Control apparatus and method to control a high-power electric motor

Abstract

Control apparatus and corresponding method for controlling a high power electric motor, preferably of the order of megawatts, preferably of or associated with a shredding plant which is preferably usable for shredding very bulky and heavy objects and is provided with a rotating shredding member connected to the rotor of the electric motor, where a control circuit is configured to control the electric motor so that it can operate selectively in different operating modes.

Claims

1. A control method to control a high power electric motor associated with a shredding plant for shredding bulky and heavy objects and provided with rotating shredding means connected to a rotor of said electric motor, by means of a control apparatus which comprises electric power supply means associated with said electric motor to selectively power the motor with a voltage and a current so that said electric motor operates at a selected temperature, supplies said power, rotates at a selected rotation speed and applies to said rotating shredding means a determinate torque necessary for shredding said objects, and a control circuit connected to said electric power supply means and configured to control said electric motor so that the motor selectively operates at said temperature, supplies said power, rotates at said rotation speed and applies said torque to said rotating shredding means, the method comprising a first step in which said control circuit controls said electric motor so that the motor operates in a first operating mode, with a constant rotation speed, and with a power limited by a set maximum power value, and wherein, upon reaching said maximum power value, selecting the supply of said voltage and said current so that said electric motor can selectively operate in one of either a second operating mode with a control at constant power, and with a torque limited to a selected value greater than or equal to a nominal torque value, or a third operating mode, with a control at constant torque, and with a temperature of said electric motor limited to a selected value.

2. The control method as in claim 1, wherein in said second operating mode, increasing the torque applied to said electric motor by increasing the electric current absorbed by said electric motor.

3. The control method as in claim 1, wherein upon reaching said maximum power value, selecting and maintaining said second operating mode until the torque of said electric motor reaches a maximum transmissible torque value corresponding to a predefined reachable maximum temperature.

4. The control method as in claim 1, wherein in said third operating mode maintaining said torque constant and decrease said rotation speed and said power.

5. The control method as in claim 1, further comprising controlling said electric motor with said first operating mode until the power absorbed by said electric motor reaches a set maximum power level comprised between 105% and 115% of the nominal power of said electric motor.

6. The control method as in claim 1, wherein in said third operating mode the maximum torque, corresponding to the maximum operating temperature of said electric motor, is set at a value comprised between 140% and 160% of the nominal value of said torque.

7. The control method as in claim 1, wherein upon reaching a torque limit correlated to said selected temperature value in said third operating mode, or when the third operating mode has remained operational for a predefined period of time, providing to reduce the electric power supplied to said electric motor so that said electric motor returns to function in said second operating mode.

8. The control method as in claim 1, further comprising making said electric motor work with a constant magnetic field flux value, substantially equal to a nominal flux value, in each of said operating modes.

9. The control method as in claim 1, further comprising detecting, substantially continuously, the temperature of said motor and to send on each occasion the value detected to said control circuit, in order to suitably regulate said voltage and said current to take full advantage of a potential of said electric motor, at the limit of a thermal capacity, without compromising correct functioning.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) These and other aspects, characteristics and advantages of the present disclosure will become apparent from the following description of one embodiment, given as an example, which does not restrict the field and scope of protection, with reference to the attached drawings wherein:

(2) FIG. 1, as seen above, is a known graph that schematically shows the relation between the rotation speed (ω) of the rotor of an electric motor and the power (P), torque (T) and temperature (t) parameters of the rotor;

(3) FIG. 2 is a block diagram of a control circuit according to the present disclosure associated with a shredding plant;

(4) FIG. 3 is a graph that briefly represents the trend of the rotation speed (ω) of the rotor of the electric motor used in the shredding plant of FIG. 2, in relation to the corresponding power (P), torque (T) and temperature (t°) parameters of the rotor according to the three modes, A, B and C, for controlling the electric motor according to the present disclosure, respectively at a constant rotation speed (ω), at constant power (P) and constant torque (T);

(5) FIG. 4 is a graph that briefly represents an example of the trend over time (t) of the following three parameters in the three operating modes, first A, second B and third C, for controlling the electric motor of FIG. 3: rotation speed (ω) (upper dashed curve), in revolutions per minute (rpm); power (P) (lower dash-dot curve), as a percentage of the nominal power of the electric motor; torque (T) (central continuous line curve), also as a percentage of the nominal torque of the electric motor;

(6) FIG. 5 is a schematic representation of the control circuit of FIG. 2;

(7) FIG. 6 is a graph that represents the trend over time (t) of the power (P) supplied by an electric motor controlled by a control circuit of the present disclosure (lower curve with solid black line), compared to that controlled according to the previous state of the art (upper gray curve).

DETAILED DESCRIPTION

(8) We will now refer in detail to the possible embodiments of the present disclosure, shown in the attached drawings. These examples are supplied by way of illustration of the present disclosure and shall not be understood as a limitation thereof.

(9) Before describing these embodiments, we must also clarify that the present description is not limited in its application to details of the construction and disposition of the components as described in the following description using the attached drawings. The present description can provide other embodiments and can be obtained or executed in various other ways. We must also clarify that the phraseology and terminology used here is for the purposes of description only, and cannot be considered as limitative.

(10) With reference to FIG. 2, a control apparatus 10 according to the present disclosure is shown associated with a shredding plant 11 of a known type, having an electric motor 12, the rotor of which is connected, for example by means of an extension, to a rotating drum, or mill, 13, provided with crushing hammers, of a known type and not shown in the drawings.

(11) The rotating drum 13 is associated upstream with a device 14 for introducing objects, or material, to be shredded and downstream with a device 15 for collecting the shredded pieces.

(12) The shredding plant 11 is connected to electric power supply means 16 of a known type, consisting for example of a medium voltage (MV) electricity grid, capable of supplying indicatively an electric voltage from 11 to 20 KV with a power (P) of up to about 10-15 MW.

(13) Between the electric power supply means 16 and the electric motor 12 there is interposed an MV/LV electric transformer 17, configured to transform the electric voltage from medium (MV) to low (LV), indicatively, in the example given here, to a value comprised between about 300 and 700 V, with a power of about 3-3.5 MW.

(14) The control apparatus 10 comprises a control circuit 20, connected to the electric transformer 17 and provided with selection means 21 controlled by a programmable electronic control unit 22, for example a PLC, also on the basis of suitable feedback signals coming from the electric motor 12 and indicative at least of the torque T, the power P and the rotation speed ω.

(15) According to some embodiments, the programmable control unit 22 can receive the feedback signals detected and/or monitored in real time by one or more detection devices 25 associated with the electric motor 12. According to some embodiments, the detection device/s 25 can comprise sensors suitable to detect one or more of either torque, rotation speed, absorbed electric current, or motor temperature.

(16) The detection devices 25 comprise in particular at least temperature sensors suitable to detect the temperature of the electric motor 12 and send the detected values to the control circuit 20 and/or to the programmable electronic control unit 22.

(17) The selection means 21 comprise, for example, an inversion circuit, or inverter, 23 which in turn preferably comprises a plurality of thyristors 24 (FIGS. 2 and 5).

(18) The control circuit 20 is configured to actively manage the control parameters of the electric motor 12 (power P, torque T, rotation speed ω) thanks to the inverter 23 and the programmable electronic control unit 22. All this in a dynamic manner, taking full advantage of the potential of the electric motor 12 in terms of percentage of the maximum torque that can be supplied in permanent thermal regime.

(19) In particular, the control circuit 20 is capable of separating the electric motor 12 from the electric power supply means 16, that is, from the electricity grid, so as to be able to control the currents at exit from the inverter 23 and power the electric motor 12 in a controlled manner, preventing stresses on the kinematic chain and disturbances on the electricity grid itself.

(20) In particular, the control apparatus 10 allows to operate in three different control operating modes.

(21) A first operating mode A basically provides a control at constant rotation speed ω, with a limit on power P.

(22) In this first operating mode A, the rotation speed w is the control parameter and is kept constant. Therefore, its value is numerically fixed through automation and can be selectively set to try to achieve maximum productivity, since the rotation speed w is directly proportional to the productivity of the shredding plant 11.

(23) For set values of high rotation speed w based on the formula P=T.Math.ω, the torque T and the power P also increase, up to a set power limit Pmax, which is the limit that cannot be passed in order to not damage the electric motor 12.

(24) The power P absorbed by the electric motor 12 reaches the set maximum power level Pmax, preferably comprised between 105% and 115%, for example 110% of the nominal power P of the electric motor 12, which corresponds to point A in FIG. 3. Beyond this limit, the electric motor 12 would be damaged. In fact, it would be possible to go over this limit only with an oversizing of the electric motor, as is done in the prior art. However, the present disclosure does not provide this oversizing but provides to pass, in an automatic and programmed manner, to a subsequent second operating mode B.

(25) The second operating mode B basically provides a control at constant power P, with a limit on the torque T.

(26) In this second operating mode B, the power P is the control parameter and is kept constant, so that the torque T, which increases, and the rotation speed ω, which decreases to keep the power P constant, become variable.

(27) In particular, in the second operating mode B the power P is substantially kept constant at the set maximum power value Pmax, or in any case at a value lower than this.

(28) The torque T is increased by increasing the current absorption. A higher current leads to an increase in the specific energy passing through the electric motor 12, according to the formula I.sup.2t, where “I” is the effective value in amperes of the short-circuit current and “t” is the duration of the current, with a consequent increase in the temperature t° of the electric motor 12.

(29) Therefore, advantageously, the temperature t° of the electric motor 12 is measured and its value is sent to the control circuit 20 in order to take full advantage of the potential of the electric motor 12, at the limit of its thermal capacity so as not to compromise its correct functioning.

(30) Therefore, by keeping the power P below, or at most equal to, the set maximum value Pmax, it is possible to increase the torque transmitted by the electric motor 12 to the rotating drum 13 as a function of the temperature t° of the electric motor 12, up to at a maximum torque level T set to correspond to the maximum temperature t° max that can be reached by the electric motor 12 without it being damaged, according to the characteristics of its build.

(31) In this second operating mode B at constant power (central zone of FIG. 3), the following advantages are obtained: productivity increases by 25%-30%; no current absorption peaks occur on the side of the electric power supply means 16; maximum energy is transferred on the side of the rotating drum 13; the electric motor 12 is not thermally stressed.

(32) With the control circuit 20 and with the inverters 23, moreover, it is also possible to control the switching on and off of the feeder rollers that feed the scrap to the crushing plant, as well as their speed, in such a way as to keep the engine power constant at the fixed value and transfer the maximum possible energy to the scrap.

(33) According to some embodiments, the second operating mode B with constant power limit allows to adapt the functioning of the electric motor 12 also as a function of the electric energy available. In fact, in the event a limit equal to 120% of the nominal power cannot be set, but it is necessary to limit it to 80%, in the second operating mode B it will still be possible to make the motor 12 work with a torque T variable between 100% and 150% of the nominal torque, regardless of the power.

(34) The third operating mode C provides a control substantially at constant torque T, with a limit on the temperature t° of the electric motor 12.

(35) When the maximum transmissible torque T is reached, which corresponds to the maximum temperature t° max that can be safely reached by the electric motor 12, the torque T becomes the control parameter that is kept constant, allowing the possibility for the rotation speed ω and the power P to vary, in particular to decrease.

(36) The amount of time during which the electric motor 12 can work in this condition depends on the use percentage of the torque T with respect to the nominal value, and on the previous functioning history of the electric motor 12 itself, which affects its temperature state.

(37) When the torque T limit is reached and the control is performed as a function of the temperature t°, subsequently the control method provides to limit the electric power supply to the electric motor 12.

(38) According to some embodiments, it can be provided that after a certain time interval during which operations are carried out in the third operating mode, the supply current supplied to the electric motor 12 is reduced in such a way as to bring the operating parameters of torque T, rotation speed ω and power P back within the values provided in the second operating mode B (see the graph in FIG. 3).

(39) In particular, unlike what normally occurs in known solutions, the method according to the disclosure provides to work initially with a speed control, so as to increase the power up to the maximum value Pmax, and subsequently pass to a control at constant power set at the value Pmax by progressively increasing the torque T up to the set limit.

(40) According to some embodiments, the voltage and current values supplied to the motor can be regulated in such a way as to keep the electric motor 12 in the second operating mode.

(41) The graph of FIG. 4 shows a real recording, on a functioning shredding plant 11, of the three control parameters, namely power P, torque T and rotation speed ω during a shredding process, with the three operating modes A, B and C.

(42) In particular, the graph of FIG. 4 shows at the top (upper dashed curve) the trend of the rotation speed ω in revolutions per minute (rpm), in the central part, with a continuous line, the torque T as a percentage and at the bottom, dashed-dot line, the power P also as a percentage with respect to the nominal power of the electric motor 12.

(43) The functioning of the control apparatus 10 described heretofore, which also corresponds to the control method according to the present disclosure, provides a first step in which the control circuit 20 controls the electric motor 12 according to the first operating mode A, therefore the rotation speed ω is set to increase until the maximum power threshold is reached (Pmax in FIG. 3).

(44) Upon reaching this threshold, the control circuit 20 passes to a second step in which the second operating mode B is adopted, therefore the power P remains at a fixed value, for example the maximum power value Pmax, preferably comprised between 105% and 115%, for example at 110%, of the nominal power P.

(45) The second operating mode B allows to take full advantage of the power of the electric motor 12, so that, as can be seen in FIG. 4, the lower dash-dot line never exceeds this threshold value of 110% and is kept substantially constant, considerably raising the average value of the power used compared to known solutions, until it almost reaches the nominal value.

(46) As previously described, in the second operating mode B, as the torque T transmitted increases, the temperature t° of the electric motor 12 also increases, until the maximum torque T transmissible at the maximum working temperature (t° max in FIG. 3) of the same electric motor 12 is reached.

(47) According to some embodiments, in the second operating mode B the electric motor 12 always works with a constant magnetic flux value, substantially equal to the nominal flux value.

(48) Upon reaching the maximum working temperature, the control circuit 20 passes to a third step in which the third operating mode C is adopted.

(49) According to some embodiments, the torque value T in the second B and in the third operating mode C is greater than or equal to a nominal torque value.

(50) In the graph of FIG. 4, the maximum torque T, corresponding to the maximum operating temperature (t° max) of the electric motor 12, is set to a value preferably comprised between 140% and 160%, for example 150%, of the nominal value.

(51) When the type of material to be shredded, loaded into the rotating drum 13 (FIG. 2) by means of the introduction device 14, is such that it allows to drop below the torque T limit set by the control circuit 20, there is a return to the scope of work of the second operating mode B, which is the ideal mode in which the machine should be made to work, since it is the area in which the electric motor 12 supplies the maximum power P without exerting effort, but in which it is also possible to obtain the highest productivity. This operating mode varies between point A which corresponds to the maximum rotation speed ω of the electric motor 12 and therefore of the rotating drum 13 and the lower torque T, and point C which corresponds to the maximum torque T and the minimum rotation speed ω at equal power P.

(52) In order to switch from the third operating mode C to the second operating mode B it is possible to act through the inverter 23 to reduce the electric current supplied.

(53) According to some embodiments, the method according to the disclosure provides to make the electric motor 12 work with a constant magnetic field flux value, substantially equal to the nominal flux value, in each of the operating modes A, B, C.

(54) The graph of FIG. 6 represents the result of a comparative study between the trend of the power P, limited to a maximum value of 4 MW and supplied in time (t) by the electric motor 12 controlled by the control circuit 20 (lower solid line curve), in which there is no anomalous power peak, and that of the power P supplied in time (t) by an electric motor according to the state of the art (upper dashed curve), in which it can be seen that in the latter case there are two uncontrolled power peaks, the highest of which had a value of 7.2 MW.

(55) As is evident from the graph of FIG. 6, while in the case of the prior art it is necessary to provide an oversized electric motor, with a nominal power greater than 7.2 MW, thanks to the present disclosure, with the same performance, it is possible to use an electric motor with nominal power equal to, or even lower than, 4 MW, with consequent cost savings in production and consumption.

(56) Furthermore, with the control apparatus 10 according to the present disclosure, the reaction of the control circuit 20 to the variations in the load to which the electric motor 12 is subjected is substantially immediate, of the order of a few milliseconds, compared to the time of a few seconds (from 2 to 3) of the prior art, for example with a known LRS.

(57) It is clear that modifications and/or additions of parts or steps may be made to the control apparatus 10 and method as described heretofore, without departing from the field and scope of the present disclosure as defined by the claims.

(58) It is also clear that, although the present disclosure has been described with reference to some specific examples, a person of skill in the art shall certainly be able to achieve many other equivalent forms of apparatus 10 and method for controlling a high power electric motor, preferably of the order of megawatts (MW), preferably of a shredding, or crushing, plant, having the characteristics as set forth in the claims and hence all coming within the field of protection defined thereby.

(59) In the following claims, the sole purpose of the references in brackets is to facilitate reading: they must not be considered as restrictive factors with regard to the field of protection claimed in the specific claims.