OVERLOAD PROTECTION

Abstract: Method and device for indicating overload already when the lifting starts of a load suspended in a rope. The hoist comprises a slip-ring induction motor (1). The input power together with the angular velocity of the rotary movement are metered. Then the metered values are compiled in a computer (5) by means of a suggested formula in order to estimate the hook load. The lift will be stopped, if the hook load continuously exceeds a predetermined value during a predetermined time.

Description: OVERLOAD PROTECTION OBJECT OF THE INVENTION The present invention relates to a method of and a device for indicating overload and interrupting the lifting of an overload when a hoist is used that is driven by an asynchronous induction motor. BACKGROUND OF THE INVENTION Within a wide field of activities, cranes and other hoists are being used to lift and move different work loads The work loads are hooked on to a lifting hook or similar device. Via one or more wire ropes, the hook will then be connected to a winding drum, driven by an induction motor. Between the winding drum and the induction motor may be gea mechanisms in order to adapt suitable motor revolutions per minute to convenient lifting speed. The load will be lifted when the wire is wound on the winding drum and vice versa. The induction motor may advantageously be provided with sli rings connected to fixed or variable resistances. The purpose of this is to get a smooth starting of the lifting and to limit the starting currents of the motor. The hoist is dimensioned for the rated work load. In order not to exceed this load, the lifting should be switched off by means of a switch that immediately switches off the current to the hoist motor when the rated load is exceeded. Regulations about switches are available for example in statute-book AFS 1980:19 of the Swedish Board of Occupa¬ tional Safety and Health. From § 13 is quoted: "A switch ma be designed so as to time-delay or damp its switching-off function automatically when instantaneous excessive load occurs at the start. Normally, a time delay or damping must lot last longer than 0.5 seconds." This implies that the switch has to switch off the hoisting movement during a period when the system is still being exposed to acceleration forces, and before the start resistances have been uncoupled. It is known in the art to estimate the work load at a speed of 0 or at a constant speed of movement upwards or downwards. For instance, this may be done by metering the tension in the wire ropes or the compressive forces of shafts and bearing supports. -It _Ls also possible to estimate the work load weight by measuring the input power ofthe hoist motor, the electric current in the rotor winding, the current frequencies in the rotor winding, and so on. This is clear from the Russian patent publication No. 705270 and the Japanese patent publications No. 56-53420 and 56-58623. On the other hand, no simple method is "known- that is suitable continuously to estimate the weight of the load alreday during starting up, during the acceleration and when the rotor resistances are switched on or off. No such method provides the hoist with an overload protection device that interrupts the hoisting at a very early stage v for example within half- a second, so -that Tio security risl 'will appeax -due -to overload. ' e main difficulties arise on the one hand from the fact that energy is needed during acceleration for imparting kinetic energy to rotating parts and to the work load, the hook and other movable masses and on the other hand from the need of connecting and disconnecting various resistances in the rotor circuit. BRIEF DESCRIPTION OF THE INVENTION AND DRAWINGS 'Sur r singly^ ±-fc--has tu ned out. ;t-hat it s possible to estimate the weight of the load with great accuracy and reliability already during the acceleration period at the start of a hoisting operation if the angular acceleration of the hoist motor rotor or some other rotary movement in the system is measured together with the input power of the motor sandprocessed i_a a ooτιvensLeiϊt eoπ-p ter ΛHiifc-, The invention will be described in more detail with reference to Figure 1, that shows a hoist according to the invention, and Diagrams 1 to 11, that show measured and calculated values in various lifting operations. In all < agrams, the .abscissae ' rawn up have A leng h of 4-.0 seconds. Accordingly, the " distance between the long scale lines is 0.4 seconds. DETAILED DESCRIPTION OF THE INVENTION - n Order to make the-J iveritioii clear, we -have -tried theoretically to account for the formula which is fundamental to the invention. The enclosed figure is very schematic, and shows a hoisting device that is an example o an embodiment of the invention. The hoisting apparatus comprises a slip-ring induction motor 1 with its stator winding 2 connected to a feeder cable 3 via a switch 13 and a power meter 7. The rotor gap of the induction motor is denoted by numeral 9. An instrument transformer 8 is connected for measuring the rotor frequency. A pov/er meter 7, the instrument transforme 8 and a switch 13 are connected to a computer 5. The hoist is provided with a winding drum 6, a lifting wire 11 and a lifting hook 12 in which a load 10 is suspended. The input power P. of the induction motor 1 is transmitted to the stator winding 2 via the feeder cable 3. The power may be divided up into two parts, namely P 2 and P 7 _ . . T? 7 indicates the copper losses appearing in the stator winding 2 and may be considered to be constant within the actual working range. P ? _ 4 indicates the power transmitted between stator 2 and rotor 4. Accordingly, P. = P-, + ? 2 -4 1 - 1 Further the following applies: P 2-4 -« s ' Tm (2) where : is the synchronous revolutions per minute (rpm) of the induction motor 1. T is the torque of the by motor 1. Further, the well-known equation of inertia applies: where: J = the moment of inertia of the system. ) = the angular acceleration T = the load tongue experienced by the driving motor. Equations (1) and (2) have been taken from the book "Elektriska maskiner" by Karl Erik Hallenius, 2nd Edition, almδ, Liber laromedel 1977, PP 10:38 - 10:81. Equation (3) has been taken from the book "TEFYMA; Handbok for grundlaggande teknisk fysik, fysik och matematik" by Ingelstam-Rδnngren-Sjδberg, Sjδbergs fδrlag, Stockholm/Bromma 1984, p 69. The revolutions per minute, (ύ , may be established anywhere within the systemby means of: a suitable measuring device. According to the figure, an instrument transformer 8 is used that measures the revolutions per minute by measuring the rotor circuit frequency which is proportional to the speed of rotation. In computer 5, the frequency amount will be converted into an amount of speed of rotation ω. Equations (1) to (3) result in: T.. = — P l - P 2- - J * C0 (5) U)s As P 1 , and _ 2 ω s ω s an .J j r -constant n the formulae may be simplified as follows: T L = K P L - A - B * ώ (6) where K, A, and B are constants. The equation for this load case is as follows: T χ = (Mx + Mo) (a + g) ' r (7) lα &xe. Mit = weight, -wrk load -10.- Mo = the weight of other parts suspended from the winding drum 6, such as wire 11, hook 12 etc; g = the acceleration due to gravity a = the instantaneous acceleration of the work load, and r = the imaginary radius experienced by the motor. The force F exerted on the hoisting machinery is F x = M x (a+g) (8) where a— r ~ &Q <9) If the items (7) , (8) and (9) are inserted in the equation (5) , the result will be: F χ = f ' p l " (Έ + Mo ' g ) " (Έ ~ r ' Mo ) "W The terms • j + Mo . gj and - r . Mo^ are constant during the hoisting. Insertion of' K = κ A A - -r ++ MMoO g = A - r . Mo = B 1 resulting in the following expression. F x = K l " P l ' A l " B l ώ (10) Now we have to establish constants K-, A. and B 1 as follows: 1. Reading power input P_ without load 10 and when having the following acceleration: &) = 0. The value is indicated by 2. Reading power input P.. when knowing load F and having the following acceleration: - = 0. The value is indicated by P.. From this we get the following system: 0 = K l * P a - A l " B l * ° Being a system of equations, from which A 1 and K.. may be solved. After having done this, we make a further trial loading with a known load, indicated by 10 in Figure 1. Then we can read power input as well as angular acceleration when the angular acceleration is not = 0. In this way, we can also establish constant B.. According to the formula F = K.. * P χ - A. ^ - B. ^ ' ώ, ( , the hook loading F may be calculated when knowing constants K- , P, and A., i.e. the hook loading may be estimated, Jslook K,, P, and A., i.e. the hook loading may be estimated. Hook loading -F - consists o£ th -.real load 10 -pl-us the dynamic addition of the load, but in this application, the dynamic addition is small as compared with the real load 10. The formula may also be written as follows: a(F χ - A χ ) = P-. - b ' ύ = c * P χ where a, b, and c are constants. This formula may be termed as a subtraction of the angular acceleration of the motor from the power input of the motor, one or both parameters having a scaling factor (b or c) . Examples In the actual examples we used an overhead crane with following data: " Rated load -15 tons Mains voltage U. 500 V Rated output of the motor 35 kW (constant) Synchronous rpm of the motor 750 Lifting speed max 10 m/min The rotor was provided with 4 resistances 9 connected in series, which we could uncouple in 5 steps. As instrument we used a personal computer HP 9816, 5, connected to power meter 1 ifor input power,* meter " for measuring the rotor frequency. The values of constants K. , A., and B.. was established as described under A - K and the values were fed to the computer. Under paragraph J, we made two test hoistings with har loads, load — 0 and a load of 10 5TJ0 togs, "The load Px estimated by the computer, turned out to correspond to the actual load. The estimated load F comprises the static load plus the dynamic addition from the acceleration; accordingly it is the actual hook load. The dynamic addition is so small in .relation to the "Static-Jaoad, that -it. s not -'clear -from the diagrams. As is clear from paragraph J, the overhead crane was in working order with the load limiting device operating. A. At first, with five different work loads we establishe the power input when the lifting speed had stabilized, i.e. at an acceleration = 0. Measurement Work load weight Total power input No (tons >) (kW) 1 0 29.12 2 2.5 33.7 3 5.1 37.4 4 10.5 45.1 5 15.6 53.7 Load weight 10 = _ Fx when a = 0. g The values were also entered into Diagram 1 with load 10 along the horizontal axis and the power along the vertical axis. According to the diagram, the power without work load amounted to 29.12 kW, and the power is directly proportiona to the load weight 10. If these values are put into equation 10 above, the constants K.. and A., will be defined as 6.3 and 183 resp. B. At the known work load of 15.6 tons, we measured the power input at 3 different regulating steps with 2, 1 and 0 resistances, 9, respectively, switched into the rotor circuit. The power input (kW) on the vertical and the time (seconds) when measuring, will be clear from diagram 2. It can be seen that the power input on step 1 amounts to about 54 kW at a constant lifting speed. When switching off one of the resistances 9, step 2, the power input will increase drastically, since the motor, 1, gives a bigger Tm. The system will accelerate up to a new operating point, in which Tm ■ = T L_i. The power input will adjust to the same power, P, when constant speed again is obtained as will also be the case when the second resistance, 9, is switched off, step 3. Accordingly, the power input is independent of resistance alterations in the rotor circuit at constant lifting speed, but it is increasing during acceleration because of the acceleration forces. C. The power input of the hoist motor was measured from .start-up until having increased the lifting speed to a. steady state. The hoist was run without work load. The following facts are clear from Diagram 3, drawn by the computer and showing the power on the vertical and time on the horizonal: a) a short current peak appeared just when starting. b) a shorter period with constant power occured due to the fact that the hoist brake was not yet released. c) A period of about 4 seconds with falling power followed; then the power stabilized. According to continued testing, the lifting speed was stabilized to constant speed at the same time. D. Diagram 4 shows- a- corresponding- 1 hoisting of the work load of 10.5 tons. E. Diagram 5 shows the rotor current as a function of time from start-up until having stabilized at a minimum value. In addition to the fact, that the size of oscillations decreased, the time between two currents peaks increased; accordingly, the slip between the angular velocity of rotor 4 and the synchronous angular velocity of motor 1 decreased. -F„ Then we ooruLd jdef ne.the * slip S by .analyzing the variations of the current. The slip S indicated as the synchronous rpm -0 minus the actual rpm#, divided by the synchronous rpm accordingly O s s = τ) —s Q " Diagram 6 shows the power input as a function of time from start-up on the one hand and the slip at corresponding times on the other. It is clear that the angular velocity of the rotor nears a constant value; accordingly, the angular velocity increases asymptotically from zero at start 35 when- starting. G. According to the formula stated above, >s * = < s . the curve according to Diagram 6 was converted into a means to trace the alterations of the angular velocity when starting the hoist. The angular velocity at different times is clear from Diagram 7. 5 H. By having the computer derivate the curve according to Diagram 7, we had the angular acceleration drawn up according to Diagram 8. I. According to the equation (10) above: F = K 1 P.. -A.. _B 1 ' CO or when Fx = 0 (empty 10 hoisting) 15 As mentioned above, K- was established to 6.3 and A. to 183; now B_ can be established as follows: 2 Δ 0 U B- = 6 ' 3 * P l ' 183 (Diagram 9) 1 CO The interferences at the end are due to the fact, that we are dividing by a small value. When hoisting, the acceleration nears asymptotically zero. 25 With the use of the values established for K. , A. and B_ together with formula (10) , curves have been drawn for empty hoisting. Diagram 10, and for a known work load of 10.6 tons. Diagram 11. As is clear from the curves, there are some interferences at the start of the hoisting, for 0 example, due to some mechanical gaps in the system and due to the fact that the break does not release instantaneously, but already after 0.1 to 0.2 seconds, the value of the load weight is obtained. Accordingly, switch 13, connected to the computer, will be triggered already 0.1 to 0.2 seconds after 5 the starting of the hoisting operation if the work load 10 exceeds the permissible value. To facilitate this, the computer could preferably be programmed to provide about 10 estimations a second. Accordingly, according to the invention, an overload protecting device, can be obtained which will .stand ,up. o .most .standards.,--and which may be adapted to different standards by a simple reprogramming of the computer. If a standard prescribes that overload may appear, however, only during a predetermined period for example 0.5 seconds, the computer can be programmed to initiate the switching off of the motor*s -supply current, provided that all calculations during such a period indicate overload. The time concerned is usually 0.2 - 1.0 seconds, and at least three calculations should be done during the period. . Alternatively the computer can be programmed to initiate switching-off when.,a preset number of calculations have estimated overload, provided-that at least this "number of calculations are made during the predefined period of time.

Claims: CLAIMS 1. A method of indicating overload and interrupting the lifting of an overload when a hoist is used that is driven by an asynchronous induction motor (1), c h a r a c t e r z e d by predetermining a limit value of a formula that comprises a subtraction of the angular acceleration of the motor from the power input of the motor with predetermined scaling factors, and switching off the supply current of th motor if this limit value is obtained. 2. A method of indicating overload and interrupting the lifting of an overload when a hoist is used that is driven by an asynchronous induction motor (1), c h a r a c t e r z e d by first metering the power input of the motor and the rotation velocity of the motor in a test-lifting- cycle and defining the constants K. , A. and B_ in the following formula: F x = K l ' P l - A l - B l • « where F is hook load, P. the power input of the motor and angular acceleration of the motor, the metering being carried out on load 0 and acceleration 0, on a known load and acceleration 0, and on a known load and known acceleration, and then in use metering the input power of the motor and rotation velocity of the motor and using the formula to estimate the hook load several times a second, and switching off the lift if the estimated hook load reaches a predefined value. 3. A method according to claims 1 or 2, c h a r a c t e r i z e d by the step of estimating the hook load at least 3 times a second. 4. A method according to claim 3, c h a r a c t e r i z e d by estimating the hook load at least 5 times a second. 5. A method according to anyone of the preceeding claims c h a r a c t e r i z e d in that the hoisting is not interrupted until overload has been indicated in a preset number of estimations. 6. A method according to claim 5, c h a r a c t e r i z e d in that the hoisting is not interrupted until all.-estimations within a predefined period have indicated overload. 7. A method,according to claim 6, c h a r a c t e r i z e d in that said period is between 0.2 to 1.0 seconds long. 8. A protection device for a hoisting apparatus with an asyncronσus induction motor (1) , the device being arranged to indicate overload and to switch off the supply current to the motor when overload occurs, c h a r a c t e r i z e d in that a power meter (7) is connected to the input line (3) of the motor (1) and a meter (8) is provided for metering the rotation velocity, a computer (5) is connected - to the power meter (7 and to the meter {8} of the speed of the rotation velocity, and arranged to make calculations on a formula several times a second, said formula comprising a subtraction of the angular acceleration from the input power with programmed scaling factors and the computer (5) being programmed to initiate a supply current switch off if a predefined value of the calculation is reached.

Inventors: OLVERUD BJOERN, SVEDLUND TOMMY RAGNAR, OLVERUD, BJOERN, SVEDLUND, TOMMY, RAGNAR
Applicants: SSAB SVENSKT STAL AB, SSAB SVENSKT STAAL AB
Application year: 1987
Main classification: G01L3
IPC codes: B66B5, G01L3, G01L5, H02H7