Excitation current-limited power generator

Abstract

Device comprising: a digital interface (124) designed for coupling with a motor control unit (108); a controller (310) which is configured to couple with an excitation current input of a current generator (106), wherein the excitation current controls a current generated by the current generator (106); a frequency sensor (308) designed to measure the rotational speed of the power generator (106); and a memory (312) which stores a limit communicated through the digital interface (124) and a first permanent limit, wherein the controller (310) is configured to limit the excitation current to the lower of the first permanent limit and the communicated limit, wherein the memory (312) further stores a second permanent limit and a rotation threshold associated with the second permanent limit, wherein the controller (310) is further configured to limit the excitation current to the lower of the second permanent limit and the communicated limit when the rotational speed of the current generator (106) is greater than the rotation threshold.

Description

Technical field

[0001] The present invention relates generally to power generation and in particular to embodiments, techniques and mechanisms for an excitation current-limited power generator. background

[0002] Power generators (e.g., alternators) typically contain a moving coil within a stator coil. The output current of the stator coil can be changed by altering the excitation current flowing through the moving coil. Some types of power generators, such as alternators, are commonly used in applications involving internal combustion engines, for example, passenger cars, allowing the power generator to be connected to the engine's drivetrain.

[0003] The publications International Rectifier: IRVR101 - LIN Controlled Alternator Voltage Regulator. USA, 2003. pp. 1-15, Freescale Semiconductor: Alternator Regulator with LIN - 80310 - AUTOMOTIVE ALTERNATOR REGULATOR. USA, 2012. pp. 1-31, DE 10 2005 036 220 A1 and DE 10 2004 059 151 A1 disclose systems in which a limit for the excitation current of a power generator is received via an interface. A default limit may also be provided, which is used when no communicated limit is available.

[0004] Power generators can experience reduced performance under harsh environmental conditions, such as cold temperatures and low engine speeds. Rapid engine speed cycles, such as acceleration and deceleration, can cause an internal combustion engine to operate periodically at lower speeds. Furthermore, research efforts for passenger cars have increasingly focused on reducing engine speeds to improve fuel efficiency. This has exacerbated problems associated with operating power generators at lower engine speeds. The performance of power generators in modern, fuel-efficient vehicles can be further degraded in colder climates.

[0005] Deteriorating performance of a power generator can cause it to exceed its maximum rated output or result in the generator experiencing counter-torque from the combustion engine. Such reduced performance can damage the power generator over time.

[0006] It is a task to provide ways to avoid or at least mitigate such problems. Summary of the invention

[0007] A device according to claim 1, a method according to claim 10 or 15, and a system according to claim 16 or 18 are provided. The dependent claims define further embodiments.

[0008] Technical advantages are generally achieved through embodiments of the present application that describe techniques and mechanisms for an excitation current-limited power generator.

[0009] According to some embodiments, a device is provided. The device includes a digital interface configured for coupling with an engine control unit (ECU), a coupled controller configured for coupling with an excitation current input of an alternator (hereinafter referred to as a generator, in particular an AC or three-phase generator), wherein the excitation current controls the current generated by the alternator, a frequency sensor configured to measure the rotational speed of the alternator, and a memory that stores a limit communicated through the digital interface and a first permanent limit, wherein the controller is configured to limit the excitation current to the lower of the first permanent limit and the communicated limit, respectively.

[0010] In some embodiments, the controller is further configured to update the first communicated limit with a value received from the ECU. In some embodiments, the value received from the ECU is selected to limit the current generated by the alternator. In some embodiments, the value received from the ECU is selected to limit the counter-torque generated in the alternator. The memory further stores a second permanent limit and a rotation threshold associated with the second permanent limit, the controller further being configured to limit the excitation current to the lower of the second permanent limit and the communicated limit when the alternator speed is greater than the rotation threshold. In some embodiments, the first permanent limit is higher than the second permanent limit.In some embodiments, the first permanent limit is lower than the second permanent limit. In some embodiments, the controller is further configured to limit the excitation current to the first permanent limit in response to a loss of connection between the digital interface and the ECU. In some embodiments, the device also includes the alternator. In some embodiments, the alternator comprises a moving coil in stator coils.

[0011] According to some embodiments, a method is provided. The method comprises receiving a communicated limit for excitation current in an alternator via a digital interface, determining a permanent limit for the excitation current in the alternator, limiting the excitation current to the communicated limit in response to the communicated limit being lower than the permanent limit, and limiting the excitation current to the permanent limit in response to the communicated limit being higher than the permanent limit.

[0012] In a first alternative of the method, determining the permanent limit comprises determining an alternator speed and selecting a permanent limit from one or more permanent limits, wherein the one or more permanent limits each correspond to a lower and upper speed threshold, and the alternator speed lies between the lower and upper speed thresholds of the selected permanent limit. In some embodiments, the one or more permanent limits comprise a first permanent limit and a second permanent limit, wherein the upper speed threshold of the first permanent limit is lower than the upper speed threshold of the second permanent limit. In some embodiments, the first permanent limit is higher than the second permanent limit. In some embodiments, the first permanent limit is lower than the second permanent limit.In a second alternative of the procedure, the procedure further includes receiving an updated permanent boundary via the digital interface and storing the updated permanent boundary in memory.

[0013] According to some embodiments, a system is provided. The system includes an engine control unit (ECU), a digital interface coupled to the engine control unit, a power generator coupled to the digital interface, the power generator having a power controller which includes a memory configured to store a communicated limit received from the digital interface and a first permanent limit, an excitation current input, and a regulator coupled to the excitation current input, the regulator being configured to control current output from the alternator by varying the excitation current, and the regulator being configured to limit the excitation current to the lower of the first permanent limit and the communicated limit.

[0014] In some embodiments, the alternator further comprises a rotation sensor coupled to an output of the generator, the rotation sensor measuring the rotational speed of the generator. In a first alternative of the system, the memory is further configured to store a second permanent limit, the first permanent limit being associated with a first rotational threshold and the second permanent limit being associated with a second rotational threshold, the generator being configured to limit the excitation current to the first permanent limit when the generator's rotational speed is lower than the first rotational threshold, and the generator being configured to limit the excitation current to the second permanent limit when the generator's rotational speed is lower than the second rotational threshold.In a second alternative of the system, the power generator is configured to update the first permanent limit stored in memory with an updated permanent limit received from the ECU via the digital interface. Brief description of the drawings

[0015] For a better understanding of the present invention and its advantages, reference is now made to the following descriptions in conjunction with the accompanying drawing, which shows: Fig. 1 a motor vehicle system; Fig. 2A and Fig. 2B Exemplary current and torque curves; Fig. 3 a detailed view of a power control system; Fig. 4 excitation current curves; Fig. 5 a generator overcurrent protection method; Fig. 6 output current curves; and Fig. 7A and Fig. 7B permanent limits to protect an alternator.

[0016] Corresponding numbers and symbols in the various figures generally refer to corresponding parts, unless otherwise indicated. The figures serve to clearly illustrate the relevant aspects of the embodiments and are not necessarily drawn to scale. Detailed description of illustrative embodiments

[0017] The manufacture and use of embodiments of the present application are discussed in detail below. It is understood, however, that the concepts disclosed herein can be implemented in a wide variety of specific contexts and that the specific embodiments discussed herein serve only to illustrate, and not to limit, the scope of protection of the claims. Furthermore, it should be obvious that various modifications, substitutions, and amendments can be made herein without departing from the essence and scope of protection of the present application as defined by the appended claims.

[0018] Techniques and mechanisms for an excitation-current-limited power generator are disclosed herein. Various embodiments incorporate self-protection mechanisms in a power generator that limit the excitation current applied to the moving coil of the power generator. The excitation current can be limited based on a measured rotational speed of the power generator. Excitation current limiting thresholds can be selected to limit maximum current output, measured in amperes (A), and / or counter-torque, measured in newton-meters (Nm). Various self-protection mechanisms can be applied independently of a motor control unit (ECU), which can also control the excitation current or the supply voltage level of the moving coil.

[0019] Various designs can offer advantages. Limiting the current output of the power generator allows it to operate in harsher environmental conditions, such as cold climates, without exceeding its maximum output. Ensuring the generator does not exceed its maximum output can prevent damage to the generator or to equipment driven by it, such as vehicle loads. Limiting the torque output of the power generator allows it to operate at lower speeds, such as the low engine speed of a passenger car, without damaging the generator or producing undesirable side effects, such as humming, in the vehicle's engine. This can improve the performance, reliability, comfort, and lifespan of fuel-efficient vehicles.

[0020] Fig. Figure 1 shows a motor vehicle system 100 that can be used in a motor vehicle or other passenger vehicles, such as a car or truck. The motor vehicle system 100 includes vehicle loads 102, a battery 104, an alternator 106, and an ECU 108. Although this discussion is presented in connection with passenger vehicles, it should be obvious that embodiments described herein can be applied to any power generator with moving magnets.

[0021] The vehicle loads 102 contain the devices required to operate the vehicle system 100. Examples of vehicle loads 102 include the ignition, spark plugs, climate control, and entertainment systems within the vehicle system 100. The battery 104 provides an initial charge for the vehicle loads 102 during ignition of the vehicle system 100. The battery 104 can, for example, be a 12V battery. Furthermore, the battery 104 acts as an electrical buffer for the output of the alternator 106.

[0022] The alternator 106 contains a moving coil 110, stator coils 112, rectifier diodes 114, and a power controller 116. The moving coil 110 is positioned inside the stator coils 112 and contains a rotating magnetic field (not shown). Rotation of the magnetic field generated by the moving coil 110 in the stator coils 112 produces an electric current at the outputs of the stator coils 112. Three stator coils 112 can be provided, so that the generated power is three-phase AC power. The rectifier diodes 114 rectify the generated three-phase AC current to produce DC power. The DC power is supplied to drive the vehicle loads 102 and to charge the battery 104.

[0023] The power controller 116 includes a voltage level input 118, a speed input 120, an excitation current output 122, and a digital input / output (1 / 0) 124. The power controller 116 is coupled to various components of the alternator 106 in such a way that it can control the amount of current generated by the alternator 106. Controlling the current output can be achieved by varying the excitation current of the moving coil 110. The power controller 116 can be, for example, an application-specific integrated circuit (ASIC) or a state machine.

[0024] The voltage level input 118 is coupled to the rectifier diodes 114 in such a way that the power controller 116 can measure the DC power output of the alternator 106. The voltage level input 118 allows the power controller 116 to determine whether the output of the alternator 106 has been exceeded. Such a protection mechanism allows the power controller 116 to reduce generation by reducing the excitation current of the moving coil 110.

[0025] The speed input 120 is coupled to one of the stator coils 112. As discussed above, several stator coils 112 can be provided, so that the stator coils 112 generate three-phase power. By coupling the speed input 120 to one of the stator coils 112, the power controller 116 can measure the frequency of one of the three-phase AC outputs. The frequency output of each of the stator coils 112 corresponds to the speed of the alternator 106. Accordingly, the power controller 116 can determine the speed (in RPM) of the alternator 106 by measuring the frequency of one of the signals from the stator coils 112.

[0026] The excitation current output 122 is coupled to the moving coil 110. As discussed above, the output current of the alternator 106 can be controlled by varying the excitation current of the moving coil 110. Accordingly, the power control 116 can control the output current of the alternator 106 via the excitation current output 122. The power control 116 can limit the excitation current of the moving coil 110 in response to communication from the ECU 108 (discussed below) or in response to a self-protection feature (also discussed below).

[0027] The digital I / O 124 allows the alternator 106 to communicate with other devices in the vehicle system 100, such as the ECU 108. The digital I / O 124 is capable of bidirectional digital communication. Examples of such a communication system include a LIN (Local Interconnect Network). Communication via the digital I / O 124 can be performed in defined time windows, such as every 100 milliseconds (ms). The digital I / O 124 can be shared with other devices in the vehicle system 100, so that the power controller 116 can communicate only with the ECU 108 during part of the time windows. Accordingly, communication with the ECU 108 may have lower throughput and higher latency.

[0028] The ECU 108 is coupled to the alternator 106 and other devices (not shown) in the vehicle system 100, enabling it to control and monitor parameters of these devices. The ECU 108 communicates with the alternator 106 via the digital I / O 124. Because the ECU 108 can measure many parameters in the vehicle system 100, it can access more information than the power controller 116 and, in some situations, be able to precisely control the output current or counter-torque generated by the alternator 106. In some embodiments, the ECU 108 can control the generation by the alternator 106 by communicating an excitation current limit to the power controller 116.

[0029] To control the output of alternator 106, ECU 108 should be capable of supporting this functionality. As discussed above, the digital I / O 124 between alternator 106 and ECU 108 may be slow, potentially preventing the ECU from responding to sudden changes in conditions, such as engine speed. Consequently, brief current output spikes and / or back-torque spikes may occur in alternator 106 before ECU 108 communicates a new excitation current limit to power control 116. Furthermore, digital I / O 124 may fail, resulting in a temporary or permanent loss of communication with alternator 106. During these periods of lost communication, alternator 106 may be damaged. Over its lifetime, these brief current and / or back-torque spikes can cause wear on alternator 106, reducing its lifespan.Furthermore, such peaks can also damage the on-board power supply of the vehicle system 100, such as the vehicle loads 102 and / or the battery 104.

[0030] The Fig. 2A and Fig. Figure 2B shows exemplary current and torque output curves for an alternator at different excitation currents. As in Fig. As can be seen in the diagram, the maximum rated current output of the alternator is approximately 315 A. Fig. 2A is shown by the dashed line. Each of the alternator's output current curves increases with RPM and eventually converges to a final output current. For example, at the lowest excitation current, the output current gradually increases until it delivers approximately 75 A at approximately 8000 RPM. Conversely, at the highest excitation current, the output current increases steeply until it delivers approximately 350 A at approximately 8000 RPM. In some embodiments, the alternator can be operated at a higher excitation current when driven at a lower RPM to quickly achieve sufficient current output. In some embodiments, the alternator can be operated at a lower excitation current when driven at a higher RPM to avoid excessive current output.

[0031] As in Fig. As can be seen in 2B, the maximum rated torque output of the alternator is approximately 20 Nm, as shown in Fig. Figure 2B is represented by the dashed line. Each of the alternator's output torque curves exhibits a peak at lower RPM, the amplitude of which is proportional to the excitation current. For example, at the lowest excitation current, the output torque peaks at approximately 2 Nm at approximately 3000 RPM. Conversely, at the highest excitation current, the output torque peaks at approximately 27 Nm at approximately 3000 RPM. In some embodiments, the alternator can be operated at a lower excitation current when driven at a lower RPM to avoid torque peaks that could damage the alternator. In some embodiments, the alternator can be operated at a higher excitation current when driven at a higher RPM, as there is a reduced risk of torque peaks at higher RPM.

[0032] The Fig. 2A and Fig. 2B represents a compromise between current and torque peaks that can occur at higher excitation currents and achieving sufficient current output, which is difficult to achieve at lower excitation currents. These peaks can occur over relatively short periods, so the ECU 108 may not respond to them in a timely manner. Accordingly, in some embodiments, the power control 116 can perform self-protection of the alternator 106 independently of the ECU 108 to avoid current and / or torque peaks that could cause damage.

[0033] Fig. Figure 3 shows a detailed view of the power controller 116. The power controller 116 includes a bus 302, a communication controller 304, a battery sensor 306, a frequency sensor 308, a controller 310, a memory 312, and a master logic unit 314. Devices in the power controller 116 can optionally be connected to the bus 302.

[0034] Although they are shown as functional blocks, it should be obvious that the battery sensor 306, the frequency sensor 308, and the regulator 310 may also contain other components for coupling the power control 116 with components in the alternator 106. These devices may include, for example, converters, analog-to-digital converters, digital-to-analog converters, registers, amplification circuits, support circuits, and the like.

[0035] The communication controller 304 is coupled to the digital I / O 124 and connects the alternator 106 to external devices, such as the ECU 108. As discussed above, the digital I / O 124 can be a bidirectional digital interface, such as a LIN. Thus, the communication controller 304 can, for example, be a LIN controller.

[0036] The battery sensor 306 is connected to the voltage level input 118, so that the power controller 116 can measure the output voltage level of the battery 104 and the DC power output from the alternator 106. The battery sensor 306 can, for example, be an analog-to-digital converter. In some embodiments, the analog-to-digital converter can be a 10-bit ADC.

[0037] The frequency sensor 308 is connected to the speed input 120, enabling the power controller 116 to measure the frequency of the AC output signal from one of the stator coils 112. The frequency sensor 308 can, for example, include a linear oscillator, such as a resistor-capacitor oscillator, which is used to detect the frequency of the AC waveforms. The speed of the alternator 106 can thus be determined according to the measured frequency and the number of phases rectified by the rectifier diodes 114. By relating the frequency of the AC signal to the motor speed, the power controller 116 can determine the speed of the alternator 106 independently of any motor speed parameters that can be communicated via the communication controller 304.

[0038] The regulator 310 is coupled to the excitation current output 122, allowing the power controller 116 to vary the excitation current of the moving coil 110. By varying the excitation current, the regulator 310 can thus regulate and control an output current of the alternator 106. The regulator 310 can, for example, be a voltage regulator.

[0039] Memory 312 can be volatile memory, such as random access memory (RAM), or non-volatile random access memory (NVRAM), such as an EEPROM. In some embodiments, memory 312 contains both RAM and NVRAM. The NVRAM can be implemented using fuses, electronic fuses (e-fuses), or one-time programmable memory (OTP). Memory 312 is used to store limiting parameters (sometimes referred to as "limits"). Limits are maximum excitation current values ​​that the controller 310 should observe when varying the excitation current supplied to the moving coil 110. One or more limits can be contained in memory 312, and the applied limit can be determined according to different states of the alternator 106.

[0040] In some embodiments, a first type of current limit (sometimes referred to as a "communicated limit") of the power controller 116 can be communicated via the communication controller 304 and stored in the memory 312. A communicated limit can be considered immediately when determining the current supplied to the moving coil 110. A communicated limit can be stored in RAM or NVRAM portions of the memory 312.

[0041] In some embodiments, a second type of current limit (sometimes referred to as a "permanent limit") can be considered when determining the current supplied to the rotating coil 110. The permanent limit is stored in NVRAM portions of memory 312, so that it persists in the power controller 116 even after a loss of power or communication with the ECU 108. Accordingly, the power controller 116 can be able to read the permanent limit from the NVRAM and limit the excitation current to the permanent limit during periods of lost or missing communication. For example, the excitation current can be limited during vehicle ignition before the ECU 108 has sent a communicated limit to the power controller 116. A permanent limit can be pre-programmed in the NVRAM.In some embodiments, a permanent limit of the power controller 116 can be communicated from time to time via the communication controller 304, and the permanent limit can be stored in the NVRAM. It should be obvious that multiple permanent limits and / or communicated limits can be stored in the memory 312.

[0042] In some embodiments, each of the permanent limits stored in memory 312 can be assigned to a motor speed threshold, measured in RPM. The controller 310 can select different permanent limits to apply to the excitation current according to the speed determined by the frequency sensor 308. For example, a first permanent limit can be assigned to a first speed threshold, and a second permanent limit can be assigned to a second speed threshold that is higher than the first. The controller 310 can apply the first permanent limit when the measured speed is lower than the first speed threshold, and then apply the second permanent limit when the measured speed is lower than the second speed threshold. Hysteresis can be included with each different applied speed threshold.

[0043] In some embodiments, the controller 310 can consider both a permanent limit and a communicated limit when limiting the excitation current. The controller 310 can prioritize the lower of the permanent limit and the communicated limit, so that the permanent limit is not exceeded. In other words, if the communicated limit is lower than a permanent limit currently being applied, then the controller 310 can allow the excitation current to be reduced to the communicated limit. However, if the communicated limit is higher than the permanent limit, then the controller 310 cannot allow the excitation current to be increased beyond the permanent limit. The ECU 108 can thus apply a lower communicated limit that allows the ECU 108 to perform torque management for the alternator 106. However, the ECU 108 cannot override the alternator 106 with a higher permanent limit.Such a protection mechanism can protect the alternator 106 and allow faster response times when the output current is rapidly increased. In some embodiments, the output of the alternator 106 can be switched on or off when the voltage level measured by the battery sensor 306 falls below the limit set by the regulator 310.

[0044] The master logic unit 314 is the main processing pipeline for the power controller 116. It contains functional units and / or circuit arrangements for performing start sequences, controlling the controller 310 and the communication controller 304, and optimizing, testing, and troubleshooting the power controller 116. The master logic unit 314 can also include functionality for interacting with the battery sensor 306 and the frequency sensor 308. The master logic unit 314 can select a permanent limit from the memory 312 based on the measured rotational speed and can determine whether the permanent or communicated limit should be applied to the controller 310.

[0045] Fig. Figure 4 represents the excitation current required to achieve an ideal output current for an alternator at various engine speeds. An ideal output current can be one that approaches the peak output current. As shown by the solid line, at lower engine speeds, the alternator does not exceed the peak output current, even at maximum excitation current, because lower engine speeds may not generate enough power to exceed the peak output. However, as engine speed increases, the permanent limit for the excitation current is lowered to prevent the alternator from exceeding the peak output.

[0046] In some embodiments, the memory can contain a permanent limit for the motor speed, corresponding to each data point forming the solid line. The power control can thus contain sufficiently permanent limits for the excitation current so that the alternator does not exceed the peak output current at any given motor speed. For example, assuming the alternator has a similar power characteristic to the one in Fig. If the response behavior shown in Figure 4 were to occur, then the alternator storage would contain six permanent limits (from approximately 6 [A] to approximately 4 [A]) at a respective speed threshold (from approximately 0 RPM to approximately 6500 RPM) to ensure that the alternator does not exceed the peak current output.

[0047] In some embodiments, the memory can contain a relatively smaller number of permanent limits. Thus, the power control can limit the excitation current so that the alternator does not exceed the peak output current at most engine speeds. The dashed line in Fig. Figure 4, for example, represents an embodiment in which only two permanent limits (PLIM1 and PLIM2) are stored in the memory. When only two permanent limits are used, the alternator may consequently exceed or fall below the peak current output at some engine speeds (a narrow band between approximately 2500 RPM and 3500 RPM), but generally operates at or near an ideal output. More or fewer permanent limits could be used, so that the alternator's response characteristics are a closer or wider approximation of the ideal output.

[0048] Fig. 5 represents an alternator overcurrent protection method 500. The alternator overcurrent protection method 500 can indicate operations that take place in the power control 116 when a permanent or communicated limit is applied to the excitation current generated by the controller 310.

[0049] The alternator overcurrent protection procedure 500 begins by setting a communicated limit (CLIM) (step 502). The communicated limit can be received from an ECU. The communicated limit can be updated periodically by the ECU. Next, a permanent limit (PLIM) is set (step 504). Setting the permanent limit may involve selecting a permanent limit based on a measured alternator speed. If the permanent limit exceeds the communicated limit (step 506), then the permanent limit is used to limit the excitation current (step 508). However, if the permanent limit does not exceed the communicated limit (step 506), then the communicated limit is used to limit the excitation current (step 510).If either the permanent or the communicated limit is selected, the value is forwarded to the controller (step 512). The controller can then select an excitation current for the moving coils that does not exceed the selected limit.

[0050] Fig. Figure 6 shows output current curves when one of several permanent output limits is applied to the alternator. As shown, a first permanent limit PLIM1 is applied for speeds lower than the engine speed threshold s. 12 are. A second permanent limit (PLIM2) is applied for speeds higher than the engine speed threshold s. 12Accordingly, the output current approaches the maximum rated current output when the first (higher) permanent limit is applied. Before the output current exceeds the maximum output, the second (lower) permanent limit is applied, thus reducing the output current. Therefore, the alternator's output current cannot exceed the maximum output, preventing damage to the alternator.

[0051] The Fig. 7A and Fig. 7B represents permanent limits to protect an alternator from excessive current output or excessive torque. Fig. 7A represents the application of two permanent limits, with a lower permanent limit being applied at higher engine speeds. Accordingly, Fig. 7A excitation currents for a power control system operating in an overcurrent protection mode.

[0052] Fig. 7B represents the application of two permanent limits, with a higher permanent limit being applied at higher engine speeds. Accordingly, Fig. Figure 7B shows excitation currents for a power control system operating in an over-torque protection mode. As indicated by the dashed lines, hysteresis can be applied in both current-limiting and torque-limiting operating modes.

Claims

[1] Device comprising: a digital interface (124) designed for coupling with a motor control unit (108); a controller (310) which is configured to couple with an excitation current input of a current generator (106), wherein the excitation current controls a current generated by the current generator (106); a frequency sensor (308) designed to measure the rotational speed of the power generator (106); and a memory (312) which stores a limit communicated through the digital interface (124) and a first permanent limit, wherein the controller (310) is configured to limit the excitation current to the lower of the first permanent limit and the communicated limit, wherein the memory (312) further stores a second permanent limit and a rotation threshold associated with the second permanent limit, wherein the controller (310) is further configured to limit the excitation current to the lower of the second permanent limit and the communicated limit when the rotational speed of the current generator (106) is greater than the rotation threshold. [2] Device according to claim 1, wherein the controller (310) is further configured to periodically update the communicated limit with a value obtained from the motor control unit (108). [3] Device according to claim 2, wherein the value received by the motor control unit (108) is selected to limit the current generated by the current generator (106). [4] Device according to claim 2 or 3, wherein the value received by the motor control unit (108) is selected to limit a counter-torque generated in the current generator (106). [5] Device according to one of claims 1-4, wherein the first permanent limit is higher than the second permanent limit. [6] Device according to one of claims 1-4, wherein the first permanent limit is lower than the second permanent limit. [7] Device according to one of claims 1-6, wherein the controller (310) is further configured to limit the excitation current to the first permanent limit in response to the digital interface (124) losing a connection with the motor control unit (108). [8] Device according to one of claims 1-7, further comprising the current generator (106). [9] Device according to claim 8, wherein the current generator (106) comprises a moving coil (110) in stator coils (112). [10] Procedures, including: Receiving a communicated limit for excitation current in a power generator (106) via a digital interface (124); Determining a permanent limit for the excitation current in the current generator (106); Limiting the excitation flow to the communicated limit in response to the fact that the communicated limit is lower than the permanent limit; and Limiting the excitation flow to the permanent limit in response to the fact that the communicated limit is higher than the permanent limit, where determining the permanent boundary includes the following: Determining the rotational speed of the power generator (106); and Selecting one permanent limit from several permanent limits, wherein the one or the several permanent limits each correspond to a lower and upper speed threshold, wherein the speed of the generator (106) lies between the lower and upper speed threshold of the selected permanent limit. [11] Method according to claim 10, wherein the multiple permanent limits comprise a first permanent limit and a second permanent limit, wherein the upper speed threshold of the first permanent limit is lower than the upper speed threshold of the second permanent limit. [12] Method according to claim 11, wherein the first permanent limit is higher than the second permanent limit. [13] Method according to claim 11, wherein the first permanent limit is lower than the second permanent limit. [14] Method according to any one of claims 10-13, further comprising: Receiving an updated permanent boundary via the digital interface (124); and Storing the updated permanent boundary in memory (312). [15] Procedure, encompassing: Receiving a communicated limit for excitation current in a power generator (106) via a digital interface (124); Determining a permanent limit for the excitation current in the current generator (106); Limiting the excitation flow to the communicated limit in response to the fact that the communicated limit is lower than the permanent limit; Limiting the excitation flow to the permanent limit in response to the fact that the communicated limit is higher than the permanent limit; Receiving an updated permanent boundary via the digital interface (124); and Storing the updated permanent boundary in memory (312). [16] System (100), comprising: a motor control unit (108); a digital interface (124) coupled to the motor control unit (108); a power generator (106) coupled to the digital interface (124), wherein the power generator (106) includes a power control (116) which comprises the following: a memory (312) which is set up to store a boundary communicated by the digital interface (124) and a first permanent boundary; an excitation current input; and a controller (310) coupled to the excitation current input, wherein the controller (310) is configured to control a current output from the current generator (106) by varying the excitation current, wherein the controller (310) is configured to limit the excitation current to the lower of the first permanent limit and the communicated limit, wherein the power generator (106) further comprises a rotation sensor which is coupled to an output of the power generator (106), wherein the rotation sensor is configured to measure the rotational speed of the power generator (106), wherein the memory (312) is further configured to store a second permanent limit, wherein the first permanent limit is associated with a first rotation threshold and the second permanent limit is associated with a second rotation threshold, wherein the current generator (106) is configured to limit the excitation current to the first permanent limit when the rotational speed of the current generator (106) is lower than the first rotation threshold, wherein the current generator (106) is configured to limit the excitation current to the second permanent limit when the rotational speed of the current generator (106) is lower than the second permanent limit. [17] System according to claim 16, wherein the current generator (106) is configured to update the first permanent limit stored in the memory (312) with an updated permanent limit received from the motor control unit (108) via the digital interface (124). [18] System (100), comprising: a motor control unit (108); a digital interface (124) coupled to the motor control unit (108); a power generator (106) coupled to the digital interface (124), wherein the power generator (106) comprises the following: a memory (312) which is set up to store a boundary communicated by the digital interface (124) and a first permanent boundary; an excitation current input; and a controller (310) coupled to the excitation current input, wherein the controller (310) is configured to control a current output from the current generator (106) by varying the excitation current, wherein the controller (310) is configured to limit the excitation current to the lower of the first permanent limit and the communicated limit, wherein the current generator (106) is configured to update the first permanent limit stored in the memory (312) with an updated permanent limit received from the motor control unit (108) via the digital interface (124).

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