Patent Publication Number: US-2023145914-A1

Title: Electric motor device and method for controlling a motor braking procedure for an electric motor

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is the U.S. National Phase of PCT Appln. No. PCT/DE2021/100111 filed Feb. 5, 2021, which claims priority to DE 102020105950.7 filed Mar. 5, 2020, the entire disclosures of which are incorporated by reference herein. 
    
    
     BACKGROUND 
     DE 10 2017 111 108 A 1 describes a method for braking a vehicle in the event of a transmission failure. It describes that the braking procedure is carried out via a wheel brake if a transmission fault occurs. 
     The vehicle is at least in a safe state when it is no longer in motion. Apart from the possibility of using the vehicle brakes arranged on the vehicle wheels to brake the vehicle or also in the event of a malfunction of the vehicle brakes, a braking procedure can take place via an electric motor acting as a drive element of the vehicle. The electric motor has a rotor able to rotate at a motor speed and is electrically controlled via motor connections through an inverter circuit that has a plurality of switching elements. 
     A motor braking procedure can be initiated by adopting a braking switch position of the inverter circuit, in which at least two motor connections are short-circuited. The resulting braking torque is dependent on the motor speed of the rotor. At lower motor speeds, an undesirable increase in motor braking torque can occur, which can induce excessive braking on the vehicle, which in turn can affect the safety of the vehicle. 
     SUMMARY 
     The disclosure, according to an exemplary embodiment, improves motor braking. A motor braking procedure can be controlled to allow the braking torque to be applied more consistently at lower motor speeds. The vehicle can be braked in a more stable and smoother manner. The braking of the vehicle can be carried out more safely. The vehicle can be prevented from swerving due to motor braking. 
     The electric motor can be arranged in an electric vehicle. The electric motor can be non-rotatably connected to a vehicle wheel. A transmission can be effectively arranged between the electric motor and the vehicle wheel. 
     The electric motor can have several motor phases. The motor phases can each be connected to an inverter circuit via motor connections. The electric motor can be a brushless direct current motor. The electric motor can have three motor phases. 
     The inverter circuit can be a half-bridge circuit. The half-bridge circuit can be a multiple half-bridge circuit, in particular a triple half-bridge circuit. 
     A limit speed can be less than or equal to 3000 rpm. 
     During the motor braking procedure, braking parameters, in particular an alternate switching frequency, a time period of the individual braking switch position intervals and/or a time period of the individual open switch position intervals, can be specified as a function of a motor speed and can be called up during operation of the vehicle. These braking parameters can also be controlled in an adapted manner depending on acceleration information and/or regulated to regulate the braking torque. 
     Motor braking can be initiated during emergency braking. 
     In an embodiment of the disclosure, the time period of the individual braking switch position intervals is set depending on the motor speed. Below the limit speed, the time period of the individual braking switch position intervals can be proportional to the motor speed. 
     In an embodiment of the disclosure, the time period of the individual open switch position intervals is set depending on the motor speed. Below the limit speed, the time period of the individual open switch position intervals can be proportional to the motor speed. 
     In an embodiment of the disclosure, the time period of at least one braking switch position interval differs from the time period of at least one subsequent open switch position interval. 
     In an embodiment of the disclosure, at least two motor connections are electrically short-circuited in the braking switch position. In the braking switch position, at least two switching elements assigned to the same voltage potential of the supply voltage can be closed. All switching elements assigned to the same voltage potential of the supply voltage can also be closed. 
     In an embodiment of the disclosure, the majority of the switching elements in the open switch position are in a blocking position. In the blocking position, power transmission is interrupted. 
     In an embodiment of the disclosure, all switching elements are in the blocking position in the open switch position. 
     In an embodiment of the disclosure, during motor braking at motor speeds above the limit speed, the braking switch position is adopted without interruption. 
     In an embodiment of the disclosure, the limit speed corresponds to a motor speed at which the voltage induced by the rotor due to the open switch position at the switching elements is below a limit voltage. As a result, the voltage load on the switching elements can be reduced in the open switch position. 
     Furthermore, an electric motor device is proposed for providing a drive torque for a vehicle, having an electric motor having a rotatable rotor, which is subject to a braking torque during an electric motor braking procedure and can be rotated at a motor speed, and an inverter circuit for controlling the motor braking procedure through a method with at least one of the features described above. 
     Further advantages and advantageous embodiments of the disclosure are apparent from the description of the figures as well as the figures themselves. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure is described in detail below with reference to the figures. In the figures: 
         FIG.  1   : shows an electric motor device in an exemplary embodiment of the disclosure. 
         FIG.  2   : shows a braking torque curve in the embodiment of a method in an exemplary embodiment of the disclosure. 
         FIG.  3   : shows detail A from  FIG.  2    in an enlarged view. 
     
    
    
     DETAILED DESCRIPTION 
       FIG.  1    shows an electric motor device  10  in an exemplary embodiment of the disclosure. The electric motor device  10  is arranged in a vehicle to provide a drive torque and has an electric motor  12  having a rotor  14  able to rotate at a motor speed. The electric motor  12  can output a drive torque for driving the vehicle via a transmission  16  to a vehicle wheel  18  of the vehicle. The rotor  14  can be connected to the vehicle wheel  18  in a rotationally fixed manner via the transmission  16 . 
     The electric motor  12  can be a brushless DC motor that is controllable by an inverter circuit  20 . The electric motor  12  can have three motor phases Mp. The respective motor phase Mp is connected to the inverter circuit  20  via a motor connection Ma. The inverter circuit  20  is fed by a supply voltage Ub, which is preferably a DC voltage. The supply voltage Ub can be provided by a vehicle battery. The vehicle can preferably be an electric vehicle. 
     The inverter circuit  20  is in particular a multiple half-bridge circuit, here a triple half-bridge circuit for driving the three motor phases Mp. The inverter circuit  20  has six switching elements S 1 -S 6 . An electrical motor braking procedure for triggering a braking torque by rotor  14  is preferably set by the inverter circuit  20  adopting a braking switch position in which at least two of the motor connections Ma are short-circuited via the inverter circuit  20 . In particular, all three motor phases Mp can be short-circuited by the three switching elements SI-S 3  being switched on or the other three switching elements S 4 -S 6  being switched on. 
     When the inverter circuit  20  is in an open switch position, the braking torque caused by the rotor  14  is less than in the braking switch position or no braking torque is present at all. The open switch position can be adopted when all switching elements S 1 -S 6  are switched to a blocking position. However, a rotational movement of the rotor  14  can then produce an induced voltage at the switching elements S 1 -S 6 , which loads the switching elements S 1 -S 6  and is speed dependent. The induced voltage is proportional to the motor speed. 
       FIG.  2    shows a braking torque curve in the embodiment of a method in an exemplary embodiment of the disclosure. The motor braking procedure brought about by a braking switch position of the inverter circuit adopted entirely without interruption via the motor speed n enacts a speed-dependent braking torque Nb on the rotor. The braking torque Nb is inversely proportional to the motor speed n. At low motor speeds, the braking torque Nb can therefore have high values. 
     Assuming that the electric motor is to be braked by the electric motor braking procedure starting from a high motor speed n, then the braking torque Nb will change only slightly up to a limit speed ng of 3000 rpm here and increase from the limit speed ng with increasingly lower motor speeds n. As a result, the vehicle is braked more strongly at lower motor speeds n and can even swerve due to the excessive braking torque Nb, depending on the road conditions. 
     For this reason, during the motor braking procedure at motor speeds n below the limit speed ng, the braking switch position is adopted by alternate switching S in braking switch position intervals alternating with open switch position intervals of the open switch position. The resulting braking torque curve Nb 1  is thus adjusted on average to the braking torque curve Nb 0  at motor speeds n above the limit speed ng. As a result, the braking torque Nb can be applied more consistently over the motor speeds n and a more consistent average braking torque Nbm can be achieved. The vehicle can be braked in a more stable and smoother manner, and the braking procedure of the vehicle can be carried out more safely. The vehicle can be prevented from swerving due to motor braking. 
       FIG.  3    shows detail A from  FIG.  2    in an enlarged view. The alternate switchings S between the braking switch position Sb and the open switch position Sa, which are dependent on the motor speed n, are also shown. Starting with a rotor rotating at a motor speed n above the limit speed ng and the motor braking procedure initiated by the braking switch position Sb of the inverter circuit being adopted without interruption up to the limit speed ng, the braking switch position Sb is replaced by the open switch position Sa when the limit speed ng is reached. The open switch position Sa is maintained over a first time period ta 1 . During this time, the braking torque Nb is reduced. 
     After the first time period ta 1  has elapsed, the open switch position Sa is replaced and the braking switch position Sb is adopted for a first time period tb 1 . Then, the open switch position Sa alternates with the braking switch position Sb again for a second time period ta 2 . After the second time period ta 2  has elapsed, the braking switch position Sb is adopted again for a second time period tb 2 . These alternate switchings S take place in particular until the rotor no longer rotates. 
     The first time period ta 1  is greater than the subsequent second time period ta 2  of the respective open switch positions Sa, since the braking torque Nb, starting from the motor speed n that has reached the limit speed ng, is to be reduced more than in the further course of the alternate switching S. The respective time period of the open switch position and the braking switch position, as well as the alternate switching frequency, for example 14 alternate switchings S are available here, can be selected from the required curve for the braking torque Nb and, for example, adapted to ambient conditions. 
     List of reference symbols 
       10  Electric motor device 
       12  Electric motor 
       14  Rotor 
       16  Transmission 
       18  Vehicle wheel 
       20  Inverter circuit 
     Ma Motor connection 
     Mp Motor phase 
     n Motor speed 
     ng Limit speed 
     Nb Braking torque 
     Nb 0  Braking torque curve 
     Nb 1  Braking torque curve 
     Nm Average braking torque 
     S Alternate switching 
     S 1 -S 6  Switching element 
     S a Open switch position 
     Sb Braking switch position 
     ta 1  First time period 
     ta 2  Second time period 
     tb 1  First time period 
     tb 2  Second time period 
     Ub Supply voltage