winding to reinforce the magnetic field of an exciter magnet. 
     Referring to  FIG. 4 , a diagnostic tool  100  for use with the generator  67  and automatic voltage regulator  73  electrical system is shown. Diagnostic tool  100  includes a bank  101  of three multimeters  102 ,  104  and  106 . Multimeters  102 ,  104 ,  106  are circuit analyzers which provide measurements of AC or DC voltage and circuit resistance. Two AC voltage sensing multimeters ( 102 ,  104 ) and one DC voltage sensing multimeter  106  are provided. Two terminal plugs  108  and  110  are attached to the multimeters by cabling  56 . The cabling  56  provides wire pairs corresponding to the pairs  23 A, B and C of the sense and field connection lines  23 . The cabling  56  arrangement allows the diagnostic tool to be substituted for, or introduced into, sense and field cables  23 . As illustrated in  FIG. 5 , cabling  56  may be collected in a harness to form a “T” with the top bar of the “T” connected between AVR  73  and the generator  67  with the connectors  108 ,  110  at either end of the top bar and the meter bank  101  at the base of the “T”. The connectors  108  and  110  are coded to direct appropriate connection to the generator  67  and the AVR  73 , either introduced into or replacing the sense and field lines  23 . Digital or analog readouts may be used for the multimeter bank  101 . 
     In a motor vehicle auxiliary power unit setting a 6 kilowatt generator  67  may be used. Power output is usually set somewhat below the maximum power output capacity of the auxiliary engine  47 . A typical engine choice comfortably supplies 4.5 kilowatts of electrical power. Diagnostics relating to the AVR  73  and generator  67  are performed with the APU  21  both running and with it off (“ON” and “OFF” tests). For the ON test, performed when APU  21  is running, multimeter bank  101  provides three voltage readings: the auxiliary stator  91  voltage (AC); the main stator  89  voltage (AC); and, the exciter stator  81  voltage (DC). For the OFF test meter bank  101  provides three resistance readings: main stator  89  winding resistance; auxiliary stator  91  winding resistance; and excitation stator  81  winding resistance. Nominal values depend upon the application of the device, but for a North American truck APU  21  providing 110-120 volt single phase AC, nominal values might be as follows. The DC voltage supplied by the AVR  73  to the exciter stator  81  of generator  67  is 60 volts. The AC voltage supplied from the auxiliary stator  91  to the AVR  73  is 400 volts. The “sense” voltage measured across the main stator  89  is 120 volts (AC). The resistance of the main stator  89  winding is 0.6 ohms, the resistance of the auxiliary stator  91  winding is 3 ohms and the resistance of the excitation stator  81  winding is 50 ohms Fluctuation in the measurements within a tolerance range is permissible. The nominal voltages will change for APU  21  based on its configuration, for example, if it is configured to supply 50 cycle, 220 volt current. 
     Where the DC resistance readings for a particular winding depart from the nominal values for the winding, it may be taken that the measured winding is defective. Voltage readings are subject to more variation in interpretation. When the APU  21  is running and there is voltage readings on the meters of multimeter bank  101  are zero there source of the problem could be a faulty AVR  73 , a poor or open connection between AVR  73  and exciter stator  81 , an exciter stator  81  defect, a faulty rectifier  88  on the rotor  84 , an open circuit on the rotor  84 , a loss of field magnetism on the exciter stage magnetic circuit (linking exciter stator  81  and exciter rotor  83 ) or the main stage magnetic circuit (linking main rotor  87  to main stator  89  and auxiliary stator  91 ). There could also be a problem with generator  67  circuit breakers (not shown). 
     If voltage readings are low when the APU  21  is on and no load is applied, a problem may lie with the a faulty AVR  73 , the exciter stator  81 , a defective rectifier  88  or the main rotor winding  89 . If voltage readings are higher than nominal a problem may lie with the AVR  73 . If there are excessive voltage reading fluctuations a problem is likely in loose or poor connections or a faulty AVR  73 . If voltage readings are low only when a load is applied to the generator  67  than it is likely that fault lies with the rotor rectifier  88 , the main rotor winding  87  or the exciter stator  81  winding. 
     Put another way, a faulty AVR  73  can be determined from ON and OFF tests. If only the sense (main stator  89 ) voltage is low and all other readings are within nominal ranges and adjusting the AVR  73  to bring the sense voltage within nominal ranges is unsuccessful than a faulty AVR  73  is indicated. A poor/open connection from the AVR  73  to the exciter stator  81  is indicated during an OFF test if an open circuit is seen in the exciter stator  81  circuit. An exciter stator  81  winding failure is indicated during an OFF test if resistance is otherwise outside of an acceptable range from nominal. A rotor  84  fault is indicated (either in windings or the rectifier  88 ) from ON and OFF readings if resistance readings are within nominal ranges and there are either no voltage readings or erratic voltage readings. Isolation of the problem within the rotor  84  may involve partial disassembly of generator  67 . The loss of field magnetism can be determined from the ON and OFF tests by reduced or no sense/auxiliary winding voltages, but other readings being within specifications. Intermittent readings during either ON or OFF tests indicate poor or loose connections.