Abstract:
A locomotive cab air conditioning method involves providing a multi-speed motor operable in at least a first speed state and a second speed state, the motor connected for driving a refrigerant compressor from a companion alternator output of the locomotive. An operating speed of a locomotive engine is monitored, and operation of the motor in one of the speed states is established based at least in part upon the monitored locomotive engine speed.

Description:
[0001]    This application claims the benefit of provisional application Serial No. 60/182,084 filed Feb. 11, 2000. 
     
    
     
       TECHNICAL FIELD  
         [0002]    The present invention relates generally to locomotive air conditioners and, more particularly, to a locomotive air conditioning control system providing improved operation over a range of locomotive engine speeds.  
         BACKGROUND  
         [0003]    Air conditioning (A/C) for the crew of diesel-electric locomotives has been available in many different design concepts for more than 15 years. These A/C systems are far different from those found in commercial and residential applications because no utility power source is available on freight locomotives to operate them, i.e. there are no 110 volt or 220 volt single phase outlets; there also are no 230 or 460 volt three phase outlets.  
           [0004]    The supplier of an air conditioner for a locomotive cab must operate his product from either or both of two power supplies on the locomotive: 74 volts DC and/or a three-phase variable-voltage and variable-frequency supply from the companion alternator, which changes its output from 26.7 Hz, 44.5 volts AC to 120 Hz, 200 volts AC, directly as engine speed changes from 200 to 900 RPM.  
           [0005]    Early non-hermetic systems of the 74 volt DC type used three DC motors to operate the compressor, evaporator fan motor and condenser fan motor. These motors were extremely expensive and required regular brush, commutator and DC contactor maintenance. For these reasons, such systems have been unpopular.  
           [0006]    In the 1990s, a hermetic, all - AC motor system was introduced which used solid state inverters to convert 74 volts DC to three-phase AC power to run the three air conditioning motors. Today this technology dominates the locomotive A/C industry, and accounts for almost 100% of all new locomotive (OEM) applications. The appeal of this technology is clear:  
           [0007]    (1) Hermetic—no shaft seal leakage  
           [0008]    (2) No brushes—low maintenance and better reliability  
           [0009]    (3) No contactors—improved reliability  
           [0010]    (4) Solid state, microcomputer control—improved control features.  
           [0011]    The principal disadvantage of this technology is its cost. While that cost level can be justified on new locomotives, it is difficult to justify on a retrofit package for perhaps thousands of older locomotives which need A/C to meet potential FRA rules and workforce demands. In addition, the 74 volt DC electrical system of these older locomotives may be inadequate for the additional A/C load; upgrading the 74 volt DC system is an additional major expense.  
           [0012]    Systems of the companion alternator type were popular before the introduction of 74 volt hermetic systems already described. One short coming of these products was simply that A/C performance changed as the locomotive was operated from idle to maximum speed. In particular, since the companion alternator output frequency changes from 120 Hz to 26.7 Hz as the engine changes its speed by the ratio of 900 RPM to 200 RPM, or 4.5:1, the motor would have to be selected such that at 120 Hz the compressor did not overspeed. Clearly the compressor capacity would be reduced by about  4 . 5 : 1  (78 %) at locomotive idle. Similarly the air delivery of a condenser fan would be reduced by the same amount. These two factors show that indeed such single-speed systems must have poor A/C capacity at idle. Since older locomotives are often used extensively in low speed service, crew complaints of inadequate A/C capacity have been common with this technology. Furthermore, the product uses a shaft-driven (open drive) compressor and leaks, therefore, can occur at the rotating shaft seal, compromising reliability.  
         SUMMARY  
         [0013]    In one aspect, a locomotive cab air conditioning method involves providing a multi-speed motor operable in at least a first pole state and a second pole state, the motor connected for driving a refrigerant compressor from power derived from a companion alternator output of the locomotive. A frequency or period of the companion alternator output is monitored, and operation of the motor in one of the pole states is established based at least in part upon the monitored frequency or period.  
           [0014]    In another aspect, a locomotive cab air conditioning method involves providing a multi-speed motor operable in at least a first speed state and a second speed state, the motor connected for driving a refrigerant compressor from a companion alternator output of the locomotive. An operating speed of a locomotive engine is monitored, and operation of the motor in one of the speed states is established based at least in part upon the monitored locomotive engine speed.  
           [0015]    In still a further aspect, a locomotive cab air conditioning control system for use with a locomotive cab air conditioning system including a compressor and a companion alternator associated with the locomotive engine for providing a power output which varies in frequency as the locomotive engine speed varies is provided. The control system includes a detection circuit for detecting a frequency or period of the companion alternator power output and a controller for receiving a frequency or period indicative output from the detection circuit. A multi-speed motor is connected for controlling operation of the compressor utilizing power from the companion alternator power output. The controller is connected to control a speed state of the multi-speed motor, and the controller is operable to monitor a frequency or period of the companion alternator power output and to establish a speed state of the multi-speed motor based at least in part upon the monitored frequency or period.  
           [0016]    In still a further aspect, a locomotive cab air conditioning control system for use with a locomotive cab air conditioning system including a compressor, a condenser fan motor, an evaporator fan motor, and a companion alternator associated with the locomotive engine for providing a power output which varies in frequency as the locomotive engine speed varies is provided. The control system includes a detection circuit associated for detecting a frequency or period of the companion alternator power output and a controller for receiving a frequency or period indicative output from the detection circuit. A multi-speed motor is provided for controlling operation of the compressor utilizing power from the companion alternator power output, and an inverter for providing engine-speed-independent power to the condenser fan motor and to the evaporator fan motor is included. A contactor control circuit is connected to a contact arrangement of the multi-speed motor for controlling a contact/pole state thereof, the contactor control circuit responsive to control signals received from the controller. The controller is operable to monitor a frequency or period of the companion alternator power output, and to provide an output signal to the contactor control circuit for controlling operation of the motor in one of at least two pole states based upon the monitored frequency or period. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    [0017]FIG. 1 is a graph of compressor capacity vs. compressor speed;  
         [0018]    [0018]FIG. 2 is a graph of compressor capacity vs. locomotive engine speed for a multi-speed compressor motor and a single speed compressor motor, both powered by the variable output of a companion alternator;  
         [0019]    [0019]FIG. 3 is a graph of system capacity vs. locomotive engine speed for a rooftop type system;  
         [0020]    [0020]FIG. 4 is a schematic of one embodiment of an exemplary refrigerant system;  
         [0021]    [0021]FIG. 5 is a schematic of one embodiment of a control arrangement for a locomotive cab A/C system;  
         [0022]    [0022]FIG. 6 is a schematic of another embodiment of an exemplary refrigerant system; and  
         [0023]    [0023]FIG. 7 is a schematic of another embodiment of a control arrangement for a locomotive cab A/C system. 
     
    
     DETAILED DESCRIPTION  
       [0024]    Referring to the compressor capacity curves shown in FIGS.  1 - 3 , the use of a multi-speed motor to operate the compressor of a locomotive A/C system significantly improves A/C performance at low engine speeds and can ensure that the compressor does not overspeed at high engine speeds. The graph  100  of FIG. 1 shows compressor capacity vs. compressor speed, the graph  110  of FIG. 2 shows compressor capacity vs. locomotive engine rpm of a compressor operated by a multi-speed motor and a compressor operated by a single speed motor, and the graph  120  of FIG. 3 shows system capacity vs. locomotive engine rpm for a rooftop type locomotive A/C system.  
         [0025]    In one embodiment, as will be described in further detail below, the multi-speed motor operates by changing its number of poles by connecting the consequent pole stator winding in delta configuration for low speed operation and in wye configuration for high speed.  
         [0026]    Referring to FIG. 4, a schematic depiction of one embodiment of a refrigerant system  10  is shown. The relative locations of the condenser fan  12  and compressor  14  in such a system are shown. The illustrated system  10  may be a locomotive cab rooftop configuration, but could also be some other type of configuration such as sub-base or side mount.  
         [0027]    Referring to FIG. 5 one embodiment of a power and control system arrangement  20  is depicted. The three-phase companion alternator  22  generates variable frequency output AC power on lines L 1 , L 2 , L 3 , with the frequency varying in proportion to the locomotive engine speed (rpm). The companion alternator  22  is inductively coupled via transformer arrangement  23  with a frequency detector circuit  24  which provides a signal indicative of the frequency or period of the companion alternator signal to a controller  26 . The companion alternator output is also provided through a 3-phase bridge  28  to inverter  30 , the bridge  28  providing variable voltage DC to the inverter  30 . The inverter  30  also receives a control input in the form of a Hi/Lo speed command from the controller  26 . The inverter provides a 55 volt AC power signal output to the evaporator fan  34  and the condenser fan  36 , regardless of the variable voltage DC, by varying a PWM signal to account for the changing DC input. A contactor control circuit  38  is provided for controlling the operating state of the compressor motor  39  based upon a control signal received from controller  26  via line  27 . In particular, the number of operating poles of the compressor motor  39  is changed by controlling the state of the compressor motor control contacts  40 , as will be explained in further detail below with reference to FIG. 7. Power to switch the compressor motor control contacts is also received from the output of the controller  26  through the contactor control circuit  38  as shown by line  42  but could be provided from another source. Power for the electronic components such as controller  26 , frequency detector  24 , and contactor control circuit  38  could be derived from the companion alternator output or could also be derived from another source such as the 74 volt DC supply commonly available in locomotive applications.  
         [0028]    The controller  26  also receives input data from a CRAT (cab return air temperature) thermistor  44  and an OAT (outside air temperature) thermistor  46 . A heater interlock arrangement may also be provided to the controller  26  to ensure that A/C does not operate when heaters are energized. A control switch  48  provides an input to the controller  26  for selecting any one of five operating modes of the unit: OFF, LOW VENT, HIGH VENT, LOW COOL, and HIGH COOL. An HPS (high pressure switch)  50  and an LPS (low pressure switch)  52  provide discrete control signals in response to refrigerant circuit pressure and are intended to halt A/C operation respectively in the event of either excessively high or low system pressure.  
         [0029]    In one embodiment of operation, at engine speeds less than a threshold speed in the range of about 300 to about 500 RPM (as determined by controller  26  from detector circuit  24 ) the motor  39  is connected for high speed (2 pole) operation; and at engine speeds above the threshold speed (as determined by controller  26  from detector circuit  24 ), the motor  39  is automatically connected for low speed (4 pole) operation by the controller  26  according to the output effected on line  27  to the contactor control circuit  38 . The threshold speed can be selected as desired for a given implementation, and in some implementations might be outside the specifically noted range.  
         [0030]    Since the frequency of the companion alternator output changes directly with locomotive engine speed, the period of the output voltage changes inversely with engine speed. A timer in controller  26  may monitor this period and cause appropriate switching to occur as previously described. Each switch point may be provided with a few RPM of hysteresis to ensure chatter-free speed changes. For example, while a change from two-pole operation to four-pole operation might occur at an established period of the companion alternator  22  output (representative of the threshold engine speed), a switch back to two-pole operation may be prevented unless the period of the companion alternator  22  output signal falls below the established period by a predetermined or threshold amount in order to prevent rapid switching between motor states when the locomotive is operating around the switching speed. It is recognized that some other indicator of locomotive engine speed could likewise be monitored to control the switching of the motor states or speeds.  
         [0031]    The inverter  30  can maintain a constant fan speed, independent of engine speed, since it produces 55 VAC motor power regardless of the engine speed.  
         [0032]    The proposed system may use a shaft-driven (open drive) compressor, similar in concept to those found in earlier non-hermetic systems. Refrigerant leakage at the rotary shaft seal, if a concern, can be mitigated by providing seal lubrication and extra refrigerant. In particular, because air conditioners may set unused for weeks or months on a locomotive, the shaft seal can dry out in that time and cause refrigerant to leak. Logic can be built into the controls to run the compressor for a few seconds every day to keep the seal lubricated, thereby extending its useful life. Further, even with such a regular seal lubrication scheme, it is likely that the shaft seals will leak at some time. Such leakage need not result in a immediate loss of performance if extra refrigerant is carried in the system. A receiver  15  (FIG. 4) connected immediately after the condenser coil accomplishes this purpose and may improve A/C capacity in high ambient conditions. These two measures may be used to extend the maintenance interval of the AC system.  
         [0033]    Referring now to FIG. 6, a schematic depiction of another embodiment of a refrigerant system  200  is shown. The refrigerant system  200  is substantially the same as system  10  of FIG. 4, with the exception that a compressor bypass path  202  is provided having a normally closed solenoid valve  204 , or other flow control device, positioned therealong. The bypass path  202  can be used to substantially equalize compressor head pressure prior to changing motor pole or speed states as will be described in more detail below. Referring to FIG. 7, another embodiment of a power and control system arrangement  220  is depicted. Like numerals reflect similarities between system  220  and system  20  of FIG. 5. System  220  includes an opto-isolator arrangement  222  between the companion alternator  22  and the frequency detector circuit  24 , using diode  224  and light sensitive transistor  226 , to provide electrical isolation of the companion alternator output from the frequency detector circuit  24 . A 2 pole R-C filter  228  is also provided to remove noise from the companion alternator output. LED  224  turns on and off with a frequency which corresponds to the frequency of the companion alternator output, and thus transistor  226  switches between on and off states in a corresponding manner, enabling circuit  24  to detect the frequency or period of the companion alternator output.  
         [0034]    The controller  26  of system  220  also includes an additional output  230  which is used to control the solenoid valve  204  along bypass path  202 . In particular, the solenoid valve  204  may be opened each time a transition between pole states of motor  39  is made in order to reduce head pressure. For example, when it is desired to switch from a 2 pole high speed state to a 4 pole low speed state, or visa versa, operation of the compressor motor  39  may be halted for an established time period and the solenoid valve  204  may be opened during at least part of that time period in order to substantially equalize the pressure. The compressor motor  39  can then be started again in its new pole state. In one embodiment the established time period may be between about 5 and about 25 seconds while in another embodiment the time period may be between about 10 and about 20 seconds, but it is recognized that this time period could vary from those ranges depending upon the particular application or system. The solenoid valve  204  may be closed immediately before restarting the compressor motor, simultaneous with restarting the compressor motor, or after restarting the compressor motor.  
         [0035]    With respect to switching of the motor speed states, in the illustrated embodiments compressor motor  39  comprises a single winding motor which is controlled by changing its number of poles by connecting the consequent pole stator winding in delta configuration for low speed operation and in wye configuration for high speed operation. In particular, referring to FIG. 7, the HI  1  contact controller controls HI contacts  230 , the HI  2  contact controller controls HI contacts  232 , and the LO contact controller controls contacts  234 . Table  236  shows the manner in which the contacts are controlled in order to achieve desired low speed and high speed operation. It is recognized that other multi-speed motor arrangements could be utilized.  
         [0036]    Although the invention has been described above in detail referencing the preferred embodiments thereof, it is recognized that various changes and modifications could be made without departing from the spirit and scope of the invention.