Classifications

• • H—ELECTRICITY
  • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
  • H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
  • H02H6/00—Emergency protective circuit arrangements responsive to undesired changes from normal non-electric working conditions using simulators of the apparatus being protected, e.g. using thermal images
  • H02H6/005—Emergency protective circuit arrangements responsive to undesired changes from normal non-electric working conditions using simulators of the apparatus being protected, e.g. using thermal images using digital thermal images

Definitions

• the invention relates to thermal overload protection for protecting electrical devices, and particularly electric motors, from overheating.
• Electric motors are utilized in several applications for driving various moving parts. An electric motor often has an associated control unit for adjusting and monitoring the operation of the electric motor, the speed of rotation, for example.
• An electric motor may temporarily operate also overloaded, but if it becomes overheated as the loading continues, this may result in damage to the motor. Damage to the isolation of the stator coiling caused by overheating is the most critical.
• 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 on modelling the heating of the motor by using an RC equivalent circuit.
• thermo relay thermo relay
• a known solution is a thermal safety switch arranged inside or in connection with the motor, the switch tripping after a given temperature limit and interrupting the current flow through the electric motor.
• a more advanced version is an electronic unit that measures the temperature of the electric motor with temperature sensors and triggers a shut-off of the motor. This alternative manner is directly based on temperature detection with various sensors. The problem is the difficulty of placing the sensors correctly. Such a protection reacts relatively slowly.
• numerical protection data is processed in a numeric format, i.e. digitally. Analogical measurement data are converted with an A D converter into digital.
• the actual measurement and protection functions are implemented by means of a microprocessor.
• the thermal overload protection measures the root mean square (rms) values of the phase currents (load currents) of a motor or another object to be protected (e.g. a cable or a transformer), and calculates the temperature-dependent operating time. This thermal operating time may be accordant with standard I EC 60255-8:
• the thermal time constant ⁇ is determined as the time required of the object to be protected to reach a temperature ⁇ , which is a given portion (e.g. 63%) of a steady-state temperature ⁇ s , when the object to be protected is supplied with constant current.
• the operating current l p is the highest allowed continuous current, which also corresponds to the highest allowed temperature, i.e. the steady-state temperature ⁇ s . This highest allowed temperature is the trip level.
• the relative value of the thermal load on the object to be protected relative to a full (100%) thermal load can be calculated from the phase currents. The trip occurs when the relative thermal load reaches a 100% value.
• the object of the invention is thus to provide a method for thermal protection of electrical devices and an apparatus for implementing the method, allowing the calculation associated with the protection to be lightened and the technical requirements of the processors and peripheral circuits to be lowered.
• the object of the invention is achieved with a method and system that are characterized in what is stated in the independent claims. Preferred embodiments of the invention are described in the dependent claims.
• the invention enables the calculation of the thermal load with a less efficient processor and less memory, which, in turn, lower the power consumption, production costs and physical size of the device.
• the calculation can be implemented with a simple and transferable code, which does not require a mathematics processor or mathematical libraries.
• the thermal load can be calculated with nearly the accuracy of a 64-bit floating-point number calculation, even if the processor used 32-bit fixed-point arithmetic.
• FIG. 1 is an exemplary block diagram illustrating the overload protection according to an embodiment of the invention
• Figure 2 is an exemplary signal diagram illustrating the operation of the device of Figure 1
• Figure 3 is an exemplary flow diagram illustrating the operation of the device of Figure 1.
• a thermal overload protection is coupled between an electric motor M or other electrical device to be protected and a three-phase mains current supply L1 , L2 and L3.
• S1 is a main mains switch, e.g. manually controlled
• S2 is a release switch controlled by the overload protection and controlled with a trip signal TRIP.
• the overload protection 1 measures the current load of each phase L1 , L2 and L3 of the mains current supply of the motor M with a current measurement unit 10, which is based on current transformers, for example.
• the overload protection 1 may comprise a measuring unit 11 for measuring phase voltages.
• the overload protection 1 preferably comprises a user interface, i.e.
• the overload protection 1 may comprise a data communication unit 15 connected to a local area network (e.g. Ethernet), a bus, a field bus (e.g. Profibus DP) or another data communication medium 17.
• a local area network e.g. Ethernet
• a bus e.g. Profibus DP
• another data communication medium e.g. Profibus DP
• the most essential function is related to the protection and control unit 16.
• the overload protection 1 is implemented with a microprocessor system, the majority of the above units being implemented with suitable microprocessor software and peripheral circuits, such as memory circuits.
• the measuring values provided by the current and voltage-measuring units are converted into numerical, i.e. digital values with digital/analog converters (A/D).
• the microprocessor system employs fixed-point arithmetic, preferably 32-bit arithmetic.
• a suitable processor type is for instance a general- purpose processor having a 32-bit RISC instruction set, such as ARM 7/9 or the M68k series.
• the overload protection 1 protects the motor M from overheating and from any damage caused thereby. The protection is based on calculating the thermal load on the motor on the basis of measured phase currents. In the following, the general operation of the protection will be explained by means of the example of Figures 2 and 3.
• Phase conductors L1 , L2 and L3 are connected to the motor M by closing switches S1 and S2.
• the current- measuring unit 10 measures the currents of the phases (step 31 , Figure 3), and the control unit 16 calculates the thermal load on the motor M on the basis of the phase currents by using fixed-point arithmetic (step 32).
• Factor C is preferably a trip-class factor t ⁇ , which indicates the longest starting time set on the motor relative to the actual starting time of the motor. Factor C may be for instance 1.7 (x actual starting time).
• Y represents Y/100% of the nominal current
• thRes ( ( ⁇ T* ( i 2 /C ) +ROUNDING) /MSEC ) + ( ( ( (MSEC*SCALING) - ( ( ⁇ T*SCALING) / (R*C) ) ) /S PARTI ) *th) /SP ⁇ RT? ) +thFract
• MSEC e.g. 1000
• ROUNDING corresponds to decimal rounding. MSEC scales milliseconds into seconds. SCALING is accuracy scaling. The product of terms SPART1 and SPART2 represents the scaling of a time unit (preferably milliseconds), split into two parts to maintain calculation accuracy.
• This quotient ⁇ is saved as parameter thFract and employed in the calculation the next time. Calculation accuracy on 0 to 100% thermal load is better than 0.1% of the thermal load.
• the graph of Figure 2 represents the calculated thermal load ⁇ as a function of time t. When the motor M is started from cold state, it begins to warm up. In the same way, the calculated thermal load ⁇ increases as a function of time.
• the control unit 16 may give an alarm to the operator for instance via the user interface 12-14 or the communication unit 15 (steps 35 and 36 in Figure 3).
• the control unit 16 may also continuously or after a given level calculate the remaining time to trip (time-to-trip) and communicate it to the operator (steps 33 and 34 in Figure 3).
• the control unit 16 activates a trip signal TRIP, which controls the switch S2 to open, whereby the motor M is disconnected from the three-phase supply L1 , L2 and L3 (steps 37 and 38 in Figure 3).
• the protection 1 may prevent a restart until the motor is cooled to a given level (restart inhibit) or for a given time (steps 39 and 40 in Figure 3).
• start-up signal TRIP is again connected inactive and switch S2 is closed.
• the operator may control the control unit 16 into an override state, wherein the Trip level is double (override Trip level).

Landscapes

• Protection Of Generators And Motors (AREA)
• Control Of Electric Motors In General (AREA)
• Emergency Protection Circuit Devices (AREA)

Applications Claiming Priority (2)

Publications (1)

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• 2004
  • 2004-02-02 FI FI20040154A patent/FI118659B/fi not_active IP Right Cessation
• 2005
  • 2005-02-01 EP EP05708146A patent/EP1719231A1/de not_active Withdrawn
  • 2005-02-01 WO PCT/FI2005/000066 patent/WO2005074089A1/en active Application Filing
  • 2005-02-01 CN CNB2005800038328A patent/CN100539347C/zh not_active Expired - Fee Related
  • 2005-02-01 CA CA2554117A patent/CA2554117C/en not_active Expired - Fee Related

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