Overload protected motor control

Description

[0001] The invention relates to a method for controlling an electrical machine, wherein the motor current is limited as a function of the motor speed of the electrical machine to prevent thermal overload of the electrical machine. Furthermore, the invention relates to a power converter and an electric drive.

[0002] Electric drives that incorporate a power converter and an electric motor are designed electrically and thermally for their specific application. This means that the electrical components of the power converter and electric motor are designed for the loads encountered in such a way that they are not electrically or thermally overloaded. This results in electrical losses in both the power converter and the electric motor. In the event of an overload, temperatures inside the power converter or the electric motor exceed permissible limits. This accelerates aging or causes damage to the electric motor, leading to failure.

[0003] The present application deals with the thermal protection of the electrical machine.

[0004] The term "electrical machine" encompasses both motors and generators. For the sake of simplicity, terms referring to the motor, such as motor current or motor speed, are understood to refer generally to the electric machine and not limited to motor operation.

[0005] The characteristics of a drive with regard to torque and speed are usually specified in a speed / torque characteristic map. This shows the maximum permissible torque versus speed. In addition, permissible values ​​for different operating modes are often also specified. The permissible operating range under different boundary conditions of the electric machine is therefore typically specified by the speed / torque characteristic map.

[0006] A thermal engine model ensures that engine load does not lead to an unacceptably high engine temperature. Temperature sensors in the engine can be used for this purpose, for example.

[0007] A drive arrangement for a portable working device is known from EP 1 935 228 A1. This drive arrangement proposes a drive arrangement for a hand-held working device for landscaping, forestry, and horticulture, comprising an electric motor, a storage device for electrical energy, and a control device for the electric motor fed from the energy storage device. The electric motor has a specific, in particular linear, characteristic curve between the electrical current consumed by the electric motor and the rotational speed of an output shaft of the electric motor. The control device has current-limiting means that limit the current flowing through the electric motor to a maximum value in order to protect the electric motor from overload.The control device limits the current consumption of the electric motor in a range of low speeds, depending on the speed, to values ​​that are significantly lower than the characteristic values ​​resulting from the characteristic curve of the electric motor for these speeds.

[0008] WO 2013 / 098102 A2 discloses a method for operating an electric machine and an electric machine. This disclosure relates to a method for operating an electric machine, in particular a motor vehicle, wherein the electric machine is operated in an operating range limited by the speed and / or torque of the electric machine. It is provided that the operating range is limited to an operating range optimized for the electric machine, and that operating points outside the optimized operating range are approached for only a limited period of time.

[0009] EP 2 438 808 A1 discloses a power tool with an electric drive motor. This disclosure relates to a hand-held electric chainsaw whose electric drive motor has a motor-specific characteristic curve of the consumed motor current versus the motor speed. A control unit regulates the current flowing through the electric drive motor below an engagement speed to values ​​that are below the motor-specific characteristic curve.In order to create an operating point perceptible to the user in a working range of the motor chain saw in a predetermined speed range, it is provided to specify a control characteristic curve of the electrical input power of the electric drive motor over the speed above the engagement speed, according to which the input power in the predetermined speed range is reduced to an approximately equal average power in such a way that in the predetermined speed range the torque of the drive motor increases as the speed decreases.

[0010] Systems and methods for estimating the stator winding temperature of electric motors are known from US 2011 / 279074 A1. The electric motor system comprises an electric motor including a stator with windings and a rotor configured to operate at motor speed; a cooling system including coolant configured to cool the rotor and the stator, the coolant having a coolant flow rate and a coolant temperature; an inverter module coupled to the electric motor and configured to supply current to the windings based on inverter control signals; a current-controlled torque controller coupled to the inverter module and configured to generate the inverter control signals in response to a reduced torque command.and a temperature estimation controller coupled to the current-controlled torque controller and configured to generate the reduced torque command based on an initial torque command and an estimated stator winding temperature. The temperature estimation controller is configured to estimate the estimated stator winding temperature based on the engine speed and the coolant flow rate.

[0011] The invention is based on the object of improving the protection of an electrical machine against thermal overload.

[0012] This object is achieved by a method having the features of claim 1. Furthermore, this object is achieved by a power converter with a control device configured to control an electrical machine using such a method. The object is further achieved by an electric drive comprising such a power converter and an electrical machine, in particular a permanent-magnet synchronous machine.

[0013] Further advantageous embodiments of the invention are mentioned in the dependent claims.

[0014] The invention is based, among other things, on the finding that protection against thermal overload is ensured in a particularly simple manner if a first maximum permissible torque is taken from the speed / torque characteristic field for standstill, i.e. speed n=0, and a second maximum permissible torque is taken for the nominal speed nn. The relationship M = k T ⋅ I A first current limit value I 1 is determined for the first permissible torque value and a second current limit value I 2 is determined for the second permissible torque value. The current of the electrical machine is limited to these values ​​for standstill and for the rated speed nn. For the values ​​between standstill and rated speed, the limit value is linearly interpolated as a function of the motor speed. In other words, a straight line is formed in the speed / torque characteristic curve that runs through the first and second current limit values. The values ​​of this straight line represent the current limit value for the corresponding motor speed between standstill and rated speed. Mathematically, the current limit value I GW is determined by linear interpolation as a function of the motor speed n as I GW , 1 = I 1 − n n n I 1 − I 2 .

[0015] It has been shown that the respective current limit values ​​can be easily calculated in real time, depending on the speed, using linear interpolation, or by forming a straight line in the characteristic curve. This technical solution is therefore particularly easy to implement and offers reliable protection.

[0016] Furthermore, the use of temperature sensors can be dispensed with. Temperature sensors mounted in the motor are subject to high mechanical and thermal stress. As a result, they are often subject to a high failure rate and / or are expensive. Replacing defective sensors is complex and therefore not only expensive, but also associated with significant downtime for the electric motor.

[0017] The proposed method also works without temperature sensors. Optionally, the proposed method can also be implemented with one or more temperature sensors. The electrical machine is therefore temperature-sensor-free. The electrical machine can be reliably protected while simultaneously increasing machine availability.

[0018] The particular advantage of the invention lies in the simple and cost-effective implementation of the thermal overload protection function. This monitoring is not only resource-efficient in terms of the required computing power, but also provides reliable and robust protection for the electrical machine against thermal overload.

[0019] The motor current is limited to a value on a second straight line as a function of the motor speed for values ​​above the rated speed, whereby the second straight line runs through the second current limit as a function of the motor speed and assumes the value zero at the maximum speed of the electrical machine. This corresponds to a linear interpolation of the limitation between the rated speed nn and the maximum speed n max . It has been shown that the thermal behavior of the electrical machine changes above the rated speed. Therefore, it has proven advantageous to use a second straight line for thermal protection in this speed range, the gradient of which differs from the first straight line. The current limit, which then results from this second straight line, is determined by linear interpolation as I GW , 2 = I 2 ⋅ n max − n n max − n n .

[0020] The thermal protection is significantly better due to the use of the second straight line and reliably protects the electrical machine from overload.

[0021] In an advantageous embodiment of the invention, the first current limit and / or the second current limit depend on the permissible temperature increase in the electrical machine. Depending on the permissible temperature increase, different first and second current limits can be specified. If, for example, the materials used inside the electrical machine allow a greater temperature increase, the first and second current limits can also assume higher values ​​without thermally overloading the electrical machine. Likewise, the ambient conditions at the site of use, such as the ambient temperature, determine the permissible temperature increase. Alternatively, the current limit can therefore be specified not only as a function of the permissible temperature increase, but also as a function of the ambient temperature.Since the influence of the ambient temperature has only a slow effect on the service life, it has proven advantageous to specify the current limit value as a function of an averaged ambient temperature, in particular as a function of an ambient temperature averaged over one hour.

[0022] In a further advantageous embodiment of the invention, the first current limit value and / or the second current limit value depend on the operating mode of the electrical machine. The operating mode of the electrical machine is defined in IEC 60034-1 or EN 60034-1, as well as in other national standards. The operating mode, designated, for example, with an abbreviation S1..S10, has a significant influence on the load and thus the heating of the electrical machine. Especially in the case where the electrical machine is designed without a temperature sensor, the electrical machine can be utilized much more effectively by taking the operating mode into account. Due to the known load profile, temperature fluctuations can be predetermined, for example using a temperature model of the motor. At the same time, the current limitation reliably prevents unacceptably high heating of the electrical machine, even for the current load case.

[0023] In a further advantageous embodiment of the invention, the second current limit value is specified for a nominal speed of 6000 rpm. A nominal speed of 6000 rpm has proven advantageous for a large number of applications. In addition, the cooling of the electric machine up to such a speed can be achieved in a simple manner, for example by means of a fan on the shaft of the electric machine. The resulting first straight line therefore has only a slight gradient. The second current limit value assumes a value in the range 60..70% of the first current limit value. Only above this value does the current limit value, defined by the second straight line, drop sharply. The maximum speed in this case is, for example, 10000 rpm or even lower.

[0024] In a further advantageous embodiment of the invention, the electrical machine is operated in continuous operation mode S1. During continuous operation, heating is significantly greater than during short-term operation, intermittent operation, or periodic operation. It has been shown that, for design purposes in continuous operation, performance is primarily determined by thermal behavior. This makes reliable protection against thermal overload all the more important. During short-term operation, intermittent operation, or even periodic operation, thermal behavior plays a much lesser role. Performance is then determined more by the maximum currents occurring, i.e., the electrical operating variables.

[0025] During continuous operation, the electric machine reaches a higher temperature than in the other operating modes. Therefore, the first and second current limits are also lower than in the other operating modes. This reliable method for protecting the electric machine offers the advantage of being able to utilize the electric machine more effectively in terms of current and torque. This allows the electric machine to be operated both economically and safely.

[0026] In a further advantageous embodiment of the invention, the electrical machine is operated at a permissible temperature increase of 100K. The materials used and the ambient conditions of the electrical machine make operation of the electrical machine at a temperature increase of 100K particularly economical. It has been shown that this temperature increase can be maintained in a particularly simple manner using the proposed method. Even if inaccuracies in the method result in only a partially accurate reduction in the permissible current, it has been shown that these inaccuracies do not impair the service life of the electrical machine, especially at the permissible temperature increase of 100K. Therefore, the use of the proposed method is particularly advantageous for electrical machines with a permissible temperature increase of 100K.

[0027] In a further advantageous embodiment of the invention, the electrical machine is designed as a permanent magnet synchronous machine or a reluctance machine. It has been shown that temperature monitoring is particularly advantageous for synchronous machines and reluctance machines.

[0028] The invention is described and explained in more detail below with reference to the exemplary embodiments shown in the figures. They show: FIG 1 an electric drive FIG 2 a speed / torque characteristic and FIG 3 a characteristic with current limit values

[0029] The FIG 1 shows an electric drive 3. This has a power converter 2 that supplies an electric machine 1 with electrical energy. A control device 4 is used to control, monitor, and protect the electric machine 1, for example, against thermal overload.

[0030] The FIG 2shows a typical torque / speed characteristic of a converter-fed electrical machine 1. To avoid repetition, reference is made to the description of FIG 1 and the reference symbols introduced there. The permissible torque M is specified there as a function of the speed n of the electrical machine 1. The maximum torque M max is the maximum permissible torque on the shaft of the electrical machine 1. It typically results from mechanical and / or electrical limitations in the design of the electrical machine 1.

[0031] Below are values ​​for permissible torques for different operating modes of the electrical machine 1. This is, for example, continuous operation with a 100K temperature increase. This operating mode is then designated S1 (100K). In general, a permissible torque value can be specified for each operating mode of the electrical machine 1 as a function of the speed. For this purpose, FIG 1 generally a curve denoted by S. For the different operating modes, sets of permissible torques then result.

[0032] The FIG 3 shows a diagram that indicates the permissible current as a function of the speed. To avoid repetition, please refer to the description of the Figures 1 and 2 , as well as the reference symbols introduced there. For current and torque, the linear relationship applies as a first approximation M = k T ⋅ I .

[0033] In this way, permissible currents and permissible torques can be easily converted into one another with sufficient accuracy. Using the example of continuous operation S1 at a temperature increase of 100K, designated as S1 (100K), the first current limit value I1 is given when the electrical machine 1 is at a standstill and the second current limit value I2 is at a rated speed nn of 6000 rpm. The intermediate limit values ​​in the speed range between standstill and rated speed are linearly interpolated. In other words, these limit values ​​form a first straight line 11. It can be seen that the current limit line shown in the characteristic curve roughly corresponds to a straight line. This shows that protection against thermal overload can be implemented safely and reliably using the first straight line 11 and at the same time does not take into account the high design reserves of the electric drive 3. For the range above the rated speed, the current limit values ​​​​decline sharply.It has proven advantageous to describe this range of current limits by a second straight line 12, which runs through the second current limit I1 and, at the maximum speed nmax, through the point zero. Although the deviation from the characteristic curve is greater in this speed range, these points are reached less frequently during operation. Thus, the thermal effects, particularly on the service life of the electric machine 1, are negligible.

[0034] In summary, the invention relates to a method for controlling an electrical machine, wherein the motor current is limited as a function of the motor speed of the electrical machine to prevent thermal overload of the electrical machine. To improve protection against thermal overload, it is proposed that a first current limit value be specified for the standstill of the electrical machine and a second current limit value be specified for a rated speed of the electrical machine, wherein the motor current is limited as a function of the motor speed for values ​​between standstill and rated speed to a value of a first straight line, wherein the first straight line is formed as a function of the motor speed by the first current limit value and the second current limit value. Furthermore, the invention relates to a power converter with a control device, configured to control an electrical machine by means of such a method.The invention further relates to an electric drive comprising such a power converter and an electric machine, in particular a permanent magnet synchronous machine.

Claims

1. Method for regulating an electric machine (1), wherein to prevent a thermal overload of the electric machine (1) the motor current is limited in dependence upon the motor rotational speed (n) of the electric machine (1), wherein a first current limit value (I1) is predefined for the electric machine (1) at a standstill and a second current limit value (I2) is predefined for a rated rotational speed (nn) of the electric machine (1), wherein the motor current is limited to a value (IGW,1) of a first straight line (11) in dependence upon the motor rotational speed (n) for values between standstill and rated rotational speed (nn), wherein the first straight line (11) is formed as a function of the motor rotational speed (n) by the first current limit value (I1) and the second current limit value (I2), wherein the motor current is limited to a value (IGW,2) of a second straight line (12) in dependence upon the motor rotational speed (n) for values above the rated rotational speed (nn), wherein the second straight line (12) runs as a function of the motor rotational speed (n) by way of the second current limit value (I2) and assumes the value of zero at maximum rotational speed (nmax) of the electric machine (1).

2. Method according to claim 1, wherein the first current limit value (I1) and / or the second current limit value (I2) depend upon the permissible temperature increase in the electric machine (1).

3. Method according to one of claims 1 or 2, wherein the first current limit value (I1) and / or the second current limit value (I2) depend upon the mode of operation of the electric machine (1).

4. Method according to one of claims 1 to 3, wherein the second current limit value (I2) is predefined for a rated rotational speed (nn) of 6000 per minute.

5. Method according to one of claims 1 to 4, wherein the electric machine (1) is operated in the mode of operation S1 continuous operation.

6. Method according to claim 5, wherein the electric machine (1) is operated with a permissible temperature increase of 100K.

7. Method according to one of claims 1 to 6, wherein the electric machine (1) is embodied as a permanently excited synchronous machine or reluctance machine.

8. Current converter (2) with a regulating apparatus (4), configured for regulating an electric machine (1) by means of a method according to one of claims 1 to 7.

9. Electric drive (3) having a current converter (2) according to claim 8 and an electric machine (1), in particular a permanently excited synchronous machine.