Design and implementation of brushless doubly fed induction machine with new stator winding configuration

Abstract

Brushless doubly fed induction machine has acquired relevance as a wind electric generator because of its relative merits over doubly fed induction generator. However, the main drawback of brushless doubly fed induction machine is the presence of torque ripples due to spatial and time harmonics caused by two stator windings and complex rotor structure. In this article, a special design of brushless doubly fed induction machine using delta-star connection in one of the two stator windings is proposed to reduce the torque ripples. Simulation of the new brushless doubly fed induction machine design is performed in ANSYS Maxwell software, and the results when compared with the conventional winding design validated the effectiveness of the new design in minimizing the torque ripples. Prototype of the new brushless doubly fed induction machine has been fabricated and tested in laboratory. Tests have been conducted in both synchronous and asynchronous modes of brushless doubly fed induction machine. Simple induction and cascade connections have been tested in asynchronous motoring mode. Motoring as well as generating conditions have been tested in synchronous mode. Test results show that the new brushless doubly fed induction machine has not only the desired characteristics for wind turbine generator but also capabilities suited for variable torque–variable speed motor applications as well as constant speed applications.

Keywords

Brushless doubly fed induction machine synchronous power winding asynchronous power winding delta-star winding simple induction mode cascade mode synchronous mode

Introduction

In recent years, generation of electricity from renewable energy sources has gained importance because of its minimal environmental impacts. Among the various renewable energy sources, the potential of offshore wind farm is enormous. With the advancement in technology, offshore wind farms are becoming more efficient and less expensive (Yogesh et al., 2016). For medium and large wind turbines in offshore, permanent magnet synchronous generator (PMSG) and doubly fed induction generator (DFIG) are preferred because of higher power conversion efficiency. Main drawbacks of these generators are the presence of slip rings in DFIG and the usage of permanent magnets in PMSG, which make the system cost effective requiring more maintenance (Strous, 2016). An alternative to avoid these drawbacks is the use of brushless doubly fed induction machine (BDFIM), which has robust construction with the absence of slip rings and use of fractionally rated converters making the system cost-effective with superior controllability (Serous et al., 2017). In spite of the advantages, BDFIM has demerits such as more complex structure, larger dimensions compared to conventional machines of the same rating, spatial harmonics in air gap due to two stator windings, and complex rotor structure which have impact on the speed–torque characteristics of the machine (Logan et al., 2014; Serous et al., 2017). Published literature on BDFIM dealt with mathematical modeling, causes and origin of torque ripples, design of rotors for reducing torque ripples, optimal design and development of BDFIM, and control algorithms of BDFIM for maximum power point tracking as variable speed wind electric generator (Gorginpour et al., 2013; Resmi and Vanitha, 2015; Roberts et al., 2004; Suresh et al., 2015). Tohidi et al. (2014) compared the low-voltage ride through the capability of BDFIM as a generator with that of DFIG. Han et al. (2017) carried out power/torque density optimization-based design procedure for dual stator BDFIM to eliminate the harmonic-related problems and analyzed the same with finite element method. F. Zhang et al. (2018) developed a new hardware of rotor named hybrid rotor for BDFIM to reduce harmonic distortion compared to radial magnetic barrier rotor. Moros and Gerling (2014) carried out the optimization of electrical machine with wye-delta winding to reduce ohmic resistance, saving in conductor material and even heat distribution without causing any negative impact on the machine. In Kumaresan and Subbiah (2003), squirrel cage induction generator with delta-star winding is proposed for wind turbine generator to enhance the generation of real power and reduce consumption of reactive power. Onur Misir and Ponick (2014), Misir et al. (2016, 2017), Raziee et al. (2017), and Kasten and Hofmann (2011) discuss the use of delta-star winding in standard induction motors for industrial purposes for improving efficiency by incrementing the fundamental winding factor. Lei et al. (2011) present the impact of delta-star winding in the reduction of spatial harmonics and stray losses in electrical machines.

All the published research related to delta-star winding has been carried out in standard induction machine only and not in BDFIM. This article involves design, analysis, and development of a three-phase, 400-V, 3.5-kW, 2/6 pole BDFIM with two stator windings, delta-star in one and star in another, along with nested-loop rotor. Simulation of the new BDFIM design is done in finite element method–based ANSYS Maxwell software and the results when compared with the conventional winding design validated the effectiveness of the new design in minimizing the torque ripples. The author’s design of star/delta-star BDFIM gained 20% reduction in torque ripple when compared with the conventional delta/delta design. Prototype of the new BDFIM has been fabricated and tested in laboratory. Tests have been conducted in simple induction mode, cascade mode, and synchronous mode of BDFIM in motoring as well as generating conditions. Results of motoring mode showed a wide range of speed–torque combinations of BDFIM in different modes using different combinations of windings and connections, so that it can be used in applications where wide speed range is required. Results of star/delta-star BDFIM operated in generator mode show a speed change up to 67%; thus, it is better suited for wind power applications than DFIG as the latter has much less operating range of shaft speed.

Construction and operation of BDFIM

BDFIM has two stator windings with different numbers of pole pairs in the same casing, which are connected independently to different three-phase sources. The stator winding connected directly to the grid is hereby named as synchronous power winding (SPW), and the other stator winding that is connected to the grid via fractionally rated bi-directional converter is named as asynchronous power winding (APW). The numbers of pole pairs are selected such that direct electromagnetic coupling between the two stator windings is avoided so that the mutual inductance and hence harmonics can be reduced leading to less torque ripples. This also helps in avoiding any unbalanced magnetic pull on the rotor (Lei et al., 2011).

In this study, three pole pairs for SPW and one pole pair for APW have been selected resulting in zero mutual inductance between the two stator windings. The rotor of BDFIM is designed as a nested-loop type, which cross couples magnetic fields produced by the two stator windings and runs at synchronous speed (Roberts, 2004). To obtain the desired cross-coupling, the currents induced by SPW and APW in the rotor bars should be at the same frequency; to achieve this, the number of rotor nests should be equal to the sum of pole pairs of two stator windings. The flow of real power in the stator windings depends on whether the machine is motoring or generating in synchronous mode or asynchronous mode (Abdi et al., 2013a; McMahon et al., 2006).

In synchronous mode, the two stator windings are connected separately to two different power sources, one directly to the grid and the other to a variable voltage–variable frequency source. The synchronous speed of BDFIM in synchronous mode is

Ns = \frac{120 (f_{S P} - f_{A P})}{P_{S P} + P_{A P}}

(1)

where f_AP is the frequency of supply given to APW, f_SP is the frequency of supply given to SPW, P_SP is the number of poles in SPW, and P_AP is the number of poles in APW. If the phase sequence of SPW is the same as that of APW, positive sign is to be used; otherwise, negative sign is to be used.

There are two types of asynchronous mode in which BDFIM can operate, namely the simple induction mode and the cascade induction mode. In simple induction mode, one of the two stator windings is open-circuited and the other is excited by the three-phase grid source. In this case, the machine acts similar to an induction machine with synchronous speed given by either P_SP or P_AP poles, depending on whether SPW or APW is connected. In cascade induction mode, one stator winding is short-circuited and the other is excited by the three-phase grid source. Here, the machine acts as an induction machine with synchronous speed given by (P_SP + P_AP) poles.

Design of BDFIM

Machine specifications

The design of electrical machines in fact presents a mathematically indeterminate problem with many solutions, as the number of equations is less than the number of unknowns. The presence of two stator windings and a special type of rotor construction make the design of BDFIM slightly different from a normal induction machine. A three-phase, 400-V, 3.5-kW, 2/6 pole BDFIM has been designed, constructed, and used in this work. The major specifications and design parameters of the machine are given in Table 1. Nested-loop rotor structure houses the copper bars in the rotor slots as conductors. The stator inner diameter is fixed as 190 mm by considering the dimension of standard stator stamping. Wire cutting method is used to increase the slot area in stator to accommodate both SPW and APW. Double-layer lap winding is used for both. Skewing by one stator slot pitch is provided in the stator. The space provided for SPW and APW are based on the power flow in each winding, which depends on the fraction of total number of poles. Powers through SPW and APW are calculated as 2.625 and 0.875 kW, respectively, and accordingly, the stator and rotor slots are designed. Standard output equation and design equations are used to find the machine dimensions (Boldea, 2009; Han et al., 2016; Sawhney, 2013).

Table 1.

Design parameters of BDFIM.

Design parameters	SPW	APW
Rated power	3.5 kW
Rated voltage	400 V
Average flux density	0.45 T
Number of phases	3
Power factor	0.8
Current density in stator windings	4 A/mm²
Current density in rotor bars	3 A/mm²
No. of poles	6	2
Rated current (A)	5.6	7
Frequency (Hz)	50	10–20
Rated output (kW)	2.625	0.875
No. of conductors per slot in stator	54	34 (Delta), 20 (Star)
Insulation class	F
Axial length (mm)	127
Air gap length (mm)	0.5
Stator outer diameter (mm)	304
Stator inner diameter (mm)	190
Number of stator slots/rotor slots	36/40
Stator core depth (mm)	12
Rotor core depth (mm)	16.45
Rotor bar depth (mm)	30
Rotor bar width (mm)	7
Shaft diameter (mm)	40

BDFIM: brushless doubly fed induction machine; SPW: synchronous power winding; APW: asynchronous power winding.

Winding connections

Double-layer, lap, full-pitch, star winding is used for the 6-pole SPW, which is of the conventional type, whereas double-layer, lap, short-pitch, delta-star winding is used for the 2-pole APW. In delta-star winding, a conventional 60° phase spread is divided into two parts as star and delta and is connected in parallel to a single three-phase supply, which is shown in Figure 1.

Figure 1.

Star/delta-star connection in SPW/APW.

While three-phase winding and six-phase winding have spread factors as 0.827 and 0.955, respectively (Misir et al., 2017), the delta-star winding has a spreading factor of 0.987, which is almost equal to that of a 12-phase winding; it gives higher output power and also improves the waveform of magnetomotive force, thereby reducing harmonics. In order to avoid circulating currents in APW, electromotive forces induced in the delta and star windings need to be at the ratio of 1.732:1 (Moros and Gerling, 2014). Figure 1 shows the two stator winding connections of BDFIM. Table 2 provides the coils allotted for delta and star windings in APW and Table 3 provides the coils allotted for SPW winding. Figure 2 shows the connections of the nested-loop rotor.

Table 2.

Delta-star double-layer, lap winding connection for 3 phases, 2 poles, and 36 slots.

Section	Coil numbers with polarity
Delta winding
R1-R2 Top layer	1, 2, 3, 4, 5, 6, –19, –20, –21, –22, –23, –24
R1-R2 Bottom layer	1, 2, 3, 4, 35, 36, –17, –18, –19, –20, –21, –22
B1-B2 Top layer	13, 14, 15, 16, 17, 18, –31, –32, –33, –34, –35, –36
B1-B2 Bottom layer	11, 12, 13, 14, 15, 16, –29, –30, –31, –32, –33, –34
Y1-Y2 Top layer	25, 26, 27, 28, 29, 30, –7, –8, –9, –10, –11, –12
Y1-Y2 Bottom layer	23, 24, 25, 26, 27, 28, –5, –6, –7, –8, –9, –10
Star winding
R1′-R2′ Top layer	1, 2, 3, 4, 5, 6, –19, –20, –21, –22, –23, –24
R1′-R2′Bottom layer	1, 2, 3, 4, 35, 36, –17, –18, –19, –20, –21, –22
B1′-B2′ Top layer	13, 14, 15, 16, 17, 18, –31, –32, –33, –34, –35, –36
B1′-B2′ Bottom layer	11, 12, 13, 14, 15, 16, –29, –30, –31, –32, –33, –34
Y1′-Y2′ Top layer	25, 26, 27, 28, 29, 30, –7, –8, –9, –10, –11, –12
Y1′-Y2′ Bottom layer	23, 24, 25, 26, 27, 28, –5, –6, –7, –8, –9, –10

Table 3.

Double-layer, lap winding connection for 3 phases, 6 poles, and 36 slots.

Section	Coil numbers with polarity
Star winding
R1-R2 Top layer	1, 2, 3, 4, 5, 6, –19, –20, –21, –22, –23, –24
R1-R2 Bottom layer	1, 2, 3, 4, 35, 36, –17, –18, –19, –20, –21, –22
B1-R2 Top layer	13, 14, 15, 16, 17, 18, –31, –32, –33, –34, –35, –36
B1-B2 Bottom layer	11, 12, 13, 14, 15, 16, –29, –30, –31, –32, –33, –34
Y1-Y2 Top layer	25, 26, 27, 28, 29, 30, –7, –8, –9, –10, –11, –12
Y1-Y2 Bottom layer	23, 24, 25, 26, 27, 28, –5, –6, –7, –8, –9, –10

Figure 2.

Nested-loop rotor bars.

Analysis of BDFIM design using ANSYS Maxwell software

BDFIM designs in the past used star and delta windings in SPW and APW. Delta-star winding is considered in the present design of 2/6 pole BDFIM for APW in order to address the major issue of spatial harmonics and torque ripples. The newly designed BDFIM is simulated and analyzed in ANSYS Maxwell software. Figure 3(a) and (b) gives the flux density distribution of BDFIM when the two stator windings are connected in delta/delta (both windings in delta) and star/delta-star (SPW in star and APW in delta-star) configurations, respectively. The BDFIM design had chosen the average flux density as 0.45 T as suggested in Lei et al. (2011). The software analysis found that the maximum flux density in star/delta-star configuration is more than that of delta/delta configuration, giving more output. Table 4 gives the maximum flux density in different parts of BDFIM, in the two configurations as obtained from the simulation. It is seen that all values are within the limits and star/delta-star gives better performance than delta/delta. Distributions of flux density for the two configurations in the radial air gap of BDFIM operated in synchronous mode are shown in Figure 4(a) and (b). The two profiles are compared based on total harmonic distortion (THD) and the respective values are given in Table 5. In Gorginpour et al. (2011), it is found that the THD in the space harmonics is coming as 39.87 for 2/4 pole configuration BDFIM with the standard winding connection. It is found that star/delta-star performance is better than that of the other from Table 5.

Figure 3.

Flux density distribution of BDFIM with two stator windings connected in (a) delta/delta and (b) star/delta-star.

Table 4.

Flux density distribution in BDFIM.

Winding connection	B (design)	B (simulation)
Winding connection	B (design)	Star/delta-star	Delta/delta
B_max in stator teeth (T)	1.4–2.1	1.95	1.7
B_max in rotor teeth (T)	1.5–2.2	2	1.75
B_max in stator yoke (T)	1.1–1.7	1.45	1.35
B_max in rotor yoke (T)	1.2–1.7	1.65	1.35
B_max in air gap (T)	0.65–0.82	0.75	0.68

BDFIM: brushless doubly fed induction machine.

Figure 4.

Radial air gap flux density distribution in synchronous mode: (a) delta/delta and (b) star/delta-star.

Table 5.

Air gap flux density THD in BDFIM.

Winding connection	Delta/delta	Star/delta-star
Air gap flux density THD (%)	28.74	21.98

THD: total harmonic distortion; BDFIM: brushless doubly fed induction machine.

Figure 5(a) and (b) shows the torque of 2/6 pole BDFIM on load in the synchronous mode for delta/delta and star/delta-star configurations, respectively. Table 6 shows the % torque ripples of BDFIM in the two configurations. It proves that when the machine is connected in star/delta-star, the torque ripple is less by 20%.

Figure 5.

Torque ripple in synchronous mode: (a) delta/delta and (b) star/delta-star.

Table 6.

Torque ripple percentage in BDFIM.

Connection	Delta/delta	Star/delta-star
T_max (N m)	26	23
T_min (N m)	16	17
T_mean (N m)	20.5	20.5
T_ripple (%)	48.78	29.26

BDFIM: brushless doubly fed induction machine.

Fabrication of BDFIM

Designed machine is fabricated using the stator and rotor stampings, as shown in Figures 6 and 7, respectively. Figure 8 shows the stator core with 2-pole windings and 6-pole windings and Figure 9 shows the nested-loop rotor. Twelve terminals are taken out from the delta-star (APW) winding and six terminals are taken out from the star (SPW) winding. The rotor winding consists of nests; the number of nests is equal to the sum of pole pairs of stator windings, which is 4. As the number of rotor slots is 40, the number of loops inside each nest is 5. Figure 10(a) shows the complete prototype model of BDFIM coupled to a DC machine for testing and Figure 10(b) shows the stator winding terminals of BDFIM.

Figure 6.

Stator stamping.

Figure 7.

Rotor stamping.

Figure 8.

Stator core with two windings.

Figure 9.

Nested-loop rotor.

Figure 10.

(a) Prototype of BDFIM and (b) stator winding terminals.

Testing of BDFIM

Asynchronous modes of operation

The prototype BDFIM has first been tested in the two asynchronous modes of operation. Figure 11 provides the speed–torque and speed–power plots of BDFIM in simple induction and cascade induction modes of operation of the fabricated BDFIM when tested as motor on load. Torque is calculated from the measured values of power output of the BDFIM and corresponding shaft speed. BDFIM output power is measured as the power absorbed by the generator coupled with it in the test setup. The induction mode with SPW gives a maximum torque of 11 N m and a speed range between 666 and 982 r/min, whereas the induction mode with APW gives the maximum torque as 6 N m and speed range from 1496 to 2500 r/min. SPW in cascade mode gives the highest torque as 14 N m and the speed ranges from 738 to 758 r/min. APW in cascade mode gives the maximum torque of 34 N m with a range of speed from 195 to 750 r/min. It is observed that BDFIM can be operated in a wide range of speed and torque under different asynchronous modes of operation. The load test has been performed for BDFIM power output up to 1 kW. While in cases (a), (b), and (c), the output power increased with decreasing speed, the power reaches its peak and then decreases further with decreasing speed in (d). BDFIM shows here capability to operate at different combinations of speed and torque by changing winding connections and configurations.

Figure 11.

Speed–torque and speed–power plots of BDFIM: (a) simple induction mode with excited SPW and open APW, (b) simple induction mode with excited APW and open SPW, (c) cascade induction mode with excited SPW and shorted APW, and (d) cascade induction mode with excited APW and shorted SPW.

Synchronous mode of operation

Synchronous motoring mode

To operate in synchronous motoring mode, BDFIM is started in the cascade induction mode and then switched over to synchronous mode. SPW is connected to the three-phase, 415-V, 50-Hz grid supply, and APW is connected to a grid powered rectifier–inverter unit, where the inverter is operated in variable voltage, variable frequency (VVVF) mode.

Figure 12 shows the single line diagram of the test setup of BDFIM in synchronous motoring mode. Separately excited DC machine coupled to BDFIM acts as a generator which is loaded by a resistor. Figure 13 presents speed–torque and speed–power plots of BDFIM in synchronous motoring mode with varying load. Results show that the power drawn by the motor increases when the load is increased and correspondingly, the rotor speed decreases. The motor is tested for load up to 1.4 kW that yielded a slip of 7%.

Figure 12.

Test setup of BDFIM during motoring mode.

Figure 13.

Speed–torque and speed–power plots of prototype BDFIM in synchronous motoring mode.

The synchronous motoring test has also been carried out for different load levels while maintaining the speed constant by suitably varying the APW frequency; Figure 14 shows speed–torque and speed–power plots of BDFIM in synchronous motoring mode for various loading conditions when speed is maintained constant. The results confirm the suitability of BDFIM in constant speed–variable torque applications too. Torque ripple measurement in the prototype could not be carried out due to the absence of torque transducer in the test setup.

Figure 14.

Speed–torque and speed–power plots of prototype BDFIM in synchronous motoring mode maintaining constant speed at various loads.

Synchronous generating mode

The DC machine coupled with the BDFIM is operated as the prime mover for BDFIM in the generating mode of operation. SPW is directly connected to three-phase 415-V, 50-Hz grid supply and APW is connected to a variable resistance load. Figure 15 shows the single line diagram of the test setup of BDFIM in synchronous generating mode.

Figure 15.

Test setup of BDFIM during generating mode.

The load test has been carried out and the result is provided in Figure 16 as a plot of generated power against shaft speed. With the increase in power input to the generator, the power delivered to the grid increased as well as the shaft speed. That the speed varies with power is the desired feature for use as wind turbine generator. A variation in shaft speed above 30% has been observed in the test, which is similar to the published results in the literature (Benelghali et al., 2012; Mahvash et al., 2017). The load test had constraints of prime mover capacity and current rating of winding; otherwise, further increase in APW frequency could yield yet higher speed for the generator. Shaft speed variation of 40% by 6 MW brushless doubly fed induction generator (BDFIG) is reported in Abdi et al. (2013b). A higher loading on the present star/delta-star BDFIG to match its rated capacity has been carried out in the computer-simulated model. Figure 16 shows the profile of generated power variation when plotted against the shaft speed based on this simulation results. It shows a variation from 850 to 1420 r/min in shaft speed, which means 67% change in speed between the no load and full load conditions of the generator. Such variability highly suits a wind turbine.

Figure 16.

BDFIM characteristics in synchronous generation mode.

Conclusion

This study has proved that the spatial harmonics and torque ripples in BDFIM can be reduced by appropriate choice of stator winding design and connection. The authors’ design of star/delta-star BDFIM gained 20% reduction in torque ripple when compared with the normal delta/delta design. Versatility of BDFIM as multi-characteristic motor and wind turbine generator has been major findings of the study reported here. It can assume variable torque–variable speed as well as variable torque–constant speed applications as proved in the laboratory testing of the fabricated prototype of BDFIM. The combinations of torque and speed exhibited by the prototype even promise its probable fitness in electric vehicles too. While wind turbines need shaft speed variation in line with wind speed variation, the former is decided by the coupled generator. Therefore, the star/delta-star BDFIG developed here with a possible speed change up to 67% is better suited for wind power applications than DFIG or delta/delta BDFIG as the latter has much less operating range of shaft speed.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This study was financially supported by the Amrita Vishwa Vidyapeetham, Coimbatore.

ORCID iD

R Resmi

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