Method for operating an electric motor, and electric motor

Abstract

The invention relates to a method for operating an electric motor (10) as at least one drive element of a vehicle, having the steps of: • providing an electric motor (10) which has at least one stator (40) with three stator phases and a rotor (42) that can rotate relative to said stator and has an excitation coil, • electrically operating the rotor (42) by energizing the excitation coil, and • electrically operating (44) the stator (40) via an inverter (46) with a three-phase phase current (16, 1, 2, 3) which corresponds to a d-current (Id) and a q-current (Iq) in the rotor-fixed d-q coordinate system (56) of the rotor (42).

Description

[0001] METHOD FOR OPERATING AN ELECTRIC MOTOR AND ELECTRIC MOTOR

[0002] The invention relates to a method having the features according to the preamble of claim 1. Furthermore, the invention relates to an electric motor.

[0003] The field of application of the invention is a drive system for an electric or hybrid vehicle with a battery, an inverter and an electric motor controlled by the inverter.

[0004] The electric motors in such drive systems heat up considerably during operation due to losses and therefore require active cooling. Oil cooling, for example, is used for this purpose; the heat energy emitted by the electric motor is dissipated via a cooling medium, such as oil. Electric or hybrid vehicles also have a battery, the temperature of which generally must be controlled in order to operate it within an efficient operating range. For example, battery performance can be significantly reduced at low ambient temperatures. Therefore, it is generally known to use the waste heat from the electric motor to heat the battery. However, this approach leads to a dilemma: If the operation of the electric motor is made more efficient, less waste heat is generated. However, as a result of such a desirable optimization, there may not be enough waste heat available to heat the battery.Therefore, it may be necessary to provide additional heating units to heat the battery even when using an efficient drive system, which entails additional effort and increases the vehicle weight.

[0005] Permanent magnet synchronous motors (PMSMs) are typically controlled with the goal of achieving maximum efficiency, reliability, and consistent performance by, among other things, influencing both the motor's magnetic field strength and its torque through targeted control with phase currents. However, PMSM control is also used to generate thermal energy, for example, to heat or warm vehicle components, particularly in cold environments. For this purpose, it is known to apply a d-current when the rotor is stationary, which generates thermal energy through copper losses, while the q-current is adjusted such that no torque acts on the stationary rotor. For example, DE 10 2021 003 611 A1 recognized that the isolated application of a d-current in a single polarity when the rotor is stationary can lead to uneven heating, which could cause potential damage or overheating in parts of the power electronics.Therefore, the d-current is applied with a harmonic triangular shape with alternating positive and negative current values ​​and is accompanied by a zeroing of the q-current when the rotor is at standstill to ensure uniform loading and heating of the high-side and low-side switches of the inverter.

[0006] The separately excited synchronous machine is enjoying increasing popularity because, unlike a permanent magnet synchronous machine, it is constructed without a permanent magnet in the rotor and thus does not require rare earth elements. This saves costs and reduces supply chain dependencies.

[0007] In battery-powered vehicles, battery performance is limited, especially in cold conditions. This potentially limited battery performance is particularly crucial for charging; only a battery within the optimal temperature range can be charged quickly. To condition the battery in cold conditions, it is usually heated using an auxiliary electric heater.

[0008] A concept developed by the applicant optimizes the thermal behavior of the entire vehicle over a driving cycle to improve cycle energy consumption. The engine's waste heat is used to transport heat to the battery via the cooling circuit.

[0009] In contrast to a permanent magnet synchronous machine (PMSM), in a separately excited synchronous machine, the permanent magnet in the rotor is replaced by a current-carrying coil. In the simplest case, the energy for the coil is transmitted via slip rings, but can also be transmitted contactlessly via so-called transmitters. As with permanent magnet systems, the primary idea behind energy harvesting is to heat the battery and, if necessary, the vehicle's interior in cold outside temperatures. Ideally, drive motors with improved oil cooling, known as slot cooling, are used for this purpose. The dissipated energy in the form of heat is used to heat the battery. This allows the battery to operate at a more favorable operating point.To quickly bring a cold engine and battery to a favorable operating point, several known concepts (Tesla: US 2018 083 509 A1, BMW: US 10 183 580 B2) attempt to generate additional waste heat in the traction motor and thus ultimately warm the battery. Compared to an auxiliary electric heater, for example, in the battery, this allows existing hardware to be utilized and, in certain cases, an improvement in overall efficiency can be achieved. By eliminating additional components, a weight advantage can be achieved in addition to the cost advantage.

[0010] The known methods generate significantly higher losses in the electric motor by selecting different target currents, without noticeably affecting the motor's torque. Applying constant additional currents (DCI - Direct Current Injection) is the simplest approach and can be applied efficiently to a rotating motor.

[0011] For the warm-up phase, there are so-called high-frequency methods that impose high-frequency currents. This can result in desired torque modulations, or if the excitation is selected so that the constant torque hyperbola is not exceeded, torque modulations can be largely avoided (Mercedes-Benz Group: DE 10 2021 003 611 A1, DE 10 2021 003 612 A1, DE 10 2021 003 621 A1). High-frequency methods are also capable of generating iron losses and eddy current losses and are therefore a complement or alternative to constant auxiliary currents.

[0012] The as yet unpublished German patent application DE 10 2023 116 682 proposes controlling the additional heat loss by appropriately controlling the motor with an inverter with multiple levels in the PWM output signal. These so-called multi-level inverters make it possible to significantly reduce losses in the electric motor and thus enable highly efficient systems. For energy harvesting, the multi-level inverter is operated in the energetically less favorable two-level mode.

[0013] In the as yet unpublished German patent application DE 10 2023 125 487, it is proposed for a permanent magnet synchronous motor (PMSM) to impose a d-current on the stator during a rotor standstill in order to generate heat, the course of which has a time-alternating trapezoidal or rectangular shape.

[0014] The control of a separately excited synchronous machine offers further improved possibilities for energy harvesting, especially during standstill. Strategies for this are to be defined.

[0015] The invention is based on the object of enabling efficient operation of a separately excited synchronous machine and heating of the battery supplying the separately excited synchronous machine without additional components.

[0016] The object is achieved by a method having the features according to claim 1.

[0017] The method according to the invention for operating an electric motor as at least one drive element of a vehicle therefore provides:

[0018] • Providing an electric motor having at least one stator with three stator phases and a rotor rotatable relative thereto with an excitation coil,

[0019] • electrical operation of the rotor by energizing the excitation coil,

[0020] • electrical operation of the stator via an inverter with a three-phase phase current which corresponds to a d-current and a q-current in the rotor-fixed dq coordinate system of the rotor, wherein it is provided that during a first operating state a rotor standstill of the rotor is brought about and at least one d-current is applied to the stator for the targeted generation of a heat loss at the stator, alternating in time with the application of a current to the rotor for the targeted generation of a heat loss at the rotor, for heating at least one vehicle component.In a preferred embodiment of the invention, it is provided that when a d-current is applied to the stator, a q-current is additionally or alternatively applied to the stator and when a current is applied to the rotor, a direct current and additionally or alternatively a high-frequency alternating current are applied to the rotor for the targeted generation of heat loss.

[0021] In a preferred embodiment of the invention, it is provided that the application of the current to the stator is maintained in a current supply interval of the stator for a predetermined stator current supply period, i.e. is kept constant, and is then reduced to zero again, and wherein the application of the current to the rotor is maintained in a current supply interval of the rotor for a predetermined rotor current supply period and is then reduced to zero again.

[0022] The current strengths with which the rotor and stator are energized in the energization interval are predetermined and can, for example, be the maximum permissible current strength for the selected electric motor, be based on this and be selected somewhat lower, or be determined beforehand in tests.

[0023] In a preferred embodiment of the invention, it is provided that the current is impressed on one of the two rotors or stators before the current is reduced to zero on the other of the two stators or rotors.

[0024] In a preferred embodiment of the invention, it is provided that the current profiles of the three phase currents of the stator and the current profile of the rotor have a trapezoidal shape that alternates over time between the phase currents of the stator and the current profile of the rotor.

[0025] In a preferred embodiment of the invention, the first operating state is present when the battery temperature of the vehicle battery that at least partially supplies the electric motor is lower than a predetermined battery temperature limit. In a preferred embodiment of the invention, no torque is generated in the first operating state while the rotor is at a standstill.

[0026] In a preferred embodiment of the invention, it is provided that in each successive energization interval of the stator, the strength of the energization of the individual phases is changed in order to supply the stator phases of the stator with the same amount of heat loss over several energization intervals in order to heat the stator phases of the stator evenly.

[0027] In a preferred embodiment of the invention, it is provided that the value of the rotor position angle is changed for each successive energization interval while the actual rotor is still at a standstill, so that for each stator phase, in each two successive energization intervals of the same stator phase, a different strength of the energization is achieved.

[0028] According to the invention, an electric motor is also provided as at least one drive element of a vehicle, comprising at least one stator and a rotor rotatable relative thereto, wherein the electric motor is designed to be operated using one of the aforementioned methods.

[0029] The advantage of alternating the stator coils and the rotor coil when stationary is that it avoids the torque that could cause the vehicle to roll away, while simultaneously allowing for approximately uniform heating of the rotor and stator, thus minimizing the mechanical stress on the components. Alternating from stator to rotor current and back helps to heat the rotor and stator evenly, reducing thermal stresses that can negatively impact component aging.

[0030] Further advantages and advantageous embodiments of the invention are the subject of the following figures and their description. They show in detail:

[0031] Figure 1 A method for generating waste heat in a separately excited electric motor.

[0032] Figure 2 A method for electric motor operation in a specific embodiment of the invention.

[0033] Figure 3 A method for generating waste heat of a separately excited electric motor in a special embodiment of the invention.

[0034] The rotor's excitation coil and the stator coils are designed to be controlled in a manner suitable for energy harvesting, especially at standstill. It is proposed to control the coils alternately at standstill to prevent a moment that could cause the vehicle to roll away, while simultaneously allowing for approximately uniform heating of the rotor and stator, thus minimizing mechanical stress on the components.

[0035] Figure 1 shows a method for generating waste heat from an electric motor. The method for generating waste heat uses an electric motor 10, which is, in particular, a drive element 12 of a vehicle. The electric motor is a separately excited synchronous motor, whose rotor 42 therefore does not have a permanent magnet, but rather a coil supplied with direct current on, for example, a ferromagnetic coil core—which is not explicitly shown in Fig. 2—to generate a magnetic field. Power electronics 14, comprising an inverter, supplies the electric motor 10 with a three-phase phase current 16. In addition, the coil of the rotor is supplied with direct current (not explicitly shown). The power electronics 14 is controlled depending on a control signal 18 from a control unit 20.The control unit 20 receives a rotor position signal 22 from a position sensor of the electric motor 10 or a sensorless rotor position signal 22 and exchanges control data 24 with a thermal control system 26. The thermal control system 26 transfers the thermal energy generated by the electric motor 10 as waste heat 28 to a coolant, which transfers the thermal energy to vehicle components 30, here to a vehicle battery 34, to achieve a required minimum operating temperature. Figure 2 shows a method for electric motor operation in a specific embodiment of the invention. The method for electric motor operation 36 is applied to an electric motor 10 used as the drive element 12 of a vehicle 38. The electric motor 10 comprises a stator 40 and a rotor 42 rotatable relative to the stator.The electric motor 10 is preferably a separately excited synchronous motor, which is electrically controlled by an electrical drive 44 with a field-oriented control by power electronics 14 with a three-phase phase current 16. In addition, the coil of the rotor is supplied with direct current (not explicitly shown). The power electronics 14 has an inverter 46 with a three-phase bridge circuit 48 with a high-side switch 50 and low-side switch 52 of a respective stator phase 54. The three-phase phase current 16 of the stator 40 corresponds in a dq coordinate system 56 of the rotor 42 to a d-current Id for influencing the magnetic flux and a q-current Iq for controlling a motor torque. The d-current Id is generated in a first operating state 60 when the rotor is at a standstill, i.e. when the rotor speed is zero, by alternately switching the high-side switch 50 and the low-side switch 52 of at least one stator phase 54.The first operating state 60 occurs when, for example, the temperature of the vehicle battery 34 is lower than a predetermined battery temperature limit value, so that an energy harvesting mode is activated: In the first operating state 60, while the rotor 42 is at a standstill, at least one d-current Id is impressed 58 to specifically generate a heat loss for heating at least one vehicle component, for example the vehicle battery 34. In the first operating state 60, while the rotor is at a standstill, the impressed d-current Id has a trapezoidal shape that alternates with a zero-current period (Fig. 3, period after b) (cf. Fig. 3).

[0036] Figure 3 shows a possible control strategy. The excitation current 5 of the rotor 42 is shown in the upper diagram, the stator currents 1, 2, 3 in the lower diagram. The curve shown corresponds to a stationary motor. At time 0, the stator 40 is supplied with a current which results in the values ​​shown in the three stator phases. With a known rotor angle, a target current Id is set. Since the excitation current in the rotor is zero during this period (Fig. 3, period before c) and therefore no appreciable torque can be generated in the rotor, a target active current Iq in the stator 40 can also be considered. Which target current pairs (Id, Iq) are particularly effective can depend on the motor selected and must be determined through testing.

[0037] In temporal alternation, the phase currents 1, 2, 3 are reduced to zero (period from a to b), while instead the excitation current 5 in the rotor 42 is increased (period from c to d). The current curve 5 of the rotor 42 shows a representation of current intensity against time as in Fig. 3, i.e. a trapezoidal curve, interrupted by periods without energization of the rotor coil. During these periods without energization of the rotor, the current curve of the three stator phases 1, 2, 3 is also trapezoidal. The change from stator to rotor energization and back helps to heat the rotor 42 and the stator 40 evenly in order to reduce thermal stresses, which in turn can have a negative effect on component aging, without exerting any appreciable torque on the rotor.

[0038] Alternatively, the rotor coil can be energized with high-frequency alternating current during the energization interval, in addition to or as an alternative to the direct current 5, resulting in increased heating due to iron and eddy current losses in the coil core. In this case, the envelope of the high-frequency alternating currents would be trapezoidal.

[0039] Ideally, a cooling medium flows through the rotor 42, just like the stator 40. In this case, the current flow intervals for the rotor excitation current and the stator excitation current can be selected to be the same or at least similar, as shown in Fig. 3. If the rotor is not cooled, the rotor excitation current flow interval will have to be significantly shorter than the stator current flow interval to achieve uniform heating.

[0040] The gradient for the build-up and decay of the current for the stator phase currents 1, 2, 3 is defined, for example, by points a and b. These must be determined through testing to ensure that possible reluctance effects of the motor do not lead to noise and torque inputs. This also applies to the excitation current in the rotor: there it is points c and d. As an alternative to the sequence shown, point b can occur after c, so that the excitation current 5 in the rotor is already rising even though the stator phase currents 1, 2, 3 have not yet decayed. This can ultimately generate slightly more heat. However, it must again be determined through testing that the resulting torque fluctuations do not have a negative influence.

[0041] Should excessive torque be generated at the rotor 42 or torque be transmitted to the drive wheels of the vehicle, which would lead to a possible rolling, a braking intervention is initiated in good time by a parking lock or a parking brake.

[0042] In order to heat the stator phases of the stator 40 evenly, the strength of the current supply 1, 2, 3 of the individual phases should be changed in each successive current supply interval of the stator 40 - unlike what is shown in Fig. 3.

[0043] For example, for this purpose only the value of the rotor position angle can be changed for each successive current supply interval, for example as if the rotor were fictitiously rotated while the actual rotor is still at a standstill, so that for each stator phase, in each two successive current supply intervals of the same stator phase, a different current intensity 1, 2, 3 is achieved, since the commutation of the stator phases depends on the value of the rotor position angle.

[0044] Reference symbols a Time b Time c Time d Time

[0045] 1 Stator phase current

[0046] 2 Stator phase current

[0047] 3 Stator phase current

[0048] 5 Excitation current of the rotor

[0049] 10 Electric motor

[0050] 12 Drive element

[0051] 14 Power electronics

[0052] 16 phase current

[0053] 18 Control signal

[0054] 20 Control unit

[0055] 22 Rotor position signal

[0056] 24 Control data

[0057] 26 Heat control system

[0058] 28 Heat loss

[0059] 30 vehicle components

[0060] 32 Vehicle interior Vehicle battery

[0061] Procedure for electric motor operation

[0062] vehicle

[0063] stator

[0064] Rotor electric operation

[0065] Inverter

[0066] Bridge circuit

[0067] High-side switch

[0068] Low-side switch

[0069] Motor phase dq coordinate system

[0070] Imprinting first operating state

Claims

Patent claims 1 . A method for operating an electric motor (10) as at least one drive element of a vehicle, comprising: • Providing an electric motor (10) which has at least one stator (40) with three stator phases and a rotor (42) rotatable relative thereto with an excitation coil, • electrical operation of the rotor (42) by energizing the excitation coil, • electrical operation (44) of the stator (40) via an inverter (46) with a three-phase phase current (16,1,2,3) which corresponds to a d-current (Id) and a q-current (Iq) in the rotor-fixed dq coordinate system (56) of the rotor (42), characterized in that during a first operating state (60) a rotor standstill of the rotor (42) is brought about and at least one d-current (Id) is impressed (58) on the stator (40) for the targeted generation of a heat loss at the stator (40), alternating in time with an application of current to the rotor (42) for the targeted generation of a heat loss at the rotor (42), for heating at least one vehicle component (30).

2. Method according to claim 1, characterized in that when impressing (58) a d-current (Id) on the stator (40) additionally or alternatively a q-current (Iq) is impressed on the stator (40) and when impressing a current on the rotor (42) for the targeted generation of a heat loss at the rotor (42) a direct current and additionally or alternatively a high-frequency alternating current is impressed.

3. Method according to one of the preceding claims, characterized in that the application of the current to the stator (40) is maintained in an energization interval of the stator (40) for a predetermined stator energization period and is then reduced to zero again and wherein the application of the current to the rotor (42) in a energization interval of the rotor (42) is maintained for a predetermined rotor energization period and is then returned to zero.

4. Method according to one of the preceding claims, characterized in that an application of the current to one of the two rotors (42) or stators (40) begins (c) before the reduction of the current to zero at the other of the two stators (40) or rotors (42) is completed (b).

5. Method according to one of the preceding claims, characterized in that the current profiles of the three phase currents (1, 2, 3) of the stator (40) and the current profile (5) of the rotor (42) have a trapezoidal shape (1, 2, 3, 5) alternating in time between the phase currents (1, 2, 3) of the stator (40) and the current profile (5) of the rotor (42).

6. Method according to one of the preceding claims, characterized in that the first operating state (60) exists when the battery temperature of the vehicle battery (34) which at least partially supplies the electric motor (10) is lower than a predetermined battery temperature limit value.

7. Method according to one of the preceding claims, characterized in that in the first operating state (60) no torque is generated during the rotor standstill.

8. Method according to one of the preceding claims, characterized in that in each successive energization interval of the stator, the strength of the energization of the individual phases is changed in order to supply the stator phases of the stator with the same amount of heat loss over several energization intervals in order to heat the stator phases of the stator evenly.

9. Method according to claim 8, characterized in that the value of the rotor position angle is changed for each successive energization interval while the actual rotor (42) is still at a standstill so that for each stator phase, in each of two consecutive current supply intervals of the same stator phase, a different current supply intensity is achieved.

10. Electric motor (10) as at least one drive element (12) of a vehicle (38), comprising at least one stator (40) and a rotor (42) rotatable relative thereto, wherein the electric motor (10) is designed to be operated by a method for its operation according to one of the preceding claims.

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