CA1299644C - Apparatus for overload protection - Google Patents
Apparatus for overload protectionInfo
- Publication number
- CA1299644C CA1299644C CA000591265A CA591265A CA1299644C CA 1299644 C CA1299644 C CA 1299644C CA 000591265 A CA000591265 A CA 000591265A CA 591265 A CA591265 A CA 591265A CA 1299644 C CA1299644 C CA 1299644C
- Authority
- CA
- Canada
- Prior art keywords
- signal
- motor
- load
- overload
- providing
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Fee Related
Links
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
-
- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
- H02H7/00—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions
- H02H7/08—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions for dynamo-electric motors
- H02H7/0833—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions for dynamo-electric motors for electric motors with control arrangements
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
- H02H7/00—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions
- H02H7/08—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions for dynamo-electric motors
- H02H7/085—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions for dynamo-electric motors against excessive load
- H02H7/0856—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions for dynamo-electric motors against excessive load characterised by the protection measure taken
- H02H7/0857—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions for dynamo-electric motors against excessive load characterised by the protection measure taken by lowering the mechanical load of the motor
-
- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
- H02H7/00—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions
- H02H7/08—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions for dynamo-electric motors
- H02H7/085—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions for dynamo-electric motors against excessive load
- H02H7/0856—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions for dynamo-electric motors against excessive load characterised by the protection measure taken
- H02H7/0858—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions for dynamo-electric motors against excessive load characterised by the protection measure taken by reversing, cycling or reducing the power supply to the motor
Landscapes
- Protection Of Generators And Motors (AREA)
Abstract
Case 3009 IMPROVED APPARATUS FOR OVERLOAD PROTECTION
Abstract of the Disclosure Apparatus for permitting limited operation in the overload region of an electric motor by controlling motor load when operating in that region. A sensor senses the current supplied to drive the motor, or senses the electrical energy supplied to the motor, or senses some other load related parameter, and provides a signal representing motor load to-an accumulated thermal overload circuit which provides an output signal representing the accumulated value of any motor thermal overload. An overload capacity circuit has set into it the maximum thermal overload capacity the motor can withstand and it provides an overload capacity signal representing this. A desired thermal reserve circuit has set into it a value representing the thermal reserve and it provides a thermal reserve signal representing this. A summing means receives the overload capacity signal, the accumulated thermal overload signal, and the desired thermal reserve signal, and provides a net signal representing the amount by which the desired thermal reserve signal plus the output signal exceed the overload capacity signal.
This net signal is used to reduce the motor load in proportion to the value of the net signal.
Abstract of the Disclosure Apparatus for permitting limited operation in the overload region of an electric motor by controlling motor load when operating in that region. A sensor senses the current supplied to drive the motor, or senses the electrical energy supplied to the motor, or senses some other load related parameter, and provides a signal representing motor load to-an accumulated thermal overload circuit which provides an output signal representing the accumulated value of any motor thermal overload. An overload capacity circuit has set into it the maximum thermal overload capacity the motor can withstand and it provides an overload capacity signal representing this. A desired thermal reserve circuit has set into it a value representing the thermal reserve and it provides a thermal reserve signal representing this. A summing means receives the overload capacity signal, the accumulated thermal overload signal, and the desired thermal reserve signal, and provides a net signal representing the amount by which the desired thermal reserve signal plus the output signal exceed the overload capacity signal.
This net signal is used to reduce the motor load in proportion to the value of the net signal.
Description
Case 3009 IMPROVED AP ARATUS FOR OVERLOAD PROTECTION
Background of the Invention This invention relates to apparatus for protecting electric motors from damage due to overloads, and in particular it relates to apparatus for protecting motors from damage due to overloads by controlling the motor while it is operating in a limited overload regionO
Electric motors have a thermal rating which should not be exceeded. If the thermal rating is exceeded, the life of the motor may be reduced or the motor may fail. However most motors are designed with a short term overload capability, that is, they may operate for a short time duration at loads in excess of the normal steady state rating before the thermal rating of the motor is exceeded. For example, a motor may be designed to operate for two hours at a load rating of 110%, and to operate for perhaps 60 seconds at a load rating of 150%.
In the past, overload conditions have been controlled in several ways. A simple way to control an overload situation in a motor and one which permits the motor to handle~a short time overload, is to use a temperature sensor or thermal overload protector which can determine when a designated temperature has been . ' .
reached in the motor and which then disconnects the motor from its supply of power.
The motor may also be disconnected by a device which monitors motor current and provides an analog representation of the internal thermal level of the motor. When a predetermined thermal level is reached, the motor is disconnected. Such a device is described, for example, in Canadian Patent No. 983,094 - Boothman et al, issued February 3, 1976. This arrangement may permit limited operation in the overload region before the motor 10 is disconnected.
It will be apparent that there are drive systems where it may be undesirable or even dangerous to disconnect a mot~r. Thus, it may not be desirable to disconnect a motor when it is required to provide a continuing operation, such as in a process that is expensive to shut down or in a system where the motor provides a critical control facility. For example, an ore grinding mill should not be shut down when overloaded if the shut down can be avoided as the load may set and be difficult to remove. Also, in a drive system for a ship, such as an icebreaker, when the captain may be maneuvering the vessel in ice (where temporary overloads are a normal occurr~nce), it would not be desirable to shut down the motor.
It is, of course, possible to control the overload in another way and this involves reducing the load. Canadian Patent No. 1,202,673 - Leuthen, issued April 1, 1986 describes one such control. In this patent a control is described which varies the frequency of the AC power supplied to the motar, as well as the amplitude, to control motor speed. A
sensor senses motor temperature, and when the temperature exaeeds a predetermined level, the frequency and amplitude of the power supplied to the 6~
motor are reduced in proportion to the excess of the temperature over the predetermined level. This reduces the speed of the motor by an amount related to the degree of overheating. There is also described an arrangement for reducing motor speed by an amount related to the amount by which motor current exceeds a predetermined level.
The prior apparatus does not keep track of the accumulated overload, set a thermal reserve value within the overload region, and reduce the load on the motor when the accumulated thermal overload plus the thermal reserve approaches the motor's thermal overload capacity.
Summary of the Invention The present invention provides an overload envelope within which the motor may operate. The load on the motor may be determined by sensing a suitable parameter indicative of load. For example, a current sensor providing a signal corresponding to current supplied to the motor could represent motor load, or a sensor measuring electrical energy supplied to the motor could provide a signal representing motor load.
Circuitry accumulates a representation of any thermal overload which occurs and motor operation is permitted in the overload region until the representation of accumulated thermal overload, plus a rspresentation of a value for a thermal reserve, approaches a representation for thermal overload capacity. The load on the motor is then reduced gradually, by reducing motor speed or motor load, until the representation of accumulated overload is reduced to a predetermined level, for example to a level corresponding to rated load or to a level slightly below rated load. Normal operation then resumes.
It is therefore an object of the invention to provide an improved apparatus for controlling overload in an electric motor.
It is another object of the invention to provide an improved apparatus which permits operation of an electric motor within an overload envelope above rated load but below the thermal overload limit capacity of the motor.
Accordingly there is provided apparatus for providing a control signal for controlling an electric motor comprising, an electric motor connected to drive a load, a source of electric power connected to supply the motor, sensor means for sensing the motor load and providing a load signal representing this, an accumulated thermal overload circuit for receiving the load signal and providing an output signal representing the accumulated value of any thermal overload on the motor, the accumulated value increasing with the time the overload persists and the degree of overload, and decreasing with the time the load is less than rated load and the amount by which it is less than rated load, an overload capacity circuit having a value corresponding to the maximum overload capacity and providing an overload capacity signal representing this, a desired thermal reserve circuit having a value corresponding to the thermal reserve desired and providing a thermal reserve signal representing this, summing means for receiving the output signal, the overload capacity signal and the thermal reserve signal and providing a net signal representing the amount by which the desired thermal reserve signal with the output signal exceed the overload capacity signal, and a circuit mean~ for receiving the net signal, and providing a control signal for reducing motor load.
In accordance with a broad aspect of the present invention there is provided an apparatus for providing a control signal for controlling an electric motor driving a load. The apparatus includes sensor means for sensing a parameter represenkative of motor load and providing a load signal representing this parameter. The apparatus also includes means responsive to the load signal for provi~ing an accumulated thermal output signal representing accumulation of any thermal overload on the motor, the accumulated thermal output signal increasing proportionally as a function of time and degree of motor overload and decreasing proportionally as a function of time and degree of motor load below rated load. The apparatus includes means for providing an overload capacity signal representative of the maximum overload capacity of the motor and means for providing a thermal reserve signal representative of desired thermal reserve for the motor. A summing means is provided for receiving the accumulated thermal output signal, the overload capacity signal and the desired thermal reserve signal and providing a net signal representing the amount by which the desired thermal reserve signal with the output signal exceed the overload capacity signal. There is also provided means for receiving the net signal and providing a control signal for use in reducing the motor load.
In accordance with another aspect of the present invention there is provided a method of controlling an electric motor driving a load comprising the steps of:
sensing a parameter representative of motor load and providing a load signal representing this parameter, generating an accumulated thermal output signal representing accumulation of any thermal overload on the motor, the accumulated thermal output signal increasing proportionally as a function of time and degree of motor overload and decreasing proportionally as a function of time and degree of motor load below rated load, providing an overload capacity signal representative of the maximum overload capacity of the motor, providing a thermal reserve signal representative o~ desired thermal reserve for the motor, summing the accumulated thexmal output signal, the overload capacity signal and the desired thermal reserve signal to provide a net signal representing the amount by which the desired thermal reserve signal with the output signal exceed the overload capacity signal, and providing a control signal corresponding to the net signal for use in reducing the motor load.
Brief Description of the Drawinqs The invention will be described with reference to the accompanying drawings, in which Figure 1 is a schematic block diagram of apparatus according to the invention, Figure 2 is a schematic block diagram showing an alternative arrangement which is in digital form, and Figure 3 is a schematic diagram indicating an analog version of a portion of the apparatus of Figure 1, and Figure 4 is a schematic diagram showing another portion of the apparatus of Figure 1.
Description of the Preferred Embodiment Referring to Figure 1, there is shown in schematic block form a motor 10 connected to drive a load lOA. The motor can be a DC motor, an AC induction motor or a synchronous motor. A load sensor 11 senses the load on motor 10 which is connected to a source of supply 13. The sensor 11 may conveniently be a current sensor which senses the current supplied to motor 10.
However it may be desirable to sense other parameters . ~ .
.
~ CASE 3 009 than current to obtain a siynal representative of motor load. For example, in an AC motor it may be desirable to sense the kilowatts of energy supplied to the motor, or current and power factor. Any param~ter or parameters which provide a good representation of load may be used. The load sensor 11 provides a signal representing motor load to a circuit 12 which accumulates a value representing the amount of motor overload, that is, it accumulates a value representing lo any overload with respect to time and, of course, reduces this accumulated value when the load is below rated load (or alternately when the load falls to a level that is slightly below rated load). Thus, the output from accumulated thermal overload circuit 12 will be zero if the motor has not been overloaded or if it has been run for a sufficient length of time below rated load after being overloaded. An overload capacity circuit 14 is set to provide a signal representing the overload capacity of the motor 10.
This is the overload-time value which is the maximum the motor 10 can stand without damage. ~ thermal reserve circuit 15 provides a signal representing the desired thermal reserve. The desired thermal reserve is, in effect, a safety factor which is determined to permit the motor to run in an overload condition but to prevent the motor reaching its thermal overload capacity. The signal representing the thermal reserve may be a constant signal representing a constant thermal reserve, or it may be ~aried within limits according to the rate of accumulation of overload if desiredO
The three signals from circuits 14, 12 and 15, representing overload capacity, accumulated thermal overload and desired thermal reserve, are applied to a summing point 16. When the sum of the signals representing the accumulated thermal overload C~SE 3009 plus the desired thermal reserve, equals and exceeds the signal representing overload capacity, then there will be an output from summing point 16 applied to a detector 17. The signal applied to detector 17 will be proportional to the amount by which the sum of the signals representing accumulated thermal overload plus the desired thermal reserve exceeds the signal representing thermal overload capacity. In accordance with the positive and negative convention used for the signals in Figure 1, this signal from summing point 16 would be a negative signal, but those skilled in the art will be aware that other conventions could be used. The detector 17 detects change in the sign of the signal which is applied to it. That is, detector 17 does not pass a signal that is positive, and the signal is positive when, for example there is no accumulated thermal overload. The signal remains positive until the signal representing accumulated thermal overload with the signal representing thermal reserve equal the signal representing overload capacity. When detector 17 detects a negative signal from summing point 16 (with the sign convention used in Figure l), the signal representing accumulated thermal overload plus the signal representing thermal reserve will have exceeded the signal representing overload capacity. Detector 17 then passes this negative signal to a proportional control circuit 18. The negative signal received by proportional control circuit 18 is proportional to the excess of the accumulated thermal overload and thermal reserve over the overload capacity. The proportional control circuit 18 provides a correcting control signal proportional to the signal it receives and in a form suitable for reducing the load on the motor 10.
The load control signal from proportional control 18 may be used to either reduce motor speed and ~ CASE 3009 g thus reduce load as is indicated by control 20 (i~ the motor is an induc~ion motor or synchronous motor this may be ~one by reducing the frequency of the power supplied to drive the motor as is done ~or example in the aforementioned Canadian Patent No. 1,202,673 and if the motor is a DC motor this may be done by reducing armature current), or it may be used to reduce the actual load such as by reducing or temporarily stopping the feed to a grinding mill as is indicated by control 21.
Referring now to Figure 2, there is shown a block schematic form of the invention in a digital version where the load sensor llA, which may be a current sensor, and if so provides a signal representing the current drawn by motor 10 (Fig.l~, however the current signal provided is in digital form, that is, the signal is a series of pulses representing motor load. This signal is applied to a rated load circuit 22. The rated load circuit 22 provides a digital signal on conductor 23 when the signal from sensor llA represents a value above rated load, and this signal on conductor 23 is proportional to the amount by which the rated load is exceeded. The rated load circuit 22 provides a digital signal on conductor 24 when the signal from sensor llA represents a value below rated load (or is below rated load by a predetermined amount), and may also be proportional to the amount by which the signal represents a value below rated load or below a predetermined value.
A counter 25 receives the signals from conductors 23 and 24 and it accumulates a count which represents the thermal load. If the motor has been running below rated load, there will be a digital signal on conductor 24, but as the accumulated count in counter 25 is zero, the signal from conductor 24 is ignored. When the motor runs above its rated load there is a proportional signal representing this on conductor 23, and counter 25 increases its count accordingly. If the motor load should now be reduced below its rated load, there will be no siynal on conductor 23 but there will be a signal on condutor 24 representing the amount by which the motor is below rated load (or a predetermined value below rated load). The count in counter 25 is reduced by the signal on conductor 24 until the count becomes zero or until the motor load increases above rated load.
~ounter 25 provides a signal on conductor 26 to adder 27. Adder 27 receives the digital signal from conductor 26, which represents an accumulated overload, and it also receives a signal from overload capacity circuit 14A and desired thermal reserve circuit 15A.
In the Figure 2 version these signals are digital signals. As before, the overload capacity circuit 14A
is preset in accordance with a particular installation to provide a digital signal representing the overload capacity of the motor. The desired thermal reserve circuit 15A is set to provide a reserve or margin of safety in accordance with conditions existing at a particular installation.
~hus, adder 27 receives digital signals representing overload capacity, desired thermal reserve, and accumulated thermal overload. When the signals representing accumulated thermal overload and desired thsrmal reserve exceed the signal representing overload capacity, a signal is provided on conductor 28 which is proportional to the excess. The proportional control 18A receives this signal and converts it to a proportional signal in a form for reducing the load on the motor 10 (Figure 1).
Referring to Figure 3, there is shown a schematic diagram of an analog version of part of the apparatus of Figure 1, that is, showing a suitahle a~
~ C~SE 3009 circui~ ~or summing point 16 and detector 17. As before, the overload capacity signal is set for the particular installation and is on conductor 30. The thermal reserve si~nal is set in accordance with the conditions in mind for the particular installation and is on conductor 31. The signal representing accumulated thermal overload is on conductor 32. The three conductors 30, 31 and 32 are connected throuyh respective isolating resistors 33, 34 and 35 to a conductor 38 which carries the sum of the signals (this is the summing point 16 of Figure 1).
The operational amplifier 36 with feedback resistor 39 and diode 37 will not pass a signal when the conductor 38 is positive. However, when the signal representing accumulated thermal overload and the signal representing desired thermal reserve exceed the signal representing overload capacity, the conductor 38 becomes negative. There will then be a signal on conductor 40 and this will be proportional to the excess. This signal may be applied to the proportional control 18 (Figure 1) for reducing motor load.
Referring now to Figure 4, there is shown analog circuitry suitable for providing a signal representing accumulated thermal overload. Thls particular circuit requires that two reference signals be set into the circuit. These two signals are an overload level signal on conductor 41 and an underload level signal on conductor 42. The signal on conductor 41 representing overload level is conveniently just above the maximum normal continuous load level, such as perhaps 105% of normal load. It may, of course be set closer to the normal 100% load.
The signal on conductor 42 representing underload level is conveniently just below normal continuous load, such as perhaps 95% of normal load. This signal may also, if desired, be set closer to 100% load, or less , - C~SE 3009 than 95% if conditions make it desirable to do so.
The signal on conductor 43 is the load signal representing the sensed motor load. As explained in connection with Figure 1, the signal on conductor 43 is obtained from a load sensor such as, for example, a current sensor, an energy sensor, a sensor determining KVA, or a sensor for any desired other load-related parameters. The portion of Figure 4 comprising resistors 44, 45 and 46, amplifier 47 and diode 48, is an overload detector. If there is no signal on conductor 43, the signal on conductor 41, which is a positive signal, will cause the output of amplifier 47 to be negative. This negative output on conductor 50 is blocked by diode 48. If the load signal on conductor 43 (a negative signal) representing load, exceeds the signal on conductor 41, then the output of amplifier 47 which is on conductor 50 will be positive and will correspond to the degree of overload. This positive signal is passed by diode 48 to resistor 51.
The portion of Figure 4 comprising resistors 51 and 52, capacitor 53, amplifier 54 and zener diode 55 performs an integrating function. A
positive signal through resistor 51 will cause the output on conductor 32 to increase negatively as long as the signal on conductor 51 remains positive. If the signal on conductor 51 drops to zero, the output signal on conductor 32 will remain at the level it attained while the signal was positive. The zener diode 55 prevents the output signal on conductor 32 from exceeding a predetermined negative value which might drive amplifier 54 into saturation.
The circuit comprising amplifier 57 with resistors 58, 60 and 61, and diode 62 comprises an underload detector which operates in a generally similar manner to the overload detector. When the load is greater than, say 95~ load, the negative signal on conductor 43 will be greater than the positive underload level signal on conductor 42. The oUtput of amplifier 57 will be positive and thiS is blocked by diode 62. When the load signal on conductor 43 becomes less than 95% load (as is represented by the underload level signal on conductor 42), the output from amplifier 57 will be negative and this will be passed by diode 62 to resistor 52. The signal passing through resistor 52, which is now negative, is proportional to the degree of underload. This causes the output from amplifier 54 to decrease. That is, the signal on conductor 32 becomes less negative. In this manner the signal on conductor 32 represents the accumulated overload.
It is believed the preceding description will provide an understanding of the invention to those skilled in the art.
Background of the Invention This invention relates to apparatus for protecting electric motors from damage due to overloads, and in particular it relates to apparatus for protecting motors from damage due to overloads by controlling the motor while it is operating in a limited overload regionO
Electric motors have a thermal rating which should not be exceeded. If the thermal rating is exceeded, the life of the motor may be reduced or the motor may fail. However most motors are designed with a short term overload capability, that is, they may operate for a short time duration at loads in excess of the normal steady state rating before the thermal rating of the motor is exceeded. For example, a motor may be designed to operate for two hours at a load rating of 110%, and to operate for perhaps 60 seconds at a load rating of 150%.
In the past, overload conditions have been controlled in several ways. A simple way to control an overload situation in a motor and one which permits the motor to handle~a short time overload, is to use a temperature sensor or thermal overload protector which can determine when a designated temperature has been . ' .
reached in the motor and which then disconnects the motor from its supply of power.
The motor may also be disconnected by a device which monitors motor current and provides an analog representation of the internal thermal level of the motor. When a predetermined thermal level is reached, the motor is disconnected. Such a device is described, for example, in Canadian Patent No. 983,094 - Boothman et al, issued February 3, 1976. This arrangement may permit limited operation in the overload region before the motor 10 is disconnected.
It will be apparent that there are drive systems where it may be undesirable or even dangerous to disconnect a mot~r. Thus, it may not be desirable to disconnect a motor when it is required to provide a continuing operation, such as in a process that is expensive to shut down or in a system where the motor provides a critical control facility. For example, an ore grinding mill should not be shut down when overloaded if the shut down can be avoided as the load may set and be difficult to remove. Also, in a drive system for a ship, such as an icebreaker, when the captain may be maneuvering the vessel in ice (where temporary overloads are a normal occurr~nce), it would not be desirable to shut down the motor.
It is, of course, possible to control the overload in another way and this involves reducing the load. Canadian Patent No. 1,202,673 - Leuthen, issued April 1, 1986 describes one such control. In this patent a control is described which varies the frequency of the AC power supplied to the motar, as well as the amplitude, to control motor speed. A
sensor senses motor temperature, and when the temperature exaeeds a predetermined level, the frequency and amplitude of the power supplied to the 6~
motor are reduced in proportion to the excess of the temperature over the predetermined level. This reduces the speed of the motor by an amount related to the degree of overheating. There is also described an arrangement for reducing motor speed by an amount related to the amount by which motor current exceeds a predetermined level.
The prior apparatus does not keep track of the accumulated overload, set a thermal reserve value within the overload region, and reduce the load on the motor when the accumulated thermal overload plus the thermal reserve approaches the motor's thermal overload capacity.
Summary of the Invention The present invention provides an overload envelope within which the motor may operate. The load on the motor may be determined by sensing a suitable parameter indicative of load. For example, a current sensor providing a signal corresponding to current supplied to the motor could represent motor load, or a sensor measuring electrical energy supplied to the motor could provide a signal representing motor load.
Circuitry accumulates a representation of any thermal overload which occurs and motor operation is permitted in the overload region until the representation of accumulated thermal overload, plus a rspresentation of a value for a thermal reserve, approaches a representation for thermal overload capacity. The load on the motor is then reduced gradually, by reducing motor speed or motor load, until the representation of accumulated overload is reduced to a predetermined level, for example to a level corresponding to rated load or to a level slightly below rated load. Normal operation then resumes.
It is therefore an object of the invention to provide an improved apparatus for controlling overload in an electric motor.
It is another object of the invention to provide an improved apparatus which permits operation of an electric motor within an overload envelope above rated load but below the thermal overload limit capacity of the motor.
Accordingly there is provided apparatus for providing a control signal for controlling an electric motor comprising, an electric motor connected to drive a load, a source of electric power connected to supply the motor, sensor means for sensing the motor load and providing a load signal representing this, an accumulated thermal overload circuit for receiving the load signal and providing an output signal representing the accumulated value of any thermal overload on the motor, the accumulated value increasing with the time the overload persists and the degree of overload, and decreasing with the time the load is less than rated load and the amount by which it is less than rated load, an overload capacity circuit having a value corresponding to the maximum overload capacity and providing an overload capacity signal representing this, a desired thermal reserve circuit having a value corresponding to the thermal reserve desired and providing a thermal reserve signal representing this, summing means for receiving the output signal, the overload capacity signal and the thermal reserve signal and providing a net signal representing the amount by which the desired thermal reserve signal with the output signal exceed the overload capacity signal, and a circuit mean~ for receiving the net signal, and providing a control signal for reducing motor load.
In accordance with a broad aspect of the present invention there is provided an apparatus for providing a control signal for controlling an electric motor driving a load. The apparatus includes sensor means for sensing a parameter represenkative of motor load and providing a load signal representing this parameter. The apparatus also includes means responsive to the load signal for provi~ing an accumulated thermal output signal representing accumulation of any thermal overload on the motor, the accumulated thermal output signal increasing proportionally as a function of time and degree of motor overload and decreasing proportionally as a function of time and degree of motor load below rated load. The apparatus includes means for providing an overload capacity signal representative of the maximum overload capacity of the motor and means for providing a thermal reserve signal representative of desired thermal reserve for the motor. A summing means is provided for receiving the accumulated thermal output signal, the overload capacity signal and the desired thermal reserve signal and providing a net signal representing the amount by which the desired thermal reserve signal with the output signal exceed the overload capacity signal. There is also provided means for receiving the net signal and providing a control signal for use in reducing the motor load.
In accordance with another aspect of the present invention there is provided a method of controlling an electric motor driving a load comprising the steps of:
sensing a parameter representative of motor load and providing a load signal representing this parameter, generating an accumulated thermal output signal representing accumulation of any thermal overload on the motor, the accumulated thermal output signal increasing proportionally as a function of time and degree of motor overload and decreasing proportionally as a function of time and degree of motor load below rated load, providing an overload capacity signal representative of the maximum overload capacity of the motor, providing a thermal reserve signal representative o~ desired thermal reserve for the motor, summing the accumulated thexmal output signal, the overload capacity signal and the desired thermal reserve signal to provide a net signal representing the amount by which the desired thermal reserve signal with the output signal exceed the overload capacity signal, and providing a control signal corresponding to the net signal for use in reducing the motor load.
Brief Description of the Drawinqs The invention will be described with reference to the accompanying drawings, in which Figure 1 is a schematic block diagram of apparatus according to the invention, Figure 2 is a schematic block diagram showing an alternative arrangement which is in digital form, and Figure 3 is a schematic diagram indicating an analog version of a portion of the apparatus of Figure 1, and Figure 4 is a schematic diagram showing another portion of the apparatus of Figure 1.
Description of the Preferred Embodiment Referring to Figure 1, there is shown in schematic block form a motor 10 connected to drive a load lOA. The motor can be a DC motor, an AC induction motor or a synchronous motor. A load sensor 11 senses the load on motor 10 which is connected to a source of supply 13. The sensor 11 may conveniently be a current sensor which senses the current supplied to motor 10.
However it may be desirable to sense other parameters . ~ .
.
~ CASE 3 009 than current to obtain a siynal representative of motor load. For example, in an AC motor it may be desirable to sense the kilowatts of energy supplied to the motor, or current and power factor. Any param~ter or parameters which provide a good representation of load may be used. The load sensor 11 provides a signal representing motor load to a circuit 12 which accumulates a value representing the amount of motor overload, that is, it accumulates a value representing lo any overload with respect to time and, of course, reduces this accumulated value when the load is below rated load (or alternately when the load falls to a level that is slightly below rated load). Thus, the output from accumulated thermal overload circuit 12 will be zero if the motor has not been overloaded or if it has been run for a sufficient length of time below rated load after being overloaded. An overload capacity circuit 14 is set to provide a signal representing the overload capacity of the motor 10.
This is the overload-time value which is the maximum the motor 10 can stand without damage. ~ thermal reserve circuit 15 provides a signal representing the desired thermal reserve. The desired thermal reserve is, in effect, a safety factor which is determined to permit the motor to run in an overload condition but to prevent the motor reaching its thermal overload capacity. The signal representing the thermal reserve may be a constant signal representing a constant thermal reserve, or it may be ~aried within limits according to the rate of accumulation of overload if desiredO
The three signals from circuits 14, 12 and 15, representing overload capacity, accumulated thermal overload and desired thermal reserve, are applied to a summing point 16. When the sum of the signals representing the accumulated thermal overload C~SE 3009 plus the desired thermal reserve, equals and exceeds the signal representing overload capacity, then there will be an output from summing point 16 applied to a detector 17. The signal applied to detector 17 will be proportional to the amount by which the sum of the signals representing accumulated thermal overload plus the desired thermal reserve exceeds the signal representing thermal overload capacity. In accordance with the positive and negative convention used for the signals in Figure 1, this signal from summing point 16 would be a negative signal, but those skilled in the art will be aware that other conventions could be used. The detector 17 detects change in the sign of the signal which is applied to it. That is, detector 17 does not pass a signal that is positive, and the signal is positive when, for example there is no accumulated thermal overload. The signal remains positive until the signal representing accumulated thermal overload with the signal representing thermal reserve equal the signal representing overload capacity. When detector 17 detects a negative signal from summing point 16 (with the sign convention used in Figure l), the signal representing accumulated thermal overload plus the signal representing thermal reserve will have exceeded the signal representing overload capacity. Detector 17 then passes this negative signal to a proportional control circuit 18. The negative signal received by proportional control circuit 18 is proportional to the excess of the accumulated thermal overload and thermal reserve over the overload capacity. The proportional control circuit 18 provides a correcting control signal proportional to the signal it receives and in a form suitable for reducing the load on the motor 10.
The load control signal from proportional control 18 may be used to either reduce motor speed and ~ CASE 3009 g thus reduce load as is indicated by control 20 (i~ the motor is an induc~ion motor or synchronous motor this may be ~one by reducing the frequency of the power supplied to drive the motor as is done ~or example in the aforementioned Canadian Patent No. 1,202,673 and if the motor is a DC motor this may be done by reducing armature current), or it may be used to reduce the actual load such as by reducing or temporarily stopping the feed to a grinding mill as is indicated by control 21.
Referring now to Figure 2, there is shown a block schematic form of the invention in a digital version where the load sensor llA, which may be a current sensor, and if so provides a signal representing the current drawn by motor 10 (Fig.l~, however the current signal provided is in digital form, that is, the signal is a series of pulses representing motor load. This signal is applied to a rated load circuit 22. The rated load circuit 22 provides a digital signal on conductor 23 when the signal from sensor llA represents a value above rated load, and this signal on conductor 23 is proportional to the amount by which the rated load is exceeded. The rated load circuit 22 provides a digital signal on conductor 24 when the signal from sensor llA represents a value below rated load (or is below rated load by a predetermined amount), and may also be proportional to the amount by which the signal represents a value below rated load or below a predetermined value.
A counter 25 receives the signals from conductors 23 and 24 and it accumulates a count which represents the thermal load. If the motor has been running below rated load, there will be a digital signal on conductor 24, but as the accumulated count in counter 25 is zero, the signal from conductor 24 is ignored. When the motor runs above its rated load there is a proportional signal representing this on conductor 23, and counter 25 increases its count accordingly. If the motor load should now be reduced below its rated load, there will be no siynal on conductor 23 but there will be a signal on condutor 24 representing the amount by which the motor is below rated load (or a predetermined value below rated load). The count in counter 25 is reduced by the signal on conductor 24 until the count becomes zero or until the motor load increases above rated load.
~ounter 25 provides a signal on conductor 26 to adder 27. Adder 27 receives the digital signal from conductor 26, which represents an accumulated overload, and it also receives a signal from overload capacity circuit 14A and desired thermal reserve circuit 15A.
In the Figure 2 version these signals are digital signals. As before, the overload capacity circuit 14A
is preset in accordance with a particular installation to provide a digital signal representing the overload capacity of the motor. The desired thermal reserve circuit 15A is set to provide a reserve or margin of safety in accordance with conditions existing at a particular installation.
~hus, adder 27 receives digital signals representing overload capacity, desired thermal reserve, and accumulated thermal overload. When the signals representing accumulated thermal overload and desired thsrmal reserve exceed the signal representing overload capacity, a signal is provided on conductor 28 which is proportional to the excess. The proportional control 18A receives this signal and converts it to a proportional signal in a form for reducing the load on the motor 10 (Figure 1).
Referring to Figure 3, there is shown a schematic diagram of an analog version of part of the apparatus of Figure 1, that is, showing a suitahle a~
~ C~SE 3009 circui~ ~or summing point 16 and detector 17. As before, the overload capacity signal is set for the particular installation and is on conductor 30. The thermal reserve si~nal is set in accordance with the conditions in mind for the particular installation and is on conductor 31. The signal representing accumulated thermal overload is on conductor 32. The three conductors 30, 31 and 32 are connected throuyh respective isolating resistors 33, 34 and 35 to a conductor 38 which carries the sum of the signals (this is the summing point 16 of Figure 1).
The operational amplifier 36 with feedback resistor 39 and diode 37 will not pass a signal when the conductor 38 is positive. However, when the signal representing accumulated thermal overload and the signal representing desired thermal reserve exceed the signal representing overload capacity, the conductor 38 becomes negative. There will then be a signal on conductor 40 and this will be proportional to the excess. This signal may be applied to the proportional control 18 (Figure 1) for reducing motor load.
Referring now to Figure 4, there is shown analog circuitry suitable for providing a signal representing accumulated thermal overload. Thls particular circuit requires that two reference signals be set into the circuit. These two signals are an overload level signal on conductor 41 and an underload level signal on conductor 42. The signal on conductor 41 representing overload level is conveniently just above the maximum normal continuous load level, such as perhaps 105% of normal load. It may, of course be set closer to the normal 100% load.
The signal on conductor 42 representing underload level is conveniently just below normal continuous load, such as perhaps 95% of normal load. This signal may also, if desired, be set closer to 100% load, or less , - C~SE 3009 than 95% if conditions make it desirable to do so.
The signal on conductor 43 is the load signal representing the sensed motor load. As explained in connection with Figure 1, the signal on conductor 43 is obtained from a load sensor such as, for example, a current sensor, an energy sensor, a sensor determining KVA, or a sensor for any desired other load-related parameters. The portion of Figure 4 comprising resistors 44, 45 and 46, amplifier 47 and diode 48, is an overload detector. If there is no signal on conductor 43, the signal on conductor 41, which is a positive signal, will cause the output of amplifier 47 to be negative. This negative output on conductor 50 is blocked by diode 48. If the load signal on conductor 43 (a negative signal) representing load, exceeds the signal on conductor 41, then the output of amplifier 47 which is on conductor 50 will be positive and will correspond to the degree of overload. This positive signal is passed by diode 48 to resistor 51.
The portion of Figure 4 comprising resistors 51 and 52, capacitor 53, amplifier 54 and zener diode 55 performs an integrating function. A
positive signal through resistor 51 will cause the output on conductor 32 to increase negatively as long as the signal on conductor 51 remains positive. If the signal on conductor 51 drops to zero, the output signal on conductor 32 will remain at the level it attained while the signal was positive. The zener diode 55 prevents the output signal on conductor 32 from exceeding a predetermined negative value which might drive amplifier 54 into saturation.
The circuit comprising amplifier 57 with resistors 58, 60 and 61, and diode 62 comprises an underload detector which operates in a generally similar manner to the overload detector. When the load is greater than, say 95~ load, the negative signal on conductor 43 will be greater than the positive underload level signal on conductor 42. The oUtput of amplifier 57 will be positive and thiS is blocked by diode 62. When the load signal on conductor 43 becomes less than 95% load (as is represented by the underload level signal on conductor 42), the output from amplifier 57 will be negative and this will be passed by diode 62 to resistor 52. The signal passing through resistor 52, which is now negative, is proportional to the degree of underload. This causes the output from amplifier 54 to decrease. That is, the signal on conductor 32 becomes less negative. In this manner the signal on conductor 32 represents the accumulated overload.
It is believed the preceding description will provide an understanding of the invention to those skilled in the art.
Claims (12)
1. Apparatus for providing a control signal for controlling an electric motor, comprising an electric motor connected to drive a load, a source of electrical power connected to supply the motor, sensor means for sensing a parameter representative of motor load and providing a load signal representing this, an accumulated thermal overload circuit means for receiving said load signal and providing an output signal representing the accumulated value of any thermal overload on the motor, said accumulated value increasing with the time the overload persists and the degree of the overload, and decreasing with the time the load is less than rated load and the amount by which the load is less than rated load, an overload capacity circuit means having a value corresponding to the maximum overload capacity of the motor and providing an overload capacity signal representing this, a desired thermal reserve circuit means having a value corresponding to the thermal reserve capacity desired and providing a desired thermal reserve signal representing this, a summing means for receiving said output signal, said overload capacity signal and said desired thermal reserve signal and providing a net signal representing the amount by which said desired thermal reserve signal with said output signal exceed said overload capacity signal, and a circuit means for receiving said net signal, and providing a control signal for use in reducing the motor load.
2. Apparatus as defined in claim 1 in which said sensor means is a current sensor for sensing current provided to said motor from said source of electrical power, said current sensor providing said load signal.
3. Apparatus as defined in claim 1 in which said sensor means is an energy sensor means for sensing the amount of electrical energy provided by said source of electrical power to said motor, said energy sensor means providing as an output said load signal.
4. Apparatus as defined in claims 1, 2 or 3 in which the motor is connected to drive an ore grinding mill having a feed providing ore to said mill, and in which said control signal reduces the feed to said mill.
5. Apparatus as defined in claims 1, 2 or 3 in which said motor is a synchronous motor and said source of electrical power is at a predetermined and controllable frequency, and in which said control signal reduces said frequency below said predetermined frequency to reduce the motor load.
6. Apparatus as defined in claims 1, 2 or 3 in which said motor is a DC motor and in which said control signal reduces armature current to reduce motor load.
7. Apparatus for providing a control signal for controlling an electric motor for enabling said motor to operate under overload conditions in a controlled manner, comprising an electric motor connected to a load for driving said load, a source of electrical power connected to said motor for providing power to run said motor, sensor means for sensing the motor load and providing a motor load signal representing the load on said motor, an accumulated thermal overload circuit means for receiving said motor load signal, accumulating a value of any thermal overload, and providing an output signal of a first sign representing the accumulated value of any thermal overload on said motor, said output signal increasing with the time the motor is operating in an overload condition and the degree of the overload, and decreasing with the time the overload is less than a predetermined value no greater than rated load and with the degree the load is less than said predetermined value, an overload capacity circuit having set therein a value corresponding to the maximum thermal overload capacity of said motor for providing an overload capacity signal representing this, said overload capacity signal being of a second sign opposite to said first sign, a desired thermal reserve circuit having therein a value corresponding to the thermal reserve capacity for said motor with its load, for providing a desired thermal reserve signal representing this, said desired thermal reserve signal being of said first sign, summing means for receiving said output signal, said overload capacity signal, and said desired thermal reserve signal, and providing a net signal representing the algebraic sum of said signals, said net signal being of said second sign when said output signal with said desired thermal reserve signal is less than said overload capacity signal and being of said first sign when said output signal with said desired thermal reserve signal exceed said overload capacity signal, and circuit means for receiving said net signal, determining when said net signal is of said first sign, and providing in response thereto a control signal proportional to the excess of said output signal and said desired thermal reserve signal over said overload capacity signal for proportionally reducing the load on said motor.
8. Apparatus as defined in claim 7 in which said electric motor is a synchronous motor and in which said source of electric power connected to said motor is a DC fed inverter for providing electric power of variable frequency and amplitude to run said motor, and in which said control signal for proportionally reducing the load on said motor controls said inverter to proportionally reduce at least the frequency of power supplied to said motor for reducing load on said motor.
9. Apparatus for controlling an electric motor driving a load, comprising sensor means for sensing a parameter representative of motor load and providing a load signal representing this parameter, means responsive to the load signal for providing an accumulated thermal output signal representing accumulation of any thermal overload on the motor, the accumulated thermal output signal increasing proportionally as a function of time and degree of motor overload and decreasing proportionally as a function of time and degree of motor load below rated load, means for providing an overload capacity signal representative of the maximum overload capacity of the motor, means for providing a thermal reserve signal representative of desired thermal reserve for the motor, summing means for receiving said accumulated thermal output signal, said overload capacity signal and said desired thermal reserve signal and providing a net signal representing the amount by which said desired thermal reserve signal with said output signal exceed said overload capacity signal, and means for receiving said net signal and providing a control signal for use in reducing the motor load.
10. Apparatus as defined in claim 1 in which said sensor means is a current sensor for sensing current provided to said motor from said source of electrical power, said current sensor providing said load signal.
11. Apparatus as defined in claim 1 in which said sensor means is an energy sensor means for sensing the amount of electrical energy provided by said source of electrical power to said motor, said energy sensor means providing as an output said load signal.
12. A method of controlling an electric motor driving a load, comprising the steps of:
sensing a parameter representative of motor load and providing a load signal representing this parameter, generating an accumulated thermal output signal representing accumulation of any thermal overload on the motor, the accumulated thermal output signal increasing proportionally as a function of time and degree of motor overload and decreasing proportionally as a function of time and degree of motor load below rated load, providing an overload capacity signal representative of the maximum overload capacity of the motor, providing a thermal reserve signal representative of desired thermal reserve for the motor, summing said accumulated thermal output signal, said overload capacity signal and said desired thermal reserve signal to provide a net signal representing the amount by which said desired thermal reserve signal with said output signal exceed said overload capacity signal, and providing a control signal corresponding to the net signal for use in reducing the motor load.
sensing a parameter representative of motor load and providing a load signal representing this parameter, generating an accumulated thermal output signal representing accumulation of any thermal overload on the motor, the accumulated thermal output signal increasing proportionally as a function of time and degree of motor overload and decreasing proportionally as a function of time and degree of motor load below rated load, providing an overload capacity signal representative of the maximum overload capacity of the motor, providing a thermal reserve signal representative of desired thermal reserve for the motor, summing said accumulated thermal output signal, said overload capacity signal and said desired thermal reserve signal to provide a net signal representing the amount by which said desired thermal reserve signal with said output signal exceed said overload capacity signal, and providing a control signal corresponding to the net signal for use in reducing the motor load.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000591265A CA1299644C (en) | 1989-02-16 | 1989-02-16 | Apparatus for overload protection |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000591265A CA1299644C (en) | 1989-02-16 | 1989-02-16 | Apparatus for overload protection |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1299644C true CA1299644C (en) | 1992-04-28 |
Family
ID=4139632
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000591265A Expired - Fee Related CA1299644C (en) | 1989-02-16 | 1989-02-16 | Apparatus for overload protection |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1299644C (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN112190986A (en) * | 2020-09-04 | 2021-01-08 | 广东新环环保产业集团有限公司 | Turnover-plate type single-rail mud scraper |
-
1989
- 1989-02-16 CA CA000591265A patent/CA1299644C/en not_active Expired - Fee Related
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN112190986A (en) * | 2020-09-04 | 2021-01-08 | 广东新环环保产业集团有限公司 | Turnover-plate type single-rail mud scraper |
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