Apparatus for displaying thermal condition of motor controller

A method and apparatus for remotely displaying information about heat in a motor having an associated motor-starting contactor with an overload relay system is provided. The apparatus includes a suitably programmed microprocessor which obtains a signal representative of current in the motor, squares the current and multiplies by time. This actual I.sup.2 t value is then used to calculate a signal representative of a relationship between the actual I.sup.2 t value and a target value at which the contacts of the motor-starting contactor will open due to an overload current situation. This relationship is preferably a percentage which is then displayed on either an LED bar type display or a digital display. The display may also include a "trip" indicator and a "reset allowed" indicator.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to motor starting contactors with an overload relay 
system and, more specifically, the invention relates to an apparatus and 
method for displaying certain conditions of the motor controller and the 
associated motor. 
2. Background Information and Description of the Prior Art 
Electromagnetic contactors are switch devices which are especially useful 
in motor-starting, lighting, switching and similar applications. A 
motor-starting contactor with an overload relay system is often called a 
motor controller. A contactor usually has a magnetic circuit which 
includes a fixed magnet and a movable magnet or armature with an air gap 
therebetween when the contactor is opened. An electromagnetic coil which 
is controllable upon command interacts with a source of voltage which may 
be interconnected with the main contacts of the contactor for 
electromagnetically accelerating the armature towards the fixed magnet, 
thus reducing the air gap. The armature has a set of bridging contacts, 
the complements of which are fixedly disposed within the contactor case 
for being engaged by the movable contacts as the magnetic circuit is 
engaged and the armature is moved. The load and voltage source therefor 
are usually interconnected with the fixed contacts and become 
interconnected with each other as the bridging contacts make with the 
fixed contacts. 
When the system is used as a motor controller with an overload relay 
device, the overload relay function senses current which is related to the 
input current being drawn by the motor. This then is provided to an 
electromagnetic or electrothermal device calibrated to cause the main 
contacts to open under certain overload current situations. Additionally, 
raw data concerning the current is supplied to a microprocessor. 
Typically, the microprocessor calculates the heat generated in the motor 
by calculating current squared multiplied by time, or I.sup.2 t. It is 
necessary when supplying the current to a microprocessor that it be 
converted from an analog to a digital signal for effective utilization by 
the microprocessor. An apparatus for use with an electromagnetic contactor 
and the background information concerning an exemplary electromagnetic 
contactor is provided in U.S. Pat. No. 4,757,420 issued Jul. 12, 1988 and 
which is assigned to the assignee of the present invention. 
As discussed in the U.S. Pat. 4,757,420 the current is measured and a 
representative signal is generated for input to the microprocessor. As 
mentioned above, prior to supplying this signal to the microprocessor it 
is necessary to convert the analog representation thereof to a digital 
signal for effective utilization by the microprocessor. The microprocessor 
thereafter performs various calculations in order to determine from the 
current representative input signal, the heat in the associated motor. 
This value is then used to communicate with the motor controller to open 
the contacts of the motor controller in overload current situations or for 
other purposes within the system. An operator of the system, however, does 
not have a convenient way of determining the status of the system at any 
particular point. For example, the operator does not know whether the 
system is near a trip condition because the operator has no direct 
information about the thermal condition of the motor. Thus, the only 
indication of the thermal condition that the operator has is when the 
motor controller trips in response to a current overload situation. If the 
operator were aware of the current status of the thermal condition of the 
motor, the operator could take steps to avoid an early trip of the motor 
controller. In other words, an operator who is activating a motor starter 
in order to effectuate a particular task could slow the activations down 
or space them apart at greater time intervals in order to allow a further 
cooling of the motor between activations. At present, there is no method 
by which an operator can visually check the system to determine the 
status. There remains a need, therefore, for a device which would allow an 
operator to visually check the status of the motor controller and the 
associated motor in order to control activation of the system to more 
efficiently operate the motor and the motor starter. 
In addition, after a motor has tripped, typically a safety feature is built 
into the motor controller such that it cannot be reset until the motor 
cools down to a certain percentage of the trip value. The operator, 
however, is not aware of the degree of cooling within the motor. This 
means that the operator would simply continue to press a reset button 
until the device resets. There remains a need for a device which indicates 
whether a reset is allowed which would thereby indicate that the motor has 
cooled to a sufficient temperature to allow operation thereof after a trip 
condition has occurred. There remains a further need for a device which 
indicates that a trip has in fact occurred and a device which indicates 
that the system is ready to be reset and thereby reactivated. 
SUMMARY OF THE INVENTION 
These and other needs are satisfied by the device of the present invention 
in which a method and apparatus is provided for remotely displaying 
information about the thermal condition of a motor and the status of the 
associated motor-starting contactor. An associated microprocessor is 
suitably programmed, as stated above, to perform calculations to determine 
the thermal condition in the motor. More specifically, the microprocessor 
is programmed to generate a signal which is the square of the current 
multiplied by time. This is an actual signal representative of the heat in 
the motor at a particular instant in time. In accordance with the present 
invention, the microprocessor is additionally suitably programmed to 
determine a relationship between this actual signal and a target trip 
amount. In a preferred embodiment of the invention, the relationship is 
determined to be the actual signal as a percentage of the target value. 
This percentage signal is then converted into an appropriate bit pattern 
and correspondingly displayed on preferably a LED bar type display which 
ranges vertically from 0% at the bottom of the bar display to 100% at the 
top of the display. An operator can visually check the display device to 
determine the percentage value and if, for example, it is determined that 
the system is operating at a percentage approaching the trip condition, 
then the operator can make adjustments in the activation of the 
motor-starter to thereby more efficiently operate the system. 
In accordance with another embodiment of the invention, an indicator light 
is provided to notify the operator that the motor-starting contactor has 
tripped in response to a current overload condition. Additionally, an 
indicator light can be provided which informs the operator that a reset of 
this system is allowed once the system has cooled after a trip to an 
appropriately lower temperature in accordance with safety considerations.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, a prior art motor contactor is shown in order to 
provide the background for the disclosure of the present invention. 
Further details about the system are set forth in the U.S. Pat. No. 
4,757,420. A cross section of a three phase electrical contactor or 
controller 10 is shown in FIG. 1. For purposes of simplicity of 
illustration the construction features of only one of the three poles will 
be described and it should be understood that the other two poles are the 
same. Contactor 10 has terminals 14 and 16 for interconnection with an 
electrical apparatus, or a circuit or a system to be serviced or 
controlled by the contactor 10. Terminals 14 and 16 are spaced apart and 
interconnected internally with conductors 20 and 24 respectively. 
Conductors 20 and 24 are terminated by appropriate fixed contacts 22 and 
26, respectively. Interconnection of contacts 22 and 26 will establish 
circuit continuity between terminals 14 and 16 and render the contactor 10 
effective for conducting electrical current therethrough. 
A separate coil control board 28 is securely disposed within the housing of 
contactor 10. Coil control board 28 includes a coil or solenoid assembly 
30 which may include an electrical coil or solenoid 31. Spaced away from 
the coil control board 28 and forming one end of the coil assembly 30 is a 
spring seat 32 upon which is securely disposed one end of a kickout spring 
34. The other end of kickout spring 32 resides against base 12 until 
movement of a contact carrier 42 causes bottom portion 42A to engage 
spring 34 and compress it against seat 32. 
A fixed magnet 36 is strategically disposed within a channel 38 which is 
radially aligned with the solenoid or coil 31. Axially displaced from the 
fixed magnet 36 is a movable armature 40 which is movable in the channel 
38 relative to the fixed magnet 36. At the end of the armature 40 and 
spaced away from the fixed magnet 36 is the longitudinal extending 
electrically insulating contact carrier 42 upon which is disposed an 
electrically conducting contact bridge 44. Contact 46 is disposed on one 
radial arm of contact bridge 44 and on another radial arm of contact 
bridge 44 is a disposed contact 48. Contact 46 abuts contact 22 and 
contact 48 abuts contact 26 when a circuit is internally completed between 
the terminal 14 and 16 as contactor 10 closes. This occurs with electrical 
energization of the coil or solenoid 31 by electrical power provided at an 
externally accessible terminal (not visible in FIG. 1). This energization 
could be in response to a contact closing signal available externally at 
the externally accessible chamber. This generates a magnetic flux path 
through fixed magnet 36, the air gap 58 and armature 40. As is well known 
to those skilled in the art, such a condition causes the armature 40 to 
longitudinally move within channel 38 in an attempt to shorten or 
eliminate air gap 58 and to eventually abut magnet 36. This movement is in 
opposition to, or is resisted by, the force of compression of kickout 
spring 34 in initial stages of movement and is further resisted by the 
force of compression of the contact spring 56 after the contacts have been 
closed. 
Also enclosed within the contactor 10 is a overload relay printed circuit 
board 60 upon which are disposed current sensors which may be 
current-to-voltage transducers or transformers 62. The information sensed 
is utilized advantageously in a manner described hereinafter for providing 
useful circuit information for the contactor 10. 
More particularly and referring additionally to FIG. 2, current sensor 80 
of the overload relay circuit board generates a signal representative of 
the current in the motor starting contactor and this signal is the input 
signal to microprocessor 82. Microprocessor 82 would include an analog to 
digital converter as discussed in detail in U.S. Pat. No. 4,757,420 or it 
may include any conventional analog to digital converter known to those 
skilled in the art. This would convert the analog current sensor signal to 
a digital signal usable by the microprocessor. The microprocessor is 
suitably programmed in a manner understood by those skilled in the art to 
perform the following calculation: as each sample of current is obtained 
from sensor 80 the current signal is squared and multiplied by the time 
elapsed in taking the current sample to generate an I.sup.2 t signal. This 
I.sup.2 t signal is the actual value of heat in the motor controller at 
that instant in time. The microprocessor would also be programmed to have 
stored in memory a target value for I.sup.2 t appropriate to the system 
being used and the application in which it is being used. In accordance 
with the present invention the microprocessor is programmed to determine a 
relationship between the actual value of I.sup.2 t and the target value. 
This percentage is then the output signal which is then used for display 
84. 
Either the overload microprocessor may be used to perform the calculation 
in accordance with the present invention, or the information could be 
transmitted by the overload microprocessor to another microprocessor based 
unit and that unit may then perform the calculation and drive the display. 
More specifically, referring to FIG. 3, the current sensor in the overload 
relay 83 transmits the raw data regarding the current drawn by the motor 
to the microprocessor 85 as discussed above. Microprocessor 85 performs 
the I.sup.2 t calculations. In this embodiment of the invention, however, 
it is desired to remotely display the percentage information. Thus, a 
microprocessor in a network interface 87 is used to perform the 
calculation of the relationship between the actual signal and the target 
value. This network interface unit 87 is then used to drive an associated 
display device 89 as discussed in greater detail below. 
As discussed further hereinafter, display 84 (FIG. 2) also contains 
appropriate circuitry to conform the digital signal of the I.sup.2 t 
percentage into a bit pattern suitable to illuminate the various segments 
of a display device appropriate to display an understandable reading. In 
addition, the signal generated by the microprocessor may also be used to 
activate an associated control device 86 which may be an electromagnetic 
or an electrothermal device which forces contacts 44 and 48 opened in the 
event of a current overload situation. 
Referring now to FIG. 4, a block diagram of the circuit for preparing the 
display signal is shown. Input device 90 receives the signal from the 
microprocessor as a digitized signal for driving the display. This input 
may be protected if desired by suitable input protection circuit 92 which 
may contain several subcircuits including suitable 
resistor-diode-capacitor combinations as would be understood by those 
skilled in the art. A protected signal is then directed to an integrated 
circuit 94 which may be an MC14489 integrated circuit available from 
Motorola, Inc. of Schaumburg, Ill. which prepares the suitable bit pattern 
depending upon the value of the input to input device 90. The circuit 94 
generates an appropriate signal for input to the various input ports of a 
suitable display device 96 which may either be an LED bar type display 
which displays the percentage signal in a bar graph configuration or a 
digital display. In the case where device 96 is an LED bar display the 
appropriate number of lights are illuminated to reflect the percentage of 
the target value of I.sup.2 t. For example, if few lights are illuminated, 
this means that a low percentage of the target I.sup.2 t value exists and 
the operator at that stage need not take further action. If, on the other 
hand, many of the lights are illuminated, this shows that the I.sup.2 t 
value is approaching the target and the operator may then decide to take 
appropriate action. If all of the lights are illuminated, this indicates a 
trip situation. 
Additionally, display 96 may contain two separate LED's (not shown in FIG. 
4). The first LED is a trip indicator which lights when the actual I.sup.2 
t signal from the input 90 is 100% of the target signal which indicates 
that a trip condition has been reached. Additionally, a second LED can be 
provided as part of display 96 which is illuminated when the motor has 
cooled a sufficient amount such that a reset of the system is appropriate. 
As would be understood by those skilled in the art, the motor controller 
is preset to trip at a particular I.sup.2 t value. It cannot thereafter be 
reset until the I.sup.2 t value falls to at least a certain percentage of 
the trip value as dictated by safety concerns. A typical value may be 
about 50% of the total I.sup.2 t trip target value. The second LED is 
illuminated when the I.sup.2 t value reaches the predetermined safe reset 
position such as, for example, 50%. This informs the operator that a reset 
of the system may proceed and the operator would then activate an 
associated reset button to reactivate the equipment. 
Referring to FIG. 5, an example of a display panel in accordance with the 
present invention is shown. An LED bar graph display 100 is shown in the 
embodiment of the FIG. 4 and the percentages are indicated on the right 
hand side of the bar graph 100. The "TRIP" indicator light 102 provides 
information to the operator that the motor contactor has tripped. The 
"RESET ALLOWED" light 104 indicates that the system has returned to a 
condition under which reset is allowed and further operation can 
thereafter continue. 
A flow chart describing the method of the present invention is shown in 
FIG. 6. The program of the microprocessor is initiated with the start 
signal 110. The microprocessor obtains current samples from the current 
sensors 80 of the contactor 10 as shown in step 110. For a particular 
instant in time the current value for I is squared as is shown as step 
114. As shown in step 116 this is then multiplied by the time elapsed in 
taking the sample. This number is then the actual I.sup.2 t value 
indicating the thermal condition of the contactor at that point. The 
target value for I.sup.2 t is then retrieved from memory as shown in step 
118. A percentage is then calculated of the actual I.sup.2 t compared to 
the target value as shown in step 120. This percentage is then converted 
to a suitable bit pattern usable by a display device such as display 100 
of FIG. 4. This step is shown in step 122. The information is displayed as 
shown in step 124 and, as mentioned hereinbefore, the display may be an 
LED bar type display or a digital display. 
In accordance with another aspect of the invention the microcomputer may be 
additionally suitably programmed to calculate estimated time to trip and 
estimated time to reset. 
It should be understood that the present invention discloses a method and 
apparatus for remotely displaying the thermal condition or the level of 
heat in a motor such that an operator can read the display and take action 
to avoid an overload trip which would result in down time until a reset 
can be performed. Specifically, the operator could activate the system in 
a different manner if he or she were aware that the device was close to 
the trip condition. This is particularly useful in plugging of 
plug-reversing type duty. The I.sup.2 t may displayed on either an LED bar 
type display or a digital display. In addition, a trip LED and a reset 
allowed LED could also be provided. 
While specific embodiments of the invention have been described in detail, 
it will be appreciated by those skilled in the art that various 
modifications and alternatives to those details could be developed in 
light of the overall teachings of the disclosure. Accordingly, the 
particular arrangements disclosed are meant to be illustrative only and 
not limiting as to the scope of the invention which is to be given the 
full breadth of the appended claims and any and all equivalents thereof.