Patent Publication Number: US-11646681-B2

Title: Motor branch circuit health monitoring method

Description:
PRIORITY 
     The present U.S. patent application is a continuation of U.S. patent application Ser. No. 16/446,050, filed Jun. 19, 2019, and claims priority under 35 U.S.C. § 120. The disclosure of the above priority application is incorporated herein, in its entirety, by reference. 
    
    
     BACKGROUND 
     Technical Field 
     The present disclosure relates to motor protection and, more specifically, to techniques for dynamically setting and applying a current imbalance threshold for motor protection, based on measures of voltage imbalance. 
     Description of the Related Art 
     Three-phase induction motors have three windings in the stator, which when connected to power lines supplying alternating voltage and current in three-phases, cause magnetic flux to rotate in a positive sequence direction within the stator. The rotor within the stator has an arrangement of closed-loop coils that can rotate and have current induced in them by the rotating magnetic field of the stator windings, forcing the rotor to rotate in the same direction as the positive sequence direction of the magnetic flux within the stator. 
     As long as the power supply voltages and currents are equal in magnitude in the three-phases, i.e. balanced, the magnetic flux rotates in the positive sequence direction within the stator. However, voltages and currents may occasionally become unbalanced in the three-phases of the power supply lines. For example, such imbalances can result from faults in a distribution transformer or unbalanced distribution of single phase loads on the same branch circuit, such as a momentary current draw by starting-up large electrical machinery or by a heavy arc welder. While three-phase motors may continue to operate with unbalanced voltages and currents, such continued operation can result in less efficient operation and can potentially damage the motor. 
     SUMMARY 
     In accordance with one embodiment described herein, a method, apparatus and computer program product monitor the health of a three-phase induction motor or other type of three-phase load. An expected threshold current unbalance is calculated as a function of an expected ratio of current unbalance to voltage unbalance for the three-phase motor or other type of three-phase load. Diagnostic information is generated based on measured current unbalance and measured voltage unbalance. A determination is made as to whether a measured current unbalance exceeds the expected threshold current unbalance. Protection is activated for the three-phase induction motor or other type of three-phase load, based on whether the measured current unbalance exceeds the expected threshold value. 
     A method to monitor the health of a three-phase load, comprises:
         calculating, by a three-phase protective device, an expected current unbalance, as a product of a measured voltage unbalance times an expected ratio of current unbalance to voltage unbalance for a three-phase load;   calculating, by the three-phase protective device, an expected threshold current unbalance indicating a potential fault caused by unbalanced currents, as the expected current unbalance plus a value of unbalance tolerance/sensitivity;   determining, by the three-phase protective device, whether a measured current unbalance exceeds at least one of the expected current unbalance or the expected threshold current unbalance; and   providing, by the three-phase protective device, an action for the three-phase load, based on the determination.       

     A method to monitor the health of a three-phase induction motor as a three-phase load, comprises:
         calculating, by a three-phase protective device, an expected current unbalance for a three-phase induction motor, as a product of a measured voltage unbalance times an expected ratio of current unbalance to voltage unbalance for a three-phase induction motor;   calculating, by the three-phase protective device, a expected threshold current unbalance indicating a potential fault caused by unbalanced currents, as a calculated expected current unbalance of the three-phase induction motor plus a value of unbalance tolerance/sensitivity for the three-phase induction motor;   determining, by the three-phase protective device, that a measured current unbalance exceeds at least one of the expected current unbalance or the expected threshold current unbalance; and   providing, by the three-phase protective device, at least one of diagnostic information or protection for the three-phase induction motor, based on the determination.       

    
    
     
       DESCRIPTION OF THE FIGURES 
       Example embodiments are depicted in the accompanying drawings that are briefly described as follows: 
         FIG.  1    is an example functional block diagram of the motor protection relay for monitoring the health of a three-phase motor, in accordance with one embodiment described herein. 
         FIG.  2    is an example flow diagram of an example overall method, including a threshold calculation process monitoring the health of the three-phase motor, implemented as computer program code executable by a processor in the motor protection relay. 
         FIG.  3    is an example logic block diagram for generating a warning signal or a fault signal in response to a detected current phase unbalance at which the motor protection relay will be tripped, implemented as computer program code executable by a processor in the motor protection relay. 
         FIG.  4    is an example graph illustrating an example diagnostic and protection function of the motor protection relay of  FIG.  1    and the method of  FIG.  2   . 
     
    
    
     DETAILED DESCRIPTION 
     Voltages and currents within three-phase induction motors may occasionally become unbalanced in the three-phases of the power supply lines, e.g., as a result of faults in a distribution transformer or unbalanced distribution of single phase loads on the same branch circuit, such as a momentary current draw by starting-up large electrical machinery or by a heavy arc welder. A three-phase motor may continue to operate with unbalanced voltages and currents, however its efficiency is reduced by both increased current and increased resistance due to heating. The stator winding with the highest current will have the greatest overheating, resulting in deterioration of the insulation of the stator winding. During power supply unbalance, currents flow through the stator windings in a negative sequence direction, resulting in induction of negative sequence voltage in the rotor coils, abnormal current flow, and overheating. 
     Under conditions of balanced voltages and currents, where the motor operates at a continuous load for a sufficient time, its operating temperature reaches thermal equilibrium. Each motor has a characteristic safe maximum operating temperature permitted by the motor design, above which permanent damage may begin to occur to insulating layers and other components. Under conditions of current phase unbalance, the stator windings generate heat at an increased rate causing a faster rise in the temperature of the surrounding core. The time interval necessary to raise the temperature of the core to the maximum operating temperature is the trip time after the detection of the unbalance condition. To avoid permanent damage to the motor, conventional protection relays set a fixed threshold for detected current phase unbalance at which the relay is prematurely tripped at an interval that is shorter than the trip time. However, such a fixed threshold is typically optimal under certain conditions, and at other times may result in either under-protection (e.g., when the threshold is set too high) or nuisance trips (e.g., where the threshold is set too low, resulting in false positives). 
     As such, one embodiment described herein provides a method, apparatus and computer program product for monitoring the health of a three-phase induction motor. The method and apparatus calculate an expected threshold value as a function of an expected ratio of current unbalance to voltage unbalance for the three-phase motor. The method and apparatus determine whether a measured current unbalance exceeds the expected threshold value. The method and apparatus generate diagnostic information or activate protection of the three-phase induction motor, if the measured current unbalance exceeds the expected threshold value. 
       FIG.  1    is an example functional block diagram of the motor protection relay  100 , in accordance with one embodiment described herein. The relay  100  receives three-phase power Φ 1 , Φ 2 , and Φ 3  and conducts three-phase currents I 1 , I 2 , and I 3 , through optional switches S 1 , S 2 , and S 3  to the power input nodes N 1 , N 2 , and N 3  of the three-phase induction motor  102  in a branch circuit. The stator windings W 1 , W 2 , and W 3  are respectively connected between the power input nodes N 1 , N 2 , and N 3 . The rotor within the stator is driven into rotation when the optional switches S 1 , S 2 , and S 3  are closed, delivering three-phase currents I 1 , I 2 , and I 3 , to the stator windings W 1 , W 2 , and W 3 . The optional switches S 1 , S 2 , and S 3  may be located inside the housing of the relay or they may be located outside of it. The trip actuator  130  controls the on/off state of the optional switches S 1 , S 2 , and S 3 . The current transformers CT 1 , CT 2 , and CT 3  associated with the protection relay  100 , are inductively coupled to the currents I 1 , I 2 , and I 3 , and output a sensing current I 1 ′, I 2 ′, and I 3 ′ that are measured by a microprocessor  110  and memory  112  in the protection relay, using known conventional techniques. The current transformers CT 1 , CT 2 , and CT 3  may be located inside the housing of the relay or they may be located outside of it. Three voltage taps V 1 , V 2 , and V 3  respectively connect the voltages of the three phases to the microprocessor  110  and memory  112  in the protection relay. 
     The memory  112  stores a motor protection component  115 , shown in greater detail in  FIG.  2   . The motor protection component  115  includes the following sub-components: Measured Voltage Unbalance Component  118 , Expected Current Unbalance Calculation Component  120 , Expected Threshold Calculation Component  122 , Measured Current Unbalance Determination Component  124 , Diagnostic Information Component  126 , and Activate Protection Component  128 . The memory  112  also stores load application parameters and in particular, expected ratio of current unbalance to voltage unbalance values  140  for various load types. Example values  140  of the expected ratio of current unbalance to voltage unbalance for an example type of three-phase induction motor  102  may be 6:1, for an example resistor may be 1:1, or for an example drive may be 20:1. The memory  112  receives user entry of load type information  150 , such as motor, resistor, or drive. The memory  112  receives user entry values of unbalance tolerance/sensitivity  152  for the three-phase motor  102 , such as low 5%, medium 10%, or high 15%. 
     Example values for the expected ratio of current unbalance to voltage unbalance  140  for three phase induction motors may be provided by the manufacturer or may be determined based on testing by the user. A table with different values for the expected ratio  140  may be based, for example, on the class of the motor, the motor design, its efficiency classification (1E1, 1E2, 1E3, or 1E4). 
     Example user entry of load type information  150  for three-phase single-speed, cage-induction motors may include a specification for having 2, 4, or 6 poles (3,000; 1,500; and 1,000 RPM at 50 Hz), having a rated output between 0.75 and 375 kW, having a rated voltage up to 1000 V, and a rating on the basis of either duty type S 1  (continuous duty) or S 3  (intermittent duty) with a rated cyclic duration factor. 
     Example user entry of the unbalance tolerance/sensitivity  152  for a three phase induction motor is primarily related to the priority in the user&#39;s application. A large tolerance in load sensitivity  152  would correspond to the user&#39;s preference to prioritize continued operation during a non-critical problem that is manifested by unexpected unbalance. A small tolerance in load sensitivity  152  would correspond to the user&#39;s preference to prioritize immediately stopping to diagnose a problem that is manifested by unexpected unbalance. 
     In an example embodiment, the motor manufacturer may include with the motor, a memory chip that stores the motor&#39;s parameters for values  140  of the expected ratio of current unbalance to voltage unbalance for each type of three-phase motor  102 . The motor&#39;s parameters are accessible by the micro-processor  110  in the motor protection relay  100 . 
     In another embodiment, the three-phase power Φ 1 , Φ 2 , and Φ 3  may supply a three-phase alternating current I 1 , I 2 , and I 3  to a three-phase heater with heater coils W 1 , W 2 , and W 3 , functioning as another type of three-phase load, being respectively connected between the power input nodes N 1 , N 2 , and N 3  and neutral, in the star or wye configuration shown in  FIG.  1   . The three-phase heater generates resistive heat when the switches S 1 , S 2 , and S 3  are closed, delivering three-phase currents I 1 , I 2 , and I 3  to the heater coils W 1 , W 2 , and W 3 . An example value  140  of the expected ratio of current unbalance to voltage unbalance for an example three-phase heater may be a ratio of 1 to 1. Example user entry of the load type information  150  may be as a resistor. Example user entry of the unbalance tolerance/sensitivity  152  for an example three-phase heater is primarily related to the priority in the user&#39;s application. A large tolerance in load sensitivity  152  would correspond to the user&#39;s preference to prioritize continued operation of the three-phase heater during a non-critical problem that is manifested by unexpected unbalance. A small tolerance in load sensitivity  152  would correspond to the user&#39;s preference to prioritize immediately turning off the heater to diagnose a problem that is manifested by unexpected unbalance. The principle of operation of the motor protection component  115  is also applicable to monitoring the health an example three-phase heater. The measured current unbalance of an example three-phase heater may be analyzed to determine whether it exceeds a calculated expected threshold current unbalance as a function of the expected ratio of current unbalance to voltage unbalance for the three-phase heater. Diagnostic information or activation of protection may be performed for the three-phase heater, if the measured current unbalance exceeds the expected threshold value. 
       FIG.  2    is an example flow diagram of an example overall method, including a threshold calculation process monitoring the health of the three-phase motor  102 , implemented as computer program code executable by the processor  110  in the motor protection relay  100 . 
     The Motor Protection Component 
     The motor protection component  115  stores at  202  the load application parameters for the expected ratio of current unbalance to voltage unbalance  140 . The motor protection component  115  receives at  204  the user entry of load type information  150 , for example the particular type of induction motor  102 . The motor protection component  115  receives at  206  the user entry of unbalance tolerance/sensitivity  152 . The user sets the unbalance tolerance/sensitivity based on how the user intends to operate the motor, either very conservatively to minimize wear, or aggressively to extract maximum performance at the possible sacrifice of useable life of the motor. The motor protection component  115  performs a look up to obtain the value of the expected ratio of current unbalance to voltage unbalance  140  for the particular type of motor  102 . 
     The Measured Voltage Unbalance Component 
     The Measured Voltage Unbalance Component  118 , which is a sub-component of the motor protection component  115  of  FIG.  2   , measures the line-to-line voltages V 1 , V 2 , and V 3  applied to the nodes N 1 , N 2 , and N 3  of the three-phase motor  102  and determines a percent unbalance  400  in the measured voltages. A general definition for measuring the line-to-line voltage unbalance is provided in “Definitions of Voltage Unbalance”,  IEEE Power Engineering Review , Volume: 21, Issue: 5, May 2001. The Line Voltage Unbalance Rate is the ratio of the maximum voltage deviation of any of the voltages V 1 , V 2 , and V 3  from the average phase voltage magnitude, divided by the average phase voltage magnitude Vavg. Using this definition, the calculation of percent unbalance  400  in the measured voltages may be:
 
percent unbalance measured voltages=100*{max[(V1−Vavg),(V2−Vavg),(V3−Vavg)]}/Vavg.
 
     Other definitions of voltage unbalance may be used to calculate the percent unbalance  400  in the measured voltages V 1 , V 2 , and V 3  (see Anwari, et al., “New Unbalance Factor for Estimating Performance of a Three-Phase Induction Motor With Under- and Overvoltage Unbalance”,  IEEE Transactions on Energy Conversion,  25(3), pp. 619-625, October 2010). 
     The Measured Voltage Unbalance Component  118  may also detect variations in the power quality when a measured phase voltage V 1 , V 2 , or V 3  deviates from the prescribed range, such as resulting from a momentary unbalanced distribution of single phase loads on the same branch circuit. Since such momentary deviations in power quality may occur randomly and frequently, the Measured Voltage Unbalance Component  118  may be repeatedly invoked in a loop to detect power quality variations to be factored into the diagnosis performed by the Diagnostic Information Component  126 . 
     The Expected Current Unbalance Calculation Component 
     The Expected Current Unbalance Calculation Component  120 , which is a sub-component of the motor protection component  115  of  FIG.  2   , performs the expected current unbalance calculation by using the percent unbalance in the measured voltages and the value of the expected ratio of current unbalance to voltage unbalance  140  for the particular type of motor  102 . The calculation of the Expected Current Unbalance is the percent unbalance in the measured voltages times the expected ratio of current unbalance to voltage unbalance  140 . Reference to  FIG.  4    shows a graph of voltage unbalance (abscissa) to current unbalance (ordinate) for the particular type of motor  102 . The figure shows the straight line trace  405  of the expected current unbalance  402  as a function of the voltage unbalance (abscissa) and the slope of the straight line trace is the particular expected ratio of current unbalance to voltage unbalance  140 .  FIG.  4    shows that the Expected Current Unbalance  402  on the ordinate corresponds to the percent unbalance in the measured voltages  400  on the abscissa. 
     The Expected Threshold Calculation Component 
     The Expected Threshold Calculation Component  122 , which is a sub-component of the motor protection component  115  of  FIG.  2   , performs the calculation of the Expected Threshold current unbalance using unbalance tolerance/sensitivity  152 . The calculation of the Expected Threshold current unbalance is the Expected Current Unbalance  402  plus the unbalance tolerance/sensitivity  152 .  FIG.  4    shows the straight line trace  412  of the Expected Threshold current unbalance as a function of the voltage unbalance (abscissa) for the user-selected value of unbalance tolerance/sensitivity type  152 .  FIG.  4    shows that the Expected Threshold current unbalance  404  on the ordinate corresponds to the percent unbalance in the measured voltages  400  on the abscissa. 
     The Measure Current Unbalance Component 
     The Measure Current Unbalance Component  124 , which is a sub-component of the motor protection component  115  of  FIG.  2   , measures currents I 1 , I 2 , and I 3  flowing in the corresponding stator windings W 1 , W 2 , and W 3  of the three-phase motor  102  and determines a percent unbalance  406  in the measured currents. The computation of the percent unbalance  406  in the measured currents may be approximated by adapting the formula for computation of the measured voltage unbalance  400  described above for the Measured Voltage Unbalance Component  118 :
 
percent unbalance in measured currents=100*{max[(I1−Iavg),(I2−Iavg),(I3−Iavg)]}/Iavg.
 
     Other definitions of current unbalance may be used to calculate the percent unbalance  406  in the measured currents I 1 , I 2 , and I 3 .  FIG.  4    shows the percent unbalance  406  in the measured currents on the ordinate of the graph. 
     The Diagnostic Information Component 
     The Diagnostic Information Component  126 , which is a sub-component of the motor protection component  115  of  FIG.  2   , compares at  210  the measured current unbalance  406  with the expected current unbalance  402  and the expected threshold current unbalance  404 . The Diagnostic Information Component  126  monitors the health of a three-phase induction motor and may issue a warning at  212  if the measured current unbalance  406  exceeds the expected current unbalance  402 , as is illustrated in the logic block diagram  FIG.  3   . A warning may be issued if the measured current unbalance  406  is greater than, for example, a predetermined fraction of the expected threshold current unbalance  404 , as is illustrated in  FIG.  3   . The Diagnostic Information Component  126  may repeatedly loop back to the Measured Voltage Unbalance Component  118 , as shown in  FIG.  2   , to detect power quality variations to be factored into the diagnosis. 
     The logic block diagram  FIG.  3    shows that the Diagnostic Information Component  126  may use different values for the expected ratio of current unbalance to voltage unbalance, depending on whether the motor  102  is in a start state or a run state. The expected ratio  140  may generally have a starting ratio  140 (S) when starting and it may generally have a running ratio  140 (R) when running at its designed speed, between unloaded and fully loaded. In some embodiments the difference between the lower ratio  140 (S) when starting and the higher ratio  140 (R) when running, may be considered small enough to approximate the two ratios as being substantially equal. In some embodiments, the starting ratio  140 (S) and/or the running ratio  140 (R) may have other values, such as a relationship between the measured current and the nominal (name plate) current. 
     The logic block diagram  FIG.  3    shows the Diagnostic Information Component  126  receiving the measured current unbalance  406  from the Measure Current Unbalance Component  124 . For example, the measured current unbalance  406  may be input to logic blocks  342 (S) and  312 (S) for the motor start state, and to logic blocks  342 (R) and  312 (R) for the motor run state. For example, the start state signal value  302  closes the switch  310 (S) and the value of the starting ratio of current unbalance to voltage unbalance  140 (S) is applied to logic blocks  342 (S) and  312 (S) for the motor start state. The logic block  312 (S) determines whether the measured current unbalance  406  exceeds the expected current unbalance  402 (S), using the lower expected ratio  140 (S). If the measured current unbalance  406  exceeds the expected current unbalance  402 (S), then logic block  312 (S) outputs a current unbalance warning  320 (S) for the motor start state, which may be displayed to operating personnel. The logic block  342 (S) determines whether the measured current unbalance  406  is greater than the expected threshold current unbalance  404 (S), using the lower expected ratio  140 (S). If the measured current unbalance  406  is greater, then logic block  342 (S) outputs a combined voltage and current unbalance warning  350 (S) for the motor start state, which may be displayed to operating personnel. 
     For example, the run state signal value  304  closes the switch  310 (R) and the value of the running ratio of current unbalance to voltage unbalance  140 (R) is applied to logic blocks  342 (R) and  312 (R) for the motor run state. The logic block  312 (R) determines whether the measured current unbalance  406  exceeds the expected current unbalance  402 (R), using the higher expected ratio  140 (R). If the measured current unbalance  406  exceeds the expected current unbalance  402 (R), then logic block  312 (R) outputs a current unbalance warning  320 (R) for the motor run state, which may be displayed to operating personnel. The logic block  342 (R) determines whether the measured current unbalance  406  is greater than the expected threshold current unbalance  404 (R), using the higher expected ratio  140 (R). If the measured current unbalance  406  is greater, then logic block  342 (R) outputs a combined voltage and current unbalance warning  350 (R) for the motor run state, which may be displayed to operating personnel. 
     After the Diagnostic Information Component  126  has performed a diagnosis of the health of a three-phase induction motor by measuring current unbalance and measuring voltage unbalance, it may repeatedly loop back to the Measured Voltage Unbalance Component  118 , as shown in  FIG.  2   , to detect any variations in the power quality when a measured phase voltage V 1 , V 2 , or V 3  deviates from the prescribed range, such as resulting from a momentary unbalanced distribution of single phase loads on the same branch circuit. 
     The Activate Protection Component 
     The Activate Protection Component  128 , which is a sub-component of the motor protection component  115  of  FIG.  2   , activates protection if the measured current unbalance  406  is greater than the expected threshold current unbalance  404 , such as by causing the microprocessor  110  to trip the actuator  130 . The Activate Protection Component  128  may delay activating protection pending the expiration of a time out interval, after which the activation of protection may commence, as is illustrated in  FIG.  3   . The duration of the time out interval may be shorter for a start state operation for the motor due to higher currents, and a longer time out interval when the motor is running under load. 
     The logic block diagram  FIG.  3    shows that the Activate Protection Component  128  may use different values for the expected ratio of current unbalance to voltage unbalance, depending on whether the motor  102  is in a start state or a run state. The expected ratio  140  will have a lower ratio  140 (S) when starting and it will have a higher ratio  140 (R) when running at its designed speed, between unloaded and fully loaded. 
     The logic block diagram  FIG.  3    shows the Activate Protection Component  128  receiving the measured current unbalance  406  from the Measure Current Unbalance Component  124 . For example, the measured current unbalance  406  may be input to logic block  330 . The logic block  330  determines whether the measured current unbalance  406  exceeds the expected threshold current unbalance  404 . The start state signal value  302  and the value of the lower expected ratio  140 (S) are input to an AND logic block, and if both values are true, the expected threshold current unbalance  404  uses the lower expected ratio  140 (S). If logic block  330  determines that the measured current unbalance  406  exceeds the expected threshold current unbalance  404 , using the lower expected ratio  140 (S), then an enabling signal  332  is applied to AND logic block  334 (S) for the motor start state. The starting delay logic block  336 (S) will delay outputting a current unbalance fault signal  340 (S) and the activating protection pending the expiration of a short time out interval T 1 , after which the activation of protection may commence, as is illustrated in  FIG.  3   . The duration of the time out interval T 1  may be shorter for the start state operation for the motor due to higher currents, and a longer time out interval when the motor is running under load. 
     For example, the run state signal value  304  and the value of the higher expected ratio  140 (R) are input to an AND logic block, and if both values are true, the expected threshold current unbalance  404  uses the higher expected ratio  140 ((R). If logic block  330  determines that the measured current unbalance  406  exceeds the expected threshold current unbalance  404 , using the higher expected ratio  140 (R), then the enabling signal  332  is applied to AND logic block  334 (R) for the motor run state. The running delay logic block  336 (R) will delay outputting a current unbalance fault signal  340 (R) and the activating protection pending the expiration of a longer time out interval T 2 , after which the activation of protection may commence, as is illustrated in  FIG.  3   . The duration of the time out interval T 2  may be longer for the run state operation for the motor than for the start state operation. 
     If the measured current unbalance does not exceed the expected threshold current unbalance  404 , then the health of the three-phase induction motor is acceptable. 
       FIG.  4    is an example graph illustrating an example diagnostic and protection function of the motor protection relay of  FIG.  1    and the method of  FIG.  2   . The figure shows a graph of voltage unbalance (abscissa) to current unbalance (ordinate) for the particular type of motor  102 . The figure shows the straight line trace  405  of the expected current unbalance  402  as a function of the voltage unbalance (abscissa) and the slope of the straight line trace is the particular expected ratio of current unbalance to voltage unbalance  140 . The Expected Current Unbalance  402  on the ordinate corresponds to the percent unbalance in the measured voltages  400  on the abscissa. The figure shows the straight line trace  412  of the Expected Threshold current unbalance as a function of the voltage unbalance (abscissa) for the user-selected value of unbalance tolerance/sensitivity type  152 . The line  412  represents the current unbalance expected from voltage unbalance plus an allowance for the load contribution load sensitivity. The figure shows that the Expected Threshold current unbalance  404  on the ordinate corresponds to the percent unbalance in the measured voltages  400  on the abscissa. The line  420  represents a conventional fixed threshold. The region  408  represents an area of insufficient protection wherein the load is significantly contributing to the unbalance. The region  410  is an area of nuisance trips, wherein the load may be slightly (or perfectly) balanced and still cause a trip. 
     The resulting method, apparatus and computer program product monitor the health of a three-phase induction motor or other type of three-phase load. An expected threshold current unbalance is calculated as a function of an expected ratio of current unbalance to voltage unbalance for the three-phase motor or other type of three-phase load. Diagnostic information is generated based on measured current unbalance and measured voltage unbalance. A determination is made as to whether a measured current unbalance exceeds the expected threshold current unbalance. Protection is activated for the three-phase induction motor or other type of three-phase load, based on whether the measured current unbalance exceeds the expected threshold value. 
     In the preceding, reference is made to various embodiments. However, the scope of the present disclosure is not limited to the specific described embodiments. Instead, any combination of the described features and elements, whether related to different embodiments or not, is contemplated to implement and practice contemplated embodiments. Furthermore, although embodiments may achieve advantages over other possible solutions or over the prior art, whether or not a particular advantage is achieved by a given embodiment is not limiting of the scope of the present disclosure. Thus, the preceding aspects, features, embodiments and advantages are merely illustrative and are not considered elements or limitations of the appended claims except where explicitly recited in a claim(s). 
     The various embodiments disclosed herein may be implemented as a system, method or computer program product. Accordingly, aspects may take the form of an entirely hardware embodiment, an entirely software embodiment (including firmware, resident software, micro-code, etc.) or an embodiment combining software and hardware aspects that may all generally be referred to herein as a “circuit,” “module” or “system.” Furthermore, aspects may take the form of a computer program product embodied in one or more computer-readable medium(s) having computer-readable program code embodied thereon. 
     Any combination of one or more computer-readable medium(s) may be utilized. The computer-readable medium may be a non-transitory computer-readable medium. A non-transitory computer-readable medium may be, for example, but not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, or device, or any suitable combination of the foregoing. More specific examples (a non-exhaustive list) of the non-transitory computer-readable medium can include the following: an electrical connection having one or more wires, a portable computer diskette, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), an optical fiber, a portable compact disc read-only memory (CD-ROM), an optical storage device, a magnetic storage device, or any suitable combination of the foregoing. Program code embodied on a computer-readable medium may be transmitted using any appropriate medium, including but not limited to wireless, wireline, optical fiber cable, RF, etc., or any suitable combination of the foregoing. 
     Computer program code for carrying out operations for aspects of the present disclosure may be written in any combination of one or more programming languages. Moreover, such computer program code can execute using a single computer system or by multiple computer systems communicating with one another (e.g., using a local area network (LAN), wide area network (WAN), the Internet, etc.). While various features in the preceding are described with reference to flowchart illustrations and/or block diagrams, a person of ordinary skill in the art will understand that each block of the flowchart illustrations and/or block diagrams, as well as combinations of blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer logic (e.g., computer program instructions, hardware logic, a combination of the two, etc.). Generally, computer program instructions may be provided to a processor(s) of a general-purpose computer, special-purpose computer, or other programmable data processing apparatus. Moreover, the execution of such computer program instructions using the processor(s) produces a machine that can carry out a function(s) or act(s) specified in the flowchart and/or block diagram block or blocks. 
     The flowchart and block diagrams in the Figures illustrate the architecture, functionality and/or operation of possible implementations of various embodiments of the present disclosure. In this regard, each block in the flowchart or block diagrams may represent a module, segment or portion of code, which comprises one or more executable instructions for implementing the specified logical function(s). It should also be noted that, in some alternative implementations, the functions noted in the block may occur out of the order noted in the figures. For example, two blocks shown in succession may, in fact, be executed substantially concurrently, or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved. It will also be noted that each block of the block diagrams and/or flowchart illustration, and combinations of blocks in the block diagrams and/or flowchart illustration, can be implemented by special purpose hardware-based systems that perform the specified functions or acts, or combinations of special purpose hardware and computer instructions. 
     It is to be understood that the above description is intended to be illustrative, and not restrictive. Many other implementation examples are apparent upon reading and understanding the above description. Although the disclosure describes specific examples, it is recognized that the systems and methods of the disclosure are not limited to the examples described herein but may be practiced with modifications within the scope of the appended claims. Accordingly, the specification and drawings are to be regarded in an illustrative sense rather than a restrictive sense. The scope of the disclosure should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.