FI118659B - Thermal overload protection - Google Patents

Description

118659

Thermal overload protection

Background of the Invention

The invention relates to thermal overload protection to protect electrical equipment, and in particular electric motors, from overheating.

5 Electric motors are utilized in many applications to use various moving parts. An electric motor is often accompanied by a control unit which adjusts and monitors the operation of the electric motor, for example, its rotational speed.

The electric motor can also operate momentarily overloaded, but if overheated as the load continues, this can result in engine damage. The most critical is the damage to the stator winding insulation due to overheating.

Various solutions are known for protecting an electric motor against thermal overload. One known solution is based on 1..3-phase measurement of the motor current and modeling of the motor warm-up using the RC replacement 15-circuit. The oldest and most common technical implementation is a bi-metal relay (thermal relay) connected directly to the main circuit or via a current transformer.

One known solution is a thermal circuit breaker located within or adjacent to the motor which, after a certain temperature limit, trips and interrupts the flow of current through the electric motor. A more advanced version is the electronic unit 20, which measures the temperature of the electric motor with temperature sensors and trips the motor. This alternative method is based directly on the detection of temperature by separate sensors. The problem is the difficulty of getting the sensors to the right ·: ··· location. Such protection responds relatively slowly.

. ···. In numerical protection, information is processed in numerical form, that is, digitally. The analog measurement data is converted to digital by the A / D converter. The actual implementation of the measurement and protection functions is done by a microprocessor. Thermal Overload Protection measures the phase currents (load currents) of the motor or other object to be protected (eg cable or transformer): *** Rms and calculates the temperature dependent operating time. This term of operation may be in accordance with IEC 60255-8: i2 - lp2 * * t = Tin-- ·: ··: l2-Ib2:! ·. where • * · “[. · t = operating time • · τ - time constant 118659 2

Ip = load current before overload occurs I »load current

Ib = operating current (maximum allowable continuous current) 5 The thermal time constant τ is defined as the time required for the protected object to reach the temperature Θ, which is a fraction (eg 63%) of the steady-state warm patient 0S). The operating current lp is the maximum allowable continuous current which also corresponds to the maximum allowable temperature, i.e. the steady-state temperature 0S. This maximum allowable temperature is the trip 10 level. Alternatively, from the phase currents, the relative value of the thermal load of the protected object relative to the full (100%) thermal load can be calculated. Triggering occurs when the relative thermal load reaches 100%.

Thus, numerical thermal protection involves heavy computing, which requires an efficient processor and fast and expensive peripherals such as memories. The prior art solutions utilize a powerful processor, which further includes a built-in math processor, a floating point unit (FPU) or the like to perform real-time computing within a specified time. A powerful processor with library-20 functions that emulate a floating point unit has also been used. There are also implementations where the algorithm is implemented with ASIC circuits, so there is no post-programmability in them. Therefore, no change can be made to such a single purpose circuit, but a new circuit is always required if the function is to be changed. There are also implementations where power is measured / counted - counted - warmed - measured again, etc. This kind of implementation does not provide full real-time protection (no continuous measurement), but allows for a less efficient processor.

···· · *** '··· ** Brief Description of the Invention

It is therefore an object of the invention to provide a method and a device implementing the thermal ··: *** protection of electrical equipment by which the computation related to the protection · · · 30 can be lightened and the technical requirements of processors and peripherals reduced. The object of the invention is achieved by a method and system which are characterized by what is stated in the independent claims. Preferred embodiments of the invention are the subject of the dependent claims.

The invention is based on the fact that the mathematical equation or algorithm with the thermal load calculating algorithm and its operands is programmed to be suitable for an X-bit, preferably X-32, processor system using fixed-point arithmetic so that the result or intermediate result never exceeds X in the program. . The measured current is preferably scaled to a unit value in the range 0 to Y, where Y represents Y / 100% of the rated current, and preferably 5 Y-65000, whereby the calculation is independent of the actual current range.

Thanks to the invention, the thermal load can be calculated with a less efficient processor and with less memory, which in turn reduces the power consumption, manufacturing costs and physical size of the device. The computation can be accomplished with a simple and portable code that does not require a math-10 teak processor or math libraries. However, thermal load can be calculated with near 64-bit floating point accuracy even if the processor uses 32-bit fixed point arithmetic.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail with reference to exemplary embodiments, with reference to the accompanying drawings, in which:

Fig. 1 is an exemplary block diagram illustrating an overload protector according to an embodiment of the invention;

Fig. 2 is an exemplary signal diagram illustrating the operation of the device of Fig. 1; and FIG. 3 is an exemplary flow diagram illustrating the operation of FIG.

DETAILED DESCRIPTION OF THE INVENTION ... In Figure 1, a thermal overload protector is connected to the electric motor to be protected; ***; * to the electric motor M or other electrical device and to the three-phase mains supply L1, • ** | 25 between L2 and L3. S1 is the master network switch, for example, manually controlled, and S2 is the overload protection-controlled release switch controlled by the trip signal TRIP. The overload protector 1 measures the load current of each of the phases L1, L2 and L3 of the mains power supply to the motor M by a current measuring unit 10 based, for example, on current transformers. In addition, the overload protector 1 may comprise a measuring unit 30 11 for measuring phase voltages. Further, the overload protection 1 preferably comprises a human-machine-interface (HMI) 12 interface with display 13 and keypad 14. Further, the overload protection 1 may comprise a communication unit 15 connected to a local area network. (eg Et-: ··· peas), bus, fieldbus (eg Profibus DP) or other communication media 17.

118659 4

The most important function of the invention relates to the protection and control unit 16. The overload protection 1 is implemented by a microprocessor system, whereby the majority of the aforementioned units are implemented by suitable microprocessor programs and auxiliary circuits such as memory circuits. The measured values produced by the current and voltage measuring units 5 are converted into numerical or digital values by digital-to-analog converters (A / D). According to the basic principle of the invention, the microprocessor system uses solid-point arithmetic, preferably 32-bit arithmetic. A suitable processor type is, for example, a general-purpose processor with a 32-bit RISC instruction set, such as the ARM7 / 9 or M68k series.

It is to be understood that the above structure is only one example of a thermal overload protection implementing the invention.

The overload protector 1 protects the motor M from overheating and any resulting damage. The protection is based on the calculation of the thermal load of the motor based on the measured phase currents. The general operation of the shield 15 will now be explained by the example of Figures 2 and 3. The phase conductors L1, L2 and L3 are connected to motor M by closing switches S1 and S2. The current measuring unit 10 measures the phase currents (step 31, Fig. 3) and the control unit 16 calculates the thermal load of the motor M by fixed point arithmetic based on the phase currents (step 32). The mathematical yh-20 used to calculate the thermal load can be as follows for one step: i2 f AT λ ..:! · ® * = αγ * - + [ι -— J · ® * -, • ·! where "**: Θ = thermal load, preferably 0 to 200%, preferably within the range 0-2.4 ··· ··· 25 AT = thermal load calculation interval, preferably in milliseconds • M ·. · * ·. R = the cooling factor of the electrical appliance, preferably 1 ... 10 C = trip class factor i = measured load current 30 · The factor C is preferably a trip class factor te that tells the motor *: * *: The maximum set start time relative to the actual engine start time ··: The coefficient C may be, for example, 1.7 (x actual start time) In the preferred embodiment of the invention, the trip class coefficient t® is multiplied by a constant, preferably 29.5, or is calculated by the formula (1 / k) * Te * (la / ln) 2, where la = start current, In = rated current, Te = allowable start time and k = constant, constant k = 5 118659 1.22, when desired operating time curve as a trip class and t6 time combination (IEC 60947-4-1 The measured current is preferably scaled to a unit value in the range 0 to Y, where Y represents Y / 100% of the rated current, and preferably Y = 65000, whereby the calculation is independent of the actual current range.

Consider, for example, 32-bit fixed-point arithmetic. According to the invention, the mathematical equation or algorithm for calculating the thermal load described above is programmed into a processor array using 32-bit fixed-point arithmetic so that the result or intermediate result never runs above a 32-bit value when running the program in the processor system.

An example of a structured and scaled calculation equation programmed in this way is thRes = ((ΔΤ 1 (i2 / C) + ROUNDING) / MSEC) 15 + (((((MSEC1SCALING) - ((AT1SCALING / (R1C))) / SPART1) 1th / SPART2) + thFract where operand values are for example thRes = thermal load 0 to 200% corresponding to value range 0-24000 ROUNDING = e.g. 500 20 MSEC = e.g. 1000 SCALING = e.g. 10000 ... T SPART1 = e.g. SCALING / 10 SPART2 = e.g. SCALING / 100 *: **: thFract = thRes of previous calculation divided by constant, e.g.

25 constant = SCALING = 10000.

.:. ROUNDING corresponds to a decimal round. MSEC scales what-! ···. towns for seconds. SCALING is a precision scaling. The product of the terms SPART1 and • SPART2 represents a unit of time (preferably milliseconds) scaled in two to maintain computational accuracy.

• · 30 The thermal load result thRes is too * ··· 'large due to scaling (in the range 0-24000 in the example) and is scaled down to represent ·: ··: the used thermal load unit value (in the example range 0-2, 4 Θ = thRES / 10000 · • 118 118 118 6 118659 This quotient Θ is stored as thFract and is used in the next calculation. Calculation accuracy with 0-100% thermal loading is better than 0.1% thermal loading.

The graph of Fig. 2 shows the calculated thermal load Θ as a function of time t 5. When engine M starts from a cold state, it starts to warm up. Similarly, the calculated thermal load Θ increases with time. As the thermal load Θ increases to a specific set alarm level in Alarmjevel, the control unit 16 can provide an alarm to the operator, for example, via the user interface 12-14 or the communication unit 15 (steps 35 and 36 in Figure 3). The control unit 10 16 may also continuously or after a certain level calculate the time-to-trip and inform the operator thereof (steps 33 and 34 in Figure 3). As the thermal load Θ increases to a specific set trip level (preferably 100% of the motor thermal load), the control unit 16 activates a trip signal TRIP controlling the switch S2, thereby disconnecting the motor M from the three-phase supply L1, L2 and L3 (steps 37 and 38). ). If there is too little thermal power remaining after motor tripping (e.g., less than 60%), protection 1 may prevent restarting until the motor cools to a certain level (restart inhibit) or for a specified time (steps 39 and 40 in Figure 3). For start-up, the TRIP signal is again deactivated 20 and the switch S2 is closed. In one embodiment, the operator may direct the control unit 16 to an override mode with a triple double level (override trip * level).

: It will be obvious to a person skilled in the art that as technology advances, the basic idea of the invention can be implemented in many different ways. The invention and its implementation. Thus, the forms are not limited to the examples described above, but may vary within the scope of the claims.

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Claims (13)

118659 7 Apparatus for thermal overload protection of an electrical device, in particular an electric motor (M), comprising means (10) for measuring at least one load current supplied to the electrical device (M), means (16) for calculating a thermal load of the electrical device 5 based on said at least one load current (S2) for interrupting the power supply (L1.L2.L3) when the thermal load reaches a certain threshold level, said means (16) for calculating the thermal load of the electrical device comprising an X-bit, preferably X = 32, fixed point arithmetic processor system, that the processor system includes means for scaling the measured current to a unit value from 0 to Y, where Y represents Y / 100% of the nominal current, and means for calculating the thermal load by a mathematical equation programmed into the microprocessor system with its operands the intermediate result will never exceed the X-bit value. Device according to claim 1, characterized in that the mathematical equation is where Θ - thermal load, 20 ΔΤ = thermal load calculation interval; ··; R = electrical refrigeration coefficient i.i: C = trip class factor Device according to claim 2, characterized in that O is used for one or more of the following operand values ·: · 25 Θ = 0 to 200%, preferably corresponding to a value range of 0-2.4 • • a1 ΔΤ = thermal load calculation interval in milliseconds ··· R = cooling factor of the electrical equipment in the range 1 ... 10: ·. C = trip class factor • aa [·· .. i = measured current. aa *: 1 30 Device according to claim 1, 2 or 3, characterized in that "" a mathematical equation with its operands is structured such that the result or intermediate result of a ter *: 2 load calculation never exceeds 32 bits ar-:! ·. voa on »1» • · · a • aa1 · 2 • a 118659 8 thRes = ((AT * (iz / C) + ROUNDING) / MSEC) + {(((MSEC * SCALINGM (AT * SCALING / (R * C))) / SPART1) * th / SPART2) + thFract where 5 thRes = thermal loads, ΔΤ = is the thermal load calculation interval R = electrical cooling factor C = trip class factor i = measured current scaled to unit value 10 ROUNDING = rounding factor MSEC = time unit scaling SCALING = precision scaling SPART1 = partial scaling SPART2 = partial scaling 15 thFract = thermal load of previous calculation thRes divided by na constant. Device according to Claim 4, characterized in that one or more of the following operand values are used thRes = 0 to 200%, preferably corresponding to a value range of 0-24000 20 ΔΤ = thermal load calculation interval in milliseconds R = 1 ... 10 i = measured current scaled to 0- 65000 corresponding to 0- • · · 650% of rated current, *: ··: ROUNDING = 500 25 MSEC = 1000 * ·:. SCALING = 10000 SPART1 = SCALING / 10 SPART2 = SCALING / 100 ... thFract = thRes of previous calculation divided by constant 30 SCALING. Apparatus according to claim 3, 4 or 5, characterized by *: **: that C is a trip class coefficient te multiplied by a constant, preferably 29.5, or calculated by the formula:: ··· (1 / k) * Te * (la / ln) 2 where la = start current, In = rated current, Te. \ = allowable start time and k = constant, preferably k = 1.22. • · · * ··: 35 t 9 118659 A method for thermal overload protection of an electrical device, in particular an electric motor, comprising measuring at least one load current supplied to the electrical device, calculating the thermal load of the electrical device based on said at least one load current using an X bit, preferably X = 32, fixed point arithmetic processor interrupting the power supply to the electrical device when the thermal load reaches a certain threshold level, characterized in that the measured current is scaled to a unit value in the range 0 - Y, where Y 10 represents Y / 100% of the rated current, the thermal load of the electrical device is calculated by a processor equation structured so that the result or intermediate result never exceeds the X-bit value. A method according to claim 7, characterized in that the mathematical equation is ®l = AT * - + fl ——— k C {R * C) * 1 where Θ = thermal load, preferably 0 to 200%, preferably within the range 0 -2.4 AT = thermal load calculation interval, preferably in milliseconds R = cooling factor of electrical device, preferably 1 ... 10 • *: C = trip class factor * "·: i = measured current. Method according to claim 7 or 8, known the fact that a programmed mathematical equation, structured · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · ·. i2 / C) + ROUNDING) / MSEC) + ((((((MSEC * SCALING) - ((AT * SCALING / (R * C))) / SPART1) * th / SPART2) + thFract • · **: * '30 where *: * ·: thRes = thermal load, ·: ··: AT = is the thermal load calculation interval R = electrical cooling factor i C = trip class factor 35 i = measured current scaled to unit value 10 118659 ROUNDING s rounding factor MSEC = time unit scaling SCALING = precision scaling SPART1 = subscale 5 SPART2 = subtask thFract = thermal load of previous calculation thRes divided by constant. Method according to Claim 9, characterized in that one or more of the following operand values of 10 thRes to 0 to 200% are used, preferably corresponding to a value in the range 0 to 24000 Δ ter s thermal load calculation interval in milliseconds R = 1 ... 10 C = trip class factor i the measured current scaled to a unit value of 0-65000 corresponding to Οι 5 650% of the rated current, ROUNDING = 500 MSEC = 1000 SCALING = 10000 SPART1 = SCALING / 10 20 SPART2 = SCALING / 100. thFract = thRes of previous calculation divided by constant; 1 2 3 · | SCALING. A method according to claim 8, 9 or 10, characterized in that C is a trip-class coefficient t6 multiplied by a constant, preferably 25 29.5, or calculated by the formula (1 / k) 1 Te 1 (la / ln) 2, where la = start current, In = rated current, Te = allowable start time and k = constant, preferably k = 1.22. • · · • • 1 · 1 »• 1 * · • · · · · · m · 1 · 2 · · · 1» 2 • · · 3 · 118659 11