RU2227354C2 - Method of thermal protection of electric motor and arrangement for its realization - Google Patents

Abstract

FIELD: electrical devices utilized in thermal protection of electric motors. SUBSTANCE: method refers to thermal protection of electric motors against emergency conditions accompanied by inadmissible rise of temperature of their windings. In compliance with given method of thermal protection of electric motor current consumed by electric motor and temperature of windings of electric motor are measured by means of controller and electric motor is switched off when temperature of winding of electric motor reaches or exceeds upper limit. Ohmic resistance of its winding is measured in addition before switching on of electric motor and after its switching off and initial and final temperatures of its winding re computed. Data on initial temperature is used to compute changes of its temperature in process of operation of electric motor. Data on final temperature are compared with data computed on basis of measured current, if these data show discrepancy then parameters of thermal model of electric motor are corrected thanks to insertion of unit measuring resistance, intermediate relays into arrangement for thermal protection of electric motor which includes contactor with contacts sand winding, current pickups in supply circuits of electric motor, unit controlling controls and controller. EFFECT: increased accuracy of computation of temperature of winding of protected electric motor that excludes false operation and failure of thermal protection to operate. 2 cl, 1 dwg

Description

The invention relates to electrical engineering and can be used in thermal protection devices of electric motors, i.e. protection against emergency operation, accompanied by an unacceptable temperature rise of their windings. Since the most common emergency mode is motor overload, this protection is often referred to as overload protection. All thermal protection devices, depending on the emergency response method, are divided into temperature, current and temperature-current.

A known method of current protection of a motor with a dependent response characteristic [1], where time constants are set separately for both the stator winding and the magnetic circuit, is determined taking into account the temperature of the cooling air, the long-term permissible current multiplicity is determined, for the current multiplicities less than the long-term allowed, the temperature of the thermal balance of the winding is determined simulate the adiabatic heating process of the winding, etc. The specified protection method requires for its implementation a very complex device with a large number of functional elements, which leads to the large dimensions and cost of the protective device and reduces the reliability of its operation.

The closest in technical essence and adopted for the prototypes of the claimed technical solutions are a method of protecting the engine from overload and a device for its implementation, described in [2]. The specified method provides protection against engine overload, regardless of any duration of interruption of electrical power.

The method consists in measuring the current supplied from the power source to the motor, periodically digitally calculating data representing the temperature of the motor based on the measured current, shutting down the motor every time any last calculated motor temperature data at least reaches the prescribed threshold value, and is non-volatile memory of the last calculated engine temperature data before the specified engine was turned off, measuring elapsed time during which the specified engine was turned off, digitally calculating, based on the stored engine temperature data and the measured elapsed time, new data representing the engine temperature after the specified engine was in the off state, and resuming periodic digital calculation of the engine temperature data based on the specified new data and measuring current after the specified motor is turned on. When calculating data representing the temperature of the motor, it is necessary to take into account the current flow time and use the parameters of the thermal model of the protected motor.

A device for implementing this method comprises a control unit with a controller having a programmable module for periodically calculating digital data representing engine temperature; the controller also has means for issuing an output signal. In addition, the control unit contains switching means (for example, a contactor) that respond to the controller output signal and are designed to turn off the motor. The controller also has non-volatile memory; the control unit of the device also includes a timer connected to the controller; an R-C circuit may be used as the indicated timer.

The protection method described above has the following disadvantages:

1) when the engine is first turned on, its temperature is unknown, starting from which digital calculation of data representing the temperature of the running engine will be carried out; this uncertainty initially determines the uncertainty of the temperature calculation results and possible very gross errors in the operation of the protection device;

2) the results of calculating the temperature of the running engine may contain errors caused by both inaccuracy of the parameters of the thermal model of the protected engine embedded in the programmable module and those arising as a result of changes in the heat transfer conditions of the engine (ambient temperature, violation of engine ventilation, etc.);

3) the results of calculating the new temperature based on the stored data of the engine temperature and the measured elapsed time after the specified engine was in the off state, may also contain errors for similar reasons; in addition, the device must have a timer to measure the time of the engine off state, which can be of different durations.

The technical result of the claimed invention is improving the accuracy of calculating the temperature of the winding of the protected motor, eliminating the possibility of false positives and failures of protection.

The technical result is achieved in that in the method of thermal protection of the engine, which consists in measuring the current consumed by the motor, periodically calculating data representing the temperature of the motor winding, based on the measured current and its time, using the parameters of the thermal model of the protected motor, and turning off the engine when the calculated temperature data of the winding reaches or exceeds the prescribed upper limit, in addition, before switching on the motor and after turning it off, measure ohmic the resistance of its winding and from the measurement results calculate data representing the initial and final temperature of its winding; data on the initial temperature of the winding are used in calculating changes in its temperature during engine operation, data on the final temperature obtained from the measured resistance are compared with data calculated on the basis of the measured current, and if a discrepancy is found between these data, the parameters of the thermal model of the engine are adjusted.

The technical result is implemented by a device for thermal protection of the engine, comprising a contactor with contacts in the motor power supply circuit and a winding, current sensors in the motor power supply circuits, a control unit with a controller having controls, a programmable module and non-volatile memory, an additional measurement unit is added to the control unit resistance, the first intermediate relay, the contacts of which are connected between the motor power circuits and the inputs of the resistance measurement node, and the second intermediate relay, contact which is included in the power circuit of the contactor winding.

The essence of the invention is to supplement the known method of protecting the motor against overload, carried out using digital calculation of the temperature of its winding by current, time and parameters of the thermal equivalent circuit, which is accompanied by inevitable errors and deviations of the temperature values calculated in this way from the actual ones, by direct measurement of the ohmic resistance of the de-energized motor windings and calculation of the measured resistance of the actual temperature values, not -containing said errors and deviations and used for correcting the calculation results of the temperature of the motor current.

The claimed method, which should be attributed to methods of thermal current thermal protection, meets the criteria of “Novelty” and “Significant differences”, since the distinguishing features of the invention are not found in the claimed combination in known devices of a similar purpose and the use of the claimed method allows without significant complication of the protection device eliminate the serious disadvantages inherent in known thermal protection devices using known methods of protection.

Indeed, for the implementation of the proposed method of protection, it is enough to include the resistance measuring unit and an intermediate relay in the composition of the protection device [2], the contacts of which this node is connected to the power supply circuits of the motor winding; this eliminates the need for a timer in the specified device.

The drawing shows a diagram of a device for implementing the inventive method of protecting the engine, where the following notation:

1 - conclusions of the protection device,

2 - the main contacts of the contactor,

3 - winding of the contactor,

4 - control unit

5 - governing bodies,

6 - controller

7 - programmable module,

8 - non-volatile memory,

9 - node measuring resistance

10 - winding of the first intermediate relay,

11 - contacts of the first intermediate relay,

12 - winding of the second intermediate relay,

13 - contact of the second intermediate relay,

14 - current sensors,

15 - motor winding.

The device comprises a contactor with three main contacts 2, connected to the terminals of the device 1, and a winding 3, which is connected to one of the terminals 1 of the device through terminal 13 of the intermediate relay. The device includes a control unit 4 having controls 5, a controller 6 with a programmable module 7, non-volatile memory 8 and a resistance measurement unit 9. The control unit 4 also contains a first intermediate relay with winding 10 and contacts 11 and a second intermediate relay with winding 12 and contact 13. Contacts 11 of the first intermediate relay are used to connect to the inputs of the resistance measurement node 9 of the motor winding 15. Current sensors 14 are included in the power supply circuit of the motor winding 15, and their outputs are connected to the inputs of the controller 6.

The device operates as follows. When voltage is applied to the terminals of device 1, controller 6 is powered up, which is initialized and ready to execute operational commands received from control elements 5 of control unit 4. Having received a command to turn on the motor, controller 6 polls the outputs of current sensors 14 and receives a confirmation of the absence of current in power supply circuits of the motor winding 15 and includes a winding 10 of the first intermediate relay. This relay closes its contacts 11, connecting the motor winding 15 to the input of the resistance measuring unit 9. The latter measures the ohmic resistance of the winding 15 and sends the measurement result to the programmable module 7 of the controller 6, which calculates the data representing the initial temperature of the motor winding 15 and compares them with prescribed upper limit. After that, the controller 6 disconnects the winding 10 of the first intermediate relay, thereby opening its contacts 11, polls the output of the resistance measuring unit 9 and receives confirmation of disconnecting it from the motor winding 15. If the calculated data of the initial temperature of the winding 15 is lower than the prescribed upper limit, the controller 6 turns on the winding 12 of the second intermediate relay. The latter closes its contact 13, supplying power to the winding 3 of the contactor, which is turned on and by its contacts 2 connects the motor winding 15 to the terminals of the device 1, thereby including the motor to the power source.

To calculate the data representing the temperature of the winding, based on the result of measuring its ohmic resistance, the value of the ohmic resistance of the winding at normal temperature is used, recorded and stored in the non-volatile memory 8 of the controller.

In the process of engine operation, programmable module 7 of controller 6 periodically calculates the temperature of the motor winding 15 based on the current measured by sensors 14 and its time, using the parameters of the mathematical thermal model of the motor recorded in programmable module 7 of controller 6. When the calculated temperature data of the winding reach or exceed the prescribed upper limit, the controller 6 de-energizes the winding 12 of the second intermediate relay, which leads to engine shutdown; the last calculated data is stored in the programmable module 7 of the controller 6. After that, the controller polls the outputs of the current sensors 14, receives confirmation of the absence of current in the power circuits of the motor winding 15, performs the procedure of measuring the ohmic resistance of the winding 15 and calculating its temperature data from the measured resistance. The final temperature data obtained from the measured resistance, the programmable module 7 compares with the data calculated on the basis of the measured current, and if a discrepancy in these data is detected, it corrects the parameters recorded in the thermal model of the engine in order to reduce further these discrepancies. Programmable module 7 performs the same operations in all cases when the engine is switched off by the operator acting on the controls 5 of control unit 4. As a result of a number of such repeated adjustments, the parameters of the thermal model recorded in programmable module 7 are constantly updated and come closer to the actual parameters of the protected motor , taking into account all the influencing factors (ambient temperature, engine heat transfer conditions, etc.), which ensures a high degree of accuracy of the protective devices.

An experimental verification confirmed the high efficiency of the claimed method of thermal protection of the engine.

Sources of information

1. The method of current protection of the motor with a dependent response characteristic. Pat. RF №2024147, Н 02 Н 5/04, 7/08, 1994.

2. The device and method of protecting the engine from overload using the elapsed time signal, which allows to calculate data on the temperature of the engine, regardless of the temporary power interruption. Pat. US No. 5644510, H 02 H 7/00, 1997.

Claims (2)

1. The method of thermal protection of the motor, which consists in measuring the current consumed by the motor, periodically calculating data representing the temperature of the motor winding, based on the measured current and its time, using the parameters of the thermal model of the motor being protected, and turning off the motor when the calculated data of the winding temperature reaches or exceed the prescribed upper limit, characterized in that before switching on the engine and after turning it off, measure the ohmic resistance of its winding and cut tatam calculated measurement data representing an initial and final temperature of the winding; data on the initial temperature of the winding are used in calculating changes in its temperature during engine operation, data on the final temperature obtained from the measured resistance are compared with data calculated on the basis of the measured current, and if a discrepancy is found between these data, the parameters of the thermal model of the engine are adjusted. 2. A device for thermal protection of the motor, comprising a contactor with contacts in the power supply circuit of the motor windings and a winding connected to the power circuit, current sensors in the motor power supply circuits, a control unit with controls and a controller receiving power from the terminals of the device, performing operational commands from controls and having a non-volatile memory and a programmable module for calculating the temperature of the motor winding based on the current measured by current sensors, characterized in that a measurement unit for rotation, the first intermediate relay, the contacts of which are connected between the motor power supply circuits and the inputs of the resistance measuring unit, and the winding is turned on and off by the controller, the second intermediate relay, whose contact is included in the power supply circuit of the contactor winding, and its winding is turned on by the controller, the programmable module calculates the temperature of the windings of the motor, taking into account the current flow time and the parameters of the mathematical thermal model of the engine recorded in the programmable module, preserving the last calculated according to the current data of the temperature of the motor windings, calculates the temperature from the measured resistance of the motor winding, compares it with the temperature calculated on the basis of the measured value.