Generator power management

A system and method for monitoring and controlling an electrically driven transport refrigeration unit under varying operating conditions while maintain the system generator within its electric current and temperature limitations is disclosed. Specifically, the present invention the management of generator power through the combined use of controls for a suction modulation valve (an "SMV"), for diesel engine speed control, and for electronic expansion valve ("EXV") superheat settings.

FIELD OF THE INVENTION
 The field of the present invention relates to control systems for transport
 refrigeration systems. More specifically, the present invention is
 directed towards controlling the generator power requirements through the
 combined control of the suction modulation valve (the "SMV") the
 compressor cylinder unloaders, the speed of the diesel engine for the
 system, and the superheat setting of the expansion valve.
 DESCRIPTION OF THE PRIOR ART
 A common problem with transporting perishable items is that often such
 items must be maintained within strict temperature limits, regardless of
 potentially extreme operating conditions required by a high ambient
 temperature and/or other factors. These extreme conditions can cause an
 excessive power draw from the diesel engine powering the system, thus
 potentially causing unwanted system shutdowns or even adversely impacting
 the useful life of the engine. Recent inventions by the assignee of the
 present application have enabled significant cost savings through the
 implementation of an electrically driven trailer refrigeration unit from a
 synchronous permanent magnet generator. However, the use of this new
 system has a disadvantage of significant power supply limitations compared
 to prior art devices. Thus, there is a need for an efficient controller
 which optimizes power management for the draw placed upon the generators
 of such transport systems.
 Currently available controller designs sold by assignee disclose the use of
 suction modulation valves ("SMVs") to limit the maximum system current
 draw. In addition, such units use a suction modulation valve (SMV) to
 limit the maximum system current draw, but not to control the voltage. The
 SMVs of such systems close quickly, but result in pressure drop problems
 which limit peak capacity problems, and create reliability and efficiency
 issues. In addition, previously available prior art controls are
 comparatively crude to that needed for newer, power limited systems, which
 call for sophisticated, combined controls that monitor and manipulate
 superheat settings, compressor cylinder unloaders, and engine speed in
 order to prevent unacceptable power draw on the transport refrigeration
 system. The inventors of the system and process disclosed in the present
 application have found significant improvements in generator power
 management by controlling just such parameters, thus decreasing system
 component wear and tear and increasing the engine and generator life.
 SUMMARY OF THE INVENTION
 The apparatus and control method of this invention provides a refrigeration
 unit for a transport system having a controller which monitors and
 prevents power draw overload conditions for the generator in situ. For
 example, the algorithm in the controller is designed to adjust for changes
 in box temperature and speed changes. The present invention manages power
 (indirectly) by monitoring and controlling engine speed and current. The
 controller of the present system is designed to avoid overpowering the
 generator and the engine (i.e., just to maintain the generator and the
 engine unit running), and is further designed to avoid shut down (and
 potentially, damage). Such conditions are avoided by staying below a
 preselected maximum system power limit rating.
 The present invention does not maximize capacity in in the box or container
 of the transport refrigeration system, the present invention is rather
 directed towards minimizing the limits place upon current level--thus, the
 present invention seeks minimize the reduction of the optimal
 refrigeration capacity level. Power, and consequently current draw, rise
 and fall with mass flow. Thus, the controller of the present invention
 monitoring current and controls mass flow as a means to control power draw
 on the generator.
 Also, the present invention seeks to limit the temperature of the generator
 by controlling the SMV. If the generator temperature for a permanent
 magnet rotor exceeds a certain point it will demagnitize and thus require
 an expensive and time consuming overhaul. Thus, the controller of the
 present unit monitors generator temperature and restricts mass flow in the
 system (thus decreasing power draw in the event the generator temperature
 is above a preselected limit. Specifically, the generator temperature
 sensed goes above a "soft" limit, the controller further restricts the SMV
 (indirectly, by reducing the maximum allowable current, through the PID
 control in the processor). If the generator temperature sensed goes above
 a "hard" limit, then an alarm is issued, and the unit may initiate shut
 down.
 Finally, the present invention seeks to minimize or eliminate the step
 function caused by the deenergizing of each loader of the system
 compressor, which load two additional cylinders in the compressor Each
 deenergized unloader, when deenergized, has the effect of increasing mass
 flow at least 50%, and can increase current beyond the maximum current
 draw permitted. Thus, the present invention, using programming in the
 microprocessor of the controller, increases the superheat setting, which
 results in restricting the electronic expansion valve (the "EXV") and thus
 reduces the mass flow prior to the unloader being deenergized. The
 superheat setting (and thus the EXV) is then gradually reduced to its
 initial base levels once the current draw sensed is below a preselected
 limit.

DETAILED DESCRIPTION OF THE INVENTION
 The invention that is the subject of the present application is one of a
 series of applications dealing with transport refrigeration system design
 and control, the other copending applications including: "Voltage Control
 Using Engine Speed"; "Economy Mode For Transport Refrigeration Units";
 "Compressor Operating Envelope Management"; "High Engine Coolant
 Temperature Control"; "Superheat Control for Optimum Capacity Under Power
 Limitation and Using a Suction Modulation Valve"; and "Electronic
 Expansion Valve Control Without Pressure Sensor Reading" all of which are
 assigned to the assignees of the present invention and which are hereby
 incorporated herein by reference. These inventions are most preferably
 designed for use in transportation refrigeration systems of the type
 described in copending applications entitled: "Transport Refrigeration
 Unit With Non-Synchronous Generator Power System;" Electrically Powered
 Trailer Refrigeration Unit With Integrally Mounted Diesel Driven Permanent
 Magnet Generator;" and "Transport Refrigeration Unit With Synchronous
 Generator Power System," each of which were invented by Robert Chopko,
 Kenneth Barrett, and James Wilson, and each of which were likewise
 assigned to the assignees of the present invention. The teachings and
 disclosures of these applications are likewise incorporated herein by
 reference.
 FIG. 1 illustrates a schematic representation of the transport
 refrigeration system 100 of the present invention. The refrigerant (which,
 in its most preferred embodiment is R404A) is used to cool the box air
 (i.e., the air within the container or trailer or truck) of the
 refrigeration transport system 100. is first compressed by a compressor
 116, which is driven by a motor 118, which is most preferably an
 integrated electric drive motor driven by a synchronous generator (not
 shown) operating at low speed (most preferably 45 Hz) or high speed (most
 preferably 65 Hz). Another preferred embodiment of the present invention,
 however, provides for motor 118 to be a diesel engine, most preferably a
 four cylinder, 2200cc displacement diesel engine which preferably operates
 at a high speed (about 1950 RPM) or at low speed (about 1350 RPM). The
 motor or engine 118 most preferably drives a 6 cylinder compressor 116
 having a displacement of 600cc, the compressor 116 further having two
 unloaders, each for selectively unloading a pair of cylinders under
 selective operating conditions. In the compressor, the (preferably vapor
 state) refrigerant is compressed to a higher temperature and pressure. The
 refrigerant then moves to the air-cooled condenser 114, which includes a
 plurality of condenser coil fins and tubes 122, which receiver air,
 typically blown by a condenser fan (not shown). By removing latent heat
 through this step, the refrigerant condenses to a high pressure/high
 temperature liquid and flow to a receiver 132 that provides storage for
 excess liquid refrigerant during low temperature operation. From the
 receiver 132, the refrigerant flows through subcooler unit 140, then to a
 filter-drier 124 which keeps the refrigerant clean and dry, and then to a
 heat exchanger 142, which increases the refrigerant subcooling.
 Finally, the refrigerant flows to an electronic expansion valve 144 (the
 "EXV"). As the liquid refrigerant passes through the orifice of the EXV,
 at least some of it vaporizes. The refrigerant then flows through the
 tubes or coils 126 of the evaporator 112, which absorbs heat from the
 return air (i.e., air returning from the box) and in so doing, vaporizes
 the remaining liquid refrigerant. The return air is preferably drawn or
 pushed across the tubes or coils 126 by at least one evaporator fan (not
 shown). The refrigerant vapor is then drawn from the exhanger 112 through
 a suction modulation valve (or "SMV") back into the compressor.
 Many of the points in the transport refrigeration system are monitored and
 controlled by a controller 150. As shown in FIGS. 2 and 2A Controller 150
 preferably includes a microprocessor 154 and its associated memory 156.
 The memory 156 of controller 150 can contain operator or owner
 preselected, desired values for various operating parameters within the
 system, including, but not limited to temperature set point for various
 locations within the system 100 or the box, pressure limits, current
 limits, engine speed limits, and any variety of other desired operating
 parameters or limits with the system 100. Controller 150 most preferably
 includes a microprocessor board 160 that contains microprocessor 154 and
 memory 156, an input/output (I/O) board 162, which contains an analog to
 digital converter 156 which receives temperature inputs and pressure
 inputs from various points in the system, AC current inputs, DC current
 inputs, voltage inputs and humidity level inputs. In addition, I/O board
 162 includes drive circuits or field effect transistors ("FETs") and
 relays which receive signals or current from the controller 150 and in
 turn control various external or peripheral devices in the system 100,
 such as SMV 130, EXV 144 and the speed of engine 118 through a solenoid
 (not shown)
 Among the specific sensors and transducers most preferably monitored by
 controller 150 includes: the return air temperature (RAT) sensor which
 inputs into the processor 154 a variable resistor value according to the
 evaporator return air temperature; the ambient air temperature (AAT) which
 inputs into microprocessor 154 a variable resistor value according to the
 ambient air temperature read in front of the condenser 114; the compressor
 suction temperature (CST) sensor; which inputs to the microprocessor a
 variable resistor value according to the compressor suction temperature;
 the compressor discharge temperature (CDT) sensor, which inputs to
 microprocessor 154 a resistor value according to the compressor discharge
 temperature inside the cylinder head of compressor 116; the evaporator
 outlet temperature (EVOT) sensor, which inputs to microprocessor 154 a
 variable resistor value according to the outlet temperature of evaporator
 112; the generator temperature (GENT) sensor, which inputs to
 microprocessor 154 a resistor value according to the generator
 temperature; the engine coolant temperature (ENCT) sensor, which inputs to
 microprocessor 154 a variable resistor value according to the engine
 coolant temperature of engine 118; the compressor suction pressure (CSP)
 transducer, which inputs to microprocessor 154 a variable voltage
 according to the compressor suction value of compressor 116; the
 compressor discharge pressure (CDP) transducer, which inputs to
 microprocessor 154 a variable voltage according to the compressor
 discharge value of compressor 116; the evaporator outlet pressure (EVOP)
 transducer which inputs to microprocessor 154 a variable voltage according
 to the evaporator outlet pressure or evaporator 112; the engine oil
 pressure switch (ENOPS), which inputs to microprocessor 154 an engine oil
 pressure value from engine 118; direct current and alternating current
 sensors (CT1 and CT2, respectively), which input to microprocessor 154 a
 variable voltage values corresponding to the current drawn by the system
 100 and an engine RPM (ENRPM) transducer, which inputs to microprocessor
 154 a variable frequency according to the engine RPM of engine 118.
 In the present invention, the power consumed by the compressor 116 varies
 directly with the mass flow rate of the refrigerant. Suction modulation
 valve or SMV 130 is used to provide the possibility of creating a
 restriction to refrigerant, thus causing a consequent reduction in power
 draw. Power consumption in monitored indirectly by controller 150 through
 measurement of current received through I/O board 156. A proportional,
 integral and derivative control used by processor 154 based upon the
 actual generator current measured relative to the allowable generator
 current as stored in memory 156 is then used to control the position of
 SMV 130 and limit the generator current while maximizing system 100
 refrigeration capacity. Thus the SMV 130 eventually restricts mass flow
 rate to the point where the actual current rate drops below the
 predetermined maximum level as stored in the memory 156 of controller 150.
 SMV 130 is also used to limit the temperature of the generator, as input to
 the controller as GENT. If GENT is above the timed maximum generator
 temperature as defined in the configuration parameter stored in memory 156
 for a preselected time frame (preferably greater than 5 minutes), then the
 maximum allowable generator current (which is programmed in controller
 150) is reduced by 1 amp. This will, in most instances, cause the SMV 130
 to close and reduce power draw. If, however, after a preselected time
 frame (e.g., 5 minutes) the GENT value is still above the preselected
 limit, the generator current limit is further reduced by an additional,
 preselected amount (e.g., 5 amps) and held at this level for a preselected
 time (e.g., 10 minutes). If after this period the temperature is still
 above the timed maximum generator temperature, a high generator
 temperature alarm is triggered and the operator is notified through
 display 164. If the temperature is below the timed maximum generator
 temperature, processor 154 restores the maximum allowable generator limit,
 most preferably at the rate of 1 amp per minute.
 The present invention further includes an engine speed control to limit
 unnecessary power draw on the generator. As mentioned above, a preferred
 embodiment of the present invention includes a diesel engine 118 that has
 two settings--high speed and low speed. Because generator 120 has more
 available power when the engine is operating in high speed, controller 150
 allows a higher maximum current draw from the generator 120 in high speed
 mode. However, any time the system 100 requires a transition from high
 speed to low speed (e.g., capacity control, fuel savings, high ambient
 temperature, etc.) the maximum allowable current draw has to be adjusted
 to reflect the lower available power from generator 120 in low speed.
 Controller 150 accomplishes this function by immediately reducing the
 maximum current limit value used to control the SMV 130 when low operation
 is requested, but then also delay the actual speed reduction control
 signal until either: 1) the actual current draw value measures is equal to
 (or lower than) the low engine speed current limit; or 2) a preselected
 time limit (preferably at least 40 seconds) elapses from the time that the
 low speed request is received by controller 150.
 Finally, the present invention manages generator power through the control
 of flow rate in the event of compressor unloader deenergizing. As
 mentioned above and shown in FIGS. 3 and 3A, the preferred embodiment of
 the present invention includes a compressor 116 having six cylinders and
 two unloaders. Each unloader, when energized, unloads a bank of two
 cylinders. Thus, when a cylinder bank is loaded there is a step increase
 of at least 50% (i.e., 2 to 4 cylinders, or 4 to 6 cylinders) in the
 refrigerant mass flow rate, and a consequent increase in power
 consumption. To reduce risk of damage to the generator due to such a
 "spike" in power consumption, the controller 150 monitors the actual
 generator current and prevents the unloaders from deenergizing when the
 actual generator current is equal to or greater than the maximum allowable
 generator current value, as stored in memory 156.
 The present invention has a further power consumption protection mechanism
 in the unloader deenergizing process. Specifically, prior to controller
 issuing a control signal requesting a cylinder bank to be loaded,
 controller 150 first adds to the superheat setting of the EXV 144 (as
 stored in memory) by a fixed amount. The increase in superheat setting
 causes EXV 144 to close, thus restricting the mass flow of the
 refrigerant, thus dissipating any impact on power draw which the
 additional cylinder bank being loaded might have. The controller 150 then
 gradually reduces the superheat setting (i.e., controllably opens the EXV)
 to its original value. Thus, the controller 150 of the present invention
 seeks to combine controls over engine speed, superheat (EXV) settings, SMV
 settings, and maximum current draw limits in order to prevent unnecessary
 power draw on the generator 120 in a variety of operating conditions.
 It will be appreciated by those skilled in the art that various changes,
 additions, omissions, and modifications can be made to the illustrated
 embodiments without departing from the spirit of the present invention.
 All such modifications and changes are intended to be covered by the
 following claims.