Method for determining the rotor current of a separately excited synchronous machine and device for determining the rotor current

Abstract

The invention relates to a method for determining the rotor current of a separately excited synchronous machine (FSM), comprising a stator and a rotor with an excitation winding. Initially, an excitation supply voltage is connected via a controlled bridge circuit to terminals A and B of an inductive transformer. On the secondary side, this transformer is connected to the excitation winding of the FSM via a rectifier. The controlled bridge circuit can be regulated via switching parameters using a control unit. Subsequently, the primary current of the excitation winding of the FSM is sampled at terminals A and B. Then, an evaluation time for the sampled primary current is determined, and the primary current is evaluated at that time. The evaluated primary current is then converted to the rotor current. Finally, the rotor current at the evaluation time is output.

Description

[0001] The invention relates to a method for determining a rotor current by primary current measurement for an electrical machine, in particular for a separately excited synchronous machine.

[0002] To determine the rotor current, the current is determined using either temperature models or temperature or current sensors. Temperature models must be validated through experiments and, depending on the level of detail, require high computing power. The alternative use of rotating sensors is expensive, and transmitting the signals back from the rotor is complex. Additionally, a large amount of electronics is required. The achievable resolution of such systems is limited.

[0003] Due to the use of inductive power transmission, direct measurement of the rotor current is no longer possible. Typically, the rotor current is estimated, which leads to disadvantages in torque control.

[0004] CN 114079414 A discloses a sampling method and a sampling device for determining the rotor current. The electrical power of an inverter is inductively transferred to an excitation winding. The current in the primary winding of the transformer is measured and converted to the secondary current using a transfer factor.

[0005] DE 10 2010 014 173 A1 discloses a method for operating a separately excited synchronous machine, in which the machine current and the machine voltage are recorded. The rotor flux is determined using a machine model. The excitation current fed into the excitation winding is then calculated from the magnetization characteristic.

[0006] JP 2004 - 248 365 A discloses a contactless power transmission device and a contactless power transmission method for stabilizing a secondary side output voltage without providing a feedback circuit.

[0007] DE 10 2022 210 963 A1 discloses a method for determining a secondary current in a rotor of an electrical machine, in particular a separately excited synchronous machine, of an electrical arrangement, which electrical arrangement has a primary side and a secondary side, wherein an inverter is arranged on the primary side and the rotor of the electrical machine is arranged on the secondary side, wherein the primary side and the secondary side are coupled to one another by means of a transmission device, in particular a transformer.

[0008] DE 10 2005 047 548 A1 discloses a method for measuring a load current flowing through a coil or inductor, wherein the inductor is fed via a transformer or transmitter and a downstream rectifier. The document also refers to the associated device for implementing the method.

[0009] It is therefore an object of the invention to provide a method for determining rotor current and to further improve the state of the art.

[0010] For this purpose, the invention provides a method according to claim 1 and a device according to claim 10. Advantageous embodiments can be found in the subclaims and the description.

[0011] Separately excited synchronous machines (SMMs) are a type of electrical machine that is powered by an external current or voltage source. Unlike a permanent magnet synchronous machine, a SMM requires a separate current or voltage source to generate the magnetic flux in the rotor required for its operation.

[0012] The FSM comprises a stator, a coil pack within the stator, which typically contains three phases and generates a rotating magnetic field, and a rotor, which has a core and an excitation winding. The excitation winding on the rotor, also referred to as the rotor winding, is magnetized when energized. The rotor winding can be fed inductively or via slip rings. The rotor speed depends on the frequency of the stator magnetic field. The rotor follows the stator magnetic field synchronously.

[0013] The FSM has the advantage that the strength of the rotor field can be adjusted depending on the load and speed. The strength of the magnetic field, and thus the torque of the FSM, can be controlled via the current injected into the excitation winding and thus dynamically adapted to the machine's load to improve its performance. For this purpose, the rotor current can be varied depending on the load.

[0014] FSMs are advantageous because they offer high performance and efficiency and can be easily adapted to different load requirements. Another advantage is that they eliminate the need for permanent magnets, which are made from rare earth elements. Another benefit of FSMs is that full torque is available almost from standstill and can be maintained over a wide speed range. For efficient and dynamic control of FSMs, it is necessary to know the rotor current.

[0015] For this purpose, the invention provides a method according to claim 1. Advantageous embodiments can be found in the subclaims and the description.

[0016] The invention relates to a method for determining the rotor current of a separately excited synchronous machine (SMM), comprising a stator and a rotor with an excitation winding. The method comprises the following steps: - Applying an excitation supply voltage via a controlled bridge circuit to terminals A and B of the excitation winding of the FSM via an inductive transformer; - Setting switching parameters of the controlled bridge circuit via a control unit; - Sampling a primary current of the field winding of the FSM at the connection terminals A and B of the field winding; - Determining an evaluation time of the sampled primary current; - Evaluation of the primary current at the time of evaluation; - Conversion of the evaluated primary current into a rotor current at the time of evaluation; - Output of the rotor current at the time of evaluation.

[0017] The connecting terminals A and B of the excitation winding represent the mechanical connecting terminals of the FSM. Behind the connecting terminals, an impressed current—the primary current—is transferred from the stator to the rotor via an inductive transformer. The inductive transformer comprises a primary side, which corresponds to the connecting terminal side. The inductive transformer is supplied with electrical energy via the connecting terminals via a controlled bridge circuit. The inductive transformer further comprises a secondary side, which is electrically connected to the rotor, specifically the rotor winding. The secondary side of the inductive transformer is connected to the excitation winding via a rectifier.

[0018] The excitation current for the FSM can be injected into the rotor or rotor winding via the controlled bridge circuit. The controlled bridge circuit can be designed as a controlled full-bridge circuit with four semiconductor switches arranged in the controlled full-bridge circuit. Each semiconductor switch consists of a semiconductor component such as a thyristor, an IGBT, or a MOSFET. The semiconductor switches are arranged in such a way that they are capable of controlling a current in the FSM. The controlled full-bridge circuit is controlled by a control unit that sends control signals to the semiconductor switches to control their respective switching states. The controlled full-bridge circuit can also be combined with other components, such as a transformer and a rectifier, to ensure an effective power supply for the FSM.Instead of the controlled full-bridge circuit, a controlled half-bridge circuit can also be used. This consists of only two semiconductor switches.

[0019] The evaluation can also include saving the sampled values ​​of the primary current at the time of evaluation. Since the sampling is performed at up to 400 kHz, the sampled values ​​can be temporarily stored and subsequently averaged or processed. The evaluation of the sensor data from the current sensor or the stored data can be performed using a measuring device and / or a measuring computer. The measuring computer can be implemented as a standard PC or in the cloud as a combination of both.

[0020] The issue of dead times with IGBTs must also be considered, which is why arbitrarily high switching frequencies cannot be achieved. The dead time of IGBTs is the period of time during which the IGBT cannot be immediately switched back on after being switched off. This ensures that the IGBT is completely off before being switched on again. The exact dead time can vary depending on the manufacturer and model and is typically in the range of a few nanoseconds to a few microseconds.

[0021] In a controlled full-bridge circuit, only two semiconductor switches of a semiconductor switch pair may be closed at the same time. These two semiconductor switches form a bridge between the two output terminals A and B of the controlled full-bridge circuit. The two semiconductor switches that are closed at the same time are usually referred to as the "upper" and "lower" semiconductor switches because they are located on different sides of the bridge. If both semiconductor switches are connected on one side of a controlled full-bridge circuit, a short circuit occurs. Compliance with the dead time is also important when switching the semiconductor switches, because failure to comply could result in a so-called "hot path". This means that an IGBT has not yet regained its full blocking effect and could become conductive again when voltage is applied, resulting in a short circuit.In a controlled half-bridge circuit, only one semiconductor switch may be switched on at a time.

[0022] To convert a DC voltage applied to the controlled full-bridge circuit into an AC voltage, a periodic switch is made between a positive and a negative voltage level. This generates an AC voltage with a nearly rectangular waveform. The influences of inductances or other effects are not taken into account.

[0023] A switching parameter for controlling the controlled bridge circuit is a duty cycle. Alternatively or additionally, the switching parameter includes the setting of a phase for parameterizing a phase-shifted bridge circuit or the bridge circuit in phase-shifted operation. Phase-shifted operation can typically be used in full-bridge circuits.

[0024] The semiconductor switches are usually controlled by a control unit that controls the switching frequency and the duty cycle of the circuit. The duty cycle refers to the ratio between the time the semiconductor switches of the semiconductor switch pair are switched on, i.e. their duty cycle, and the total time of the switching cycle of one period. In a preferred embodiment, the duty cycle is the ratio of the duty cycle of the semiconductor switch pairs to the period of the controlled bridge circuit. The duty cycles of the semiconductor switch pairs are added together to calculate the duty cycle. The duty cycle is normally expressed as a percentage and indicates how long the semiconductor switch is switched on during one switching cycle. The duty cycle varies between 0% and 100%. The duty cycle relates to the paired semiconductor switch pairs of two semiconductor switches each.In a half-bridge, the duty cycle is related to the two semiconductor switches present. A higher duty cycle means that the semiconductor switches are switched on for longer and therefore more current flows through the circuit. The duty cycle is an important parameter when controlling the power supply of an FSM because it influences the performance and efficiency of the circuit. The duty cycle for the controlled bridge circuit of the paired semiconductor switch pairs is, for example, 100% if the respective semiconductor switch pairs are each switched on for 50% of the total period. In this case, the active phases of the semiconductor switch pairs are the same size. Even with smaller duty cycles, the ratio of the active phases of the semiconductor switch pairs can be identical in a preferred embodiment.The period duration (T) refers to the total duty cycle of the controlled bridge circuit until a switching pattern of the controlled bridge circuit is repeated at the terminals A and B.

[0025] In a preferred embodiment, a switching parameter of the controlled bridge circuit is a switching frequency, which is generated by PWM (pulse width modulation), in particular for controlling the semiconductor switches, wherein the switching frequency refers to the switching frequency of the semiconductor switches. The PWM can be implemented, for example, as unipolar or bipolar PWM. This can be controlled by a square wave or a triangular wave as the trigger signal. With a triangular wave as the trigger signal, the semiconductor switch is switched on when the trigger signal exceeds a certain threshold. The bridge circuit can also be operated as a phase-shifted bridge circuit.

[0026] In a phase-shifted bridge circuit, the switching of the four semiconductor switches is controlled in such a way that a phase shift is visible between the two legs of the bridge, or rather their output signals, between terminals A and B. This phase shift enables interleaved operation of the semiconductor switches.

[0027] By carefully controlling the timing and phase shift, a phase-shifted bridge circuit can achieve high efficiency and high power density. The high efficiency is achieved by the zero-voltage switching of the semiconductor switches.

[0028] The transfer of electrical energy between the FSM terminals A and B and the rotor's excitation winding occurs inductively. Due to this design, a transformer equivalent circuit can be used as a first approximation to explain the electrical transmission characteristics of the inductive transformer.

[0029] A voltage U is applied to the primary side of the inductive transformer via the bridge circuit via the connection terminals A and B of the FSM. AB created.

[0030] A current driven by this on the primary side consists of two components, a magnetizing current and a secondary current multiplied by a transformation ratio ü.

[0031] The formulaic relationship between a current at an output of the bridge circuit, which corresponds to the primary current I1 of the inductive transformer, and the secondary current I2 at the output of the inductive transformer, which corresponds to the rotor current, and the magnetizing current I M is: I1=IM−I2∗u¨

[0032] The magnetizing current I Mis the current required to build up the magnetic field in the inductive transformer. The primary current I1 is called the primary auxiliary current. It flows in addition to the magnetizing current I M and is usually much larger than this.

[0033] A voltage U S on the secondary side of the inductive transformer can be determined approximately, neglecting side effects such as stray inductances, as follows: Us=u¨∗UAB

[0034] The secondary current I2, which flows on the secondary side from the output of the inductive transformer into the rotor, is largely determined by a rotor inductance L R and a rotor resistance R R certainly.

[0035] This is rectified using a rectifier circuit: IS=|I2| I Sis a secondary current I2 rectified by the rectifier circuit. In a preferred embodiment, the rectifier circuit can be integrated into the rotor of the FSM. The rotor winding is supplied with the current I S fed.

[0036] Neglecting the influence of the rectifier circuit, the following applies for a voltage at the rotor U S : US=LR∗disdt+RR∗IS

[0037] When the voltage at terminals A and B on the primary side is reversed, the voltage on the secondary side of the inductive transformer also changes. The rectifier circuit on the secondary side regenerates the pure DC voltage required by the FSM.

[0038] The magnetizing current I M changes depending on the voltage U applied to the terminals AB as follows: UAB=L∗dIMdt or IM(t)=1L∫uAB(t)dt

[0039] Thus, the secondary current or magnetizing current can only change if a voltage is also applied, with the current change being proportional to the voltage level and time. Due to the high rotor inductance at high switching frequencies, the rotor current can be assumed to be constant, while the magnetizing current oscillates periodically around a zero crossing due to the comparatively small main inductance of the inductive transformer. This means that the current change in the magnetizing current when a primary voltage is applied in a time interval is just as large as the opposite current change in the magnetizing current when an opposite voltage of the same magnitude is applied. Thus, the impressed current has a zero crossing in every cycle. For example, in the first period with a positive input voltage, a current change of i max to -i maxIn the subsequent period with negative input voltage, a current change of -i max to i max As already explained, the current change occurs symmetrically to a zero crossing in each period.

[0040] The term "zero crossing" encompasses a region around the zero crossing. A zero crossing occurs when the magnetizing current is less than a threshold value, especially when the threshold value is around or equal to zero.

[0041] The magnetizing current I M is periodic around the zero crossing. Since the current change is linear in the first approximation, it reaches TEin+2 or a point in time TEin−2 one of the controlled bridge circuits each time the zero crossing. The duty cycle refers to a conducting phase of a bridge branch or a semiconductor switch pair. Thus, for the time TEin+2 or the time TEin−2 with I M = 0 can be assumed: I1u¨=−I2

[0042] In a further preferred embodiment, the evaluation time of the primary-side current, i.e. the time at which the primary current is sampled, is half the switch-on time (TEin+2,TEin−2) the controlled bridge circuit.

[0043] The primary current visible on the input side of the inductive transformer is essentially composed of the magnetizing current and the secondary current flowing into the rotor (rotor current). At the time TEin+2 or at the time TEin−2 The primary current equals the secondary current, since at this point the magnetizing current is zero. Thus, the rotor current can be measured directly.

[0044] In a preferred embodiment, the evaluation time of the primary current is thus at the time at which the magnetizing current I M The zero crossing occurs due to the main inductance of the inductive transformer of the FSM's excitation winding. The evaluation time can also be corrected by a time offset value. The offset value can have a positive or negative sign. This may be necessary if a shift in the evaluation time is necessary due to measurement delays or other influences.

[0045] In order to determine the rotor current from the measured primary current, it must be converted to the secondary current. The conversion from primary current to rotor current is done using the equation I1 = -I 2 *ü, with I1 equal to primary current and I2 equal to rotor current or secondary current and ü equal to the transformation ratio.

[0046] The transformation ratio is the ratio between the number of secondary windings N2 on the secondary side and the number of primary windings N1 on the primary side. Therefore, I1I2=N2N1=u¨ for I M = 0.

[0047] In a preferred embodiment, the primary current is evaluated only at the time of evaluation and converted into the rotor current. The evaluation can be performed using a current sensor.

[0048] The evaluation time occurs when the magnetizing current is smaller than the threshold value, in particular zero.

[0049] In a further preferred embodiment, delays in the measurement are taken into account by shifting the evaluation time by an offset value.

[0050] In a further preferred embodiment, a plurality of sample values ​​are averaged at the time of evaluation.

[0051] The invention also relates to a device for determining the rotor current of a separately excited synchronous machine (FSM), comprising a stator and a rotor with an excitation winding. This device further comprises the following features: - the FSM with terminals A and B for the excitation winding; - a controlled bridge circuit with terminals A and B for supplying the excitation winding via an inductive transformer; - a control module which is designed to control the controlled bridge circuit; - a measuring module designed to sample the primary current of the FSM; - an evaluation module which is designed to: ◯ to determine an evaluation time of the sampled primary current; ◯ to determine the primary current at the time of evaluation; ◯ convert the primary current to the rotor current at the time of evaluation; and ◯ output the rotor current at the time of evaluation.

[0052] In a further preferred embodiment, the controlled bridge circuit is designed to be single-phase.

[0053] The invention is described below purely by way of example with reference to the drawings. They show: Fig. 1 shows a controlled full-bridge circuit with the excitation winding of an FSM and inductive transformer according to an embodiment of the invention; and Fig. 2 shows a model course of the currents in the inductive transformer in an embodiment of the invention.

[0054] Fig. 1 shows a device for determining rotor current 100 comprising a controlled bridge circuit 110, which is designed as a full-bridge circuit, with an excitation winding 121 and an FSM 120. Only a part of the FSM 120 is shown, namely the excitation winding 121, an inductive transformer 122, and a rectifier 123. The excitation winding 121 is fed via the inductive transformer 122 via connection terminals A and B. The voltage supply is provided by a DC voltage source 130. The DC voltage of the DC voltage source 130 is converted into an approximately square-wave voltage by a controlled bridge circuit 110. For this purpose, the controlled bridge circuit has four semiconductor switches V1 - V4. The semiconductor switches V1 - V4 may only be connected in pairs, and only one semiconductor switch V1 - V4 may be connected in each branch. This means that only the following pairings are permitted: V1 with V4, V2 with V3.If V1 is connected to V2 or V3 to V4, this will result in a short circuit. The primary current is measured by a current sensor 140. The alternating voltage transmitted via the inductive transformer 122 is rectified via a rectifier 123, which is integrated into the FSM 120.

[0055] Fig. Figure 2 shows a model of the currents in the inductive transformer 122 and serves for illustration purposes. The primary current 220 increases in a ramp-like manner in the phase of an applied voltage. At the same time, the magnetizing current 230 through the main inductance of the inductive transformer 122 also increases. At a time TEin+2 in a positive half-wave of the secondary current 210 or at a time TEin−2 In a negative half-cycle of the secondary current 210, the magnetizing current 230 reaches its zero crossing 231. Thus, I1 = -I2 ∗ ü. The result of the difference between the primary current 220 and the magnetizing current 230 is the secondary current 210, or the desired rotor current, which is shown with a rectangular curve, neglecting side effects. The duty cycle T EINdenotes a conductive phase of a bridge branch or of a semiconductor switch pairing V1 with V4 or V2 with V3. In other words, this is the state when voltage is applied between the connection terminals A and B. A period (T) denotes the entire switch-on time of the controlled bridge circuit 110 until a switching pattern of the controlled bridge circuit 110 is repeated at the connection terminals A and B. The duty cycle for a semiconductor switch pairing V1 with V4 or V2 with V3 would be 100% in the example shown for the controlled bridge circuit 110, since the respective semiconductor switches V1 with V4 or V2 with V3 are each active for 50% of the entire period T. The active phases of the semiconductor switches V1 with V4 and V2 with V3 are the same in the example shown. Even with smaller duty cycles, the ratio of the active phases of the semiconductor switches V1 with V4 and V2 with V3 must be identical.Time is plotted on the X-axis 240, and the current in amperes is plotted on the Y-axis 250. Due to the very large rotor inductance, the secondary current 210, or the desired rotor current, can be assumed to be constant within one switching period of the semiconductor switches V1 with V4 and V2 with V3, respectively.

[0056] The invention is not limited to the described embodiments. Within the scope of the invention, all described and / or illustrated features can be combined with one another in any way, unless otherwise stated. List of reference symbols 100 Device for determining rotor current 110 Controlled bridge circuit 120 Separately excited synchronous machine, FSM 121 Excitation winding 122 Inductive transformer 123 rectifiers 130 DC voltage source 140 current sensor A connection terminal B connection terminal V1 First semiconductor switch V2 Second semiconductor switch V3 Third semiconductor switch V4 Fourth semiconductor switch 210 Secondary current / Rotor current 220 primary current 230 magnetizing current 231 zero crossing 240 time 250 amperage T EIN Duty cycle time time T period duration

Claims

[1] Method for determining the rotor current of a separately excited synchronous machine, FSM, (120) comprising a stator and a rotor with an excitation winding (121), comprising the following method steps: - Applying an excitation supply voltage via a controlled bridge circuit (110) to terminals A and B of the excitation winding (121) of the FSM (120) via an inductive transformer (122); - Setting switching parameters of the controlled bridge circuit (110) via a control unit; - Sampling a primary current (220) of the excitation winding (121) of the FSM (120) at the terminals A and B of the excitation winding (121); - Determining an evaluation time of the sampled primary current (220); - Evaluating the primary current (220) at the time of evaluation; - Conversion of the evaluated primary current (220) to a rotor current (210) at the time of evaluation; - Output of the rotor current at the time of evaluation, characterized in that - a switching parameter of the controlled bridge circuit (110) is a duty cycle and / or the setting of a phase for parameterizing the controlled bridge circuit (110) in a phase-shifted operation; - the conversion from primary current (220) to rotor current (210) is done via the equation I1 = -I2 ∗ ü, with I1 equal to primary current (220) and I2 equal to rotor current (210) and ü equal to the transmission ratio. [2] Method according to claim 1, characterized by , that: - a switching parameter of the controlled bridge circuit (110) is a switching frequency generated by a PWM. [3] Method according to any one of the preceding claims, characterized by , that: - the duty cycle is a ratio of one on-time (T) EIN ) to a period (T) of a semiconductor switch pair of the controlled bridge circuit (110). [4] Method according to any one of the preceding claims, characterized by , that: - the evaluation time of the primary current (220) at half the switch-on time (TEin+2,TEin−2) the controlled bridge circuit (110). [5] Method according to any one of the preceding claims, characterized by , that: - the evaluation time of the primary current (220) corresponds to the time at which a magnetizing current (230) through a main inductance of the inductive transformer of the excitation winding (121) of the FSM (120) has a zero crossing (231). [6] Method according to any one of the preceding claims, characterized by , that: - the primary current (220) is only evaluated at the time of evaluation and converted into the rotor current (210). [7] Method according to any of the preceding claims, characterized by , that: - a large number of sampled values ​​are averaged at the time of evaluation.

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