High Performance Brushless Permanent Magnet Traction Drives for Hybrid Electric Vehicles

Mr N Schofield and Mr M I( Jenkins, Magnetic Systems Technology Ltd., Shefield, UK

Introduction

The unpact of vehicular exhaust emissions on the environment and public health has prompted renewed interest in all-electric and hybrid-elecmc vehicles in recent years. In particular, the increasingly ~tmqent emissions legslation introduced in the state of California, USA, over the past decade has channelled research and development effort into vehicle desips producing lower tail pipe emissions. Electnc vehicles are amongst a number o f v h c l e choices. rangmg fiom those supplied with alternative fuel sources to vehicles incorporamg enhanced lean bum engine technology. proposed to satis@ mid-term legislative requirements. However. given the h t a t i o n s of present h a n q technology, it seems Uely that for urban and mdum-high performance vehicles, most electric vehicle datelopment in the immediate hture will take place aroundthehybrid-electric format, this seen as an intermediate and technologically achievable step towards a full all-electnc vehicle having realistic road performance. Brushless permanent magnet motor drive systems are candidates for application in both all-electric and hybrid-electric vehicles since they h a w the potential to offer a cost-effective solution for realising the vehcle powa/torquerequiranents. whilst at the same time being more energy efficient and comprise of a lower mass and volume than other competing technolopes for a given rating specification - essential component constraints in these vehicular applications. The paper will compare the relative merits of motor and drive systems currently in use in electric vehxle systems. It will discuss the potential ofpermanent magnet solutions considering cost and magnetisation issues. and will present details of a permanent magnet traction machine designed for hybrid-electric applications. It is widely professed that the only design criterion for automotive application$ is cost. Unfortunately. this outlook prejudices against new technologies in favour of established vehicular components due to the relatively high capital committed to specialised tooling and automation commensurate with volume production. The feasibility o f a hybrid-electric vehicle. commercially comparable with current I.C. engine v&cles, is not restricted by the vehicle component technology but by thereluctance ofthe automotive industry to embrace. what would amount to. a major capital investment not just in component tooling but. as equally essential. the overall v e h d e support infrastructure from h e h g to mamtenance and servicing requirements. While the current cost of brushless permanent maqet drive systems is hlgh compared to existing brushed dc drives the potential for cost reduction in volume production makes this the most likely technolom to succeed in future traction applications. At the systems level. the choice of traction machine of specified rat@ suitable for both all-electric and hybrid-electric v h c l e s is generalh d e t m e d by UKCX main factors: weight. efficiency and cost. It is thmefore, worthwhile reviewing the various competing technologies IO focus on their panicular suitability for fulfilling the above criteria.

Comparison of machine output coeflicient - Bwc Q

For fixed speed and load applications the maits of competing technologies can be easily quantified. leading to a well informed choice ofmachine topology. Howmer. for variable speed and in particular vehicular traction applications. where the overall machine operating specification is not as tightly defined. the choice of a suitable machme format can be quite involved. There aremany different ways of comparing the relative mmts of elecmcal maches . a few popular techniques are summarised below: (a) (a) (c) ma.ximuni speed capability. (d) (e) specific weights. (0

only the last method gives a truly representative comparison. howma. th ls approach may also he influenced by the personal preferences of the designla whch can overshadow the selection of certain parameters for one design over another. The use of machine output coefficient has tradihonally been the main tool for comparing designs. particularly of one machine ripe, The comparison is based on the anal>% of parameters at the machme airpap. where the main paranietas of interest are: D L

output coefficients based on the rotor dunensions, magnetic and electric loading peal; values of flux: density and current loadmg.

torque per unit volume or per unit copper mass,

case study comparison of specific designs for the same application specificahon.

rotor diameter ( m ) , magnetic active axial length of stator core or rotor ( m ) .

4 i l

Bave

A

the average airgap flu-density ( T ) ,

the ampere stream or ampere conductors per meter of a q a p circumference ( Am-' ) . m z IC

The ampere stream is determined fkom A = - X D

where m is the number of phases, z the number of conductors per phase. IC the current per conductor and x D the circumfaencial path around the rotor diameter.

in practical maches , construchonal or control issues reduce the effectweness of the ampere stream, to account for t h l s a number of factors need to be considered. kf k , the supply form factor, wmdmg factor and mtunal power factor (cosme ofthe angle between flux and current)

respectively, and G the 'electncal gear ratlo' apphcable to SR machmes and is the number of revolubons of the magnetic field fbr

one revolution of the rotor. From the above, machme power can be estunated fkom:

where o is the rotor speed in mechanical radans per second ( rad(,) s-' ) The terms in parentheses are also referred to as the specific electnc loadlng. Q. adoption of whch reduces (2) to:

x D'L Pe = - Bave Q . 0 L

Equation (3) can be dmved from first principle by considering the force on a current caq ing conductor in a magnetic field and defines the machine power capability in terms of the rotor geometry, the magnetic and electric loading (usualh lumped together and referred to as the machme output coefficient) and rotational speed. For a fixed geometry and shafi speed. output coefficients can be evaluated and used to asses the relative power capability of traction motor technologies. Typical values of the parameters in (2) and (3), for force air ventilated machines. are presented in tahle 1 (columns 1-4[ I] and columns G-S[?] ) for both industrial fixed speed and representative automotive traction dnves. The values in table 1 are not intended to be hard and fast indicators, however they do allow an overview ofthe machine technolog to he taken. It will benoted that the combination ofhigh magnetic and electric loading presents the hrushless permanent magnet topology more favourably than other competing formats.

Comparison ofthe traction drive train p9 a whole

Forvariablespeeddrives considerationmustbepven tothemachineandcontrollapackageas a whole. Table?[l] compares machine and controller. weight. efficiency and cost estimates for the main traction drive topologies utilised in current hybrid electric vehcle systems. Again the permanent magnet drive is the most favourable option in terms ofweight and overall efficiency. However, as an emerging technology for traction applications, permanent magnet dnves do c a m a cost overhead above the more established brushed dc and induction machine formats. To put this cost into prospective. the v h c l e designer must consider what the extra capital outlay is worth from a systems view-point. where theweight reduction and efficiency benefits of the permanent magnet dnve are reflected in the vehicle performance and fuel consumption.

Considering particular drive components individually. the induction and permanent magnet invertas are essentialh the same and. for a given output power. the permanent magnet m a c h e stator will have one half ofthe copper and iron a5 the induction machme. Therefore. the major cost area of the permanent magnet machme can he attributed to the rotor component. arismp from the current cost ofthepermanent magnet material andassen~bh costs associatedwith hgh strength magnetised components.

Permanent magnet raw material costs and the impulse magnetisation of permanent magnet components

Figure 1 illustrates the downward trend in rare-earth permanent magnet raw material cost from 1984[3]. with a more gradual reduction in recent years as the major investment overheads are recouped. This cost will continue to fall for some years to come particularly as the inception of rare-earth permanent magnets into new application areas increase. licencing restrictions become relaxed Chinese material becomes readdy available in commercial grades and. arguably the major factor associated with high material cost the relatively low volume of manufacture is mcreased As an aside. it is worth

noting that it took some 25 years to reach the ultimate cost reductions in the processing of high volume sintered ferrite magnets.

While thepumanent magnet material cost is somewhat market d n v a onearea where cost reductlon in volune manufacture can be addressed is in the assembly of the traction machme permanent magnet rotor. This requnes a number of manual handling operations which, due to the nature of the high strength magnetised matenal. requrres a hgh degree of skill. A major step towards automating t h ~ s procedure is through the post-assembly magnetsation of fully pre-assanhled unmagnetised components.

Capacitor-dscharge impulse magnetisation of the magnets which provide the magnetic field excitation in pmianent magnet machines and devices is an important production step, particularly for rare-earth magnets and especially in automotive applications where low unit cost is a major concun. By using advanced analysis and simulation techques. magnetising systems can be designed to optimise device electro-magnetic performance whle remaining mechanically and thermally robust. Figure (2) illustrates (a) a typical production impulse magnetising fLvture for an 8-pole bonded NdFeB magnet. together with (b) representative field patterns fiom a variety of multi-pole magnetised rare-earth magnets. The fixture incorporates water cooling, temperature monitoring and protection circuits which automatically set the operating voltage and firing rate of the capacitor-clscharge unit. Magnetising systems suppiid by MST range fiom a 1 kJ unit for the 4-pole mapetisation of a 2.2mm &meter x 3mm long magnet to a 100 kJ unit for the &pole magnetisation of a 250mm diameter x 350” long rare earth magnet. The applications are varied including automotive peripheral machines to electro-magnetic enagy storage systems suitable for both industrial and electric vehicle support services. For traction drives the ability to post magnetise a fully assanbledpmanent magnet rotor significantly reduces the constructional effort and hence cost. particularly in volume manufacture. It also facilitates the recovery and repair of demagnetised components to full performance capability with relative ease.

MST Permanent magnet traction drives

MST have provided a number ofhigh performance traction drives for application in hybrid-electric vehicle systems. One important attribute of a permanent magnet drive is the lnherent peak power capabilih. making them particularly suited to urban traction applications w h e e t h e v h c l e load is of a dynamic nature, demanding high peaks o fpowa for shon duration over a low continuous base raring. The MST TD7 m a c h e provides a peak torque profile of 170” fiom 0 to 4krpm followed by a peak power profile of 70kW fiom 4krpm to the top speed of 12lirpni u1 a machine package of 43kg and envelope as illustrated in figure 3. With a 60”C, 61 /min coolant inlet. this peak rating equates to a 3 min.. 3: 1 duty rating above the continuous rating of 1 IONm. 35kW respectiveh as defined in British Standards. Typical test performance data is presented in table 3 . For automotive traction applications a desirable feature of the machine des ip is its robustness both mechanically and electromagnetically, havins a high resilience to demagnetisation particularly under dnve fault condtions. This is of particular importance when the drive motor is operated at a speed above the base level fixed hy the dc Id supply voltage. i.e. m the field weakening reion, h a e the motor back-emf is greater than the effective phase voltage available from the inverter. However. since the bacli-emf is lagging in phase with-respect-to the supph voltage hy an aneie controlled via the drive inverter, a motor current will flow and torque produced. If the inverter power devices are disabled at speeds above base. through loss of the dnve control circuits or device failure the motor back-emf will force a regenerative current into the battery via the inverter bridge fly-back diodes. For a g i ~ e n dc supply and motor speed. the magnitude and phase angle of this fault current relative to the niotor hack-enif will he determined by the motor winding, invertor. supply cables and battery impedance‘s. Typically the total circuit inductance will be relatively low, hence the fault current will have a hgh torque producing componnent which niust be restricted IO a short time duration so as not to critically influence the vehicle stability. Protection against this failure mode CM be enhanced by a a t i o n of component redundancy into the drive control circuits and duplication of critical supplies. Should the failure arise purely in the control circuits a second level of monitomg can beincludedwhich detects the fault occurrenceandnuns onthebottomthreepowadmices oftlieinvater bridge. effectiveh short-circuiting the motor terminals. Additionally. as an ultimate back-up, an in-line IUptUMg capacity ( H R C ‘ ) h e should also be included in the dc link supply in the went of the power devices failing to open circuit. While short circuiting the motor terminals at high speed appears somewhat reckless. it does present a nice solution to the drive train fault ifthe m a c h e and drive are rated to s w i v e the event. For the MST TD7 machine the sliart circuit current and resulting braking torque at the motor shaft can be calculated f?om standard equivalent ctrcuit analysis. The validin. of h s approach can be justified by the effectively linear machine parameters over the operating range of interest and near harmonic fiee sinusoidal phase back-emfs. Nqlecting non. wkdageand fictional losses. themotorrms shon circuit current and shaft braking torque can he calculated from (4) and (5) respectively:

& W e

TR- + ( L we )- motor r m s short circuit current. I,, =

3 p R % ’ w , and short circuit shaft braking torque, Tsc = R’ + (Lee)' where. for the particular machine of interest. p is the number ofpole-pairs = 4 , ko the machine back-emf constant = 0.0484 Vs rad(e)-’ R thephaseresistance = 7.06 mR @ 150°C L the phase inductance = 177 pH @; 150°C

We

Note: shaft speed in rpm, urpm = - The simulated results of (4) and ( 5 ) are illustrated in figure 4 (solid lines) against data measured on test (broken h e s ) . It will be noted that the torque at high speed is relatively low and the current hi i ted by the circuit inductance to be within the dnve continuous ratmg.

@, 150OC

the rotational fiquency in electrical radians per second - rad(e) s-l 60

2 x p W e

Summary

Information has been presented on the methods for assessing traction machme performance for selection as all- and hybrid-electric vehxle dnves. The suitability of pamanent magnet h v e s has been &scussed together with post assembly mqetisation as a technique for reducing cost in volume manufacture. A representative 70kW peak brushless permanent mapnet traction machinemanufactured by MST Ltd. has been presentedwith some discussion of it’s performance and fault tolerance.

References

[ l ] West.J.G.U’.:*DC, induction. reluctance and pm motors for electric vehicles’. IEE (‘olloquium on Motors and D n v a for Battay Powered Propulsion, 15 April 1993, Digest No.19931’080. [2] Harris, M.R.: ’Comparitive electro-magnetic parameters for alternative motor types‘. ihid [3] ‘Permanent mapets - 1993 update’, Wheeler Associates. Permanent magnet conwitants. Kentuck?. USA.

Table 1. Comparison of traction machine performance indicators.

U'eight (based on DC = 1 .OOpu)

Motor Controller Drive Motor Controller Drive 1 Efficiency (O 0 ) 1 Drive Cost

IDC Machine 1.000 0.125 1.125 80 98 78 I 100

Inverter fed IM 0.500 0.250 0.750 90 93 84 1 100

Brushless PM

SR (8/6)

Table 2. Comparison of traction drive weights, efficiencies and cost (1992).

0.250 0.250 0.500 97 93 90 1 so 0.375 0.250 0.625 94 90 85 150

SpeedO<rpm) Power (kw) Torque (Nm) Phase current(A) Efficiency (YO) 2.0 34.5 164.6 348 83.8

3.0 54.3 172.7 389 86.4

I

~ ~ ~~~

I I I 4.0 70.2 167.7 365 90.4

- t I I 1 , I l I I I I I I I I

5.0 I 57.3 I 109.4 I 360 I 93.3

6.0 I 65.5 I 104.3 I 338 I 89.9

7.0 I 65.1 I 88.8 I 3 14 I 88.4

Table 3. Test data from the MST TD7,7OkW peak, traction drive system. -

- m f E

Figure 2. Production magcetising Gxture.

I

I 1

I I I I Y.W.". I,"."?. ia" -.d. ,L---,zs 1: y"--- '

Figure 3. 70kW peak, TD7 traction machine.

ShaftTomw I---I 200

Figure 4. Shaft torque and phase current at short circuit

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