CROSS REFERENCE TO RELATED APPLICATION

This application claims priority under 35 U.S.C. § 119 from German Patent Application No. DE 10 2020 123 957.2, filed Sep. 15, 2020, the entire disclosure of which is herein expressly incorporated by reference.

BACKGROUND AND SUMMARY OF THE INVENTION

The invention relates to a method for identifying a fault in a rotor of an electric motor and to a controller for an electric motor.

Electric motors are used, for example, in vehicles for the drive in order to convert electrical energy into propulsion. For this purpose a rotating magnetic field is generated at a stator of the electric motor, with the result that the magnetic rotor of the electric motor rotates in the magnetic field of the stator. The torque provided by the electric motor and the power of the motor are in this case dependent on the rotational speed of the rotor and thus on the magnetic field that is applied to the stator.

Field-oriented control processes for actuating electric motors are known from the prior art.

In the case of separately excited electric motors, that is to say in electric motors that have an electromagnet such as a coil through which current flows as rotor, electrical faults can arise at the rotor, for example a shorted coil. These electrical faults influence the torque generated by the electric motor.

Furthermore, the magnetization of the permanent magnets in the case of permanently excited electric motors can reduce over time.

Such faults currently cannot be identified or can be identified only with a great degree of outlay.

It is therefore the object of the invention to provide a method and a controller that identify the occurrence of such a fault in a simple manner.

The object is achieved by a method for identifying a fault in a rotor of an electric motor, which comprises a stator, by the following acts:

a) determining a characteristic variable of the rotor and an electrical characteristic variable of the stator,

b) measuring a current value of the electric motor,

c) calculating an initial setpoint voltage value for the stator based on a setpoint current value, the measured current value and the determined characteristic variables of the rotor and the stator,

d) checking, based on the calculated initial setpoint voltage value, whether there is a fault, in particular a shorted coil, at the rotor, and

e) operating the electric motor based on the setpoint voltage value by way of the controller if no fault has been identified in act d), or

f) providing a fault message by way of the controller and/or disconnecting the electric motor by way of the controller if a fault has been identified in act d).

The invention is based on the basic idea of being able to identify a fault at the rotor on account of the electromagnetic coupling between the rotor and the stator by virtue of the voltage values of the stator being evaluated. For this, the controller calculates an initial setpoint voltage value at the stator and adjusts this setpoint voltage value by way of corresponding current control at the stator. If the controller has to readjust the setpoint voltage value to a very great extent, that is to say above a threshold value, the controller identifies that there is a fault at the rotor and provides a corresponding fault message and/or disconnects the electric motor. A simple option for identifying a fault at the rotor is provided in this way.

In this case, the controller identifies, in particular, an electrical fault in a separately excited electric motor, for example a shorted coil in the rotor of a synchronous machine or a breakage of a rod of a squirrel-cage rotor of an asynchronous machine.

As an alternative, the controller can identify a magnetic fault in a permanently excited electric motor, for example a decreasing magnetization of the magnets of the rotor.

Determining a characteristic variable is understood according to embodiments of the invention to mean that the characteristic variable is read out from a memory, calculated or measured and provided to the controller.

The characteristic variable of the rotor can be an electrical or a magnetic characteristic variable. The controller can determine the initial setpoint voltage value based on a mathematical model. This makes it possible to determine the setpoint voltage value precisely.

One aspect of the invention makes provision for the controller to identify the fault based on an increased voltage control value compared with the fault-free operation of the electric motor. This makes it possible to determine the electrical fault reliably.

The controller can verify or falsify a presence of the fault by changing the initial setpoint voltage value. This prevents the controller from disconnecting the electric motor and/or from providing a fault message even though there is no fault at the rotor.

In this case, the change of the initial setpoint voltage value if the controller falsifies the electrical fault can lead to a modified setpoint voltage value, based on which the electric motor is actuated.

For example, the controller increases or decreases the initial setpoint voltage value and thereby obtains the modified setpoint voltage value.

In one configuration of the invention, the controller determines in a rotor-fixed coordinate system two setpoint voltage values for the stator and, based on an increased voltage control value for one of the two setpoint voltage values, verifies the fault. Therefore, no additional sensor system or diagnostics system is necessary to check for the presence of the fault.

It is conceivable that the controller verifies the presence of the fault of the rotor if one of the two voltage control values still has an increased value due to the change of the setpoint voltage values.

Furthermore, the presence of the fault can be falsified if both voltage control values have an increased value due to the change of the setpoint voltage values. In this way, the controller can distinguish between a fault of the coordinate system (incorrect position of the rotor) and an actual electrical fault at the rotor.

A further aspect of the invention makes provision for the controller to provide the fault message and/or disconnect the electric motor only in the case of a verified fault. Confusion over the presence of an electrical fault is therefore prevented by the controller.

As an alternative or in addition, provision can be made for the controller to operate the electric motor using the calculated setpoint voltage value if the identified fault has been falsified by the controller. This prevents unnecessary disconnection of the electric motor.

The controller can determine at least an inductance and/or a resistance as electrical characteristic variable of the rotor and the stator. In this way, the controller can determine the setpoint voltage values based on a mathematical model.

In general, it is also conceivable for the controller to determine a magnetic energy density of the rotor.

In one configuration of the invention, the electric motor is a separately excited three-phase machine, in particular a separately excited three-phase synchronous machine, which is controlled by the controller. In this case, the controller measures in particular at least four current values, that is to say three current values of the coils of the stator and one current value of the coil of the rotor.

As an alternative, the electric motor can also be a permanently excited three-phase synchronous machine or a three-phase asynchronous machine.

In order to provide a specific torque at the electric motor, the controller can additionally determine the setpoint voltage value based on a torque demand and/or a measured angular velocity of the rotor.

The object of the invention is furthermore achieved by a controller for an electric motor, wherein the controller is designed to execute a previously described method. For the features and advantages of the controller, reference is made to the above statements regarding the method.

It is conceivable that the controller and the electric motor are part of a vehicle.

For example, the vehicle is an electric vehicle, with the result that the electric motor actuated by the controller is the only drive motor of the vehicle.

As an alternative, the vehicle can also be a hybrid electric vehicle, according to which the propulsion of the vehicle is generated by the electric motor and an additional internal combustion engine.

Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.

