Abstract:
A system for cooling a power transformer which generates heat, when driving a load, includes cooling devices located about the transformer which are powered to remove excessive heat from the transformer. The cooling devices may include fans to blow air onto the transformer and pumps for circulating a coolant about the transformer. The cooling devices of interest have a motor (e.g., a fan motor or a pump motor) which is energized in response to given temperature (heat) conditions. In systems embodying the invention, the currents flowing through the motors of cooling devices are sensed and monitored to determine whether the cooling devices are functioning correctly and to substitute functional cooling devices for those which are malfunctioning. The importance of sensing the motor currents and substituting operational cooling devices for defective ones is that a temperature rise due to a failure of a cooling device is not immediately detectable due to the large thermal constants associated with the transformer assembly. Sensing the currents in the motors enables the early detection of fault conditions in the cooling system. It also enables the monitoring of operating conditions and running time of the cooling devices to aid in the maintenance and operation of the cooling system.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention claims priority from provisional application Ser. No. 61/132,604 for Transformer Cooling Monitor And Control System filed Jun. 21, 2008 whose teachings are incorporated herein 
         [0002]    This invention relates to apparatus and methods for monitoring and controlling the cooling system of power transformers. 
         [0003]    Power transformers designed to distribute large amounts of power, such as substation and distribution class power transformers generally include cooling systems to remove heat generated when large loads are applied to the transformers (i.e., when large currents are drawn from and through the transformer). The cooling systems are designed to remove heat to help keep the transformer and its components below predetermined critical temperatures. Maintaining the transformer temperature below a critical value enables the transformer to handle a designed load capacity or to increase the power handling capability of the transformer. 
         [0004]    The cooling systems may include cooling fans to circulate air over the transformer. Alternatively, the transformer may be contained within a liquid (e.g., oil) filled tank with oil pumps being used to circulate the fluid through radiators attached to the tank and cooling fans circulating air over the radiators. The operation of the cooling system is vital for the transformer to deliver its designed power capacity. If the cooling is compromised, the transformer temperature may rise above desired values. Such a rise in temperature may result in the outright failure of the power transformer and at a minimum will result in some damage and an accelerated loss of life. That is, over time excessive heating will reduce transformer life and lead to premature failure which will affect the ability of a utility company to supply uninterrupted supply of power to its customers and will cost the operating utility significant replacement costs. 
         [0005]    Problems with prior art systems may be explained with reference to  FIGS. 1 ,  1 A and  2 , which show a housing  100  enclosing a power transformer  120 . As is known in the art, the primary and secondary windings of the transformer have some resistance (R). As current (I) flows through the windings, heat is generated which is a function of the winding resistance multiplied by the square of the current (i.e., I 2 R). A considerable amount of heat may be generated by, and within, the power transformer, particularly when the load is increased and more current flows through the transformer&#39;s primary and secondary windings. 
         [0006]    The heat generated within the transformer causes a rise in the temperature of the windings and in the space surrounding the windings and all around the transformer. When the temperature rises above a certain level many problems are created. For example, the resistance of the (copper) transformer windings increases as a function of the temperature rise. The resistance increase causes a further increase in the heat being dissipated, for the same value of load current, and further decreases the efficiency of the transformer. With increased temperature the transformer may also be subjected to increased eddy current and other losses. The temperature rise may also cause unacceptable expansion (and subsequent contraction) of the wires. Also, the insulation of the windings and other components may be adversely affected. Temperatures above designed and desirable levels result in undesirable stresses being applied to the transformer and or its components. This may cause irreversible damage to the transformer and its associated components and at a minimum creates stresses causing a range of damages which decrease its life expectancy. 
         [0007]    It is therefore desirable and/or necessary to maintain the temperature of the power transformer below a predetermined level. 
         [0008]    In  FIGS. 1 and 1A  the transformer  120  may be cooled by immersing the transformer in a liquid (e.g., oil) and having the liquid flow through pipes  110  extending through the radiators (e.g.,  2  and  41 ). Pumps (not shown) may be used to circulate the liquid (oil) through the radiators where the liquid may be subjected to cooling by means of cooling fans  6  and  7 . A bank of cooling fans  6  and  7  (three fans are shown in bank  6  in  FIG. 1 ) may be used to selectively blow air, or any other suitable coolant, over radiators (e.g.,  2  and  41 ) to cool the liquid as it passes through the radiators.  FIGS. 1 and 1A  show: (a) a sensor  42  designed to sense the winding temperature which is coupled to a winding temperature control module  4  having an indicator for displaying the transformer winding temperature; and (b) a sensor  82  designed to sense the top oil temperature coupled to a top oil temperature control module  8  with an indicator for displaying the temperature of the top oil. The signals from sensors  42  and  82  are processed by their respective modules. When predetermined temperature levels are reached, the cooling fans  6  and  7  are powered by signals generated by and within fan motor control modules  4  and  8  in response to the signals generated by temperature sensors  42  and  82 . 
         [0009]      FIG. 2  illustrates circuitry, which may be contained in a control cabinet  3  attached to housing  100 , for applying power to the fan motors to drive the fans. Control module  4  includes means for processing signals from sensor  42  and to generate a command signal applied to a motor winding control circuit  421  which, in turn, functions to control (turn-on and turn-off) switch  6 S which then applies power to the motors (FM 1 , FM 2 , FM 3 ) of cooling fans  6 A,  6 B and  6 C In a similar manner, control module  8  includes means for processing signals from sensor  82  and to generate a command signal to a motor winding control circuit  821  which, in turn, functions to control switch  8 S which then applies power to the motors (FM 4 , FM 5 , FM 6 ) of cooling fans  7 A,  7 B and  7 C. 
         [0010]    Admittedly, the prior teaches the use of cooling systems to protect a power transformer from excessive temperatures. However, a problem with known prior art systems, as illustrated in  FIGS. 1 ,  1 A and  2 , is that, in the event the cooling system fails, the temperature limits will be reached and/or exceeded before any corrective action can be taken. For example, if fan control switch  6 S or  8 S fails and/or in the event that a fan motor fails, the cooling of the power transformer is partially or wholly compromised. There is no provision which indicates the failure of the cooling device until the rise in temperature exceeds given limits and an alarm is sounded. Due to the large mass of the transformer system (there is a large thermal coefficient), by the time an alarm is sounded and corrective action is taken, the temperature of the transformer and associated components may rise considerably above desired and or design limits resulting in damage to the system. 
         [0011]    Clearly, the prior art does not address the problem which arises when malfunctions and failures of the cooling system are not detected early and quickly. The prior art also does not address the need to monitor the functionality of the cooling system components. These problems and other drawbacks present in the prior art are overcome in systems embodying the invention. 
       SUMMARY OF THE INVENTION 
       [0012]    A power transformer generates heat when supplying power to a load. Typically, several cooling devices are mounted on or about the power transformer and are operated (e.g., turned-on or energized) to remove excessive heat from the transformer so as to try to maintain the temperature of the transformer below predetermined levels. The cooling devices may include: (a) fans to blow a gaseous coolant (e.g., air) onto the transformer or onto radiators carrying a liquid coolant in contact with the transformer; and/or (b) pumps for circulating a liquid coolant (e.g., oil) about the transformer. The cooling devices of interest have a motor (e.g., a fan motor or a pump motor) which is energized in response to given temperature and/or heating conditions. In accordance with the invention, the currents flowing through the motors of cooling devices are sensed and monitored to determine whether the cooling devices are functioning correctly. The importance of sensing the motor currents is that it provides an immediate indication of the malfunction of its corresponding cooling device. This is highly significant since a failure of the cooling devices to perform its intended task is not immediately detectable due to the large thermal constants associated with the relatively massive power transformer assembly. Sensing the currents in the motors of the cooling devices enables the early detection of fault conditions. It also enables the monitoring of the operating conditions of the cooling devices for proper maintenance and operation of the entire cooling system. 
         [0013]    In accordance with the invention the current in the motors of cooling devices (e.g., fans and/or fluid circulating pumps) is sensed to determine the operability of the cooling devices and to provide an early indication if, and when, a cooling device is malfunctioning. 
         [0014]    Systems embodying the invention include means for sensing the current flowing through the motors of N sets of cooling devices for determining whether the cooling devices are functioning properly and to enable the substitution of a device which is functioning properly for one which malfunctioning. The N sets of cooling devices may be intended to be powered in a given sequence under normal conditions, in response to predetermined temperature conditions. In the event the malfunction of a cooling device is detected, means responsive to the sensed motor currents cause the immediate powering of another one of the N sets of cooling devices for the set including the malfunctioning cooling device; where N is an integer equal to or great than two (2). 
         [0015]    Furthermore, in accordance with the invention, each motor of a cooling device is controlled (turned on and off) in response to (a) a first signal responsive to the temperature conditions pertaining to the power transformer; and (b) a second signal responsive to the functionality condition (conduction) of the motor. 
         [0016]    Systems embodying the invention having more than one cooling device (e.g., multiple cooling fans or pumps) may include means for selectively testing their operability and means for switching an operable cooling device for a malfunctioning cooling device. 
         [0017]    Recognizing that the motor of a cooling device (e.g., a fan motor or a pump motor) is malfunctioning enables corrective action to be taken before critical temperatures are exceeded. This results in an earlier alert system if the sensed current indicative of a malfunction is sensed. That is, if there is a malfunction of the cooling system, there is no need to wait for the long thermal time constant of the transformer and its associated equipment to remediate problems with the cooling system. 
         [0018]    Systems embodying the invention may also include applying cooling in stages. For example, for sensed temperature above a first level and below a second level a first set of cooling fans is turned on, then for temperatures above the second level and below a third level a second set of cooling fans is turned on, then for temperatures above the third level and below a fourth level a third set of cooling fans is turned on. In addition, the current level drawn by the fan motors in each set is sensed such that if any one of the fans is malfunctioning, another one of the fans is turned on instead. 
         [0019]    Still further, the currents in the motors of the cooling devices may be processed such that in the event the fan motor currents are outside a prescribed range (above or below given limits), an alarm condition may be generated including alerting an operator to the potentially dangerous condition. 
         [0020]    Systems embodying the invention may also include means for monitoring the length of time the motors are operated and the current drawn by the motors to determine when preventative maintenance and/or replacement of the motors is in order. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    In the accompanying drawings, which are not drawn to scale, like reference characters denote like components; and 
           [0022]      FIG. 1  is a simplified drawing of a prior art housing containing a power transformer with cooling fans mounted on radiators and including transformer winding and oil temperature indicators; 
           [0023]      FIG. 1A  is a simplified drawing of a prior art system showing a power transformer immersed in oil within a housing, as shown in  FIG. 1 , with cooling fans for cooling a liquid flowing through the radiators and control means for controlling the operation of the cooling fans; 
           [0024]      FIG. 2  is a simplified diagram of a prior art control system responsive to winding and oil temperature suitable for use in the system of  FIGS. 1 and 1A ; 
           [0025]      FIG. 3A  is a simplified drawing of a system showing a power transformer immersed in oil within a housing with cooling fans for cooling a liquid flowing through the radiators and means for sensing the fan motor currents and control means for controlling the operation of the cooling fans in accordance with the invention; 
           [0026]      FIG. 3B  is a simplified drawing illustrating the sensing of fan motor current and the operation of cooling fan motors in accordance with the invention; 
           [0027]      FIG. 4A  is a simplified drawing of a system showing a power transformer immersed in a liquid coolant (e.g., oil) within a housing with a pump and pump motor for circulating the liquid and cooling fans for cooling the liquid flowing through the radiators and means for sensing the pump motor and fan motor currents and control means for controlling the operation of the pump motor and cooling fans, in accordance with the invention; 
           [0028]      FIG. 4B  is a simplified drawing illustrating the sensing of pump motor and fan motor currents and the operation of a pump motor and cooling fan motors in accordance with the invention; 
           [0029]      FIG. 5  is a more detailed block diagram of a transformer monitoring and cooling system embodying the invention; 
           [0030]      FIGS. 5A and 5B  are more detailed circuit diagrams of portions of the circuit of  FIG. 5 ; and 
           [0031]      FIG. 5C  is a partial logic diagram illustrating some of the functions performed in circuits embodying the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0032]    As shown in  FIGS. 3A ,  3 B,  4 A and  4 B, cooling systems embodying the invention include cooling devices, which when energized (“powered”), tend to maintain the temperature of an associated power transformer,  120 , below predetermined values. Cooling devices used to illustrate the invention include cooling fans  6 , 7  for blowing a gaseous coolant and pump(s) for circulating a cooling liquid about a power transformer. These cooling devices have motors whose currents can be measured. However it should be appreciated that the invention may be practiced with any cooling device whose current and/or voltage and/or power usage can be sensed. As noted above, if there is a loss of coolant due to the failure of a cooling device there may be an uncontrolled rise in the temperature of the transformer and/or the oil circulating around the transformer and/or components associated with the transformer resulting in catastrophic failure of the transformer and/or it associated components. This application aims at resolving problems where loss of cooling occurs due to the failure of the fans and/or pumps to operate as intended. 
         [0033]    As shown in  FIGS. 3A and 3B , systems embodying the invention differ form prior art systems in that they include means  190  for sensing the current(s) drawn by the motors of cooling fans  6  and  7 . The fan motor (FM) currents are sensed by means of a current transformer CT 12 , connected in series with the fan motors, whose output is fed to current sensor  190  and then to a module  210 . The presence as well as the amplitude of the fan motor current (s) can be determined. The amplitude can be determined with processing circuitry in module  190  or in module  210 . In the embodiment of  FIGS. 3A and 3B  it is assume that fan motor control module  210  is programmed to determine whether the fan motors are operating as intended (e.g., whether when energized a current flows and whether the amplitude of the current is within a prescribed range) and providing cooling to the transformer. 
         [0034]      FIG. 3B , which illustrates a simplified version of the system operation, shows an AC power source  212  supplying its voltage between terminals  214  and  218 . Three fan motors (FM 1 , FM 2 , FM 3 ) are shown connected via respective switches (S 1 , S 2 , S 3 ) between node  214  and an intermediate node  216 . Node  216  is then connected via the primary winding of a current sensing transformer CT 12  to terminal  218 . The secondary winding of CT 12  is shown connected to cooling fan current sensor  190  which is connected to control module  210 . Current sensor  190  and module  210  include circuitry for: (a) sensing the presence and amplitude of the sensed current; (b) processing, analyzing and storing the sensed data; and (c) producing signals for energizing predetermined switches/devices and sounding alarms, if necessary. Sensor  190  and module  210  are shown as separate circuits. However, they may be part of the same module or integrated circuit 
         [0035]    The turn-on of switches S 1 , S 2  and S 3  is initiated by signals generated by temperature sensors  42  and/or  82  which are supplied to module  210  which is designed and programmed to respond to these signals. Sensors  42  and  82  may include any probe capable of sensing temperature and providing an appropriate signal to processing circuitry contained in module  210 . 
         [0036]    For purpose of example assume that when the temperature (T) is above a temperature T 1  and below a temperature T 2  switch S 1  is to be closed supplying power to the FM 1  and activating fan  6 A. If the temperature (T) rises above T 2 , switch S 2  is to be turned on (closed) supplying power to FM 2  and activating fan  6 B. If the temperature keeps on rising and reaches a level T 3 , then switch S 3  is to be closed and power is supplied to FM 3  activating fan  6 C. It is assumed that the temperature T 2  is greater than T 1 , T 3  is greater than T 2  and T 4  is greater than T 3 . This describes the sequential activation of the fans, assuming they are all operating correctly. If the temperature rises above a level T 4 , an alarm is sounded to indicate the existence of an excessive condition. [Note: Three fans are shown for purpose of example only. There maybe more or less than three fans. Also, each one of FM 1 , FM 2 , FM 3  may include a set of fans connected in parallel, as illustrated by FM 1 A and FM 1 B drawn in dashed lines in parallel with FM.] 
         [0037]    However, in accordance with the invention, additional controls are place on the turn-on and turn off of the switches supplying power to the cooling devices, as discussed below. Assume now that S 1  is closed and FMi is to be powered. The current through FM 1  is sensed by CT 12  and processed in circuits  190  and  210 . If the sensed current through FM 1  is within a predetermined range, FM 1  is determined to be operational and S 1  is closed. If there is a malfunction in S 1  or in FM 1 , the current through CT 12  will reflect either; (a) an undercurrent condition (e.g., a partial or full open circuit) with the current being below a first value or (b) an overcurrent condition (e.g., a partial or full short circuit) with the current being above a second value. If a malfunction is sensed by sensor  190 , it produces a corresponding output signal which is then supplied to module  210 . Circuits  190  and  210  are designed and programmed to recognize the type of fault condition to enable a range of corrective actions to be undertaken. If the fault is significant, switch SI is opened removing power from FM 1 . Concurrently, switch S 2  is turned-on supplying power to FM 2  and activating fan  6 B and an alert signal may be produced indicating the nature of the fault. The corrective action taken can be supplied to the user (e.g., the entity having responsibility for the operation of the transformer). Also, the fault condition will be supplied to processing circuitry (not shown) tracking the condition of the cooling system and monitoring when needed maintenance is to be performed. 
         [0038]    Likewise, if there is a malfunction in S 2  or FM 2 , the sensed current through CT 12  will be below or above a predetermined value. The sensed signal is sent to circuits  190  and  210  which are designed and programmed to recognize the type and nature of the fault condition. If the fault is significant, switch S 2  is turned off removing power from FM 2 . Concurrently, a signal is generated to turn-on S 3  supplying power to FM 3 , activating fan  6 C, and alarms or alerts similar to those described above will be instituted and recorded. Thus, fault sensing of the cooling fans and correction for defective fans can be conducted automatically and the transformer power producing system is kept operational until an operator decides to take appropriate action. In brief, the current drawn by the fan motors is sensed such that, if any one of the fans is defective, another one of the fans is turned on instead. In addition, while remedial action is being taken an alarm may be generated to alert an operator to the potentially dangerous condition. 
         [0039]    A significant feature of the system is that circuits  190  and  210  can be programmed to periodically and selectively test the operability of all the fan motors individually. That is, module  210  can be programmed to turn-on switch S 1  (and turn off S 2  and S 3 ) and test for the presence and level of the current through FM 1  sensed by CT 12 . Then S 2  can be turned on and S 1  and S 3  turned off to test the operability of FM 2 . Then S 3  can be turned on and S 1  and S 2  can be turned off to test the operability of FM 3 . This mode of operation permits the testing of each fan motor and the determination of its operating conditions and whether any fan motor is not operating correctly. This testing can be done on a regular basis to determine the operability of the cooling system. This enables preventive action to be taken at low cost and with little effort. 
         [0040]      FIGS. 4A and 4B  illustrate that the transformer  120  may be contained within a housing  100  and a liquid coolant (e.g., oil) may be circulated about the transformer and radiators  2  and  41  by means of a pump  401  which is operated by a pump motor (PM)  402 . One pump is shown but there may be more than one. Similarly to the operation of the fan motors discussed above, the pump motor  402  may be energized by means of the turn-on of a switch S 10  connected between the motor  402  and terminal  214 . The current though the pump motor  402  may be sensed by means of a current transformer CT 412  whose primary winding is connected in series with motor  402  between the motor and terminal  218 . (Note that the current transformer in this instance and in the case of the fan motors may be located above or below the motor whose current it is sensing.) The pump motor is normally energized by closure of switch S 10  which applies power to the motor. The closure of switch S 10  is normally controlled by a pump motor processor control  410  in response to temperature signals from probes  42 ,  82  and/or any other suitable input (Tothers in  FIG. 5 ). When switch S 10  is closed a current flows through the motor. If the motor is operating as intended, the current level will normally be within a given range. If the motor is defective and/or if switch S 10  is not functioning and /or if the pump  402  is malfunctioning, the sensed motor current will be outside the given range. 
         [0041]    The current through the pump motor is sensed by CT 412  which supplies the sensed signal to current sensor  490  and module  410  for processing the output of CT 412  in a manner similar to that conducted by circuits  190  and  210 , describe above. The sensor  490  includes processing circuitry for sensing the current level of the pump motor. If the current level of the pump motor is too high or too low there is an immediate detection of the problem condition and, depending on the extent of the fault condition, corrective actions are taken long before the resulting thermal conditions (e.g., overheating) are sensed. If more than one pump is used to service the system, they can be operated in a similar manner to that described for the fans. 
         [0042]    As shown in  FIG. 4B , systems embodying the invention include respective timer circuits ( 262 ,  462 ) to which are in turn connected to respective indicators ( 264 ,  464 ). These devices monitor the length of time devices are operated and enable an operator to schedule maintenance needs for the system. 
         [0043]    It has been shown that, in accordance with the invention, circuitry operating the switch for energizing the motor of a cooling device may be designed to perform the following functions:
       1—turn-on the switch to power the motor when a given temperature is reached;   2—turn-off the switch to remove power from the motor in the event of a malfunction of the motor and, concurrently, turn on the motor of another non-defective device; and   3—Selectively turn on the switch and apply power to the motor to test the operability of the motor for maintenance purposes and independently of temperature conditions.       
 
         [0047]    The system shown in  FIG. 5  is an expanded version of  FIGS. 3B and 4B  in that it shows two sets of fans (MAi, MBi) and two current transformers (CT 12 A and CT 12 B) to sense the currents in their corresponding sets of fans. Like the previous figures,  FIG. 5  illustrates the turning on of cooling devices in a predetermined sequence and the concurrent sensing of the “operability” of the cooling devices in order to substitute “good” devices for malfunctioning devices. 
         [0048]    Circuit  501  of  FIG. 5 , which corresponds generally to circuits  210  and  410 , is responsive to signals from temperature sensors ( 42 ,  82 ) to produce control signals to turn on corresponding cooling devices, if the cooling devices are not defective.  FIG. 5A  shows how a portion of circuit  501  may be configured to produce signals indicative of the need to provide cooling (i.e., a predetermined temperature has been reached). Thus, signals from a sensor  42  (winding temperature) are applied to a measuring circuit  16  and signals from senor  82  (top oil temperature) are applied to a measuring circuit  15 . The output of circuit  15  is applied to the non-inverting inputs of comparator circuits  20  and  24 . The output of circuit  16  is applied to the non-inverting inputs of comparator circuits  21  and  23 . A reference signal Tref 1  is applied to the inverting input of comparator  23 ; a reference signal Tref 2  is applied to the inverting input of comparator  21 ; a reference signal Tref 3  is applied to the inverting input of comparator  24  and a reference signal Tref 4  is applied to the inverting input of comparator  20 . These reference signals may be determined by the transformer manufacturer or the operator of the transformer to set the temperature(s) at which the first and second stage of cooling are applied to the transformer. 
         [0049]      FIGS. 5 and 5A  show two stages of cooling; one stage of cooling is provided by a first set/bank of fans MA and the second stage of cooling is provided by a second set/bank of fans MB. The first set of fans MA is activated when switch SA is closed. The second set of fans MB is activated when switch SB is closed. 
         [0050]    Assuming that the cooling devices are all operating correctly, Switch SA is closed when a signal from sensor  42  exceeds reference signal Tref 1  or when a signal from sensor  82  exceeds reference signal Tref 3 . When Tref 1  is exceeded, the output of comparator  23  goes from a logic “0” condition to a logic “1” condition which signal is applied to an OR gate  26  whose output is used to enable switch SA whose closure causes power to be applied to the first set of fans MA. The first set of fans may also be activated when a signal from sensor  82  exceeds a reference signal Tref 3 . When that occurs, the output of comparator  24  goes from a logic “0” condition to a logic “1” condition which signal is applied to OR gate  26  whose output is fed to gating circuit  503  whose output controls switch SA which will be enabled and power the first set of fans MA (if these fans are not malfunctioning). 
         [0051]    When the signal at the output of circuit  16  exceeds Tref 2 , the output of comparator  21  goes from a logic “0” condition to a logic “1” condition which signal is applied to OR gate  25  whose output is fed to gating circuit  503  whose output controls switch SB which will be enabled and power the second set of fans MB (if these fans are not malfunctioning). Likewise, when the signal at the output of circuit  15  exceeds Tref 4 , the output of comparator  20  goes from a logic “0” condition to a logic “1” condition which signal is applied to OR gate  25  whose output is fed to gating circuit  503  whose output controls switch SB which will be enabled and power the second set of fans MB (if these fans are not malfunctioning). 
         [0052]    The above describes the intended normal operation of the cooling fans in stages as a function of increases in temperature, when additional cooling is required and for the condition that the cooling devices are all functioning as intended. 
         [0053]    As already noted, in circuits embodying the invention, the application of power to cooling devices is a function of: (a) the temperature level requirement; and (b) the operability of the cooling device. Thus, in order for any of the switches SA and SB to be enabled gating signals have to be generated which indicate that their corresponding cooling devices are operational (“working”). The gating signals are generated by sensing the currents flowing in the motors of the cooling devices. In  FIG. 5 , motor currents are shown to be sensed by current transformers CT 12 A, CT 12 B, and CT 412 . The outputs of the current transformers are supplied to respective precision rectifier amplifiers ( 26 A,  26 B,  26 C) for initially processing and digitizing the sensed signals. The outputs of the rectifier circuits ( 26   i ) are then supplied to respective current detection circuits ( 38   i ) which function to determine whether the sensed current signal is either: (a) within a prescribed range; (b) an undercurrent (below the prescribed range which is indicative of a full or partial open circuit condition); or (c) an overcurrent (above the prescribed range which is indicative of a full or partial short circuit condition). Each one of the current detection circuits ( 38 A,  38 B,  38 C) may be as shown in  FIG. 5B . Each circuit includes a comparator  28  to which is supplied an overcurrent reference  27 , and a comparator  30  to which is supplied an undercurrent reference  29 . The values of the reference levels may be dictated by the motor manufacturers and/or derived from the specifications of what constitutes acceptable or non acceptable operation of the components. The two comparators determine whether the sensed motor current is either: (a) within a prescribed range; (b) too low, i.e., below a predetermined level, indicative of one type of malfunction, such as an open circuit; or (c) too high low (i.e., above a predetermined level, indicative of another type of malfunction, such as a short circuit. The outputs of the comparators are fed to additional circuitry such as timers (e.g., one-shots)  31 ,  32  and flip-flops  35 ,  36  whose outputs are fed to an OR gate  37  to produce an output shown as Mi. For purpose of illustration when Mi is a logic “1” it signifies that the sensed motor current is within an acceptable range (indicative of operability) when Mi is a logic “0” it signifies that the sensed motor current is outside an acceptable range (too low or too high) indicative of a malfunction. Note that the nature of the malfunction, whether the current is too high or too low, may be obtained by using the output of the flip flops  35  and  36 . Use of this feature is not explicitly shown, though it may be used to practice the invention. 
         [0054]    The outputs (e.g., Mi) generated by detection circuits ( 38   i ) may be combined with a selected output signal (TA, TB or TC) of the temperature processor ( 501 ,  210 ) in a gating arrangement  503  to control the sequencing of the switches applying power to the motors and to generate appropriate alarm signals as outlined in  FIG. 5C . 
         [0055]      FIG. 5C  outlines some of the function which can be performed using the various circuits shown in  FIGS. 3A ,  3 B,  4 A,  4 B,  5 ,  5 A and  5 B for the condition of 3 sets of fans (MA, MB, MC) which are intended to be turned-on in sequence and for 3 different temperature levels (T 1 , T 2 , T 3 ). 
         [0056]    The temperature of pertinent points/parts of the system is sensed by temperature sensors (e.g.,  42 ,  82 ) which are coupled to corresponding temperature sensing modules ( 210 ,  410 ,  510 ) to produce signals (TA, TB or TC) to indicate whether the temperature is above a first level (T 1 ), a second level (T 2 ) or a third level (T 3 ). If there are no defects, when TA is a logic 1 switch SA is to be closed, when TB is a logic 1 switch SB is to be closed, and when TC is a logic 1 switch SC is to be closed. However, in accordance with the invention these switches will only be closed if no malfunction of the cooling devices is detected. 
         [0057]    The system also includes means [modules  190 ,  490 ,  26 ( i ) and  38 ( i )] for sensing and storing information regarding the status of the motors operating the cooling devices and for producing signals indicative of the functioning or malfunctioning of the devices. For ease of illustration, the signal for motor MA is also shown as MA, motor MB as MB and motor MC as MC. Also, if a motor is functioning within its prescribed range its corresponding signal (Mi) is defined as a logic “1”; if it is operating outside its prescribed specification its corresponding signal is defined as a logic “0”. 
         [0058]    The gating circuitry  503  may be an integrated circuit (IC) microprocessor or any discrete logic circuit which includes the circuitry needed to perform the functions shown in  FIG. 5C  and  FIGS. 3A ,  3 B,  4 A, and  4 B.
   1. TURN-ON OF SA AND POWERING MA:   Thus, when TA is a logic “1” (indicating that cooling is required) and MA is a logic “1” (indicating that MA is functional) an AND type circuit  507  produces a signal to turn-on switch SA and power motor MA. If MA is logic “0” (indicating that MA is malfunctioning) the switch SA may be turned off (whether there is an undercurrent or overcurrent condition).   2. TURN-ON OF SB AND POWERING MB:   (a) However, note that the need for cooling which exists is taken care of as follows. When TA is a logic “1” and if MA is a logic “0”, [MA( BAR) is a logic “1” ] indicating that motor MA is malfunctioning, the output of an AND type circuit  509  produces a signal applied to an OR type circuit  510  to turn-on switch SB and power motor MB. Concurrently, an Alarm 1 circuit may also be activated to record and report the malfunction of motor MA.   (b) When TB is a logic “1” and MB is a logic “1” an AND type circuit  511  produces a signal coupled via OR circuit  510  to turn-on switch SB and power motor MB.   3. TURN-ON OF SC AND POWERING MC:   (a) When TA is a logic “1” and if MA and MB are a logic “0”, indicating that motors MA and MB are malfunctioning, the output of an AND type circuit  513  produces a signal applied to an OR type circuit  514  to turn-on switch SC and power motor MC. If MA and MB are logic “0” (indicating that MA and MB are malfunctioning) the switches SA and SB may be turned off (whether there is an undercurrent or overcurrent condition). Concurrently, an Alarm 2 circuit may also be activated to record and report the malfunction of motors MA and MB.   (b) When TB is a logic “1” and if MB is a logic “ 0 ”, indicating that motor MB is malfunctioning, the output of an AND type circuit  515  produces a signal applied to OR type circuit  514  to turn-on switch SC. If MB is logic “0” (indicating that MB is malfunctioning) the switch SB may be turned off (whether there is an undercurrent or overcurrent condition). Concurrently, an Alarm 3 circuit may also be activated to record and report the malfunction of motor MB.   (c) When TC is a logic “1” and MC is a logic “1” an AND type circuit  517  produces a signal coupled via OR circuit  514  to turn-on switch SC and power motor MC.   
 
         [0068]    Although it may not have been explicitly shown for all instances, It should be noted that when a cooling device is found to be defective, particularly when the defective condition is due to a short circuit condition, that the switch applying power to the defective cooling device will be disabled to prevent the application of power to the device. 
         [0069]    The information pertaining to a defective cooling device may be stored in memory and the device turned off until it is replaced. Or the operability of the device may be tested periodically to determine whether its defective condition has changed. 
         [0070]    The invention has been illustrated using cooling devices having motors and using means (e.g., current transformers) to sense the current in the motors. It should be appreciated that the invention may be practiced with any cooling device whose current and/or voltage and/or power usage can be sensed to determine the operability or malfunctioning of the device. 
         [0071]    The invention has been illustrated using radiators. But any other type of heat exchanger can be used to practice the invention.