New Self-Excited Synchronous Machine with Tooth Concentrated Winding

Transcription

1 New Self-Excited Synchronous Machine with Tooth Concentrated Winding Gurakuq Dajaku 1) and Dieter Gerling 2), IEEE 1 FEAAM GmbH, D Neubiberg, Germany 2 Universitaet der Bundeswehr Muenchen, D Neubiberg, Germany gurakuq.dajaku@unibw.de Abstract This paper presents a novel and cost effective self-excitation method for the current excited synchronous machines where the high winding space harmonics are used for excitation of the rotor field winding. According to this method, the new machine design is characterized with simple tooth concentrated windings for both the stator and the rotor, and it doesn t need any additional auxiliary winding in the stator and its corresponding power supply and control unit compared with the conventional brushless excitation system. Using the new machine concept, an 18-teeth/1-poles machine for HEV applications is designed and investigated. The obtained results for the electromagnetic torque and the rotor field current response demonstrate the applicability of the new technique. Keywords: Tooth concentrated winding, synchronous machine, self-excited, low manufacturing costs, high efficiency. I. INTRODUCTION In the last years permanent magnet (PM) synchronous machines are challenging other electric machines in different industry and automotive applications due to their high energy density, compact size, high efficiency, and wide speed operation range. However, the main disadvantages of this machine types are the high costs and the fault tolerant problems. The permanent magnet material is the most expensive components in the electric machine and their price in the lasts years is always going increasing. On the other side different possible fault tolerance problems, such as short circuit case, limits the viability of the PM machines in some specific application. Therefore, the current excited synchronous AC machines are looking to be a good alternative solution for the future. By the current excited synchronous AC machines the field winding which is supplied with a DC current is used to generate the stationary magnetic field in the rotor. The direct current required for field excitation is furnished by the excitation system. Generally, two methods are commonly utilized for the application of the direct current to the rotor of a synchronous motor: 1. Brush-type systems apply the output of a separate DC generator (exciter) to the slip rings of the rotor, 2. Brushless excitation systems utilize an integral exciter and rotating rectifier assembly that eliminates the need for brushes and slip rings. The brushless excitation is called often as self-excitation. G. Dajaku is Senior Scientist with FEAAM GmbH, D Neubiberg, Germany ( Gurakuq.Dajaku@unibw.de). D. Gerling is Full Professor at the University of Federal Defense Munich, Institute for Electrical Drives, D Neubiberg, Germany ( Dieter.Gerling@unibw.de). A. Brush-type excitation Before brushless excitation machines, an excitation was traditionally implemented using slip-rings with carbon brushes. The distinguishing feature of the brush-type machines is that stationary brushes are used to transfer the DC exciting current to the rotating generator field. Current transfer is made via rotating slip rings (collector rings) that are in contact with the brushes, Fig 1. Each collector ring is a hardened-steel forging that is mounted on the exciter shaft. Two collector rings are used on each exciter, each ring is fully insulated from the shaft and each other. The inner ring is usually wired for negative polarity, the outer ring for positive polarity. The main disadvantage of brush-type excitation is that the slip-ring excitation equipment is significantly larger and expensive. Additionally, slip rings need regular maintenance and the carbon dust from the brushes is harmful for the machine. Fig. 1: Brush-type excited current synchronous machine. B. Brushless excitation system The self-excitation method for the current excited synchronous AC machines is not new and is widely used by the wind power generators. Fig. 2 shows the basic concept for the brushless excitation method. Based on the self-excitation concept the air-gap harmonics are used to supply the rotor field winding. Several patents are granted according to this excitation method [1] [3]. The concepts presented in these inventions are the same however they differ only on the realization; the stator contains two winding systems; the main three-phase winding which is generally a distributed overlapped winding, and an additional winding (auxiliary or field winding) to generate high harmonics in the air-gap. Since the both stator windings are located in the same stator core, they require a large stator volume for making place for the both winding systems.

2 These winding must be good insolated to each other which leads to further decreasing of the copper filling factor and increasing also the stator manufacturing costs. Further, the stator field winding requires an additionally power supply device or rectifier bridge. On other side, the rotor consists also of two complex winding systems that in general increase the total costs of these machine types. Fig. 2: Brushless excited current synchronous machine. To overcome the drawbacks of the conventional selfexcited synchronous machine, in this paper a novel brushless synchronous machine is presented. Fig. 6 show the basic block circuit diagram for the new machine concept. The new machine design is characterized with simple tooth concentrated windings for both the stator and the rotor, and it doesn t need any additional auxiliary winding in the stator and its corresponding power supply and control unit. The three phases ABC stator winding generate simultaneously the main working air-gap harmonic which is responsible for the electromagnetic torque, and also a specific high harmonic which is used for excitation of the rotor winding. The rotor consists of two windings; the excitation winding E and the field winding F. When the rotor rotates, in the rotor excitation windings E1 to E5 there are induced electromagnetic forces the magnitude of which depends on the magnetic field of the air-gap high harmonics. The induced electromagnetic forces in the rotor excitation winding are rectified after by the diode bridge circuit, so that a direct current I f flows in the field winding F. To avoid the high harmonics effect on the field winding, a special connection for the winding coils for the field winding is chosen. II. CONCENTRATED WINDINGS Recently, fractional slot concentrated windings (FSCW) are widely used for the design of different PM machines for several industry applications. The use of concentrated windings offers the advantage of short and less complex end-winding, high slot filling factor, low cogging torque, greater fault tolerance, and low manufacturing costs. However, the magnetic field of these windings has more space harmonics, including sub-harmonics. For the PM machines, the torque is developed by the interaction of a specific high stator space harmonic with the permanent magnets. In other side, the rest of others sub- and high harmonics, which rotate with the different speed and also in opposite directions, lead to undesirable effects, such as additional stator and rotor iron losses, eddy current loss in the magnets, and noise and vibrations, which are the main disadvantages of these winding types. To improve the magneto-motive force (MMF) winding performances of the FSCW regarding to power losses and noise problems several methods and techniques are developed and investigated in the past. References [4 to 8] show different methods for the reduction of winding suband high harmonics such as using magnetic flux barriers in the specific stator core locations [4] using multi-layer tooth concentrated windings [5, 6], or increasing the number of stator slots and using two winding systems [7, 8]. Fig. 3(a) and (b) show the winding layouts for the conventional 12- teeth/1-poles and the new 18-teeth/1-poles concentrated windings, respectively however, Fig. 4 compares the MMF winding harmonics. As well shown from the MMF spectrum, with the new winding the sub- and high MMF harmonics which are responsible for the rotor losses and also the low modes radial forces are mostly reduced down to zero. The investigations carried out on these methods show enormous improvements on the PM machine performances such as reduction of the sub- and high MMF harmonics more than 6%, reduction of radial force modes of low order, reduction of the machine losses (magnet losses, iron losses and so on), improves the cooling capability (direct cooling of coil windings), reduce the slot proximity effects, and so on. a) b) Fig. 3: a). The conventional 12-teeth /1-poles winding, b) The new 18- teeth /1-poles winding [7]. MMF [ p.u. ] Teeth/1-Poles 18-Teeth/1-Poles Harmonic Order Fig. 4: Comparison of the MMF space harmonics. III. NOVEL SELF-EXCITED SYNCHRONOUS MACHINE WITH CONCENTRATED WINDING Different from the optimization methods mentioned above where different techniques are used for reduction of MMF winding unwanted harmonics, in [9] a self-excited synchronous machine concept is presented where the FSCW are used for the stator winding and the high harmonics are used for the excitation of the rotor field winding. In following the new 18-teeth/1-poles winding is considered,

3 however, of course the new self-excited concept is useful also for any conventional concentrated winding. As well shown from Fig. 4, the new 18-teeth/1-poles winding is characterized with the 5 th and the 13 th MMF space harmonics. Therefore, according to [9] for an 1-poles self-excited synchronous machine the 5 th harmonic is used as working harmonic, however, the 13 th high harmonic is used to excite the rotor field winding. Figs. 5 and 6 illustrate the new self-excited concept according to the 18-teeth/1-poles winding. The rotor contains the excitation winding E and the field winding F, and the diode rectifier in between. The excitation and the field winding are realized using tooth concentrated coils wounded around the same rotor teeth as illustrated in the Fig. 5. The winding coils for the rotor winding are connected according to the winding schemas given in the figures 7 and 8. Further, the power supply device and the control unit are used for supplying and controlling the electric machine in a proper way. The generator and motor operation mode is possible. Fig. 5: Self-excitation concept for the 18-teeth/1-poled machine. Fig. 6: Block circuit diagram for the new self-excited synchronous machine. Fig. 7: Ten poles rotor field winding. E1 E2 E3 E4 E5 E1 E2 E3 E4 E5 Fig. 8: Rotor excitation winding. A. Reduction of harmonics effect on the rotor field winding Usually three-phase current excited synchronous machines commonly use single- or double layers, overlapping, distributed windings with q 2 ( q is the number of coils per pole per phase). This winding configuration results in more sinusoidal magneto-motive force (MMF) and electromotive force (EMF) distributions, and hence, good machine performances. However, it is characterized with many drawbacks such as large end-winding length, overlapping coils, high number of stator slots per pole and low slot filling factor which are related with higher manufacturing and material costs, high copper losses and other end-winding parasitic effects. In other side, fractional-slot concentrated windings, or modular windings, have been gaining a lot of interest in Permanent Magnet (PM) synchronous machines. This is due to the several advantages provided by this type of windings which are mentioned in previous section. Different from the distributed windings, FSCW do not produce high quality traveling fields (sinusoidal MMF). As is shown from Fig. 4, several space harmonics of closely the same magnitude are produced that travel with different speed and in different directions referred with the working MMF harmonics. In the case of the synchronous machine with wounded rotor these harmonics induces currents and generate parasitic torque components and high rotor losses compared to a conventional distributed winding. Therefore, to avoid the harmonics effect on the rotor field winding in the new machine design the winding coils for the field winding are connected in series as is illustrated in Fig. 7. As will be shown in the following analysis, by series connection, even the high harmonics induce flux components in each coil they are cancelled in the resulting induced flux-linkage. As mentioned above, the air-gap flux density components due to the MMF high harmonics rotate with the different speed (asynchronous speed) referred to the rotor synchronous speed. Analogous to the asynchronous th machines the slip of the harmonic is ωs ωr s = = 1 ( 1 s ) (1) ωs with, ωs = ωs / where, ω S is the stator electrical angular frequency, ω R is the rotor electrical angular frequency. The induced flux-linkage in the rotor field winding (one winding coil) for the case when coil pitch is equal with the rotor pole pitch is ψ () t = ψˆ cos ( s ωst) ˆ 1 (2) ψˆ = 2 r l B ξψ where, ξ represents the coupling factor between the th ψ stator MMF harmonic and the rotor field winding, π ξ ψ = sin (3) p R 2 The above expression shows that for p R = 5 (ten-poles rotor) and for = 13 the coupling factor is about 8%. Let be considered now the case when coils of the field winding are connected in series. According to eq. (2),

4 the induced flux-linkage in the k-th winding coil can be written as π ψ () ˆ k t = ψ cos s ωst+ ( k 1) (4) pr From eq. (4) and according to Fig. 7, the resulting fluxlinkage represents the sum of all induced fluxes in each coil winding and can be determined as, 2pR 2pR ψ() t = ψ () ˆ k t = cos s ( 1) b ψ ωst+ k π (5) k= 1 k= 1 pr Solving the above relation, the resulting flux-linkage in rotor field winding is 3 () 2 ˆ π ψ t = pr ψ ξ ψ sin s ωst+ 2pR 2 pr (6) = (2g 1) p and g = 1, 2, 3,... with R π ξ ψ = sin π R 2 sin p pr (7) 1 for = (2g 1) pr, g = 1, 2, 3,... = for (2g 1) pr, g = 1, 2, 3,... According to the above relations (6) and (7) it is shown that for the case when the all coil windings are connected in series only the working harmonic is induced in the rotor winding, however, the other unwanted harmonics sin ( π) ( (2g 1) p R ), even they are induced on each rotor coil, are cancelled in the resulting induced flux-linkage. B. Increasing the coupling effect for the excitation winding According to eq (3), the coupling factor for the rotor excitation winding and for the case when the coil span-angle is α x cab be written as, α x ξψ = sin 2 (8) Fig. 8 show the coupling factors vs. coil-span angle for the 5 th and the 13 th MMF harmonics. We can see here that the maximal coupling factor for the 13 th MMF harmonic can be realized when the coil-pitch of the rotor excitation winding is 7 electrical degree. Therefore, a possible realization of the rotor excitation winding with coupling factor equal to one is illustrated in Fig. 9. Of course, it is also possible to vary the coil span for the rotor excitation winding in such a way that to reduce the effect of other high harmonics. Further, different from the field winding, to ensure an induced voltage due to the high harmonics, only the symmetrical coils of the excitation winding can be connected in series, however, the coil groups should be connected directly to the diode rectifier as is illustrated in Figs. 6 and 8. Fig. 9: Rotor excitation winding with winding factor equal to one regarding to the 13 th air-gap flux density harmonic. C. Hybrid rotor design Another possible design for the new self-excitation synchronous machine is to use permanent magnets (PM) in the rotor teeth (in small amount) to improve the machine performances in low speed. The following Fig. 1 shows a hybrid rotor topology with permanent magnets inset in the rotor top teeth. Of course, other rotor topologies are also possible such as using the magnets in the rotor yoke beside the slot bottom and magnetized in the circumferential direction, Fig v=5 v= Coupling-factor Fig. 1: Self-excitation synchronous machine using additional PM in the rotor teeth alfa x [el. degree] Fig. 8: Flux-linkage coupling factors for the 5 th and the 13 th MMF harmonics. Fig. 11: Alternative hybrid rotor topology.

5 IV. EXEMPLARY MACHINE DESIGN According to the new machine concept, an 18-teeth/1- poles self-excited synchronous machine for electric vehicle application is designed and analyzed. As typical requirements for the automotive traction drive application, the following data are used: maximum DC-voltage U DC =4V, maximum current I max = 65Arms maximum short-time torque T max 51rpm, and the available machine volume: D Out =23mm, L Stack =15mm. Fig. 12 and Table-1 show the geometry and the main geometrical data of the investigated machine. If [A] T [Nm] Field Current time [ms] Electromagnetic Torque time [ms] Fig. 12: Geometries of the new 18-teeth/1-poles self-excited synchronous machine. Fig. 12: Simulation results for Is = 5A and n = 51 rpm. Active length TABLE I: MAIN GEOMETRY DATA 15 mm 4 Field Current Outer stator diameter Outer rotor diameter 23 mm 16 mm 3 Gap length 1 mm Turns per phase - Stator 16/6 Turns per phase Rotor field winding 35 If [A] 2 1 Turns per phase Rotor field winding 7 Iron Core material STEEL 33-35AP Number of rotor poles time [ms] The design and analysis of the investigated machine is performed using finite elements method (FEM). The simulations results for the electromagnetic torque and the rotor field current presented in the following Figs. 12 and 13 are determined for different operation conditions. Firstly, the machine is investigated for 5A stator current and at 51 rpm rotor speed. As well is shown, the machine torque for this operation point is about 29Nm, however, the rotor field current is 47 A. Afterwards the simulations are performed for low rotor speed (6 rpm). Since the induced rotor filed current depends on the rotor speed, during this simulation the stator phase current is increased up to 65A to have a machine torque of about 26Nm. Of course, the machine torque for the low rotor speed further can be increase by optimizing the reluctance torque component or by selecting a proper winding relation for the field- and the excitation winding. T [Nm] Electromagnetic Torque.5 1time [ms] Fig. 13: Simulation results for Is = 65A and n = 6 rpm.

6 V. CONCLUSIONS This paper presents a new self-excited synchronous machine concept where the both stator and rotor contains simple tooth concentrated windings. The three phases stator winding generate simultaneously the main working air-gap harmonic which is responsible for the electromagnetic torque, and also a specific high harmonic which is used for excitation of the rotor winding. Different from the conventional brushless excitation concept it doesn t need any additional auxiliary winding in the stator and its corresponding power supply and control unit. Using the new machine concept, an 18-teeth/1-poles machine for HEV applications is designed and investigated. For the considered stator winding the 5 th MMF harmonics is used as working (fundamental) harmonic, however, the 13 th MMF harmonic is used for the excitation of the rotor field winding. To proof the operation of the new machine concept different operation conditions are investigated. The obtained results for the electromagnetic torque and the rotor field current response demonstrate the applicability of the new technique. REFERENCES [1] H. Platzer: Brushless Synchronous Generator System, US , [2] I. Kenji, F. Takayuki: Brushless Self-Excitation Synchronous Electric Motor, JP , [3] S. Satake, Y. Hosaka, Y. Onogi, K. Inoue: Three-Phase Brushless Self-Excited Synchronous Generator with no Rotor Exciting Windings, US , [4] G. Dajaku, D. Gerling: Low Costs and High-Efficiency Electric Machines, 2nd International Electric Drives Production Conference 212 (EDPC-212), October 212, Erlangen-Nürnberg, Germany. [5] K. Ito, K. Naka, M. Nakano, and M. Kobayashi, Electric machine, US Patent US , 2 Oct 29. [6] M. Cistelecan, F. F. Ferreira, and M. Popescu, Three-phase toothconcentrated interspersed windings with low space harmonic content, 19-th International Conference on Electrical Machines (ICEM 21), September 21, Rome, Italy. [7] G. Dajaku, D. Gerling: A Novel Tooth Concentrated Winding with low Space Harmonic Content, International Electric Machines and Drives Conference (IEMDC-213), Chicago, IL, USA, May 12-15, 213. [8] G. Dajaku, D. Gerling: A Novel 24-Slots/1-Poles Winding Topology for Electric Machines, International Electric Machines and Drives Conference (IEMDC-211), Niagara Falls (Ontario), Kanada, May 15-18, 211, pp [9] G. Dajaku, Synchronmaschine, German patent application No. E212,673. February 213.