University of Connecticut OpenCommons@UConn

Honors Scholar Theses Honors Scholar Program

Spring 5-1-2018 Assisted Switched Reluctance Machines Amy Robinson [email protected]

Follow this and additional works at: https://opencommons.uconn.edu/srhonors_theses Part of the Electromagnetics and Photonics Commons

Recommended Citation Robinson, Amy, "Assisted Switched Reluctance Machines" (2018). Honors Scholar Theses. 605. https://opencommons.uconn.edu/srhonors_theses/605

Assisted Switched Reluctance Machines

University of Connecticut Honors Senior Thesis

Amy Robinson

Electrical Engineering

Advisor: Ali Bazzi

ASRM Robinson Contents

1. Abstract ...... 4 2. Introduction ...... 4 2.1 Overview of Step Machines ...... 4 3. Issues of SRMs...... 7 3.1 Torque Ripple...... 7 3.2 Current Spikes ...... 8 3.3 Control Issues ...... 9 3.4 Fixes and Uses of SRMs ...... 9 4. Assisted SRM ...... 12 4.1 Two Permanent in Yoke ...... 12 4.2 Permanent Magnets in Stator Teeth ...... 13 4.3 Permanent Magnets in the Air Gap ...... 14 4.4 Magnets bridging stator teeth in front of windings ...... 15 4.5 PM ASRM Summary ...... 17 5. Winding Replacements ...... 17 5.1 DC windings that share Phase windings space ...... 18 5.1.1 Windings Made to Increase Torque Density ...... 18 5.1.2 Windings Made to Increase Torque Density and Speed Range ...... 19 5.1.3 Windings Made to Reduce Torque Ripple and Upper Speed Limit...... 19 5.1.4 Smoothing Torque Ripple and Back EMF ...... 20 5.2 Windings that wrap around half/quarter/eight of stator back ...... 21 5.2.1 Windings Made to Redirect Stored Magnetic Field Energy ...... 21 5.3 Windings That Wrap Around Stator Yoke ...... 23 5.3.1 Concentrating Flux in Stator Yoke ...... 23 5.3.2 Concentrated Flux in C-Core ASRM ...... 24 6. Two Permanent Magnets on Stator Yoke: Deeper Review ...... 27 6.1 Original Comparison of PM and DCC ASRMs ...... 28 6.2 Simulation Results ...... 29 7. Conclusion ...... 38 8. References ...... 38

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ASRM Robinson Table of Figures

Figure 1: Parts of a Step Machine ...... 5 Figure 2: Research in ASRMs over Time ...... 10 Figure 3: Two PM in Stator Yoke ...... 12 Figure 4: 2 PM in Switched Flux Stator Yoke [7] ...... 13 Figure 5: PM Inserted into ASRM Teeth [4] ...... 14 Figure 6: ASRM with PM in Stator Yoke Between Teeth [8] ...... 14 Figure 7: Doubly Salient PM ASRM [17] ...... 15 Figure 8: Shorted Stator Teeth with PM [41] ...... 16 Figure 9: PM Between Stator Tooth Tips [3] ...... 16 Figure 10: DC Assist for Increased Torque Density [48] ...... 18 Figure 11: DC Biased DCC (Left), Angle Shift (Right) [44] ...... 19 Figure 12: DCC Optimized to Reduce Torque Ripple and Increase Speed [19] ...... 20 Figure 13: Slot/5-Pole ASRM (Left) 12-Slot/5-Pole ASRM (Right) [45] ...... 20 Figure 14: DCC ASRM for Energy Redirection [26] ...... 21 Figure 15: Equivalent Circuit for Energy Redirection [26] ...... 22 Figure 16: Four Pole ASRM [24] ...... 22 Figure 17: ASRM Layout Left [25] ...... 23 Figure 18: ASRM DCC to Concentrate Flux (Left), Flux Path in one Tooth (Right) [47] ...... 24 Figure 19: Yoke Reductions for Flux Concentration ASRMs DCC (left), PM (Right) (top [14], bottom [42]) ...... 25 Figure 20: Flux Distribution of DCC ASRM (Left) and PM ASRM (Right) [42] ...... 26 Figure 21: Flux Distribution of Cut Stator Yoke [14] ...... 26 Figure 22: Equivalent of One Phase of PM ASRM [21] ...... 28 Figure 23: Simulations and Designs: Baseline (Left), PM (Middle), DCC (Right) ...... 29 Figure 24: Flux Orientation of Assist, Baseline (Left), PM (Middle), DCC (Right) ...... 30 Figure 25: Phase Winding Configuration ...... 30 Figure 26: Baseline, 1 Amp, 200 Turns Phase A Aligned ...... 31 Figure 27: PM 1 Amp, 200 Turns, Phase A Aligned, NdFe35 ...... 31 Figure 28: DCC ASRM, DC = 2A, Turns for DCC =2000, Phase Current = 1 Amp ...... 32 Figure 29: Magnetic Flux Density of Baseline, 1 Amp ...... 32 Figure 30: Magnetic Flux Density of PM, 1 Amp ...... 33 Figure 31: Magnetic Flux Density of DCC, 1 Amp ...... 33 Figure 32: Small Current Comparison of Baseline, PM, and DCC ...... 34 Figure 33: Static Torque Results of Baseline and PM Amperage = 50 ...... 35 Figure 34: Static Torque Results of Baseline and Winding Amperage = 50 ...... 36 Figure 35: Magnetic Flux Densities at Saturation...... 37 Figure 36: DCC Reduce Effects of Saturation ...... 38

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ASRM Robinson 1. Abstract Switched reluctance machine (SRM) modifications in both control schemes and physical design have been steadily increasing in academia to improve machine performance. Assisted switched reluctance machines (ASRMs) are a type of design modification for SRMs. Permanent magnets (PMs) and electromagnetic DC coils (DCC) are being added to the SRM to improve its torque output and overall efficiency. The choice for the design modification has been evolving throughout the decades. The focus has shifted from adding DCC to ASRM to adding PMs to

ASRMs. This paper reviews the research trends of ASRMs and includes an analysis of the modified stator yoke design. Although adding PMs limit the application of machines away from extreme environmental conditions due to risk of demagnetization and increase material costs, the torque density and torque ripple reductions can out-perform DCC. PM ASRM are a good choice for energy efficiency-sensitive applications, but DCC, with proper control circuitry, can have a wider application and smaller initial build cost.

2. Introduction

2.1 Overview of Step Machines

Step machines are salient-pole motors and generators. The position and speed of the machines are discretely controlled. Teeth on both the stator and align to create short

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ASRM Robinson reluctance paths to attract flux generated by the commutating windings (Figure 1).

Figure 1: Parts of a Step Machine

When a winding is excited, a magnetic field is generated perpendicularly to the current field. The resulting flux passes out of the coil and circulates through the step machine to complete the magnetic circuit [1]. The teeth on the stator and rotor of the step machine guide the flux through the machine. Each ferrite metal tooth provides a shorter reluctance path across the air gap, and therefore attracts the flux. The flux causes the rotor to move to further shorten the reluctance path. The distribution of the flux greatly influences the torque output, torque ripple, and efficiency of SRMs [2]. Flux that is concentrated in the air gap will produce the higher torques than flux distributed throughout the machine [3]. However, strong flux fields that are permanently held in the air gap will increase holding torques on the rotor, and therefore increase torque ripples during operation [4].

The interaction of this magnetic field is analogous of two magnets held slightly apart.

Flux fields from both magnets extend toward the other because the material offers a favorable path. The fields want the shortest path possible, so a force is generated and the two magnets snap together. The same interaction occurs within the machine: when a coil generates a magnetic field that passes through the air gap to the rotor tooth, a torque is created that moves the rotor so that

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ASRM Robinson the teeth closer together. Stepped machines are useful when precision is required. The rotor can be moved to a known position or accelerated to an exact speed due to the discrete tooth-and- winding system.

Step machines come in a variety of designs and are categorized under several names based on their method of locomotion. The machines mentioned here that fall under the stepped motor category are “switched reluctance”, “doubly salient”, “memory”, and “switched flux” machines.

“Switched reluctance” refers to the changing length of the reluctance paths as a rotor moves past the stator tooth. When the rotor is unaligned with a stator tooth, the reluctance is very large as air does not carry the magnetic field as well as metal. When the rotor is aligned with a stator tooth, the distance which the magnetic flux has to travel through air shrinks, making the reluctance become very small [4].

“Doubly salient” refers to the teeth of the step machine. A salient pole is a tooth on a step machine that passes flux through it and creates torque. Because step machines feature teeth on both the rotor and the stator, it is a doubly salient machine [5].

“Memory machines” are machines that maintain a certain amount of residual magnetism after flux passes through it. They usually use ferrite material or weak magnetic material. This material is magnetized and demagnetized as the direction of the magnetic field changes [6].

“Switched flux” refers to the changing direction of the flux in a salient pole of a machine.

Although the around each salient pole excites flux in only one direction, the flux returns to the by traveling through the surrounding teeth. These surrounding teeth see the flux switching direction from a previous phase excitation. These machines usually feature

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ASRM Robinson some permanent flux device, such as PM or DCC to enhance the flux production and torque

density [7].

All three machines described above can fall under the category of the switched reluctance

machine. In compiling machine designs for this literature review, all categories of machine were

researched and are from now on only referred to as SRMs.

3. Issues of SRMs

While SRMs machines are able to provide precision motion, the discrete steps do not

provide a smooth transition between rotations. Torque ripple, current spikes, and improper

control algorithms often plague these machines to a smaller sector of industry [8].

3.1 Torque Ripple

The amount of torque produced by SRMs depends on the current and position of the

rotor. As the rotor moves between teeth, the direction of the torque produced in relation to the

rotor varies. When the teeth are completely unaligned and furthest apart from each other, the

tangential torque is at its strongest. As the teeth come aligned, the tangential torque decreases

until the teeth are completely aligned, and no tangential torque is applied to the rotor. When the

rotor is completely aligned, only perpendicular torque exists between the stator and rotor. This

torque works against the rotor’s rotational motion and holds it in place [9].

The control of the SRM is extremely important. The coil must be turned off right before

the teeth are aligned to avoid the holding perpendicular torque. Not only is the position

monitored for smooth operation, but the current within the winding must also be controlled. The

current within the winding must be completely drained in order for the torque to go to zero. The

time constant for the coil is measured and fed back into the controls to ensure the current is

dissipated before the tooth is aligned [10].

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ASRM Robinson The variation in the direction and the amount of torque moving the rotor causes vibrations. SRM are infamous for their noisy performance; vibrations are commonly cited reason

SRMs are not commercially viable [9, 11-18]. Without proper control strategies or geometry modification, torque ripple leads to poor, inefficient performance and limits the application potential for these machines.

3.2 Current Spikes

In the aligned position, current from the windings induces electric fields within the rotor.

The electric fields induce current within the coil when the rotor moves from the aligned to unaligned position. The voltage induced is known as the back electromotive force or “back

EMF”. Back EMF is caused by the transfer of electrical to mechanical energy and can affect the circuitry of the SRM. EMF is governed by the SRM’s speed, current input, and flux produced

[19].

휕휆 휕휆 푑푖 푣 = 휔 + + 푅푖 (1) 휕휃 휕푖 푑푡

휕휆 휕휆 푑푖 Where v is the source voltage, ω is the back EMF, is the change in current the 휕휃 휕푖 푑푡 occurs when the teeth λ is the flux produced by the coil, θ is the angle of the rotor, i is the current in the coil, and R is the resistance of the coil. Back EMF spikes can overwhelm the source voltage and spike the current in the winding. The presence of back EMF affects the time it takes for the winding to dissipate current, and therefore affects the control of the SRM.

Current spikes occur when the rotor moves at too high a speed, changing the flux within the teeth too rapidly, and inducing too much current. Current spikes are ameliorated with fast switching algorithms, braking mechanisms, and speed ratings for the machine [20].

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ASRM Robinson 3.3 Control Issues

Because machine often runs most efficiently when the metal of the stator and rotor are saturated with magnetic field, the torque and power produced is nonlinear with respect to current and rotor position. This nonlinearity makes the control algorithms difficult to implement, especially in cases where the output load or input power is variable [10]. The machine controls are tied closely to the physical characteristics of the teeth. The effectiveness of the control will also vary with machining imperfections [21].

3.4 Fixes and Uses of SRMs

SRMs are very useful because they do not employ permanent magnets to run [19]. The winding configuration makes them easy to cool, simple to control, and therefore very diversely applied [22]. Researchers have looked into fixing the performance issues of SRMs in both physical alterations and control refinement. The scope of this paper is for physical alterations of

SRMs.

Assisted switched reluctance machines (ASRMs) have gained momentum in waves. The basic concept of supplementing a winding with a permanent magnet has been proposed as early as 1955 [7], beginning with the switched flux machine. Since then, the pre-2000’s saw projects in the doubly salient machines with winding supplements [2, 23-26]. 2010-2017 saw permanent magnet assisted machines popularize [1-6, 8-37]. Work into DCC ASRM continued steadily into the 2000’s [38-48], but the sudden increase in interest of PM ASRM is an apparent trend.

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ASRM Robinson

Assisted Machine Publications 9 8 7 6 5 4 3 2

1 NumberPublications of

0

2005 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 Year

Permanent Magnet Winding

Figure 2: Research in ASRMs over Time

The approach in these 48 publications is trifold: 1) adding permanent magnets 2) adding windings to replace permanent magnets 3) altering the structure of the SRM to accommodate 1) and 2). The design proposals are for both application specific purposes and general improvements in torque and flux densities and ripple, cost, and size reductions.

The performance of SRMs is often compared to permanent magnet machines. Permanent magnet machines are widely used in industry. The controls of permanent magnet machines are simple and applicable to a wide range of motor sizes, making them commercially viable, and they offer the highest torque and flux densities. These machines can be made smaller and more powerful than the SRMs due to their magnetic materials [7]. Unfortunately, permanent magnet machines have large downsides as well. The magnets create holding torques (cogging) that must be overcome by the windings [41], the materials are susceptible to demagnetizations due to extreme temperatures and overcurrent [49]. The flux created by the magnets also makes position precision difficult to predict. Another issue that is often presented in earlier assisted SRM papers is the economic implications of using permanent magnets. These rare earth metals are foreign

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ASRM Robinson imports that have given China, which has a majority in the world market for permanent magnets a significant advantage [14]. However, as an increase in demand for high power density machines with the advent of electric vehicles [12-14, 38], permanent magnets are making their way back into U.S. electric machines. The increase of research interest in recent years focuses on incorporating the rare earth material in the SRMs. Permanent magnets increase the flux density in concentrated areas of the metal, allowing for an increase in torque density [12]. Windings tend to spread the flux out across the entirety of the metal piece, decreasing the flux density and potential to create torque [2].

The addition of assistive features to SRMs has two purposes. The first purpose is to reduce saturation within the ferrite metal [20]. Torque density increases when flux density increases within the machine. Although torque density is desired—as this decreases the size of the machine required for an application—increased flux density can push the SRM into the saturation region. Saturated ferrite makes the output of the SRM nonlinear and unpredictable and reduces the efficiency of the machine [50]. ASRM aim to reduce saturation by adding flux that works against the flux created by the windings. The net flux in the machine is reduced, allowing the coils to push more torque-producing current through their windings without risking saturation

[45]. The second purpose of ASRM is to decrease the time it takes to drain the inductive energy from a winding. Adding an additional flux source will help remove the energy at the end of a commutation. Both purposes deal with the same fundamental issue: reroute trapped energy within the machine [23]. By adding a flux enhancing or flux weakening component to the machine, the control of the current peaks during commutation improves. Now, currents will dissipate faster, and commutating windings can be turned on and produce torque for longer periods of time, increasing the torque density of the machine.

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ASRM Robinson Flux sources come in the form of permanent magnets or . No matter the addition of the flux source, the efficiency of the machine can increase in some operating ranges.

Electromagnets provide more flexibility within the lower speed ranges, but permanent magnets in high speed-high output applications provide more efficiency. The following sections provide a comparison of the strategies of past work to add this flux.

4. Magnet Assisted SRM

Permanent magnets have become of great interest to ASRM researchers in the last decade. Residual flux from permanent magnets can permeate the ferrite structure of SRMs and provide higher operating efficiency, higher torque, and lower torque ripple. This assistive material does not require any external power source to operate and offers a convenient way to redistribute flux in the SRM to great success.

The addition of this material, however, does put some limits on the ASRM application.

Permanent magnets are a high-cost, environment-sensitive material. High temperatures, vibrations, or opposing magnetic fields can destroy the ability of PMs to perform in the ASRM.

However, in stable environments, this assistive material is a low-maintenance way to improve the performance of an ASRM.

4.1 Two Permanent Magnets in Stator Yoke

Nakamura and Ichinokura of Tohoku University [10, 32, 35-36, 46-49] studied a permanent magnet inlaid in the stator yoke. They focused on wind turbine application and explored auxiliary windings as well as converter designs with this ASRM arrangement. The benefits of this arrangement include:

Figure 3: Two PM in Stator Yoke

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ASRM Robinson

• Ease of thermal management of magnet in outer stator

• No cogging torque—all flux remains within the stator

• Enough residual flux in the stator allows the generator to operate off-grid.

A switched flux machine aims to maintain a constant flux through the rotor at all times— the permanent magnet is not shorted through the stator like the switched reluctance machine is

[7]. The shorted magnet reduces cogging torque while decreasing risks of demagnetization of the magnets and increases the area which the BH curve is operational.

Figure 4: 2 PM in Switched Flux Stator Yoke [7]

The cons of this arrangement include large amounts of NdFeB permanent magnet are used and machining of the stator is required, which does harm some of the structural integrity.

4.2 Permanent Magnets in Stator Teeth

Cheewoo Lee led research on an E-core stator SRM which he varied by adding permanent magnets and windings to improve efficiency [4, 8]. E-core stator SRM do not have flux reversals in the teeth, making them excellent candidates for PMs, as the risk of demagnetizing the material is greatly lowered. Cheewoo Lee [8] created a novel permanent magnet-assisted SRM that featured permanent magnets drilled into the stator teeth. The pockets created by the magnet drill-holes allowed too much flux to leak out of the stator at high currents

(over 8 A per winding), and actually made the motor perform worse than if there were no permanent magnets drilled inside the motor at all.

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ASRM Robinson

Figure 5: PM Inserted into ASRM Teeth [4]

In 2016, Lee put a solid piece of permanent magnet in the stator to see its effects on the

SRM.

Figure 6: ASRM with PM in Stator Yoke between Teeth [8]

He found that the PMs lowered the reluctance torque greatly, however, increased the mutual torque enough that he concluded adding thicker and thicker PMs in the stator would increase the overall torque. He used cheap ferrite magnets for this experiment, increased the magnet volume by 6 7mmx70mmx50mm. This is about $60 addition in material costs for 10 % increase in efficiency and 0.03 Nm in increased torque.

4.3 Permanent Magnets in the Air Gap

Taobo, Yuping, Guangyou, and Yanliang [17] placed permanent magnets on the stator teeth of a PM ASRM. The idea is to increase the flux density in the air gap of the ASRM so that the most amount of torque could be produced (Figure 7)

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ASRM Robinson

Figure 7: Doubly Salient PM ASRM [17]

If the flux source is close to the air gap, more energy will be used to create torque, rather than be distributed throughout the ferrite structure and stored in the ASRM magnetic field. This basic principle showed the results the authors were looking for—high torque. However, with the addition of PM so close to the commutating windings and air gap, severe consequences were seen as well. Cogging torque in the ASRM became analogous to any permanent magnet machine. The PMs held the rotor in place and energy was wasted overcoming this flux hold to turn the rotor. From the cogging torque, an increase in torque ripple could be seen. Finally, because of the proximity of the PMs to strong, alternating flux fields, the PMs were in danger of demagnetization and overheating. The stator teeth housing the salient pole and the PM also saturated very quickly and lead to unideal performance issues.

Although the placement of the PMs did what was intended—as it increased the torque density of an SRM—the overall flux paths of this PM ASRM is not the most efficient design.

4.4 Magnets bridging stator teeth in front of windings

In an attempt to remove the possibility of cogging torque from a PM ASRM, the PM was moved from the stator tooth face to between the stator teeth. This design shorted the PM between the stator teeth to prevent the PM flux from holding the rotor in place (Figure 8).

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Figure 8: Shorted Stator Teeth with PM [41]

Although cogging torque was now ameliorated, issues of saturation and demagnetization remained [32]. Ullah performed extensive optimizations [3, 22] of his PM ASRM to reduce the saturation of the stator teeth and reduce risk of demagnetization of the PM (Figure 9)

Figure 9: PM between Stator Tooth Tips [3]

The optimizations defined operation limits for the ASRM, including bounds for operating temperature, reductions in rated speed range, input phase current, and ASRM size. However, the

PM ASRM showed higher torque production than a traditional SRM and was very successful in its aerospace application.

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ASRM Robinson 4.5 PM ASRM Summary

PM ASRM have shown across literature to create very high torque density machines with smoother torque ripple and optimized performance. However, the addition of PM detracts from the usefulness of a SRM. The beauty of an SRM is found in its lack of permanent magnets. The flexibility of non-permanent magnet machines makes it applicable to almost any situation. The low cost, simple design, large speed range, and ability to operate in extreme conditions makes

SRMs the do-all machine. The downside to SRMs is its machine-specific control scheme.

Without proper analysis of the changing induction and knowledge of its saturation limits, the performance of this motor will be greatly reduced. The next section will cover DCC as a replacement to PM ASRM. The goal is to widen the application range of SRMs with this flux assistive devices.

5. Winding Replacements

A permanent magnet can be replaced by a DC-powered electromagnet to obtain the same amount of flux [50]. DCCs are flexible assistive devices because the direction of the current in the DCC can be changed during ASRM operation to fulfill two purposes: the DCC can either create flux against the phase windings, so as to reduce saturation of the ferrite metal and increase the torque operating range of the SRM, or it can be used as a torque assist and bias the ASRM in the same direction as the phase windings. The latter intention is the strategy of PM ASRM, and so will be examined further in this section.

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ASRM Robinson 5.1 DC windings that share Phase windings space

5.1.1 Windings Made to Increase Torque Density

Liu, Zhu, and Pan 2012 [48] developed an analytical model for a three-phase DCC

ASRM. The DCC in this model wrapped around each pole and shared a slot with the phase windings (Figure 10).

Figure 10: DC Assist for Increased Torque Density [48]

These DCC stayed at a constant voltage throughout all phase current cycles so that the flux within the stator keep a constant minimum value. The purpose of DCC in this application was to increase the torque density of an already established torque-ripple-reducing phase current.

It was found that the DCC flux source could increase the torque density, however, the turn-on angle for the phase current had to be adjusted as DCC increased. A higher DCC decreased the leading angle—that is, the difference between the self-inductance peaks at the aligned position from the current phase peak. Without DCC assist, a winding would be turned-on before the current peaked so that a larger flux field could be established when the inductance peaked. With the DCC assist, the leading angle is reduced, allowing the SRM to produce torque for a longer period of rotation, thereby increasing its efficiency [48].

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ASRM Robinson 5.1.2 Windings Made to Increase Torque Density and Speed Range

Husain, Sozer, and Husain 2015 [44] presented a similar idea in their DCC ASRM. In their paper, the DCC were wound in two directions for comparison. The first configuration

(Figure 11, left) wound the DCC in the same direction as the phase windings for a DC bias. The second (Figure 11, right) alternately wound the DCC with the phase windings and against them.

Figure 11: DC Biased DCC (Left), Inductance Angle Shift (Right) [44]

The first configuration gave each phase an additional flux boost that improved the torque output. The second configuration exploited the winding phase angle of a four-phase machine to reduce the complexity of the control circuitry. Both configurations boosted the torque production of the ASRM, but with proper control of the second configuration, the DCC were also used as flux-weakening devices. Reducing the flux in the stator allows the ASRM to run smoothly over a larger speed range. Using the DCC, the authors were given the flexibility to convert a single

ASRM either a high torque density or large speed ranged machine.

5.1.3 Windings Made to Reduce Torque Ripple and Upper Speed Limit

Raminosoa, Torrey, Refaie, Grace, Pan, Grubic, Bodla, and Huh 2016 [19] analyzed a similar ASRM to those described above where DCC occupied every slot in the stator (Figure 12)

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Figure 12: DCC Optimized to Reduce Torque Ripple and Increase Speed [19]

This ASRM optimized the slot/pole combination for the least torque ripple for a given torque. The result was a sinusoidal back EMF. The power from the back EMF easily coupled with the DCC to increase torque output and reduce torque ripple. Geometry optimizations for a given application can improve the performance of ASRMs.

5.1.4 Smoothing Torque Ripple and Back EMF

Zhou and Zhu 2014 [45] used different combinations of DCC and armature winding pole overlapping to smooth the torque ripple and sinusoidal back EMF (Figure 13).

Figure 13: Slot/5-Pole ASRM (Left) 12-Slot/5-Pole ASRM (Right) [45]

It was found that the 9-Slot/5-Pole machine created the higher amplitude and smoother back-EMF, and featured a high torque and lower torque ripple than the 12-Slot/5-Pole machine.

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ASRM Robinson It is well known that a high stator tooth-to-rotor pole ratio will smooth the torque ripple in both

ASRM and SRM. The addition of the DCC

5.2 Windings that wrap around half/quarter/eight of stator back

5.2.1 Windings Made to Redirect Stored Magnetic Field Energy

Liang, Liao, and Lipo 1994 [26] proposed an ASRM in which DCC wrap around multiple poles to reduce the energy stored in the magnetic field (Phase “D” in Figure 14)

Figure 14: DCC ASRM for Energy Redirection [26]

Like in the ASRM described in the previous section, the DCC’s purpose is to provide a constant flux field so that the inductance does not go to zero when the rotor and stator teeth are out of alignment. Also, as before, the DCC allows the lead angle of the current commutation to decrease, increasing the torque density and overall efficiency. Unlike the previous design, the

DCC proposed switches on and off. The inductance within the DCC is coupled with each phase.

The DCC and phase coil are excited together using the same source and are turned off together.

EMF from the turn-off phase coil is then stored to recharge the DCC (Figure 15).

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Figure 15: Equivalent Circuit for Energy Redirection [26]

The DCC provides an independent commutation as well as medium to store inductive energy from the phase windings. This DCC creates torque from energy that would have otherwise been redirected to the source voltage and increased ripple losses. A similar analysis was performed on a four pole ASRM by Pollock and Wallace 1999 [24] (Figure 16).

Figure 16: Four Pole ASRM [24]

This ASRM did not have four phase winding coils, rather each pole had a phase winding and a DCC on either side. This configuration resulted in high losses due to uneven flux distribution in the stator suggesting that ideal ASRMs are evenly symmetric.

Li, Lloyd, and Horst 1996 [25] analyzed a similar machine, but looked at the efficiency of the ASRM in relation to the SRM. Although adding a DCC to an SRM adds copper losses, the additional flux to the stator increase the overall efficiency of the ASRM (Figure 17).

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Figure 17: ASRM Layout Left [25]

For all DCC values greater than zero, the efficiency increased in the ASRM. The additional copper losses of the DCC are insignificant to the overall gain in efficiency of the system. In the proper ratio of phase current to DCC current, the efficiency and performance of an

ASRM can be increased.

5.3 Windings That Wrap Around Stator Yoke

5.3.1 Concentrating Flux in Stator Yoke

Liu, Park, and Chen 2014 [47] rearranged the phase windings and DCC such that the phase windings wrap around an individual stator tooth vertically and a DCC provides constant inductive energy to the rotor in a modified air-gap (Figure 18).

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Figure 18: ASRM DCC to Concentrate Flux (Left), Flux Path in one Tooth (Right) [47]

This configuration reduces stray losses from the flux field and removes radial torque. The

E-core SRMs utilize a key concept of DCC ASMs: flux concentration. When the coils are wrapped around the back yoke of the stator, flux is distributed through the entire yoke and allows a higher flux density in the area. This strategy will be employed further in the stator-yoke wrapped designs.

5.3.2 Concentrated Flux in C-Core ASRM

One way to replicate the concentrated flux that the E-core ASRM champion is to reduce the stator yoke inside the electromagnet [42]. By adding slots in the yoke, more DCC turns can be accommodated, and the flux produced is distributed across a small area. This concentration at the source of the flux allows for a similar flux pattern output to PMs. This DCC strategy can be seen across a multitude of authors looking to increase the torque density of the SRM. The figures below show the design of a C-Core ASRM by two different authors [42, 14] (Figure 19).

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Figure 19: Yoke Reductions for Flux Concentration ASRMs DCC (left), PM (Right) (top

[14], bottom [42])

Both papers explored replacing PMs with DCCs in ASRMs. The flux sources bridge the area between C-core stator yoke teeth. In these papers, however, the configurations of the assistive devices favored PMs. Although the designs were made so that the DCCs and the PMs created the same flux density, the DCCs could not distribute the flux as efficiently as the PMs.

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Figure 20: Flux Distribution of DCC ASRM (Left) and PM ASRM (Right) [42]

The comparison shown in Figure 20 shows the flux completely saturating a tooth from a

DCC (left), whereas the flux from the PM (right) did not extend as low into the stator tooth. This comparison is slightly flawed—the authors performed an optimization on the saturation of the stator core and did not simulate the cut stator yoke for the DCC, as shown in Figure 19, upper.

However, the authors of Figure 19, bottom, did perform the FEA analysis on the concentrated flux stator yoke (Figure 21).

Figure 21: Flux Distribution of Cut Stator Yoke [14]

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ASRM Robinson The flux from a DCC more readily distributes across a stator tooth than the flux from a

PM due to directivity of PM flux. Despite the saturation limit placed on the DCC, the authors ran this ASRM in comparison with the PM and found that, due to the saturation, the torque ripple increased in the DCC ASRM, however, comparable torque magnitudes were reached. Further geometry optimizations could make DCC ASRMs run comparably to PM ASRMs, but due to the robustness of coils in comparison to permanent magnets, the DCC application range and reliability is far larger.

6. Two Permanent Magnets on Stator Yoke: Deeper Review

PM or DCC placed in the stator yoke has been proposed by multiple researchers [1-37].

This design offers a lot of advantages including: cogging torque reductions, decreased salient pole saturation, reductions in torque ripple, and overall efficiency improvements. The geometry placement also allows PMs to be easily cooled by heat sinks and the simple design does not add greatly to manufacture costs. The torque produced by the two magnets on the stator can be described by the following equation (21):

1 휕퐿 휕휑 푇 = 푖2 + 푖 푚 (2) 2 휃푟 휃푟

Where T is the torque, I is the current, L is the inductance, ϴr is the rotor angle, and 휑푚 is the magnetic flux of the PM. The torque produced by the phase winding is described in the first term and the torque produced by the PM assist is described by the second term. The equivalent magnetic circuit of this equation can be seen in Figure 22

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ASRM Robinson

Figure 22: Equivalent Magnetic Circuit of One Phase of PM ASRM [21]

The two PMs can also be replaced with two DCC. The magnetic flux,휑푚 is replaced by the current and inductance of the electromagnet [21].

휑퐷퐶퐶 = 퐿푌표푘푒 ∗ 푖퐷퐶퐶 (3)

It can be seen that by changing iDCC in (3), the flux in the stator yoke can be changed.

Having this flexibility over the flux assistance will allow the ASRM to achieve a large speed range. The drawback to a DCC is saturation. The flux generated by the electromagnet is concentrated over one small area of the stator yoke. This area can saturate and limit the output flux of the electromagnet. The upper limit to the current can be changed if the cross-sectional area of the yoke is increased, however, in doing so, the flux density will decrease.

To illustrate the flexibility and drawbacks of the DCC ASRM, FEA simulations were performed using Maxwell by ANSYS.

6.1 Original Comparison of PM and DCC ASRMs

Three machines were simulation on ANSYS Maxwell Magnetostatics Solver. The flux density, torque-vs-angle output, and magnetic flux density was compared across a baseline SRM, a PM ASRM, and a DCC ASRM. Transient solvers were not used as the control of ASRM is out of the scope of this paper.

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ASRM Robinson The machine geometry chosen was first proposed as a Flux Switching Machine in the early 1950’s. These machines show promise in terms of torque density and reduced cogging torque. The PMs reside in the stator, keeping the strong, permanent flux field away from rotating parts, and the PM material close to heat sinks. Using electromagnets in lieu of PM change the machining of the stator yoke to concentrate the flux within the stator, but otherwise are similarly set up. One downside of using DCC instead of PMs is the copper wiring takes up space where higher density windings for the phase control would otherwise go.

6.2 Simulation Results

Three geometry-equivalent simulations were performed to compare the magnetic flux density, flux distribution, and torque output of a traditional SRM (baseline), PM ASRM, and

DCC ASRM (Figure 23)

Figure 23: Simulations and Designs: Baseline (Left), PM (Middle), DCC (Right)

The DCC ASRM parameters were chosen so that the flux produced by the DCC matched the magnitude of the flux produced by the permanent magnets for equal comparison. The

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ASRM Robinson orientation of the assistive flux fields is shown below.

Figure 24: Flux Orientation of Assist, Baseline (Left), PM (Middle), DCC (Right)

The length of the flux paths is often cited as an influencer on the amount of torque produced. The length of the flux path varies based on winding orientation and additional flux sources. Shorted flux paths tend to create higher torques while longer flux paths store more energy in to the magnetic field of the machine. The phase windings for each of the machines can be seen in Figure 25. The windings were made so that short flux paths wrap about only a quarter of the SRM at a time.

Figure 25: Phase Winding Configuration

The phase winding configuration ensured long flux paths through the commutation poles only. The original flux paths can be seen in the Baseline flux distribution FEA analysis (Figure

26).

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ASRM Robinson

Figure 26: Baseline, 1 Amp, 200 Turns Phase A Aligned

Adding a flux source will increase the flux density in a particular flux path. For example, as seen in Figure 27, the flux strengthened in the paths created by the PM addition.

Figure 27: PM 1 Amp, 200 Turns, Phase A Aligned, NdFe35

Replacing the PM with DCC changing the flux paths slightly. The stator yoke that acts as the core for the electromagnets saturates slightly. This saturation affects the symmetry of the flux path, removing the non-assisted paths in the ASRM.

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ASRM Robinson

Figure 28: DCC ASRM, DC = 2A, Turns for DCC =2000, Phase Current = 1 Amp

The magnetic flux density was created for better visualization of the saturation in the

DCC ASRM.

Figure 29: Magnetic Flux Density of Baseline, 1 Amp

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ASRM Robinson

Figure 30: Magnetic Flux Density of PM, 1 Amp

Figure 31: Magnetic Flux Density of DCC, 1 Amp

The PMs offer a better flux distribution than DCC and do not saturate the stator yoke. In a low-amperage test of 1 A for the Phase A, it can be seen that the DCC do not aid in torque production. In fact, due to the saturation that occurs in the narrowed stator yoke, the torque ripple

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ASRM Robinson detriments the output torque.

Small Current 1 Amp, Baseline, PM, DCC =1 1Amp, DCC = 0 Amp 20 Baseline 15 DC1 10 PM DC0

5 )

0 -50 -40 -30 -20 -10 0 10 20 30 40 50

-5 Torque (Nm Torque -10

-15

-20

-25 Angle (Degrees)

Figure 32: Small Current Comparison of Baseline, PM, and DCC

The small stator yoke saturates with just phase input, as can be seen when DCC is set to 0

Amps. The ripple is still present and the overall torque is less than that of the baseline design.

Permanent magnets enhance the torque at low current inputs, whereas reducing the stator to implement DCC detracted from the torque at low phase currents.

Although the electromagnets have the potential to enhance the flux of the ASRM, the saturation greatly affected its performance. For a comparison, the ASRM and SRM were run into saturation—a phase current of 50 amps—and the static torques were measured again (Figure

33and Figure 34). It can be seen in these figures that adding PM had no effect on the output torque in these simulations. Because the stator was saturated, the flux from the PM could not add to the torque. The addition of DCC induced some ripples into the ASRM, which allowed the

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ASRM Robinson torque to peak 50 N*m higher than the baseline or PM input, however, these ripples will not provide optimal operation of the ASRM.

PM and Baseline Static Torque Results, Phase Amperage = 50 Amps 1500

1000 Baseline

PM 500

0 -50 -40 -30 -20 -10 0 10 20 30 40 50

Torque (Nm) Torque -500

-1000

-1500 Angle (Degrees)

Figure 33: Static Torque Results of Baseline and PM Amperage = 50

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ASRM Robinson DCC and Baseline Static Torque Results, Phase Amperage = 50 Amps 1500

1000 Baseline

DC2 500 DC8

0 -50 -40 -30 -20 -10 0 10 20 30 40 50

Torque (Nm) Torque -500

-1000

-1500 Angle (Degrees)

Figure 34: Static Torque Results of Baseline and Winding Amperage = 50

The magnetic flux densities are shown in Figure 35. When the stator teeth are run into saturation, no assistive element can add to the torque output.

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ASRM Robinson

Figure 35: Magnetic Flux Densities at Saturation

One interesting design note for DCC—because the flux is dependent on the direction of the current, if the direction is switched, the output flux of the DCC changes direction. This flux now opposes the phase flux of the machine. When the phase current saturates the stator tooth, the

DCC can reduce the effects of that saturation (Figure 36). Transient analysis could use the flux weakening capabilities of DCC as part of an operation strategy. Weakening the flux of the

ASRM will extend the operation limitations of the ASRM input current for a wider range of output torques outside the region of saturation.

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ASRM Robinson

Figure 36: DCC Reduce Effects of Saturation

7. Conclusion

This paper reviewed the previous work of propose ASRMs throughout time. Research on

DCC ASRMs has steadily decreased through the years, yielding to increased interest in PM

ASRM in the last decade. The emphasis of efficiency in research has increased since the 1950’s and PMs are often cited for their increase in efficiency whereas DCC are used for performance increases

The application must drive the geometry selection. High temperature, high vibration, and low speed needs should opt for DCC ASRM options, whereas efficiency sensitive applications should look to PM ASRM.

Future work should look into the transient analysis of ASRMs and the control strategies that can be placed on flux enhancing-flux weakening operation modes of DCC. Adding coils to the back of a SRM stator could be an easy way to improve the efficiency of any SRM.

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