Application of Dual Mechanical Port Machine in HybridElectrical Vehicles

Yuan Zhang, Longya XuDept. of electrical and Computer Engineering, The Ohio State University2015 Neil Ave., Columbus, OH, 43210, USA, Email: xu.12@osu.edu

Abstract - In full hybrid electrical vehicles, two separateelectric machines and two power inverters are required. Onemachine operates as a generator and the other works both as agenerator and motor. The two electric machines are connected toan intelligent gearbox called planetary gear set together withengine. In this paper, a new machine - Dual Mechanical Port(DMP) machine is proposed, which combines the functions of thetwo electric machines and the planetary gear set mentionedabove. The structure and the dynamic model of DMP areintroduced. A prototype is designed and analyzed by FEA. Thecontrol algorithms with the overall system operation performanceare evaluated by computer simulation.

Index Terms - electric machine; dual mechanical port; integratedstarter and generator; FEA; system simulation

I. INTRODUCTION Fig. 1 Geometry of the DMP machine

In the advanced full hybrid electrical vehicles, thegasoline engine only works at the speed with highest The magnetic overview of the DMP machine structure isefficiency. Usually, an intelligent gear box, called planetary illustrated in Fig. 2. As shown in the figure, the designedgear set, is used to connect the gasoline engine and two DMP is a 6-pole machine and the PM rotor is sandwiched byelectric machines together. However, the two electric the stator and inner wound rotor. In this figure, the black parts

*machinesa toper ter s make the temtmuc represent north poles and the red south poles. In operation,machines and two power inverters make the system much 1-complicated and bulky in size. In this paper, a new type of both the stator and inner rotor windings (not shown) are

electric machine is proposed - Dual Mechanical Port (DMP) connected to a common DC bus through two DC/AC inverters.electric machine. This DMP machine combines the functions The mechanical shaft of the inner rotor is coupled to the ICof the two electrical machines and the planetary gear set. engine and the mechanical shaft (not shown) of the outer PM

rotor is coupled to the differential trans-axis.

A conventional electric machine may have more than oneelectrical port but only one mechanical port. A DMP machine,however, has two mechanical ports or mechanically movableparts. The prototype designed in this paper has two rotors: anouter PM rotor and an inner wound rotor as shown in Fig. 1.The major dimensions of the DMP prototype are given in

Table 1.

l_________| stator outer rotor inner rotorOD 300 224 t200ID 226 202 60StackL 180 180 1180#o,fturns 24I PM 24

Fig. 2 DMP machine structure

1-4244-01 59-3/06/$20.00 ©2006 IEEE

II. DYNAMIC MODEL OF DMP MACHINE III. FINITE ELEMENT ANALYSIS OF THE DMP

The dynamic model of the DMP is derived in detail in In evaluating the DMP electromagnetic design, Finite[1]. Here, only the maj or equations are presented and Element Analysis (FEM) is used to compute magnetic fluxdiscussed. Eqs. (1) through (4) are the voltage equations for distribution and torque production. The currents are injectedthe stator and the inner rotor windings. in both the stator and rotor windings, and the associated

MMFs are synchronized with the PM rotor. Two calculationsVq = iq+ qs +r (1) are performed in evaluating the DMP torque capability, one

dt based on the equivalent circuit model incorporating parameters

d*ds extracted from the electromagnetic design and the otherVds idsrs + dt qs (2) directly from FEM. As shown in Fig. 3, the DMP machine

does have a very impressive torque capability.

Vqr qir r + dt + Y(Or)2dr (3) As indicated by the torque curves, the calculation resultsby the two methods are very close to each other at small and

dX*e+d2r ( )2;r 4 medium current levels. However, when the currents are going1dr' d = tr higher, we see the obvious difference between the results fromthe equivalent circuit model and FEM model. The differentresults at higher current level can be explained by the fact that

where r. is the resistance of the stator windings and rr is the FEM model can take magnetic saturation into accountresistance of the inner rotor windings.t and rtrepresent the accurately while that circuit model neglects magneticrotatingspeeds of the PM-rotor and the inner rotor Eqs (5) saturation. When the magnetic core of the machine isthrough (8) are the flux linkage equations for stator and inner saturated, the machine torque capability decreases.rotor windings.

As implied by Eqs. (9) through (11), torque capability ofe Lj + je (5) the DMP machine also relies on the relative space angles ofqs qqs qqr the stator MMF, the PM MMF and the inner rotor MMF. Fig.

m+ +Lmice;r (6) 4 shows the magnetic flux distribution at the moment ofmaximum torque production for a given current level. As

Z qr=Lriqe +L (7) indicated in the figure, the magnetic fields of the machine

,me+ Lrje + Lije (8) separated by the two air-gaps are always synchronized andldr m dr ds (8) unified; that is, the continuous flux lines go through both air-

gaps and link all three cores, one for the stator and two for thewhere Ls is the self-inductance of the stator windings. Lm is the PM and wound rotors. It is interesting to note that the fluxmutual inductance between the stator and the inner rotor lines are driven by three sources, the stator MMF, the outer

windings. .is the flux linkage producedbythePM-rotor. rotor permanent magnets, and the inner rotor MMF. Inm conditions if the outer PM and inner rotors are of different

Eqs. (9) through (11) are the three torque equations for the speeds, the electrical frequencies of the stator and inner rotorDMP machine, associated with the stator, the outer PM rotor windings have to be controlled so that all MMFs areand the inner wound rotor. synchronized to produce a unified magnetic field. The MMFs'

synchronization lays out the first principle for controlling the

Te,stator == (2driqs - 2qrids) (9)DMP machines.

kepm-rzotor - 32mm(iq +i4r (10) 822 q r

Te,wd-rotor 22 qdsr -2qs'dr) (1l)

Eq. (10) clearly indicates that the torque on the PM-rotor 400 Amp (peak)is composed of two components. The first component is = ~ aiutrubEcontributed by the interaction between the permanent magnet 200Xi XX X qXX biYFEand the stator current. Likewise, the second one is contributed A aiuorebycircitodelby the interaction between the permanent magnet and the inner Xrotor current. 0 7 4 1 8 5

Fig. 3 Electromagnetic torque on the PM rotor

iii ei oiots X X S 'Il,l~~~~111':

;~~~~~~~~~~~~~~~~~~~~~~~~~li [r 1E- Iu ':1

Fig. 5 vrl yteItutr

lmwtertMr)r

IV. CONTROL ALGORIIHMAND SYSTEM SIMULATION ~ ~ ~ ~ ~~~~~~~lul 4

IV. CONTOLALGORTHM AND YSTEMSIULATIONThe process of a hybrid electric vehicle from starting to

Because of the two mechanical and two electrical ports, coming to a stop is simulated. Torques, currents, voltages andthe DMP machine has many possible operation modes, all power flow are examined by the simulation. As shown in theshould satisfy the overall power balance equation, Eq. (12), control block diagram in Fig. 6, the DMP system control±p ± p ± p ± P = (12 consists of 2 independent speed loops, one for outer PM rotor

- e stator- m pm-rotor- e wd-rotor- m wd-rotor ( 2) ~ ~~stato

estato m,pmrotor e,wd rotor m,wd rotorand another inner wound rotor. In each speed loop, there arewhere "±" represents the power flow in or out. Subscripts "e" two current loops, one for current regulation in the q-axis andand "in" represent "electrical" and "mechanical" respectively, another d-axis. The simulated process of the DMP systemIn theory, the solutions to above equation are infinite but some could include numerous operation modes, each correspondingof them are of our particular interests, to a unique application. Also to be noticed is that the two

mechanical ports - the outer PM and inner wound rotors, are

One ossbleoluion o te abve owerbalnce attached to external mechanical loads and an IC engine

One posslble soluhon to the above power balance~~toWvel

equationgncanbeuxwrittenbuanrespectively. To highlight the DMP uniqueness in operation,the entire simulation process iS dvldedmito 6 major intervals,

Bc,ap srotor +Pm,d rotor me,stator ± ne,wd rotor = C (13) each corresponding to a featured application.

In this case, the mechanical power of the two rotors is not ima v shwv thebalanced and the remaining can be absorbed by the electrical GOterminals and sent to the common DC bus, like a battery bank, _l * afor energy storage or draining. This operation mode is much one for c r l n tq s

like hybrid traction for electric vehicles. Obviously, this d 9s l poperation mode is what we are most interested in. Des, e rr

When the DMP machine is used to drive a hybrid electric t to b ndrq 'vehicle, it can replace the two electric machines and the M 9ptHtransmission box in traditional HEVs. The inner wound rotor if .is connected to the IC engine. The PM rotor is connected to e nic adan nIC

wheels for driving purpose. Two sets of inverters are needed, Fig. 6 Block diagram of DMP machine systemone for stator windings and the other inner wound rotorwindings. However, they can share a same DC bus. Theoverall system structure is shown in Fig. 5. a) Interval I - PM rotor acceleration from 0 to 2500 rpm

In the first interval, the stator windings are powered fromthe DC power supply through a DC/AC inverter and the PMrotor and the associated vehicle mass are accelerated from 0 to2500rpm with vector control. The inner wound rotor is notexcited nor driven by any external mechanical torque. Thus,there is no electromagnetic torque between the inner and PMrotors - the inner rotor remains standstill. Through the

controller, the stator delivers electrical power to the PM rotor power to go higher speed. Therefore, the slip power controland is converted into mechanical power for the acceleration. through the inner rotor windings now has to reverse theWe have assumed some mechanical frictions on the PM rotor. direction, sending power to the inner rotor so that the rotorSo, the electromagnetic torque generated by the stator currents speed can reach 3500rpm eventually. The slip power reversalis balanced by those for acceleration and overcoming frictions, and current sequence change in Interval II are clearlyThe simulation results are shown in Fig. 7. From the top to displayed in Fig. 8. The DMP system power balance is shownbottom, plotted are PM rotor speed, inner rotor speed, stator in Fig. 9. The simulated current and power confirm thetorque, PM rotor torque and inner rotor torque. As indicated analysis stated above.by the simulation results, it takes about 6 seconds to acceleratethe PM rotor from 0 to 2500rpm. In this interval, the process MOnnnr roto \t.Ue~of a HEV being started by the electric machine is simulated. 0--L---------

0

-1

-2DO I I I ~~~~~~~~~~~~~~~~~~~~~~~~~~~~t(s)2DDD -0 20 25 30 35 40 45 50

5 10 15 2 PM rotorspeed(rprm) t(s)00 5 1 0 1 5 20 25 30 35 40 45 50 40Inrer rotor current (A) -

Ierrotorspeed (rpmr)

0L_ _ _ _ t(s) ts0510 15 20 25 30 35 4 45 5 40- -: I- -T - -F- )

500 0 515 2 5 3 5 4 5 5Stator torquje (Nm)

400 --J____ * t(s) ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~Erlargedinrnerrotorcurrent (A:)

0 5 10 15 20 25 30 35 40 45 50

500 PM rotortoru k

t(s)500 ~~~~ ~ ~~~ ~~~~~~~~~~~~~~~~~~~~~~~~t(s)1010.0210.04 10.06 10.08 10.1 10.12 10.14 10.16 10.18 10.2

0 5 10 15 20 25 30 35 40 45 50

400 -I---j---L-------- ~ nrer rotortorque(1`4 n) Fig. 8 Simulation results of inner rotor ia and Va

-200 t(s) x10 5 10 15 20 25 30 35 40 45 50 1 ---I---T Stator pawarCN

Fig. 7 Simulation results of DMP speed and torque 0----------

-1

b) Interval HI- Inner rotor acceleration from 0 to 3500rpm 65

1 1 2 25 3 35 4 45 0t(s

1 PM rotor pmvx~eThe second interval starts in 10 second and lasted for

about 0.1 sec. At the beginning of this interval, the PM rotor -1 5 10 15 0 25 3 5 4 t(s)is already at 2500 rpm. Then, a step speed command is 0 5 1 5 2 5 3 5 4 5 5

applied to the inner wound rotor to accelerate it from standstill1Innerrotor slip pxwer W

to 3500rpm to simulate IC engine starting. Notice that the 0---

inner rotor acceleration torque is given directly by the outer t(S)PM rotor, which, in turn, is from the stator. It can be observed 0 5 10 15 20 25 30 35 40 45

that the PM rotor is slowed down a little bit by the inner rotor 1 - -- -- -- - - - -- - - - ne oo -caio oe

dragging force. To stabilize the PM rotor speed, the PM rotorspeed loop requests with more stator currents for more torque ____________________________ t(s)so that the PM rotor remains at constant 2500rpm. The 0 5 10 15 2o 25 30 35 40 4.5 50

simulated stator current result clearly shows the boosted stator Fig. 9 Simulation results of DMP power balancecurrent and torque for the inner rotor acceleration and ICengine starting. c) Interval III - The engine attached to the inner rotor

starts and delivers 150 Nm torqueDuring inner rotor acceleration, it is most interesting toInntras1,wetrthengeatcedoteinrexamine the winding current of the inner rotor before and afterInntrasIIwetrthengeatcedoteinr

dthesed f200rm Before reach-ing 250 rpm, 'theine rotor and command it to deliver a mechanical torqiue of 150

actually given by the stator currents acting on the PM rotor. the peak power shoots to as high as 100kw. The stator powerTherefore, the stator is operated as a generator. On the other plot also shows that the energy recovered (area under thehand, the stator field and PM rotor have a synchronous speed speed curve) in this interval approximately equals to that inof 2500 rpm, lower than the inner rotor mechanical speed of Interval I.3500 rpm, which forces the surplus power provided by theengine absorbed by the slip power windings on the inner rotor. To highlight the power flow of the overall DMP system inIn this interval, both the stator and inner rotor are sending various intervals, the power flow in and out from bothpower to the electrical source, originated from the engine electrical and mechanical ports are plotted in Fig. 8 accordingattached. to the operational sequence discussed in the above intervals.

Notice that the stator involves in only electrical power and thed)aIndrterualmIVs-PM rotor goes to 3000 rpm from 2500rpm outer PM rotor mechanical power. This is because the stator

has no mechanical port (shaft) and PM rotor has no circuitsWith the engine outputting 55kw (at 3500 rpm and 150 (windings). The inner rotor has both mechanical port (shaft)

Nm) while both the stator and inner rotor sending power back and electric circuit (windings). Therefore, both mechanicalto the electrical power supply, speed control of the PM rotor is and electric power flow is involved by the inner wound rotor.investigated in Interval IV. This interval starts at 20 and ends In addition, if we slice any moment along the time axis, the netat approximately 26 sec., nesting inside Interval III. As power flow of the overall DMP is always zero, implying powershown, the speed control is composed of three stages: balanced by various ports, mechanical and electrical.acceleration, stable speed, and deceleration. At accelerationstage, a step speed command from 2500rpm to 3000rpm is V. CONCLUSIONSapplied to the PM rotor and the stator sends the needed powerand torque to the PM rotor, which offsets the power generation DMP machine is a brand new type of electric machine,due to the engine attached to the inner rotor. At the stage of promising in many fields as multi-source hybrid traction,the PM rotor speed stable at 3000 rpm, more power will be integrated starter and generator, variable gearboxes and sogenerated to the stator from the inner rotor and simultaneously, forth. In this paper, we have concentrated on its application inthe slip power recovery by the inner rotor windings is reduced hybrid electrical vehicle application. The dynamic model ofproportionally for reduced slip. At the final stage, with the DMP machine is established and explained. Further, aPM rotor speed return from 3000 to 2500 rpm, the stator has to prototype machine is designed and evaluated by finite elementabsorb the both the PM rotor kinematical energy and the one analysis to prove its feasibility in hybrid electrical vehicleoriginated from the engine. As expected, the stator current hits application. Several interesting operation modes are discussedthe current limits of 400 amps. The simulation results in and the corresponding control system block diagram isInterval IV clearly confirm all of these. configured. Computer simulation is conducted corresponding

to the whole process of a hybrid electrical vehicle fromstarting to coming to a stop. The control algorithms with the

e) Interval V- Engine shut down and inner rotor windings overall system operation performance are verified by computerde-excited simulation. Hardware construction of the DMP system has

At 30 sec., in Interval V, we shut down the engine and started and the latest results are to be presented in futureremove electrical excitation to the inner rotor windings. papers.Therefore, there is no electromagnetic and mechanical torque REFERENCESon the inner rotor. From simulation result it shows that theinner rotor speed drops to zero gradually because of the [1] L. Xu, "A New Breed of Electrical Machines - Basic Analysis and

Applications of Dual Mechanical Port Electric Machines", in Proc. 8thfriction torque. For the PM rotor, the speed control loop is Int. Conf: on Electrical Machines and Systems, Aug. 2005, vol. 1, pp.still active to command the stator current and electromagnetic 24-31.torque, maintaining the PM rotating at 2500rpm. In this case, [2] P. Hammond, "Energy Methods in Electromagnetism," (Book),the electromagnetic torque generated by the stator current is Clarendon Press, Oxford, 1981.balanced by the friction torque on the PM rotor. [3] S. A. Nasar, L. E. Unnerhr, "Electromechanics and Electric Machines,"

(Book), John Wiley and Sons, 1983.[4] L. Xu and W. Cheng, "Torque and Reactive Power Control of a Doubly-

f) Interval VI - PM is decelerated to 0 rpm Fed Induction Machine by Position Sensorless Scheme", IEEEIn the last interval, after the engine and the inner rotor Transactions on Industry Applications, Vol. 31, No. 3, May/June 1995,

stop, we give a negative step speed command to the PM rotor. pp.636-642.[5] Martin J. Hoeijmakers, Jan A. Ferreira, "The Electrical Variable

The electromagnetic process in this interval is of the reverse in Transmission", IEEE-IAS Annual Meeting Proceedings, Oct. 2004,Interval I. In the last interval, we recover the mechanical Seattle.energy given to the PM rotor and its associated mechanical [6] Yuusuke Minagawa, Masaki Nakano, Minoru Arimitsu and Kan Akatsu,load during acceleration. In this interval the DMP functions as "New Concept Motor with Dual Rotors Driven by Harmonic", SAEa generator. Similar to what happened in Interval I, it takes TECHNICAL, PAPER SERIES 2002-01-2857, NISSAN Motor Co.,about 6 seconds to slow down the PM rotor to zero speed andLT