DE102019130334A1 - Temperature-dependent derating of a PSM - Google Patents

Abstract

The present invention relates to a method and a device for protecting the permanent magnets of a permanent magnet synchronous machine (PSM) from demagnetization.

Description

The present invention relates to a method and a device for protecting the permanent magnets of a permanent magnet synchronous machine (PSM) from demagnetization.

In contrast to industrial drives, electrical machines in motor vehicles are not only operated at the nominal point but in a wide operating range in which speed, torque and power vary. In order to be able to offer better driving performance for a short time, e.g. for 10 seconds, the electrical machine is operated in overload, i.e. more torque is provided than is possible in continuous operation. In order not to damage the components of the electrical machine during this operation in overload, for example through excessively high temperatures in the stator winding, so-called derating functions are used in the control of the machine. The torques and powers permitted by the machine control are limited as a function of the measured or calculated temperature of the stator winding. Limiting the power leads to a reduction in the power loss and thus to a decrease in heating or even a cooling of the machine.

In the case of permanent magnet synchronous machines (PSM), the magnets in the rotor in particular must be protected from excessively high temperatures, as these can be demagnetized in combination with high opposing fields, which permanently reduces the performance of the e-machine. When designing permanent magnet synchronous machines, it is therefore important to ensure that the permanent magnets used have a sufficiently high resistance to demagnetization. The magnitude of the coercive field strength Hcj of the magnetic material must therefore be greater than the field strength of the opposing fields to be expected at the expected maximum operating temperature. This applies to both regular operation and in the event of an error.

In the event of a fault, the active short circuit (AKS) is selected as the safe state for drives with permanent magnet synchronous machines, especially at higher speeds. Since the phases of the electric machine are short-circuited via the power semiconductors in the power electronics, no high voltages can occur. In this case, however, a short-circuit current flows, which can briefly assume very high values (1.5 to 3 times the maximum current) when switching from regular operation to the AKS. This high current briefly generates a high opposing field in the PSM, which in turn can demagnetize the magnets. The amplitude of this transient short-circuit current and thus the opposing field in the machine depend on the operating state in which the machine was operated before the AKS. The highest coercive field strengths are required for motor and generator operation in the area of the maximum torque in terms of amount at a simultaneously high speed, the so-called corner point.

According to the current state of the art, the magnet selection is based on the combination of the maximum expected component temperature and the occurrence of a fault with switching of the AKS in the worst case operating point, i.e. the generator corner point.

Many magnetic materials, especially NdFeB magnets, such as those used in PSMs for traction applications, are demagnetized through the combination of high temperature and high opposing field. This results in a high requirement for the coercive field strength Hcj for the magnetic material. This requirement is classically achieved by adding the heavy rare earths dysprosium and terbium. This makes the magnet expensive.

From the DE 10 2015 218 302 A1 an electrical machine with improved short-circuit strength is known. The electrical machine is designed as a permanently excited synchronous machine for vehicles which have permanent magnets which are designed in such a way that the risk of demagnetization in the event of a short circuit is reduced. In particular, split magnets with different magnetic properties are used.

The DE 10 2017 220 685 A1 discloses a method and a device for operating an electrical machine for outputting a predetermined torque and a predetermined speed. The method is used to measure a temperature of the electrical machine and, based on this, trigger an active short circuit when a temperature threshold value is exceeded. When the temperature threshold value is exceeded, the magnets of the electrical machine are prevented from being demagnetized by reducing the maximum torque.

The WO 01/10 001 A2 describes a demagnetization-proof, permanently excited ship propulsion system. The ship's propulsion system comprises a permanently excited electric motor, the temperature and currents of which are measured, the electric motor being protected from overcurrents and excess temperatures and demagnetization being prevented in the event of a short circuit by limiting the short circuit current.

Against this background, the invention has the object of providing a method and a device with which the demagnetization of permanent magnets of a PSM can be effectively prevented, even if the magnets have a low coercive field strength.

The object is achieved according to the invention by a method with the features of claim 1 and a device with the features of claim 6. Refinements and developments of the invention emerge from the dependent claims and the description.

According to the invention, the maximum permissible torque of the PSM is limited as a function of the rotor temperature from a defined speed from which an AKS would result in demagnetization. There is therefore a rotor temperature-dependent derating to protect against demagnetization in the AKS error state.

The invention relates to a method for protecting permanent magnets of a permanent-magnet synchronous machine (PSM) in a vehicle from demagnetization, in which, when a predetermined rotor temperature of the PSM is exceeded, operation of the PSM is suppressed in areas of the speed / torque map of the PSM in which the The field strength of an opposing field generated by an active short circuit of the PSM would exceed the coercive field strength of the permanent magnets.

If the rotor temperature reaches an application-specific limit temperature below the maximum permissible component temperature, operating points with a high required coercive field strength in the AKS case are avoided by means of derating. As a result, in the event of an error that requires the AKS, only smaller opposing fields can occur than without derating. In one embodiment of the method, the limit temperature is 5 K to 25 K lower than the maximum permissible component temperature, for example 20 K lower, or also 10 K lower than the maximum permissible component temperature. In one embodiment, the maximum permissible component temperature has a value in the range from 60 ° C to 220 ° C, for example a value in the range from 80 ° C to 200 ° C, or in the range from 100 ° C to 180 ° C.

In one embodiment of the method, operation of the PSM is suppressed in areas of the speed / torque map of the PSM in which the torque is more than 70% of the maximum torque of the PSM and the speed is more than 20% of the maximum speed of the PSM . The derating does not take place by reducing the stator flux, but by regulating the speed and / or torque, which avoids critical areas of the speed / torque map of the PSM.

Operation at the corner of the speed / torque map, i.e. with maximum torque and maximum power, is only required for dynamic driving maneuvers (e.g. acceleration from 0 to 100 km / h in a minimum of time, torque vectoring for dynamic cornering).

In one embodiment of the method, the areas of the speed / torque characteristic field of the PSM to be suppressed are defined on the basis of stored tables with maximum permissible torque as a function of the speed and the rotor temperature; i.e. the derating takes place on the basis of stored tables in which the maximum permissible torque is defined depending on the speed and the rotor temperature.

In one embodiment of the method, the temperature of the rotor of the PSM is determined by means of a rotor temperature model. The temperature of the rotor or the permanent magnets of the rotor is calculated using a rotor temperature model. As soon as a certain limit temperature is exceeded, a derating function is activated, which suppresses certain operating ranges in the speed / torque map of the PSM. Suitable rotor temperature models are known in principle to the person skilled in the art, for example from US Pat DE 10 2015 114 029 A1 .

In a further embodiment of the method, the temperature of the rotor of the PSM is determined by means of a temperature of the stator of the PSM. In one embodiment, the stator temperature is measured via a temperature sensor on the stator.

The invention also relates to a device set up for carrying out the method according to the invention. to protect permanent magnets of a permanent magnet synchronous machine (PSM) in a vehicle from demagnetization.

The device according to the invention comprises means for determining a rotor temperature of the PSM, as well as a control unit set up to regulate the speed and the torque of the PSM, which is set up to regulate a torque and a speed of the PSM so that areas of the Speed / torque map of the PSM can be avoided in which the field strength of an opposing field generated by an active short circuit of the PSM would exceed the coercive field strength of the permanent magnets.

In one embodiment of the device, the control unit is set up to regulate a torque and a speed of the PSM when the specified rotor temperature is exceeded so that the torque is not more than 70% of the maximum torque of the PSM and the speed is not more than 20% at the same time. is the maximum speed of the PSM.

Another embodiment of the device additionally comprises a memory unit with at least one table in which a maximum permissible torque of the PSM is defined as a function of the speed and the rotor temperature of the PSM.

In one embodiment of the device, the means for determining a rotor temperature of the PSM comprise a rotor temperature model.

Another embodiment of the device additionally comprises means for determining a stator temperature of the PSM. In one embodiment, the means comprise at least one temperature sensor.

In one embodiment, the permanent magnets of the PSM contain neodymium, iron and boron, and optionally dysprosium. For NdFeB magnets, the maximum permissible operating temperature is no more than 120 ° C, in the presence of dysprosium no more than 200 ° C.

One of the advantages of the method according to the invention is that the probability of failure of the PSM is reduced by the rotor temperature-dependent derating. The maximum permissible rotor temperature and thus the maximum continuous output of the drive can also be increased by means of a rotor temperature-dependent derating. The robustness of the drive is increased without having to accept any losses in peak or long-term performance. Depending on the use, the maximum permissible component temperature in the life of the vehicle occurs only very rarely, and never for many drivers. Errors that require the switching of an AKS also occur very rarely in the life of a vehicle.

When developing new permanent magnet synchronous machines, magnet material with a lower required coercive field strength and thus more cost-effective material can be selected.

The drive's performance is only limited in rare cases. The vehicle user is already familiar with this behavior, e.g. from the performance characteristics of the drive when the battery is cold or repeated maximum acceleration.

Further advantages and embodiments of the invention emerge from the description and the accompanying drawing.

It goes without saying that the features mentioned above and those yet to be explained below can be used not only in the respectively specified combination, but also in other combinations or on their own, without departing from the scope of the present invention.

• 1 ein Drehmoment-Drehzahl-Kennfeld mit eingezeichneten Koerzitivfeldstärken und Derating-Bereichen.

The invention is shown schematically on the basis of an embodiment in the drawing and is described further with reference to the drawing. It shows:

• 1 a torque-speed map with drawn coercive field strengths and derating areas.

1 For an exemplary case of a torque-speed characteristic field, shows which coercive field strength Hcj in [kA / m] the magnet material must have so that the magnet is not demagnetized in the event of an active short circuit (AKS).

As can be seen, the highest coercive field strengths are required in the motor and generator area of the maximum torque in terms of absolute value at a simultaneously high speed, the so-called corner point.

The derating areas shown in the figure 100 mark the area above 70% of the maximum torque at speeds above 20% of the maximum speed. According to the invention, these areas are avoided if a defined limit temperature of the rotor winding is exceeded. As a result, in the event of a fault that requires an active short circuit, only lower opposing fields can occur, which do not cause the permanent magnets to be demagnetized.

List of reference symbols

100

Derating area

QUOTES INCLUDED IN THE DESCRIPTION

This list of the documents listed by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent literature cited

• DE 102015218302 A1 [0007]
• DE 102017220685 A1 [0008]
• WO 01/10001 A2 [0009]
• DE 102015114029 A1 [0018]

Claims (10)

Method for protecting permanent magnets of a permanent-magnet synchronous machine (PSM) in a vehicle from demagnetization, in which, when a specified rotor temperature of the PSM is exceeded, operation of the PSM is suppressed in areas of the speed / torque map of the PSM in which the field strength of an active Short circuit of the PSM generated opposing field would exceed the coercive field strength of the permanent magnets. Procedure according to Claim 1 , wherein an operation of the PSM is suppressed in areas of the speed / torque map of the PSM in which the torque is more than 70% of the maximum torque of the PSM and the speed is more than 20% of the maximum speed of the PSM. Procedure according to Claim 1 or 2 , in which the areas of the speed / torque map of the PSM to be suppressed are defined on the basis of stored tables with maximum permissible torque depending on the speed and the rotor temperature. Method according to one of the Claims 1 to 3rd , wherein the temperature of the rotor of the PSM is determined by means of a rotor temperature model. Method according to one of the Claims 1 to 4th , wherein the temperature of the rotor of the PSM is determined by means of a temperature of the stator of the PSM Device for protecting permanent magnets of a permanent-magnet synchronous machine (PSM) in a vehicle from demagnetization, comprising means for determining a rotor temperature of the PSM, as well as a control unit set up to regulate the speed and torque of the PSM, which is set up when a predetermined rotor temperature is exceeded to regulate a torque and a speed of the PSM so that areas of the speed / torque map of the PSM are avoided in which the field strength of an opposing field generated by an active short circuit of the PSM would exceed the coercive field strength of the permanent magnets. Device according to Claim 6 , in which the control unit is set up to regulate a torque and a speed of the PSM when the specified rotor temperature is exceeded in such a way that the torque is not more than 70% of the maximum torque of the PSM and the speed is not more than 20% of the maxima at the same time -Speed of the PSM. Device according to Claim 6 or 7th , which additionally comprises a memory unit with at least one table in which a maximum permissible torque of the PSM is defined as a function of the speed and the rotor temperature of the PSM. Device according to one of the Claims 6 to 8th , in which the means for determining a rotor temperature of the PSM comprise a rotor temperature model. Device according to one of the Claims 6 to 9 , which also includes means for determining a stator temperature of the PSM.

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