Torque ripple reduction in brushless wound rotor synchronous machine by two-phase excitation winding

1. Introduction

Permanent magnet synchronous machine (PMSM) is the best in compact and high torque density applications. Whereas, wound rotor synchronous machine (WRSM) is a better alternative to a permanent magnet synchronous machine in terms of cost effectiveness and control flexibility given the high prices of rare-earth magnets. Specially, WRSM is better in large capacity machines. In an application where small capacity WRSM is used due to its low-price advantage, it must be improved in terms of design or drive topology to avoid maintenance issues. One of the inherent problems of a WRSM is its assembly of brushes and slip rings. However, WRSM can still be used in small capacity machines if the brushes and slip-rings can be removed. Moreover, the volume occupied by exciter which is needed to feed DC field current is an issue especially for compact or portable applications. The problem has been addressed in many research articles providing brushless topologies for WRSM [1–5]. One way is utilizing space harmonics power by using concentrated windings. Space harmonics exist in concentrated windings of a WRSM which convert this harmonic power into heat. Instead of converting to heat this power can be used to induce voltage in the rotor winding. However, the low space harmonic power is difficult to be used for the field winding magnetization. Therefore, a design approach focused to retrieve the harmonic power is investigated in recent years, for instance, using fractional slot concentrated winding as one of the efficient methods in this regard [6,7].

Similarly, a self-excited brushless synchronous generator, employing fifth harmonic of armature MMF to excite main field winding, has been detailed in [8]. Such schemes are heavily dependent on space-harmonics and residual magnetism, which can create voltage build-up and regulation issues [9]. Secondly, current injection by an additional inverter to create harmonics from stator windings, has also been used in latest research. Distributed winding has been used in such schemes to avoid problems related to residual magnetism [10–13]. In [10], the brushless excitation is achieved by employing an additional third harmonic current injection from another inverter in the same stator winding as shown in Fig. 1. This results in a time pulsating third harmonic magnetomotive force (MMF) along with the fundamental rotating MMF in the air-gap. Fundamental component synchronizes with field winding to produce torque and the harmonic component is used to excite the rotor winding by inducing voltage in an additional harmonic winding mounted on the rotor. The induced voltage is rectified though diode rectifier which is also mounted on the rotor and the DC voltage is fed to the field winding.

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Fig. 1.

Brushless harmonic excitation synchronous machine system. (a) Schematic of an open winding machine. (b) Schematic of semi-open winding machine.

In another topology two three-phase currents are supplied to the two different portions of stator windings by two inverters. Both inverters are controlled to supply different magnitude of currents to the two different portions of stator winding to generate fundamental and a sub-harmonic component of stator MMF for torque and rotor excitation respectively as illustrated in the topology of Fig. 2 [11].

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Fig. 2.

Sub-harmonic excitation topology of a brushless WRSM.

However, such schemes are expensive as the drive requires an additional inverter for injecting the third harmonic current to the stator windings as in [10], or in case of sub-harmonic excitation scheme, the additional inverter is required to supply a different magnitude of current to generate sub-harmonic MMF as in [11]. Therefore, a new topology was recently proposed for generating a third harmonic component to excite the rotor winding from a single power source using either an inverter as described in the illustration of Fig. 3 or conventional sinusoidal power supply [14].

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Fig. 3.

Brushless WRSM topology.

The method of utilizing the space harmonics or using an extra inverter for field magnetization makes the design more complex and problematic to the efficiency of the machine. To address the aforementioned problems with brushless wound rotor synchronous machines, recently, a new topology was proposed for generating a third harmonic component to excite the rotor field winding from single power source, either an inverter or conventional sinusoidal power supply. In each case the harmonic MMF is generated by using thyristor switches as shown in Fig. 3, instead of an additional inverter as shown in Figs 1 and 2.

In [14], the 4-pole model was designed to validate the brushless topology. The 4-pole model has a high torque ripple which is a disadvantage for the brushless WRSM to be used in any suitable application. To overcome the high torque ripple problems in [14], a new topology for rotor excitation winding is proposed in this paper. The 8-pole model used in this paper with the basic rotor circuit is shown in Fig. 4. As shown in the figure, the single-phase harmonic winding, is used and is connected with field winding through a rotating rectifier. The parameters used for the design are mentioned in Table 1 below.

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Fig. 4.

Machine model and the rotor windings showing single harmonic winding.

With the single excitation winding circuit of Fig. 4, the 8-pole model results in a high torque ripple of 88%. Since the main cause of the torque ripple in this case is the ripple in the rectified current fed from the harmonic winding to the field winding, this current ripple must be decreased. For this purpose, the harmonic winding is split in two phases in such a way that a capacitor is connected to one branch.

The structure of a brushless machine theoretically uses induction motor phenomenon to generate voltage on the rotor winding for excitation. Therefore, the two-phase harmonic winding structure of the machine is same as that of a split phase induction motor where the single-phase winding is split in two branches to create a phase difference by adding a capacitor as shown in Fig. 5 [15].

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Fig. 5.

Two-phase winding connections in induction motor.

As shown in Fig. 5, there are two windings, one is called the main winding, and the second is called the start winding connected with a capacitor. However, in this single-phase induction machine a centrifugal switch is also connected with the start winding since the winding is disconnected as the machine attains 75% rated speed.

In the proposed machine the two-phase windings are on the rotor and therefore the capacitor can be placed at the end of the rotor length adjacent to end turns of the rotor windings. The mounting of capacitor can be done by adjusting the end winding turns on one end of the rotor length. The target of the capacitor is to achieve a phase difference between the two harmonic windings. This can also be achieved by using different number of turns for each harmonic winding. This type of design can solve issues of manufacturing the machine for applications which may cause heating issues for capacitor.

In the brushless WRSM, both the harmonic windings are connected throughout the operation of the machine. The current generated in the harmonic or excitation winding has unwanted higher harmonic which also get rectified and fed to the field winding, eventually the output torque of the machine contains high ripple such as 88% torque ripple in the basic model simulated in this research.

The problem can be dealt with by using a winding accordingly, that is, using two phases to achieve the third harmonic power from stator such as using capacitor in one phase to create a phase difference in the currents of the two branches. The capacitance value can be adjusted so that to achieve lowest possible torque-ripple and, with all other parameters kept the same. The location of the capacitor is at one end of the rotor length adjacent to end turns of the rotor windings. Four capacitors of one-fourth the required capacitor rating are used to adjust the capacitors in available space by connecting them in parallel. The illustration is shown in Fig. 6(a).

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Fig. 6.

(a) Rotor geometry to show the location and connection of the capacitors in parallel (b) Machine model (c) rotor winding connections diagram.

The volume of capacitor to be adjusted is an approximate calculation depending on the type of the capacitor. However, the available area per capacitor can be calculated as below. A c = π ( r 2 2 − r 1 2 ) / n

Where A _c is the area per capacitor, r ₁ is the radius of shaft, r ₂ is the radius of inner rotor and n is the number of capacitors. Figure 6 shows the machine model and the rotor winding circuit for the proposed rotor two phase topology. Two harmonic windings are connected in parallel to each other and one winding is connected to a 0.75 mF capacitor so that there is a phase difference between the two windings. This difference in the two phases helps smooth the field current after it has been rectified. Therefore, the output torque ripple is decreased.

The proposed machine is analyzed in ANSYS Maxwell 19.0 by supplying three phase balanced currents of 22 Apeak to stator armature windings with 60 Hz frequency. Due to the capacitor’s effect, harmonic winding 1 and harmonic winding 2 have difference in voltage. This voltage difference is measured across the capacitor terminals as shown in Fig. 7(a). The RMS value of voltage across the capacitor for the simulation using 0.75 mF capacitor in steady state is 1.2081 Vrms. Since we are using two windings in parallel in the rotor, the currents induced are at a phase difference to each other as shown in the simulation result in Fig. 7(b) and (c) in transient and steady-state conditions. The waveforms of currents in the two harmonic windings during transient state were shown to illustrate the phase difference between the two currents. It can be seen in Fig. 7(b) and (c) that the instantaneous currents are different in the two harmonic windings. Current in the harmonic winding 2 which has a series connected capacitor reaches zero before the current in the harmonic winding 1.

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Fig. 7.

(a) Voltage across the capacitor (b) currents in the rotor windings in transient condition and (c) currents in the rotor windings in steady state condition.

The proposed rotor harmonic winding topology is split in two phases and the phases are distributed around the rotor slots as shown in Fig. 6. Different simulations were run using different values of capacitance which is shown in the graph of Fig. 8(a). The torque ripple decreases as the capacitance value is increased to a value of 0.75 mF. However, a detailed view shows the convergence of the graph at 750 μF as in Fig. 8(b). This is due to the maximum phase difference is created at this point. Therefore, the minimum torque ripple percentage obtained in the 8-pole 48-slot model was observed to be 37% at a capacitance of 0.75 mF.

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Fig. 8.

Torque ripple variation curve with respect to capacitance (a) total curve showing the trend and (b) detailed view to show the converging value of the capacitance.

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Fig. 9.

Torque waveforms of (a) brushed WRSM (b) basic brushless WRSM topology and (c) the two-phase harmonic winding brushless WRSM topology.

Since the rotor geometry of brushless WRSM is different than brushed WRSM, it also contributes to the ripple percentage of 88% as obtained in the basic model of Fig. 4. Therefore, simulating the machine by supplying DC current to the field winding shows only 27% ripple. This means that the remaining part of the ripple is contributed by the brushless operation. Hence, using two-phase harmonic winding topology, the ripple due to brushless operation can be reduced. This is compared with the torque waveforms or brushed WRSM, basic brushless WRSM topology and the two-phase harmonic winding topology for brushless WRSM in Fig. 9. Results show that brushed WRSM has 27% torque ripple as in Fig. 9(a), the basic brushless WRSM has torque ripple of 88% as in Fig. 9(b) and the proposed brushless WRSM has torque ripple 37% as in Fig. 9(c).

In this paper, harmonic winding is split in two phases to reduce the torque ripple in a brushless wound rotor synchronous machine (BL-WRSM). The harmonic winding of the basic topology is single phase and the proposed harmonic winding is two-phase. Proposed scheme shows a reduction from 88% ripple to approximately 37% when a capacitor of 0.75 mF is connected to one branch of the harmonic winding. The proposed technique allows this machine to be used in suitable practical application. The simulation results are based on FEM analysis in ANSYS Maxwell 16 with a time-step of 0.185 ms.