Method for operating an inverter-fed electric motor

Abstract

Method for operating a converter-fed electric motor with an electromagnetically actuated brake, wherein, in generator operation, electrical energy is supplied to a winding (21) of the brake and temporarily stored there as magnetic energy and / or converted into thermal energy, wherein, during motor operation, the current supplied to the winding (21) and the associated voltage applied to the winding (21) are temporarily detected, whereby the current temperature of the winding (21) is determined taking into account the ohmic resistance of the winding (21) at a reference temperature, where in generator mode the current and / or a forecast temperature is determined only from a computational model, wherein, in motor operation, a single- or multi-stage computational model is used to determine the temperature between the times of determining the current temperature of the winding (21), wherein, in the motor case, to determine the current temperature, a measuring current exceeding the holding current for releasing the brake is supplied to the winding (21) for a period of time and at the end of the period of time the current value and the voltage applied to the winding (21) are determined, where a temperature value is predicted from the computational model, compared with a critical value and an action is carried out depending on this.

Description

[0001] The invention relates to a method for operating an inverter-fed electric motor.

[0002] DE 197 34 405 A1 discloses supplying electrical power to a brake coil winding, i.e., the excitation coil winding, during generator operation. It is also disclosed that current and voltage are measured to determine, taking the thermal time constant into account, whether the thermal load on the braking resistor exceeds a critical value.

[0003] DE 10 2004 057 915 A1 discloses a drive and a method for controlling energy flow. Starting at a critical intermediate circuit voltage, energy flow is controlled and supplied to two loads. The first load is an electromagnetically actuated brake that is released, meaning it must be supplied with a certain minimum amount of energy flow. If the intermediate circuit voltage exceeds the critical value, the energy flow dissipated to the brake coil of the brake is increased in order to load the second load, a braking resistor, only as needed.

[0004] EP 0 284 711 A2 discloses a device for detecting the winding temperature of a DC motor.

[0005] An electronic overload relay for electric motors is known from US 2006 / 0 250 154 A1.

[0006] A DC motor control is known from US 2004 / 0 195 989 A1.

[0007] The invention is therefore based on the object of better utilising the excitation coil of a drive in a system.

[0008] According to the invention, the object is achieved by the method according to the features specified in claim 1.

[0009] Important features of the method for operating a converter-fed electric motor with an electromagnetically actuated brake are that, in generator operation, electrical energy is supplied to a winding, in particular the excitation coil winding, of the brake and is temporarily stored there as magnetic energy and / or converted into thermal energy, wherein, during motor operation, the current supplied to the winding and the associated voltage applied to the winding are temporarily recorded, whereby the current temperature of the winding is determined, in particular taking into account the ohmic resistance of the winding at a reference temperature, In generator mode, the current or a predicted temperature is determined from a computational model.

[0010] In particular, when changing from motor operation to generator operation, the last determined value of the current temperature is transferred to the computational model.

[0011] The advantage of this is that the computational model allows a temperature to be determined even when highly dynamic temperature behavior occurs during generator operation. Especially during generator operation, updating the actual temperature using resistance measurement is not necessary.

[0012] Another advantage is that the starting value corresponds as closely as possible to the actual value and the temporal development can be determined by the model.

[0013] In an advantageous embodiment, in the motor case, to determine the current temperature, a measuring current is supplied to the winding for a period of time such that the holding current required to release the brake is exceeded. At the end of the period, the current value and the voltage applied to the winding are determined. This is advantageous because precise temperature determination is possible in the motor case.

[0014] If the holding current fails, i.e. if the winding is de-energized, the brake engages and thus holds the rotor shaft of the electric motor in position.

[0015] In an advantageous design, - in generator mode, the current supplied to the winding is recorded, - from this and from a resistance value of the winding which depends on the last determined current temperature, the heat power introduced into the winding is determined, which is fed into the computational model, - and from this the new current temperature is determined.

[0016] The advantage of this is that it allows for accurate temperature determination even in generator mode, although only a single model is used. The model's initial value is advantageously very close to the actual value.

[0017] In an advantageous embodiment, the computational model is implemented as a multi-stage temperature model, in particular according to a multi-stage heat equivalent circuit. This is advantageous because it enables a very precise determination of the temperature profile.

[0018] In an advantageous embodiment, the computational model is designed in several stages so that the heat transfer resistance from the winding to at least one other component of the brake and The heat transfer resistance from the other component to the environment, as well as the heat capacity of the winding and the heat capacity of the other component, are taken into account. The advantage here is that the modeling closely approximates the design of the brake in important aspects.

[0019] The modeling becomes even more precise if the computational model is designed in such a multi-level manner that the heat transfer resistance from the winding to at least one other component of the brake and the heat transfer resistance from the further component to at least one third component and the heat transfer resistance from the third component to the environment and The heat capacity of the winding and the heat capacity of the other components are taken into account. Thus, the potting compound and the cast coil body, such as cast steel or gray cast iron, are also taken into account.

[0020] In an advantageous embodiment, the input parameters of the computational model are filled with the respective current values during motor operation. This is advantageous because the model is also computable during motor operation, and thus the variables are always filled with values that are as close to reality as possible. This allows the model to continue running without significant disruptions when switching to generator operation. In particular, the computational model is also activated during motor operation, but its resulting values are not used.

[0021] However, the results can also be used advantageously. For example, a temperature forecast can also be carried out in the motor case. For example, if the temperature is predicted to fall, the absorption capacity for the power to be dissipated to the winding can also be predicted. More precisely, the temperature that will exist in, say, 30 seconds, with the same or a different operating mode, can be predicted. For this future temperature value and a generator operation that starts at that point, the permissible, just-allowable electrical power consumption can be determined. This enables timely warnings to be given in the event that the intended operating mode reaches unacceptably high temperatures. If the predicted temperature value is lower than the current one, an even greater absorption capacity can be predicted.

[0022] In an advantageous embodiment, a single-stage model is used in motor operation. The advantage of this is that it allows for very precise temperature determination in a particularly simple manner. In a first variant, a time period is determined after which the controller structure has stabilized. At the end of the time period, the temperature can then be determined from the recorded current value and the corresponding voltage value applied to the winding. This current temperature value is fed into the model. In addition, a value for the power P = R x I^2, which is dependent on the recorded current value, is fed to the model in each time step. Thus, using the model, the temperature profile can be determined from the supplied electrical power, which is converted into ohmic heat in the winding, and the temperature value determined at the end of the time period.

[0023] Likewise, in a multi-stage model, supplying a specific electrical power is advantageous in both the motor and generator modes. In motor mode, the current temperature value is also determined at the end of each of the specified time periods and used to update the model parameters.

[0024] In an advantageous embodiment, a single-stage model is applied during motor operation, taking into account only the heat transfer resistance from the winding to the environment and the heat capacity of the winding. The advantage here is that the model can be executed quickly and easily by the computer unit, making the determination process correspondingly simple.

[0025] In an advantageous embodiment, a temperature value is predicted from the computational model, compared with a critical value, and an action, in particular a warning, an alarm, a power reduction, brake activation, or shutdown, is executed based on this. The advantage here is that predictive operation is enabled.

[0026] In an advantageous embodiment, the predicted temperature value of the computational model is also used during motor operation, particularly for monitoring the drive. This is advantageous because it allows for accurate temperature determination even during motor operation, enabling predictive operation.

[0027] Important features of a converter-fed electric motor with an electromagnetically actuated brake are that, in generator operation, an electrical switch is provided to control an electrical energy flow into a winding, in particular the excitation coil winding, of the brake, in particular for intermediate storage there as magnetic energy and / or conversion into thermal energy, wherein the electrical switch determines the supply of current to the winding, wherein means are provided for detecting the current and the associated voltage applied to the winding, wherein lines are provided for supplying the signals of the means to a computing unit of the converter for determining the current temperature of the winding in motor operation or in generator operation for determining the current or predicted temperature from a computational model, wherein comparison means are provided for comparing the current temperature with a critical temperature value.

[0028] The advantage here is that the precise temperature profile determination of the multi-stage model enables the best possible utilization of the winding's absorption capacity for the power to be dissipated during generator operation.

[0029] Important features of the method for operating a system are that the system comprises a drive connected to a central controller for data exchange, wherein the drive uses the load profile that occurred in at least one period of time in the past, in particular comprising motor and generator periods, as the basis for a temperature profile forecast for a component of the drive, compares the predicted temperature with a critical value, and issues a warning to the central control system when the critical value is exceeded.

[0030] Advantageously, the winding of the brake of the drive, the stator winding of the electric motor of the drive and / or the power semiconductors of the converter of the drive feeding the electric motor can be used as components and thus the invention can be applied to various components with low thermal load.

[0031] In particular, a multi-stage model is used, thus enabling a realistic model calculation.

[0032] Further advantages emerge from the dependent claims. The invention is not limited to the combination of features in the claims. Further possible combinations of claims and / or individual claim features and / or features of the description and / or the figures will become apparent to those skilled in the art, particularly from the problem and / or the problem posed by comparison with the prior art.

[0033] The invention will now be explained in more detail with the aid of illustrations: In the Fig. 1 shows the circuit diagram of an arrangement according to the invention.

[0034] A rectifier GR is supplied from the mains, and the rectified unipolar voltage is smoothed by the intermediate circuit capacitor, i.e., capacitance C. This capacitor supplies the inverter WR to power the three-phase motor M.

[0035] The control 1 controls the switch S1 depending on the results of the calculations made within the converter 3.

[0036] The current released by switch S1 of the brake coil winding L1, which is split, i.e. with intermediate tap, is routed and determined via the shunt resistor R1.

[0037] In Fig. 2 shows a cross-section through the brake, with the rotationally symmetrical axis of the brake included in the section plane.

[0038] The coil former 20, in particular a magnetic body, is made of cast steel, cast iron, or the like. The winding 21 is made of copper wires. The coil support 22 is made of a plastic material and, after being inserted into the coil former 20, is potted together with the winding 21 using a potting compound 23. At some points, the coil support 22 touches the winding wires of the winding 21. At other points, potting compound surrounds the winding and partially also the coil support 22.

[0039] For the computational modeling of the thermal behavior of the brake coil, model assumptions are made and a two- or multi-stage thermal model is used. An example of this is shown in Fig. 3 executed.

[0040] The lower part of the Fig. Figure 3 shows the thermal model for motor operation. This model takes into account the heat transfer resistance Rth5 from the winding to the environment and the heat capacity C1 of the winding. A holding current is applied as the heat source H2 when the brake is released. Furthermore, at long intervals, for example, every 20 or 30 seconds, a larger winding current is specified as the measuring current for a period of time. This period is long enough that the coil temperature can be accurately determined from the coil's ohmic resistance. This is because when the winding current is specified, the control system must settle somewhat. This period is a few hundred milliseconds, for example, 200 ms.When the time period expires, the current is detected by means of the shunt resistor and the ohmic resistance of the winding is determined from the quotient of the voltage also detected on the winding, in particular the intermediate circuit voltage, and this detected current value.

[0041] The ohmic resistance value of the winding for a temperature of 20 °C is also stored in the processing unit located in the converter. From this value and the resistance value determined as described above, the current temperature value of the winding is determined, taking into account the change in resistance per Kelvin of temperature change as a material constant. The winding material is preferably copper, and the change in resistance per Kelvin is known and stored in the processing unit.

[0042] However, determining the ohmic resistance value of the winding for a temperature of 20 °C is difficult to achieve in mass production of drives according to the invention. Therefore, an average value is stored, from which the actual individual value may deviate considerably, for example, by 5 to 10 percent, due to production-related fluctuations.

[0043] The current temperature value determined in the manner described is fed into the model, thus updating it. At each time step, the current value is also recorded, and the power P = R x I^2 is determined from this and also fed into the model as heat output.

[0044] In this way, the temperature curve can be determined using the model, especially in the time periods between the time periods.

[0045] When the drive switches from motor to generator operation, the switch 30 switches accordingly and the upper part is used to determine temperatures. The heat source H1 of the upper part of the Fig. 3, like the heat source H2, is operated with the power determined from the current values determined in each time step. The downstream heat model, in contrast to the lower part of the Fig. 3 multi-stage. The heat capacity C2 of the potting compound, the heat capacity C3 of the coil former, the heat capacity C4 of the housing, as well as the individual resistances, such as the heat transfer resistance Rth1 from the winding to the potting compound, the heat transfer resistance Rth2 from the potting compound to the coil former, and the heat transfer resistance Rth3 from the coil former to the environment, are all taken into account. The multi-stage approach refers to the model-based separation of the heat transfers, with each heat transfer from one component to the next being assigned a heat transfer resistance and a heat capacity of the component. For simplicity, the potting compound and coil former are designed as a single component.

[0046] Since after execution Fig. 3 the potting compound and the coil former are considered as a single component, the heat capacity C2 of the potting compound and the heat capacity C3 of the coil former are connected in parallel.

[0047] In generator mode, only the model is used to determine the temperature. Therefore, no measuring current comparable to the above-mentioned measuring current for motor operation is provided in this case. The holding current for releasing the brake is superimposed by the current component supplied to the winding during generator mode to dissipate the electrical energy generated by the generator.

[0048] Since no ambient temperature value is recorded, the model is based on a high, predefined ambient temperature value for safety reasons, for example 20°C or 40°C.

[0049] The relationship Rth5 = Rth1 + Rth2 + Rth3 applies to the dimensioning of the model.

[0050] In the manner described, the temperature of the winding is determined during motor operation by measuring current and voltage. During the transition from motor to generator operation, the temperature is only measured using the model. However, the multi-stage nature of the model leads to a high degree of accuracy, allowing a reliable model value to be determined for the current temperature measurement. This allows for better utilization of the brake coil to dissipate the generator energy.

[0051] Advantageously, the model also enables a forecast, i.e., a prediction of the temperature at the winding, based on assumptions about the future load profile. This allows for preventive action such as timely shutdown, power reduction, brake activation, issuing a warning, and / or alarm.

[0052] Preferably, a load profile is assumed for the forecast that corresponds to the previous one for at least one time interval. For this purpose, the temperature is forecast, and a warning is issued if a critical value is exceeded.

[0053] In further training, different load profiles can also be used, whereby for each one it is checked whether the critical value is exceeded and accordingly a warning is assigned to each load profile or not.

[0054] Advantageously, a central control system transmits a future planned load profile to the drive connected for data exchange, which then checks whether the critical value is exceeded and reports back a warning or release accordingly.

[0055] In a further embodiment of the invention, the ambient temperature is additionally detected and taken into account, whereby the temperature determination is even more accurate.

[0056] In a further embodiment of the invention, the potting compound and coil carrier are selected and modeled as separate components, thus adding an additional stage to the model. This enables an even more precise, but also more complex, determination of the temperature profile. List of reference symbols GR rectifier WR inverter M three-phase motor C capacity 1 Control 2 computer unit 3 inverters 20 coil bodies 21 winding 22 coil carriers 23 Potting compound 30 Switching R1 shunt resistor L1 brake coil winding with center tap S1 switch C1 Heat capacity of the winding C2 Heat capacity of the casting compound C3 Heat capacity of the coil body C4 Heat capacity of the housing Rth1 Heat transfer resistance from the winding to the potting compound Rth2 Heat transfer resistance from the potting compound to the coil body Rth3 Heat transfer resistance from the coil body to the environment Rth5 Heat transfer resistance from the winding to the environment H1 Heat source / heat output source H2 heat source / heat power source

Claims

[1] Method for operating an inverter-fed electric motor with an electromagnetically actuated brake, wherein, in generator operation, electrical energy is supplied to a winding (21) of the brake and temporarily stored there as magnetic energy and / or converted into thermal energy, wherein, during motor operation, the current supplied to the winding (21) and the associated voltage applied to the winding (21) are temporarily detected, whereby the current temperature of the winding (21) is determined taking into account the ohmic resistance of the winding (21) at a reference temperature, where in generator mode the current and / or a forecast temperature is determined only from a computational model, wherein, in motor operation, a single- or multi-stage computational model is used to determine the temperature between the times of determining the current temperature of the winding (21), wherein, in the motor case, to determine the current temperature, a measuring current exceeding the holding current for releasing the brake is supplied to the winding (21) for a period of time and at the end of the period of time the current value and the voltage applied to the winding (21) are determined, where a temperature value is predicted from the computational model, compared with a critical value and an action is carried out depending on this. [2] Method according to claim 1, characterized by that the current is recorded in each time step and from this the heat power introduced into the winding (21) is determined. [3] Method according to at least one of the preceding claims, characterized bythat when changing from motor operation to generator operation, the last determined value of the current temperature or the value from the computational model activated in motor operation is transferred to the computational model which is activated in generator operation. [4] Method according to at least one of the preceding claims, characterized by that the time span is between 100 ms and 1 s. [5] Method according to at least one of the preceding claims, characterized by , that - in generator operation, the current supplied to the winding (21) is detected, - from this and from a resistance value of the winding (21) dependent on the last determined current temperature, the ohmic heat power introduced into the winding (21) is determined, which is fed to the computational model, - from which the new current temperature is determined. [6] Method according to at least one of the preceding claims, characterized by that the computational model is designed as a multi-stage temperature model. [7] Method according to at least one of the preceding claims, characterized by , that the computational model is designed in such a multi-stage manner that the heat transfer resistance (Rth1) from the winding (21) to at least one further component of the brake and the heat transfer resistance from the further component to the environment as well as the heat capacity (C1) of the winding (21) and the heat capacity (C2, C3, C4) of the further component are taken into account. [8] Method according to at least one of the preceding claims, characterized by , that the computational model is designed in such a multi-stage manner that the heat transfer resistance (Rth1) from the winding (21) to at least one further component of the brake and the heat transfer resistance (Rth5) from the further component to at least one third component and the heat transfer resistance (Rth2) from the third component to the environment as well as the heat capacity (C1) of the winding (21) and the heat capacity (C2, C3, C4) of the further component are taken into account. [9] Method according to at least one of the preceding claims, characterized by that during motor operation, parameters of the computational model are filled and updated with the respective current values. [10] Method according to at least one of the preceding claims, characterized by that the computational model is also activated during motor operation. [11] Method according to at least one of the preceding claims, characterized bythat in motor operation a single-stage model is applied such that only the heat transfer resistance from the winding (21) to the environment and the heat capacity (C1) of the winding (21) are taken into account. [12] Method according to at least one of the preceding claims, characterized by that the predicted temperature value of the computational model is also used in motor operation.

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