DESC:TECHNICAL FIELD
The present subject matter described herein, in general relates to protection of motors from overheating which is caused due to overloading, abnormal conditions like stall, locked rotor, load jam and prolonged starts and more particularly, to a system and a method for providing thermal overload protection to motors by adapting the motor characteristics.

BACKGROUND
In a power system, protection relay plays an important role for proper and uninterrupted functioning of the healthy section of the system. The main objective of the protection relay is detection and tripping/blocking of the breaker, depending upon the application, in order to isolate the faulty section of the system from the healthy part.
The motor thermal overload protection protects the electric motor from overheating. It is installed in the motor circuit to protect the motor from mechanical overload conditions when it is in the starting and running condition. Current higher than full load current, results in an excessive rise in temperature in the motor windings which in turn leads to insulation breakdown. In current practice, standard IEC60255-8 curve calculates the thermal capacity used with an exponential formula. This curve replicates the running characteristics of motor. As per the standard IEC 60255-8, starting current may indicate overload since the starting current is typically around 6 to 8 times the full load current and hence, the thermal overload relay issues a trip signal. Hence, thermal protection needs to be disabled to allow the successful starting of the motor thereby compromising on protection during starting of the motor. Thus thermal overload devices should be designed to allow motors to start by allowing high currents (typically 6 to 8 times full load current) to flow during the motor starting condition and for short duration of overloads such as a slug of product going through a system.
In cases likes high inertia loads and reduced voltage starting, the motor acceleration time may exceed the locked rotor withstand time and the starting current less than 6 times of full load current (normally 3 to 4 times of full load current). Motor may trip due to excessive start time thereby preventing the motor from successful starting.
In current practice, the control method for thermal overload relay is defined by the standard IEC 60255-8. The formula for calculating the thermal level (thereafter used as ‘thermal capacity used’ i.e. TCU) of the motor can be derived from the thermal model defined by IEC: cold curve equation and the hot curve equation.

Cold Curve:

Hot curve:

Where = operating time i.e time to trip
t = time constant
= base current (Overload factor X Full Load current)
= IEC constant
= measured motor load current
= specified load current before overload occurs
This method calculates the thermal capacity used with respect to the motor current. When motor current exceeds kIb, the thermal capacity used is accumulated. When thermal capacity used reaches 100%, then motor trips. When the motor current is below the level kIb, the thermal capacity used decreases exponentially. This accounts for motor cooling.
Limitations:
1. It cannot provide thermal protection during starting of the motor from heating due to abnormal conditions like prolonged starts, locked rotor conditions, etc.
2. Large motors and applications like high inertia loads call for the need of reduced voltage motor starting methods like autotransformer start or star-delta start where the starting current is low and the motor acceleration time exceeds the locked rotor withstand time.
3. The relay interprets it as excessive start time and issue trip to the motor.
4. The curve does not allow successful starting of motor in such applications.
Some of the prior arts in the field of present invention are as follows:
US7538992B2: This invention relates to a method for protecting an induction motor compromising the following actions. The method determines the current drawn by the motor and the state of the motor. It calculates a used thermal capacity based on the formula defined by the standard IEC 60255-8 when the motor is in active state and when it is in inactive state, it calculates the thermal level based on I2t algorithm locked rotor current and safe stall time. When the used thermal capacity attains a threshold, the relay is tripped removing current to the motor and preventing motor from overheating. A method to derive thermal time constants from desired trip limits (such as those defined by IEC standards or the thermal limit curves provided by motor manufacturers) is also presented. For example, the thermal time constants can then be used in tacking motor used thermal capacity throughout various motor states, (as shown in figure 1).
Limitations:
1. Though the curve replicates the starting and running characteristics of the motor, it does not account for prolonged starts in case of high inertia loads.
Thus, there is a need for a protection model which is designed in such a way that it protects the motor from overheating during running as well as starting conditions. Therefore, present invention provides a system and a method for thermal overload protection during motor running as well as during motor starting state by adapting itself to the motor characteristics.

SUMMARY
This summary is provided to introduce concepts related to an adaptive thermal overload protection system for motor protection and method thereof. This summary is not intended to identify essential features of the subject matter nor is it intended for use in determining or limiting the scope of the subject matter.
In one implementation, a basic object of the present invention is to overcome the disadvantages/drawbacks of the known art.
Another object of the present invention is to provide a system for thermal overload protection to motors by adapting the motor characteristics.
Another object of the present invention is to provide a method for thermal overload protection to motors by adapting the motor characteristics.
Another object of the present invention is to provide a system which allows the motor to be used to its full potential during starting and running of the motor without compromising on protection during starting.
In one implementation, a system and a method which determines the TCU and protect the induction motors from damage due to overheating. In current practice, the algorithm calculates the TCU of the motor as per IEC 60255-8 curve. According to IEC 60255-8, if the motor steady state current (i.e. during running state) causes the TCU to exceed 100%, it issues a trip command. The standard gives the protection only in case of motor running state as it is not applicable during motor starting state.
In the present system, thermal overload protection is envisaged during motor running as well as during motor starting state. Different curve regions are being followed for different states of the motor. A custom curve is utilized to provide protection during motor starting and running state. During starting state of the motor, constant curve and inverse square current curve are predominantly active and issues a trip command for TCU > 100%. Trip command generated following IEC 60255-8 curve is inhibited during starting state of the motor. During running state of the motor, IEC-60255-8 curve is active and issues a trip command for TCU > 100%.
Accordingly, in one implementation of the present invention, an adaptive thermal overload protection system (102) for protecting a motor from overheating, by tripping the motor on reaching a maximum permissible percentage of thermal capacity used (TCU) is disclosed. The adaptive thermal overload protection system comprises of a numerical relay (104) for detecting at least one thermal overload condition with a current sensing mechanism (106), and a local human machine interface (HMI) and a PC HMI (114) for providing thermal overload settings. The numerical relay (104) further comprises of a CPU controller (112) with protection algorithm logic installed (116), an analog/digital converter (108) for capturing current analog samples from said current sensing mechanism (106) and converting said analog signals into digitized form; and a data acquisition and metering section (110) taking said digitized inputs from said analog/digital converter (108) and transmitting said digitized signal to said CPU controller (112).
In another implementation of the present invention, a method for protecting a motor from overheating, by tripping the motor on reaching a maximum permissible percentage of thermal capacity used (TCU) using an adaptive thermal overload protection system (102) with a numerical relay (104) having a CPU controller (112) with a protection algorithm logic (116) is disclosed. The CPU controller (112) configured to perform the steps of:
· determining a motor current signal and a motor status for every cycle of the motor, wherein the motor status is a starting condition;
· calculating a thermal level of the motor using an inverse current squared (I2t) curve for every cycle of the motor;
· comparing the motor current with a pickup level;
· providing at least one setting parameters to obtain a constant curve;
· initiating timers, wherein the timers are initiated when the current is within a range defined for the constant curve; and
· protecting the motor from overload by issuing trip signal on detection of maximum permissible thermal overload.
In another implementation of the present invention, a method for protecting a motor from overheating, by tripping the motor on reaching a maximum permissible percentage of thermal capacity used (TCU) using an adaptive thermal overload protection system (102) with a numerical relay (104) having a CPU controller (112) with a protection algorithm logic (116) is disclosed. The CPU controller (112) configured to perform the steps of:
· determining a motor current signal and a motor status for every cycle of the motor, wherein the motor status is a running condition;
· calculating a thermal capacity used by the motor;
· comparing the motor current with a pickup level;
· providing setting parameters to obtain a constant curve;
· initiating timers, wherein the timers are initiated when the current is within a range defined for the constant curve; and
· protecting the motor from overload by issuing trip signal on detection of maximum permissible thermal overload.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The detailed description is described with reference to the accompanying figures. In the figures, the left-most digit(s) of a reference number identifies the figure in which the reference number first appears. The same numbers are used throughout the drawings to refer like features and components.
Figure 1 illustrates (PRIOR ART) a typical motor characteristics, thermal model is shown, in accordance with an embodiment of the present subject matter.
Figure 2 illustrates a typical motor characteristics, IEC 60255-8 curve and the customized thermal overload curve is shown, in accordance with an embodiment of the present subject matter.
Figure 3 illustrates an adaptive thermal overload protection system (102) for protecting a motor from overheating is shown, in accordance with an embodiment of the present subject matter.
Figure 4 illustrates a method for protecting a motor from overheating, is shown, in accordance with an embodiment of the present subject matter.
Figure 5 illustrates a method for protecting a motor from overheating in startup condition, is shown, in accordance with an embodiment of the present subject matter.
Figure 6 illustrates a method for protecting a motor from overheating in running condition, is shown, in accordance with an embodiment of the present subject matter.
Figure 7 illustrates a schematic flow diagram for thermal overload protection is shown, in accordance with an embodiment of the present subject matter.
DETAILED DESCRIPTION OF THE PRESENT INVNENTION
Preferred embodiments of the present disclosure will be described herein below with reference to the accompanying drawings. In the following description, well-known functions or constructions are not described in detail to avoid obscuring the present disclosure in unnecessary detail.
The present invention is a system and a method for thermal overload protection during motor running as well as during motor starting state.
The following description with reference to the accompanying drawings is provided to assist in a comprehensive understanding of exemplary embodiments of the invention as defined by the claims and their equivalents. It includes various specific details to assist in that understanding but these are to be regarded as merely exemplary. Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the embodiments described herein can be made without departing from the scope and spirit of the invention. In addition, descriptions of well-known functions and constructions are omitted for clarity and conciseness.
The terms and words used in the following description and claims are not limited to the bibliographical meanings, but, are merely used by the inventor to enable a clear and consistent understanding of the invention. Accordingly, it should be apparent to those skilled in the art that the following description of exemplary embodiments of the present invention are provided for illustration purpose only and not for the purpose of limiting the invention as defined by the appended claims and their equivalents.
It is to be understood that the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise.
By the term “substantially” it is meant that the recited characteristic, parameter, or value need not be achieved exactly, but that deviations or variations, including for example, tolerances, measurement error, measurement accuracy limitations and other factors known to those of skill in the art, may occur in amounts that do not preclude the effect the characteristic was intended to provide.
Accordingly, present invention is a protection strategy that avoids unnecessary trips during starting state of the motor. The proposed custom curve can be closely matched to the motor starting characteristics (rotor thermal limit curve) and motor running characteristics (stator thermal limit curve) as specified by the motor manufacturer. It can be customized to match with motor characteristics as per the load conditions by making appropriate settings for the configurable parameters of this adaptive thermal overload protection model.
Accordingly, present invention provides a system and a method for thermal overload protection during motor running as well as during motor starting state.
The custom curve comprised of three regions as follows:
1. The IEC 60255-8 region: It is predominantly used in running state of the motor. In starting state of the motor, trip command generated in this region is inhibited and is activated if motor current is less than minimum motor starting current .
2. The inverse current squared curve region: It is mainly used in starting state of the motor and when rotor current exceeds where is the start time constant and is the maximum start time. The trip time is given by the formula: .
3. The constant curve region: It is mainly used in starting state of the motor and issues a trip signal when the rotor current exceeds and is less than for the time period .
If only IEC 60255-8 curve is being followed for all types of motor characteristics, it is possible that the standard IEC 60255-8 may overlap the motor starting characteristics during conditions like high inertia load or while using motor starting techniques like autotransformer or star-delta starting as shown in figure 1. Thus IEC 60255-8 curve may cause the motor to trip even before the motor comes into running state. Also unbalance in the power system may cause rapid increase in actual TCU of the motor than TCU calculated following IEC 60255-8 curve. Thus applying correction factor (negative sequence current) in the TCU calculation will shift the calculated TCU towards actual TCU of the motor thereby protecting the motor against thermal overload in more precise manner. Thus the custom curve has been developed to ensure proper starting of the motor and provide protection during starting.
Referring now to figure 2 shows the typical motor characteristics, IEC 60255-8 curve and the customized thermal overload curve proposed in this invention. The drawing is not to scale and is intended for use in conjunction with the explanations in the following detailed description of the invention.
Encircle 1 show the curve defined as per IEC-60255-8 used in motor running state. Encircle 2 and 3 shows the constant and inverse current squared curve respectively used in motor starting condition. Encircle 1, 2 and 3 jointly defines the custom curve used for adaptive thermal overload protection.
Encircle 4 shows the motor starting characteristics for high inertia loads. Encircle 5 shows the typical motor starting characteristics in case of normal loads.
Referring now to figure 3, an adaptive thermal overload protection system (102) for protecting a motor from overheating, by tripping the motor on reaching a maximum permissible percentage of thermal capacity used (TCU) is disclosed. The adaptive thermal overload protection system comprises of a numerical relay (104) for detecting at least one thermal overload condition with a current sensing mechanism (106), and a local human machine interface (HMI) and a PC HMI (114) for providing thermal overload settings. The numerical relay (104) further comprises of a CPU controller (112) with protection algorithm logic installed (116), an analog/digital converter (108) for capturing current analog samples from said current sensing mechanism (106) and converting said analog signals into digitized form; and a data acquisition and metering section (110) taking said digitized inputs from said analog/digital converter (108) and transmitting said digitized signal to said CPU controller (112).
In one implementation, said motor selected from a group comprising of a low voltage motor, a high voltage motor, and combination thereof.
In one implementation, said numerical relay (104) is calculated for a starting condition and a running condition of the motor. The starting condition is selected from a group comprising of a direct online, a reverse direct online, a two speed, a reduced voltage, a starting of motors through drives, an applications involving normal as well as high inertia loads, and combination thereof.
Referring now to figure 4, a method for protecting motors against thermal overload by tripping the motor on detecting maximum permissible thermal overload is disclosed. The CPU controller performs the following steps:
· At step 402, the current signals are measured;
· At step 404, starting/running condition of the motor are detected;
· At step 406, inhibit condition of motor are detected;
· At step 408, the thermal overload condition, with the settings being given by the user through HMI is identified;
· At step 410, the thermal capacity used by the motor is calculated.
· At step 412, a TRIP signal when the thermal capacity used exceeds 100% is issued.
In one implementation, the method comprising the steps of capturing current analog samples per cycle and converting into digitized form using an analog/digital converter; said digitized samples from said analog/digital converter fed to a data acquisition and metering section every cycle; output from said data acquisition and metering section further fed to a CPU controller where the protection algorithm logic is processed; The protection algorithm logic is processed by the CPU controller every cycle.
The numeric relay identifies the motor status as starting or running; calculates the thermal capacity used as per the IEC or custom curve depending on the motor status, issuing a trip signal once the thermal level reaches 100%. The present method provides thermal overload protection that allows successful starting of motor without compromising on protection during starting. The motor thermal model replicates the stator thermal limit curve and rotor thermal limit curve during running conditions and starting conditions respectively and hence the motor can be used to its full potential without compromising on protection. In applications involving high inertia loads, the starting time of the motor exceeds the locked rotor withstand time. Here the starting time is calculated by the timer present in the CPU controller and the condition is checked every cycle. The proposed model introduces the constant curve to allow successful starting of motor which involves high inertia loads.
A configurable start inhibit level is provided which provide true protection on starting of the motor even if authorized start command is present and minimizes the damage to the motor. The use of combination of configurable constant curve and squared inverse current curve during starting state of the motor with negative sequence correction factor is taken into account. The inhibition of tripping which may arise due to IEC 60255-8 curves during starting state of the motor.
Referring now to figure 5, a method for protecting a motor from overheating, by tripping the motor on reaching a maximum permissible percentage of thermal capacity used (TCU) using an adaptive thermal overload protection system (102) with a numerical relay (104) having a CPU controller (112) with a protection algorithm logic (116) is disclosed. The CPU controller (112) configured to perform the steps of:
· At step 502, a motor current signal and a motor status for every cycle of the motor is determined, wherein the motor status is a starting condition;
· At step 504, a thermal level of the motor using an inverse current squared (I2t) curve for every cycle of the motor is calculated;
· At step 506, the motor current with a pickup level is compared;
· At step 508, setting parameters to obtain a constant curve is provided;
· At step 510, initiating timers, wherein the timers are initiated when the current is within a range defined for the constant curve; and
· At step 512, the motor from overload by issuing trip signal on detection of maximum permissible thermal overload is protected.
In one implementation, the method of figure 5 may further comprise of capturing the current analog samples per cycle; converting the current captured into digitized form using an analog/digital converter (108); feeding said digitized samples to a data acquisition and metering section (110) to generate an output; feeding said output generated to said CPU controller (112) where said protection algorithm logic is processed.
In one implementation, a starting time of the starting condition is calculated by a timer present in the CPU controller (112) and the condition is checked every cycle. In one implementation, the curve replicates an acceleration thermal limit of the motor. In one implementation, the motor is selected from a group comprising of a low voltage motor, a high voltage motor, and combination thereof.
Referring now to figure 6, a method for protecting a motor from overheating, by tripping the motor on reaching a maximum permissible percentage of thermal capacity used (TCU) using an adaptive thermal overload protection system (102) with a numerical relay (104) having a CPU controller (112) with a protection algorithm logic (116) is disclosed. The CPU controller (112) configured to perform the steps of:
· At step 602, a motor current signal and a motor status for every cycle of the motor is determined, wherein the motor status is a running condition;
· At step 604, a thermal capacity used by the motor is calculated;
· At step 606, the motor current with a pickup level is compared;
· At step 608, setting parameters to obtain a constant curve are provided;
· At step 610, timers are initiated, wherein the timers are initiated when the current is within a range defined for the constant curve; and
· At step 612, the motor from overload by issuing trip signal on detection of maximum permissible thermal overload is protected.
In one implementation, when the motor current exceeds the pickup level, the thermal capacity used by motor increases, and the numerical relay issues trip once it exceeds 100%.
Referring now to figure 7, the logic flow diagram for providing thermal overload protection to induction motor is disclosed.
The standard IEC60255-8 curve very often encroach the motor starting characteristics as shown in figure 1 especially in case of high inertia load applications. This often results in an unwanted tripping during start as per the application requirement. In few other applications, the motor thermal curve substantially deviates from the standard curve i.e. the motor starting characteristics and motor running characteristics do not correspond smoothly.
Thus a protection model should be designed in such a way it protects the motor from overheating during running as well as starting conditions. The invention calculates the thermal capacity used by the motor and issues a trip signal when the thermal capacity used reaches the maximum permissible value considering inhibit condition, motor starting/running condition and prolonged start condition due to high inertia loads. This model allows the motor to be used to its full potential during starting and running of the motor without compromising on protection during starting. The invention may be coded inside the relay firmware. Once coding is completed and the code is validated, it may be released as a firmware build in the relay commercial release. The flow logic used in the present invention is unique and gives a competitive advantage over other relays in the market as it aids to a novel methodology of protecting motors from overheating due to overload, load jam, locked rotor, prolonged starts, and the like.

The present invention works in the following fashion

1) Motor current signal and the motor status are computed every cycle.
2) When the motor is in starting condition,
a) The thermal level of the motor is calculated using the inverse current squared, I2t curve and it is accumulated every iteration. This curve replicates the motor acceleration thermal limit of the motor
b) If the motor current exceeds the pickup level, the thermal level increases in parabolic fashion whereas if the motor current is less than the pickup level, the thermal level deteriorates.
c) In case of prolonged starts due to high inertia loads or reduced voltage starting, the thermal model follows the constant curve. The current range along which the constant curve has to be followed can be defined by the setting parameters Imin, T and tmax.
d) The timer of tmax will be initiated when the current is within the range defined for the constant curve. The numerical relay issue trip on condition when tmax expire and the current being greater than Imin throughout.
e) If the current decreases to a level less than Imin before the timer expire, numerical relay does not issue trip.
This gives protection to the motor from locked rotor, prolonged starts, etc.
3) When the motor is in running condition,
a) The thermal capacity used is calculated using the formula defined by IEC 60255-8. If the motor current exceeds the pickup, the thermal capacity used increases and the relay issues trip once it exceeds 100%.
This gives protection to the motor from load jam and overload.
Operation of Thermal Model during motor starting state:
During starting state of the motor, there are two active curves in the present invention as below.
A. Inverse current squared curve:
This curve is operational when motor current during starting state exceeds . The curve is operational during starting state of the motor and is disabled during motor running state. This curve replicates the locked rotor thermal limit curve of the motor. A trip signal is issued when TCU (as per equation (1.2)) reaches 100%.
The time to trip is given as:

The thermal capacity is calculated as:

Where Ieq =
I2 - Negative sequence current
I - Maximum of phase currents
KN - Negative sequence factor
?t - Step time
T - Start time constant
- Maximum time allowed for current to be higher than minimum starting current.
B. Constant curve:
This curve is operational during motor starting state and when the motor current exceeds minimum start current and is less than . This curve is useful in applications employing autotransformer start or star-delta start, high inertia load applications where starting current is low but motor starting time is high.
C. IEC 60255-8 curve:
This curve is operational only when starting current of the motor is less than the minimum starting current specified by the user. The thermal capacity is calculated as per equation (2.7).
Operation of Thermal model during motor running state:
During running condition, the TCU of the motor is calculated as per the IEC 60255-8 curve. The trip time is given as:

The thermal capacity may be evaluated as follows:






Therefore equation (2.2) is satisfied at time t. The left hand side is the thermal level defined as .When is equal to 1 or 100%, the motor is tripped.
Since numerical relay is a discrete system, we redefine equation (2.2) as follows:

Differentiating both sides with respect to time :

From equation (2.3):

Substituting equation (2.5) in (2.4)

Converting equation (2.6) to a difference equation

Thus


Where Ieq =
I2 negative sequence current
I maximum of phase currents
KN Negative sequence factor
?t step time
t1 Heat rise time constant
Ib Base current i.e. Overload factor x FLA
Equation (2.7) is implemented by means of a microprocessor to compute the thermal level in the relay.
Time to Cool:
The time taken by the motor to cool down can be calculated using the formula
Time to Cool =
This parameter has its significance when the motor is stopped.
Parameter Settings:-
The following settings are provided for the user. The settings can be configured through HMI.
Parameters Range Description
Overload factor 0.1-10 Overload/Service factor of the motor
K 1.05-1.5 IEC constant
1 30-30000 s Heat rise time constant for IEC60255-8 curve
2 60-60000 s Cool time constant for IEC60255-8 curve
KN 0-20 Negative sequence factor. The factor by which the heating effect of negative sequence current is multiplied
30-30000 s Start time constant for inverse squared current curve. It can be derived from motor thermal characteristics.
0.1-1000 s The maximum time allowed for motor to carry current higher than minimum starting current.
1.05-10 FLA The minimum current for which constant curve is active. It is recommended to set this value >= steady state current drawn by the motor.
Start Inhibit level 0-99% TCU Threshold level for start of the motor. When actual TCU < set threshold TCU, then only motor is allowed to start.

An adaptive thermal overload protection system for protection of LV/MV motors from overheating by tripping the motor on reaching the maximum permissible %TCU comprises a numerical relay for detecting thermal overload condition with internal or external current sensing mechanism; a local HMI (display) and a PC HMI for providing thermal overload settings. The present system can be adapted through user settings for AC induction motors of all types including IE2 motors; for different starting conditions like direct online, reverse direct online, two speed, reduced voltage and starting of motors through drives and applications involving normal as well as high inertia loads.
The numerical relay having a CPU controller which comprises protection algorithm logic installed therein, an analog/digital converter for capturing current analog samples from a current transformer secondary and converting said analog signals into digitized form; a data acquisition and metering section taking said digitized inputs from said analog/digital converter and advancing said digitized signal to said CPU controllers.
Exemplary embodiments discussed above may provide certain advantages. Though not required to practice aspects of the disclosure, these advantages may include those provided by the following features:
One feature of the invention is that, it uses a combination of configurable constant curve and squared inverse current curve during starting state of the motor with negative sequence correction factor taken into account.
Another feature of the invention is that, it comprises of inhibition of tripping which may arise due to IEC 60255-8 curves during starting state of the motor.
Another feature of the invention is that, it includes a configurable start inhibit level to provide true protection on starting of the motor even if authorized start command is present and minimizes the damage to the motor.
Another feature of the invention is that, in applications involving high inertia loads, the starting time of the motor exceeds the locked rotor withstand time. Here the starting time is calculated by the timer present in the CPU controller and the condition is checked every cycle. The proposed model introduces the constant curve to allow successful starting of motor which involves high inertia loads.
Another feature of the invention is that, the motor thermal model replicates the stator thermal limit curve and rotor thermal limit curve during running conditions and starting conditions respectively and hence the motor can be used to its full potential without compromising on protection
Yet another feature of the invention is that, the thermal overload protection that allows successful starting of motor without compromising on protection during starting.
Still another feature of the invention is that, the numeric relay identifies the motor status as starting or running; calculates the thermal capacity used as per the IEC or custom curve depending on the motor status, issuing a trip signal once the thermal level reaches 100%.
Still another feature of the invention is that, it can be adapted through user settings for ac induction motors of all types including IE2 motors; for different starting conditions like direct online, reverse direct online, two speed, reduced voltage and starting of motors through drives and applications involving normal as well as high inertia loads.
Although implementations for the adaptive thermal overload protection system for motor protection and method thereof have been described in language specific to structural features and/or the methods, it is to be understood that the appended claims are not necessarily limited to the specific features or methods described. Rather, the specific features are disclosed as examples of an adaptive thermal overload protection system for motor protection and method thereof. It is intended that the disclosure and examples above be considered as exemplary only, with a true scope of disclosed embodiments being indicated by the following claims.
,CLAIMS:1. An adaptive thermal overload protection system (102) for protecting a motor from overheating, by tripping the motor on reaching a maximum permissible percentage of thermal capacity used (TCU), the adaptive thermal overload protection system comprising:
a numerical relay (104) for detecting at least one thermal overload condition with a current sensing mechanism (106) comprising:
a CPU controller (112) with a protection algorithm logic installed (116);
an analog/digital converter (108) for capturing current analog samples from said current sensing mechanism (106) and converting said analog signals into digitized form; and
a data acquisition and metering section (110) taking said digitized inputs from said analog/digital converter (108) and transmitting said digitized signal to said CPU controller (112); and
a local human machine interface (HMI) and a PC HMI (114) for providing at least one thermal overload settings.

2. The adaptive thermal overload protection system (102) according to claim 1, wherein said motor selected from a group comprising of a low voltage motor, a high voltage motor, and combination thereof.

3. The adaptive thermal overload protection system (102) according to claim 1, wherein said numerical relay (104) is calculated for a starting condition and a running condition of the motor.

4. The adaptive thermal overload protection system (102) according to claim 3, wherein starting condition is selected from a group comprising of a direct online, a reverse direct online, a two speed, a reduced voltage, a starting of motors through drives, an applications involving normal as well as high inertia loads, and combination thereof.

5. A method for protecting a motor from overheating, by tripping the motor on reaching a maximum permissible percentage of thermal capacity used (TCU) using an adaptive thermal overload protection system (102) with a numerical relay (104) having a CPU controller (112) with a protection algorithm logic (116), the CPU controller (112) configured to perform the steps of:
determining a motor current signal and a motor status for every cycle of the motor, wherein the motor status is a starting condition;
calculating a thermal level of the motor using an inverse current squared (I2t) curve for every cycle of the motor;
comparing the motor current with a pickup level;
providing setting parameters to obtain a constant curve;
initiating timers, wherein the timers are initiated when the current is within a range defined for the constant curve; and
protecting the motor from overload by issuing trip signal on detection of maximum permissible thermal overload.

6. The method according to claim 5 comprising the steps of:
capturing the current analog samples per cycle;
converting the current captured into digitized form using an analog/digital converter (108);
feeding said digitized samples to a data acquisition and metering section (110) to generate an output;
feeding said output generated to said CPU controller (112) where said protection algorithm logic is processed.

7. The method according to claim 5, wherein a starting time of the starting condition is calculated by a timer present in the CPU controller (112) and the condition is checked every cycle.

8. The method according to claim 5, wherein the curve replicates an acceleration thermal limit of the motor

9. The method according to claim 5, wherein the motor is selected from a group comprising of a low voltage motor, a high voltage motor, and combination thereof.

10. A method for protecting a motor from overheating, by tripping the motor on reaching a maximum permissible percentage of thermal capacity used (TCU) using an adaptive thermal overload protection system (102) with a numerical relay (104) having a CPU controller (112) with a protection algorithm logic (116), the CPU controller (112) configured to perform the steps of:
determining a motor current signal and a motor status for every cycle of the motor, wherein the motor status is a running condition;
calculating a thermal capacity used by the motor;
comparing the motor current with a pickup level;
providing setting parameters to obtain a constant curve;
initiating timers, wherein the timers are initiated when the current is within a range defined for the constant curve; and
protecting the motor from overload by issuing trip signal on detection of maximum permissible thermal overload.

11. The method according to claim 10, wherein when the motor current exceeds the pickup level, the thermal capacity used by motor increases, and the numerical relay issues trip once it exceeds 100%.