integrated starter generators

Integrated Starter Generators

Study of PM Machines

Two variants of permanent magnet (PM) based integrated starter generator (ISG) machines were compared for use during development of a new hybrid electric vehicle (HEV). Such a system design generally has high efficiency requirements, mainly regarding high power/weight ratio at minimal size. Another criteria is high reliability for high-speed operation. In a study of Ricardo, two types of ISG were compared: One with permanent magnets placed on the surface of the rotor and the other with magnets integrated within the rotor core.

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Starter Generators

1 Introduction

Ricardo was the engineering partner for the development of a new HEV for the Asian market.

By integrating the hybrid part, the fol-lowing improvements could be achieved compared to the conventional base vehi-cle: Fuel economy improvement (NEDC)was improved to 13 % less, maximum power increased 5 %, maximum torqueincreased 20 % and 0-100 km/h accelera-tion time is now 10 % faster.

An ISG machine should be capable of providing high-torque at low speeds from standstill for rapid engine starting andproviding a wide range of constant pow-er operation. In addition, it should pro-vide high-efficiency output at its opera-tional region benefiting for HEV vehicle fuel economy improvement. In practiceit is not always feasible to satisfy all thedesign requirements because they are usually contradictory with each other.

The design tradeoff measures are usually taken to balance the machine perform-ance, operation safety and control con-venience.

The design factors for an ISG PM ma-chine mainly depend on the machine parameter requirements on specifying machine pole number, permanent mag-net flux linkage, dq-axis inductance,peak torque, operational speed, etc. The mounting space restriction and stringentcost target have significant impact on the machine design as well.

Two types of PMSM machines whichare evaluated here, a surfaced mountedPM machine (SPM) and interior PM ma-chine (IPM) have been successfully usedin the Honda and Toyota’s HEV passen-ger cars [1-2]. (Control strategies of PMmachines are investigated in [3-6]). Both SPM and IPM are proved to be very com-petitive candidates for HEV applications.

The two ISG PM machines used withinthis study were both designed in similar

The Authors

Dr. Xiaojiang Chen is Technical Specialist

in Power Electronics,

Advanced Electric

Machine Control and

HEV Control Strategy

at Ricardo PLC in

Cambridge (UK).

Dr. Roger Thornton is Senior Manager in

charge of the Hybrid

Powertrain Develop-

ment and specialized

in Power Electronics

and HEV System

Integration at Ricardo

PLC in Cambridge (UK).

Murray Edington is Chief Engineer

in charge of the Hybrid

Powertrain Develop-

ment and specialized

in Power Electronics

at Ricardo PLC in

Cambridge (UK).

Russell Lewisis Principal Engineer in

charge of the Hybrid

Powertrain System

Software Development

and specialized in

Embedded Software

Development and HEV

Powertrain System

Integration at Ricardo

PLC in Cambridge (UK).

Lars Christensen is Chief Engineer for

Control & Electronics

with main focus on

SW and HW Develop-

ment for Automotive

Purposes at Ricardo

Deutschland GmbH in

Schwäbisch Gmünd

(Germany).

Table: System parameters

SPM ISG IPM ISG

Stator slot 60 18

Pole number 20 12

PM flux linkagem (Wb) 0.0250 0.027

Magnet material NdFeB NdFeB

Magnet shape Shaped Rectangular

Stator skewing Yes No

Stator outer diameter (mm) 280 265

Winding layout Distributed winding Concentrated winding

Overall length (mm) 64 65

Continuous rating current (A) 200 200

Peak rating current (A) 300 300

Continuous power (kW) 10 10

Peak power (kW) 15 15

Ld(mH) 0.105 0.151

Lq(mH) 0.105 0.278

RphR (m ) 20.5 16

Weight 12kg 13.5kg

Operational bus voltage 120~186V 120~186V

Estimated manufacturing cost >$800 About $250

ATZelektronik 04I2009 Volume 4 35

size with a 15 kW peak power and 10 kW continuous power rating.

Ricardo was completely in charge of the ISG hybrid powertrain system devel-opment including system safety design,integrated control unit hardware design and validation test, HCU control strategy and software development, ISG machinedesign, MCU control strategy and soft-ware development, system fault diagno-sis software, system integration, vehicle calibration and test validation, etc.

Extensive practical experimental tests have been carried out to evaluate the ma-chine characteristics and performance.

The SPM ISG was specially designed for the application, using a complexstructure with shaped magnets, skewed stator and complex distributed winding layout. The IPM ISG has a much simpler mechanical structure with concentrated winding layout in stator and simple sin-gle-barrier rotor structure. Both variants were tested in a dynamometer extensive-ly and installed on the real ISG vehicle toinvestigate their performance.

The IPM ISG machine was proven tobe a more cost-effective solution for the application.

2 Machine Structure

The two PM ISG machines are developed and evaluated at Ricardo for the applica-tion in a mild ISG HEV car, powered by 1.3 L gasoline engine. The machine pa-rameters are listed in Table. These two ISG machines have the same power andwinding current ratings in similar size.

The SPM ISG has a relatively complexstructure. Shaped magnets are strength-ened on its rotor surface to provide sinusoi-iidal air-gap PM flux. Its stator core is skewed intentionally for cogging torque reduction. The stator has 60 slots with a sophisticated distributed winding layout. The machinemanufacturing cost tends to be relatively higher than that of the IPM ISG.In com-mmparison, the IPM ISG machine has a sim-mmpler structure. Its rotor has the simple sin-nngle-barrier structure with rectangular-shaped magnets securely embedded intothe rotor cavities. Its stator has 18 slotswith simple concentrated winding layout. The IPM ISG has less poles than the SPM ISG, which implies much less fundamentalcontrol frequency. Low-cost lamination steel sheets are used in both the stator and rotor cores for the cost reduction purpose.

3 ISG HEV Program Overview

Both PM ISG machine were fully installed and demonstrated in a production devel-opment program of a mild hybrid vehi-cle for a crank mounted ISG. Ricardo isresponsible for the hybrid powertrain system development including:– overall system safety evaluation and

safety strategy design– “One-box” packaging hybrid control

hardware development and system validation/test including motor con-trol unit, hybrid control unit , safety control unit and DC-DC converter

– high-level HEV powertrain control sys-tem specification, strategy and appli-cation software development

– ISG machine design and performance validation

– complete motor control system speci-fication, control strategy and control software development, MCU system calibration and performance verifica-tion and test analysis

– safety critical application software specification and coding for produc-tion

– software-in-the-loop (SIL) test for HCUstrategy and function evaluation

– hardware-in-the-loop (HIL) system de-velopment for validating and testing the system hardware and software

– component and system level hard-ware and software test and validation

– hybrid vehicle performance calibra-tion and test.

The Integrated Control Unit (ICU) de-signed by Ricardo consists of four inde-pendent control units: Hybrid Control Unit (HCU), Safety Control Unit (SCU),Motor Control Unit (MCU) and 1.5 kW DC-DC converter converting electricpower from the NiMH high-voltage bat-tery to 12 low-voltage battery. The ICU module as revealed in Figure 1 has thedimension in 250 mm (L) x 235 mm (W) x 90 mm (H), Figure 1.

HCU is the HEV powertrain adminis-tration centre responsible for the hybridpowertrain management to ensure the hybrid functions including engine start/stop, regenerative braking, HV battery management and powertrain powerboosting to be delivered. Depending on driver demands, the HV battery status,and the status of the vehicle, HCU will manage to determine the operating

Figure 1: ICU module

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Starter Generators

mode of ISG machines and the torque de-livered by the electrical machine and the internal combustion engines (ICE), main-tain the SOC of the LV battery (using theDC-DC converter), maintain the SOC of the HV battery (using the MCU), interface with other controllers on the vehicle toprovide/receive information, control ex-ternal devices (e.g. pumps and fans), and handle system level faults. SCU is respon-sible for estimating the torque being pro-duced by the ISG machine, comparingthe torque estimate to driver demand so as to detect potentially hazardous unex-pected acceleration or deceleration, and

detecting driver’s door closed switch sta-tus, etc. Based on the information, the SCU can disable the ISG machine invert-tter drive, and over-ride any control from the HCU or MCU.

The SCU can also disable the HV bat-tttery supply on detection of a systemfault.The DC-DC converter converts theHV battery voltage to LV to charge the standard 12V car battery. The DC-DC con-verter is controlled from the HCU and sends fault and output current feedback to the HCU. It also recognizes its fault conditions and sends error messages to the MCU as well.

MCU accepts HCU control commandsthrough CAN to control the ISG machinecurrents to deliver the demanded torque,provide protection of the motor and the3-phase bridge against over-current, over-voltage and over-temperature faults, and transmit MCU status and fault informa-tion back to HCU. The ICU module has asingle liquid cooling system for thermalcontrol. MCU will automatically de-rateits torque output to mediate the thermalstress if a temperature limit is reachedfrom either the ISG machine stator wind-ing or MCU inverter. The close-loop speedcontrol mode is offered in MCU for theengine cranking control. The instantengine start is necessary to achieve the start-and-go function in HEV. Figure 2 indi2 -cates the practical test result of a fast en-nngine cranking control demonstrated inthe mild ISG HEV car with a warm 1.3 L gasoline engine.

4 Testing Environment

A life tester equipment was developed forlong-term performance and life endurancetest to the MCU and ISG machine. It is built with simple back-to-back configuration to simulate a dynamometer system. Two ISGmachines and two ISG ICU control modulecan be tested and evaluated simultaneous-ly. With this configuration, one ISG ma-chine system will run in speed mode, andthe other in torque mode accordingly. Fur-rrthermore once one system operates in mo-toring mode, the other will automatically run in generation to balance DC-link pow-wwer consumption and minimize the overallsystem power system consumption.

This life tester system is developed with friendly operating interface window asshown in Figure 3. All control parameters of MCU and ISG machines, such as torque,speed, temperature, mode, system faultstatus, etc, can be monitored. The testersystem runs in the specified control cycleand control profile in order to simulatethe real driving duty cycle of ISG machinesystem. The ISG speed ramping rate is managed to control energy flow betweenMCU and programmable power supplies.System protection measures against in-verter fault, over-voltage, over current,over-temperature, and fault modes are im-plemented to provide reliable and safesystem operation.

Figure 2: Rapid engine cranking start controlled by ISG PM machines

Figure 3: ISG life tester operating window


5 Test Verification of ISG Machines

Two PM ISG machines were evaluatedand verified completely on a four-quad-rant dynamometer test rig. The dy-namometer is mounted with an accurate torque sensor for measuring mechanicaltorque.

5.1 No-load LossFigure 4 illustrates the measured dy-namometer shaft loss with the PM ISGmachines inactive on the dynamometer shaft. The no-load losses were mainly con-tributed by the machine iron loss, fric-tion, and windage loss. The iron loss would be the most significant part. TheSPM ISG generates much higher iron lossthan IPM at high speed. This is mainly due to the high pole number of the SPMand higher PM flux linkage. The high no-load loss will significantly deteriorate theoverall efficiency of the SPM ISG at high speed.Further characteristics that weretested on the dynamometer were back-EMF and short-winding characteristics.

5.2 Back-EMFThe back EMF constant was determinedfor each ISG by measuring the value of the line-to-line voltage at a speed of 1000 rpm. This constant is useful to deter-rrmine the inverter DC-link voltage at un-controlled generation and compute thePM flux linkage for machine performance predication. These values are importantto estimate machine performance. Themeasured SPM back EMFs are very sinu-soidal with little harmonics. Compara-tively the IPM back EMF contains moreharmonics. The calculated back EMF con-stant of the IPM ISG is only about 2/3 of the SPM back EMF constant. This will una-voidably increase the inverter operationalsafety margin for the IPM ISG against con-trol failures especially at very high speed.

5.3 Short-winding CharacteristicsThe short-winding (SW) connection of the 3-phase terminals of ISG machinescan be employed to protect the inverteragainst the failure of field weakeningcontrol at high speed if the machineback EMF is over the inverter voltage rat-tting limit. In addition, for HEV applica-tions, the SW protection can also beadopted to remove uncontrolled charg-ing power to the HV battery.

In the SW connection, current willflow through 3 phase stator windings due to the impact of back EMF in PM ma-chines. This current will be stabilized against speed and gradually converge to a level with a constant amplitude at high speed.

The tests have shown that the steady-state IPM SW current amplitude at highspeed is within the continuous current rating circle of 200 A. that of the SPM variant is noticeably higher. This impliesthat the IPM ISG machine should present better flux-weakening capability thanthe SPM ISG.

5.4 ISG Machine ControlTorque control of PM machines requires the knowledge of the machine param-eters such as PM flux linkage and dq-axis inductance especially for the IPM ISG. The IPM reluctance torque has signifi-cant part in its overall torque generation. It has to be controlled correctly to meetthe control demand. In addition, IPMmachines are usually designed withmagnetic saturation in favor of reluc-tance torque and sizing benefit. Themagnetic saturation causes the dq-axis inductance variation against control cur-rrrent. The knowledge of dq-axis induct-ance is important for torque determina-tion and torque control of an IPM. A cost-effective method for inductance detec-tion is introduced [6].

An IPM can be designed to provide asignificant reluctance torque compo-nent, which highly relies on the dq-axis inductance difference. Its reluctance torque can bring benefit for improving machine efficiency, power density andtorque density. For IPM machine, there is

no unique solution of the control cur-rrrent vector against a torque demand. To optimize the machine control efficiency is a priority in IPM control. A so-calledMaximum-Torque-Per-Ampere (MTPA) scheme can be applied to minimize theIPM machine copper loss. The SPM torqueis almost linear to control current ampli-tude. However, due to the magnetic satu-ration and reluctance torque component,the MTPA torque curve of the IPM ISG isnonlinear to current amplitude and com-puted by numerical iteration processbased on data tables and PM flux-link-kkage.Comprehensive tests for evaluatingthe control performance of two PM ISGmachines have been carried out based onthe MCU drive hardware system andcomplete production-ready MCU code.

Figure 5 (left and right) reveal the measured control performance maps of the two PM ISG systems (machine + in-verter) in both motoring and generationcontrol. The MCU inverter DC-link termi-nals are directly connected to a Nickel-Metal-Hydride (NiMH) high-voltage bat-tery with a nominal voltage of 151 V. The DC-link voltage variation due to HV bat-tttery internal impedance and state-of-charge (SOC) variation has impact on themaximum power delivery of the ISG ma-chines. Especially in motoring control, the HV battery terminal voltage droppedbelow the battery open-circuit voltage.With the load power increase, the DC-link voltage dropped severely. The maxi-mum power output of the ISG machinehad to be derated in terms of the availa-ble voltage. However at generation con-trol, the battery terminal voltage ran above the battery open-circuit voltage.Therefore the ISG machines could deliver

Figure 4: Measured no-load shaft losses

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Starter Generators

higher generation power. The generationcontrol tends to be more efficient thanmotoring at high speed for the two ISG machine systems (machine + inverter)due to the introduction of feedback flux-weakening strategy and higher DC-link voltage available.Caused by higher fluxweakening current demand, the SPM ISGwas incapable of delivering 15 kW poweroutput at high speed. The 10 kW per-rrformance test was only carried out for the SPM ISG, as indicated in Figure 5 (left).

As shown in Figure 5 (right) the IPMISG was well able to providing 13 kW motoring power and 15 kW generation power in maximum. Both the two PM ISG machines were controlled to run upto the required maximum operationalspeed of 6000 rpm under effective flux-weakening control. Because of the loweriron loss and less flux-weakening cur-rrrent, the IPM ISG system is more efficientthan the SPM ISG system at high speed. In addition, due to much lower back EMF constant of the IPM machine, the opera-tion of the IPM ISG system will be morereliable against uncontrolled generation at high-speed control failure.

6 Conclusion

Ricardo wanted something that was up-to-date but it was important for it to be robust and to have tried and tested parts.

With more than a hundred projects inthe hybrid area, Ricardo could offerbroad experience in supporting this HEV development.

Extensive practical experimental testshave been carried out to evaluate the machine characteristics and perform-ance. The SPM ISG was specially designed with a complex individual structure.However it exhibits more iron loss and copper loss at high speed than that of the IPM ISG. The IPM ISG has much sim-pler mechanical structure offeringmuch lower manufacturing cost. Accord-ing to the test results the IPM ISG did of-fffer improved overall control efficiency over the SPM ISG.

Both the PM ISG machines have beeninstalled on a real mild ISG hybrid carand fully demonstrated to provide therequired ISG HEV function such as regen-eration-braking, powertrain power boost-tting, fast engine cranking, battery charg-ing, etc.

The low-cost IPM machine was provedto be a more viable cost-effective solution for the ISG HEV application and is finally chosen as the ISG machine in the mildhybrid vehicles.

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2006 Civic Hybrid, 2006 SAE World Congress

[2] M. Kamiya: Development of Traction Drive Motors

for the Toyota Hybrid System, 2005 International

Power Electronics Conference

[3] M.B. Michael, S.F. Torben, P. Sandholdt: Accurate

Torque Control of Saturated Interior Permanent

Magnet Synchronous Motors in the Field-Weaken-

ing Region, 2005 IEEE Industry Application Confer-rr

ence

[4] B.H. Bae, H. Patel, S. Schulz: New Field Weakening

Technique for High Saliency Interior Permanent

Magnet Motor, 2003 IEEE Industry Application

Conference, pp. 898-905, 2003

[5] N. Bianchi, S. Bolognani, M. Zigliotto: High Per-

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Electrical Scooter, 2000 IEEE Industry Application

Conference, pp. 1901-1908, 2000

[6] X. Chen, R. Thornton, M. Edington. R. Lewis, Y.

Fang: Accurate Torque Control of IPM machines for

ISG Hybrid Vehicle Applications, 14th Asia Pacific

Automotive Engineering Conference (APAC-14),

Aug 2007, Hollywood, USA

Figure 5: Measured torque-speed-efficiency map of the SPM ISG machine plus inverter(left) and measured torque-speed-efficiency map of the IPM ISG machine plus inverter (right)


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