Patent Publication Number: US-6210119-B1

Title: Reverse rotation detection compressors with a preferential direction of rotation

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a method and system for detecting reverse rotation of a compressor due to improper wiring. 
     Compressors are a major component in air conditioning and refrigeration systems. One popular type of compressors is a scroll compressor. In a scroll compressor, a pair of wraps interfit to define a plurality of compression chambers. One of the wraps is driven through an orbit relative to the other, and the compression chambers are reduced in volume such that they compress an entrapped fluid. Scroll compressors are designed to rotate in a forward direction for fluid compression. They are not intended to rotate in the reverse direction during normal operation. 
     However, if the compressor is improperly wired, or under certain operational conditions, it is possible for the compressor to operate in reverse. Scroll compressors include a motor received in a sealed compressor shell. The refrigerant leading into the compression chambers passes over the motor on its way to the compressor, cooling the motor. 
     During reverse rotation, little or no refrigerant is pumped through the compressor, and thus the motor can quickly overheat. At the same time, refrigerant is not driven through the compression chambers. Thus the pair of interfitting scroll wraps can also quickly overheat due to heat generated by internal friction between the fixed and orbiting wrap. If reverse rotation is permitted to continue for a long period of time, there is the possibility of damage to the scroll compressor wraps or the motor. 
     The prior art placed motor protection sensors or line breaks, which would cut off power to the compressor if the motor is overheated. However, this can result in continuous compressor cycling as the motor cools off and compression comes back on line. Then the motor overheats again and the compressor is cycled off. 
     This of course prevents the compressor from performing its duty of compressing refrigerant, and can lead to potential compressor damage due to continuous cycling. Also the compressor wraps can be damaged due to overheating before the compressor is cycled off. 
     For residential or commercial applications, it is sometimes possible to detect reverse rotation since it typically results in loud undesirable noise. Since the compressor in a residential or commercial application is typically near occupants, the sound may be noted and corrective measures can take place. However, in typical container refrigeration applications, such as refrigerated transport containers, the compressor and refrigerant system are not mounted near any operator who could hear the sound. Further, such systems include large fans which also generate substantial noise. This noise often masks any increase in the compressor noise. 
     A three phase compressor is driven by a motor receiving three phase power. Such a compressor can run in reverse, if the connections are miswired at the main power supply or at the connection to the compressor. 
     The problem of miswiring is especially acute if the connections must be repeatedly made. This is particularly true with three phase compressors in transport container refrigeration where electrical reconnections are frequently made thus increasing the risks of miswiring. 
     The present invention discloses methods and apparatus for detecting and responding to the detection of compressors running in reverse. 
     SUMMARY OF THE INVENTION 
     The present invention detects the occurrence of reverse rotation by comparing the suction and discharge of the compressor to the expected pressures. As an example, the system could look at the pressure differential between the compressor suction and compressor discharge. If the pressure differential is below a minimum value, then a control for the system can identify reverse rotation. If rotation were in the proper direction, then the pressure differential across the compressor will be much greater. Alternatively, the system may only look at the compressor discharge pressure, and determine whether the discharge pressure increases after start-up within a set period of time to a given minimum value. Again, if the pressure does not increase, drops, or stays the same then a determination can be made that the compressor must be running in reverse. 
     In a first embodiment, if reverse rotation is detected, the system may be shut down and/or an alarm generated. An operator then knows to correct the problem. 
     In a second embodiment, the system responds by switching the phase on any two of the three wires in the three-phase power supply. By switching these two wires, the system reverses the polarity of the power supply leading to the compressor. This will cause the compressor to begin running in the opposite direction. If the problem that caused the reverse rotation was a miswiring at the compressor, then this reversal may result in other three-phase components in the system connected to the power supply to start running in reverse. Often there are no other three-phase components, thus, it becomes a non-issue. If other three-phase components are present, such as three-phase fans, their operation in reverse is not as detrimental to system and component reliability as compressor operation in a reverse direction. Further, if the problem that caused the compressor to run in reverse has originated at the main power supply, this phase switching will universally correct that problem. 
     These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view of the first embodiment of the present invention. 
     FIG. 2 is a flow chart of the inventive system. 
     FIG. 3 is a schematic of a second embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
     FIG. 1 shows a refrigeration system  20  which receives a three phase power supply  22  having three lines  24  leading to a connector  26 . Connector  26  is connected to a system connector  28 . Supply lines  30  lead from connector  28  to a second connector  32  connected to a connector  34  having lines  36  leading to a motor  38  for a compressor  40 . Motor  38  is a three-phase motor, and there are three lines at each of the power supply stages  24 ,  30  and  36 . Compressor  40  is illustrated as a scroll compressor but can be any other compressor with a preferential direction of rotation. It is desirable for compressor  40  to rotate in one direction, and not in the reverse direction. Thus, there is a proper orientation of the lines  24 ,  30  and  36  which is achieved when the connections  26  and  28  and  32  and  34  are properly made. However, should a single connector  26 ,  28  or  32 ,  34 , be improperly connected, then the power supply to motor  38  is improper and the compressor  40  will be driven in a reverse direction from that which is desired. 
     The compressor  40  is shown schematically and is a key element for providing cooling to refrigerated transport container  42 . This transport container may be of the known type which are utilized to transport food products or other items which must be maintained at a set temperature. This known type of transport container may be shipped on a boat, carried on a railcar, and eventually transported by truck on a trailer. During this entire travel, the container  42  must be kept at a set temperature. During the connection and disconnection of the refrigeration system  20 , and further with routine maintenance, replacement, etc., it is possible that connectors  26 ,  28  or  32  and  34  may be improperly connected, thus, resulting in a reverse rotation of the compressor at start-up. It would be desirable to provide an indication to an operator of the miswiring as soon as possible. 
     As shown schematically, a suction line  44  leads to compressor  40  and a discharge line  46  leads from the compressor  40 . Pressure sensors  48  and  50  are placed on lines  44  and  46 , respectively. The pressure sensors  48  and  50  send signals to a controller  52 . Controller  52  communicates to alarm panel  54 , and to motor  38 . 
     At the start-up of the compressor, suction and discharge pressure should be equal as pressure equalization occurs over time. However, on start-up, the pressure seen at discharge line  46  should quickly increase relative to the pressure seen at suction inlet line  44 . Thus, the controller  52  may compare the signals from sensors  48  and  50 , and if a predetermined minimum differential is not established within a set period of time, the controller  52  can identify the motor as running in reverse. When the motor is identified as being run in reverse, an action is taken. In the preferred embodiment, the motor  38  is stopped, and a signal is sent to the operator through alarm panel  54 . Of course, the signal can be visual, audio, etc. 
     Alternative ways of determining that the compressor is running in reverse based upon system parameters can also be used. As an example, only a discharge pressure sensor may be used. In this case it may only be necessary to measure the discharge pressure at or before start-up, and compare it to the discharge pressure at some period of time after start-up. If there is no pronounced change in discharge pressure, the controller may identify the system as running in reverse. In addition, rather than monitoring pressure, the temperatures or other parameters inside or outside of the compressor, evaporator, or condenser could be measured. 
     FIG. 2 is a flow chart for this method. The first step is electrically connecting the system and running the system and compressor. The controller monitors the inlet and outlet pressures while the compressor is running, and shuts down and/or sends signals when a determination is made that the compressor is running in reverse. 
     FIG. 3 shows a second embodiment  60  wherein power supplies and controls are similar to those used in the first embodiment and are identified by the same reference numerals. In embodiment  60 , feeder junction  56  leads to a connector  62  connected to a connector  64  for a fan motor  66 . A similar supply  68  may lead to a connector  69  and  71  for other three phase motors  72 . The system in the second embodiment identifies reverse rotation in the same manner as the first embodiment. However, rather than shutting down the motor or signaling the reverse rotation to the operator, a phase changing member  70  is placed in the line between the power supply  22  and a feeder junction  56 . Such phase change mechanisms are known in the field of three-phase motors. When reverse rotation is detected, the phase change element  70  reverses the phase of any two of the three power supply lines. This will necessarily result in the compressor beginning to run in the opposite direction from that which it had been previously running. 
     With this system, should reverse rotation be detected at the compressor, the phase shift is made and the compressor will begin to run in the opposite direction. If the miswiring is at the main power supply line, this will correct any miswiring. However, if the miswiring is at the connection  32  and  34 , the other three phase motors  66  and  72  will now be running in reverse. Even though it may not be desirable for the fan and other system devices to run in reverse, this typically does not result in their damage or cause improper system operation. The compressor however will most probably be damaged after running in reverse for a set time period. 
     It should be understood that other fluid characteristics, in addition to discharge and/or suction compressor pressure, in the refrigeration cycle could be monitored within the scope of this invention. As an example, the temperature at the inlet or the outlet of the compressor could be sensed. Alternatively, the temperature at compressor upstream locations such as the evaporator or compressor downstream locations such as condenser can be monitored. The main aspect of this invention is the monitoring of a refrigerant system characteristic to determine when reverse rotation is occurring. 
     Preferred embodiments of this invention have been disclosed, however, workers of ordinary skill in the art would recognize that certain modifications will come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.