advances in ipm for hybrid

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Abstract—The past thirty years have been an exciting period for tremendous advances in the development of interior permanent magnet (IPM) electrical machines. Over the course of this time, interior permanent magnet synchronous machines (IPMSM) have expanded their presence in the commercial marketplace from few specialized niche markets such as machine tool servo drives to mass-produced applications including high-efficiency electric traction drives for the latest generation of hybrid-electric vehicles (HEV). Power ratings of available IPM motor drives have dramatically expanded by approximately three orders of magnitude during this period, now reaching power levels up to 1 MW ratings. What are the factors that made such impressive progress possible? Closer examination reveals that several different knowledge-based technological advancements and market forces have combined, sometimes in fortuitous ways, to accelerate the development of the impressive IPMSM drives technology that we find available today. The purpose of this paper is to provide a broad explanation of the various factors that lead to our current state-of-the-art IPM technology. This highly efficient energy conversion technology has enormous impacts on the world electrical energy supply and demand utilizing conventional fossil fuel sources like oil, coal and gas. Examples will illustrate commercial successes of Toyota’s hybrid electric vehicles like PRIUS, utilizing the latest developments in knowledge based highly efficient and smart automobiles now and in the very future.

I. INTRODUCTION Electric power system forms the backbone of modern society. Electricity and its accessibility are the greatest engineering achievements in the past century. In the 21st century, global warming has become an important issue. Carbon dioxide (Co2) gas emissions should be reduced to preserve the correct air quality as per Kyoto protocol, implemented on February 16, 2005 by most of the countries. Modern human beings, who need electric energy technologies for climate controlled home and work place environments via air conditioners and mass transportation using cars as necessities, cannot put up with the inconveniences of the past. In order to maintain and develop this energy consuming technologies, availability of sustainable energy sources and their effective uses through efficiency improvements are of paramount importance. Power electronics and electric motor drives are the enabling technologies crucial for industrial competitiveness in the world market place. One of the most valuable achievements in power electronics is to introduce degree of freedom to variable frequency from the fixed value of the generated ac power supplies. Over 60 % of the generated energy is consumed by electric motors. Variable ac speed drive, which regulates the speed of the motor by controlling the frequency, can significantly reduce the energy consumption, particularly in heavy-duty cycle fans, pumps, compressors and traction in hybrid electric vehicles. Thus improvements in efficiency of the electric motor drive systems are the most effective measures to reduce primary energy

consumption; and thereby reduce Co2 gas emissions, which cause global warming.

The objective of this invited paper is to provide a brief introduction to the recent emergence of high efficiency and high performance interior permanent magnet (IPM) synchronous motors. Highlights of IPM motor drives include wide spread application in Japanese hybrid electric vehicles, which are just one of many items of ac motor drive in passenger automobiles to save precious electric energy.

II. ANALYSIS

The principle of operation of any rotating electric motor is derived from Lorenz force. A current carrying conductor placed in a magnetic field is acted upon by a force by way of the BLI rule. For a conventional synchronous motor the stator is fed from 3-phase balanced voltage source and rotor field winding is supplied by dc excitation current through slip rings. The machine starts as an induction motor, and when it attains near synchronous speed, the dc excitation current is switched on. The rotor is snapped into synchronism and it runs at synchronous speed. The obvious disadvantage is that the motor needs two sources of power; one ac from the stator and dc through the rotor involving brushes and slip rings. Unlike in induction motor, the speed of a synchronous motor is constant irrespective of loads. But an induction machine is a singly fed motor. The rotor is squirrel cage, simple and robust. The disadvantage is that it cannot operate at synchronous speed Ns, and the rotor speed Nr decreases with load.

Thus an induction motor is an inherently inefficient motion control device, because the ideal efficiency is 1-S, where S = (Ns-Nr)/Ns. These disadvantages of both the conventional doubly fed synchronous motor and the singly fed induction motor can be overcome by means of a permanently excited singly fed IPM motor. An IPM is an induction start but synchronously run high efficiency motor. It is sometimes referred as induction-synchronous motor. It must overcome the magnet brake torque at line starting. However, there are many challenges to overcome. Some are given as follows:

• Create variation of d-q axis inductances without

varying air gap. • Vary and control of excitation of permanently

excited rotor of IPM. • Optimum variation of PM torque and reluctance

torque for specific applications. • Reduction of cost, weight and size of IPM motor. • Intelligent converter and inverter for IPM drive.

The developed power Pd in a 2-pole 3-phase salient pole

synchronous motor can be given as;

Advances on IPM Technology for Hybrid Cars

and Impact in Developing Countries

Dr. M. A. Rahman, Life Fellow, IEB and IEEE Memorial University of Newfoundland

St. John’s, Newfoundland, A1B 3X5 Canada

5th International Conference on Electrical and Computer EngineeringICECE 2008, 20-22 December 2008, Dhaka, Bangladesh

978-1-4244-2015-5/08/$25.00 (c)2008 IEEE 189

Authorized licensed use limited to: M S RAMAIAH INSTITUTE OF TECHNOLOGY. Downloaded on November 15, 2009 at 23:54 from IEEE Xplore. Restrictions apply.

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δ2sinX2X

)X(X3Vsin

XE3V

Pqd

qd2

p

d

0pd

−+= δ (1)

Where Vp is terminal voltage/per phase, Eo is excitation voltage/per phase; Xd and Xq are d-q axis reactances per phase, respectively and δ is angle between Vp and Eo. In conventional salient pole synchronous machines, the airgap length at the direct (d) axis is small and the airgap length at the quadrature (q) axis is large. Thus there exists physical variation of the airgap, which in turn causes reluctance changes of the motor as the rotor rotates. The equation (1) can be rewritten as;

δ2sinPsinP P red += δ (2) Where, Pe = [3VpEo]/ Xd and Pr = [3Vp

2 (Xd -Xq)]/ 2 Xd Xq Pe is the peak power component due to dc field excitation and Pr is the peak power component due to reluctance variation at the airgap. The latter is called the reluctance power. The contribution of each power component to the total power Pd is significant for the optimum design of a salient pole synchronous motor. For fixed parameter values it is obvious that the first term of Eqn (2) is maximum when δ is 900, and the second term of Eqn (2) is maximum for δ = 450 . The salient pole synchronous motor develops more stable power for a given excitation level, because the total developed peak power Pd peak is greater than each of the Pe and Pr components individually.

The challenge for designers for an IPM motor is to create reluctance variation of the motor by keeping airgap length constant. This has been done by inserting permanent magnets in various arrangements and magnet polarity orientations below the conduction cage of the IPM rotor such that the machine reluctance variations are made possible but keeping the airgap length uniformly constant [1]. For some specific applications the squirrel/conduction cages can be dispensed with for new IPM rotors for air conditioners and hybrid electric vehicles.

The developed torque Td is obtained by dividing Eqn (2) by angular synchronous speed. An IPM motor develops its driving torque due to both the permanent magnet excitation and reluctance variation.

Fig. 1: History of Permanent Magnet Material Developments The history of development of IPM motors is linked to the

advancement of high-energy permanent magnet materials over the past 50 years. Fig.1 illustrates the brief history of the development of permanent magnet (PM) materials. In the 1950s the most promising material was the Alnico (Aluminum Nickel Cobalt) magnet with (BH) max at around 5 MG Oe. Next, Barium Ferrite magnets came by 1960s, and Samarium Cobalt

magnets appeared in the 1970s with (BH) max at about 4 and 6 MG Oe., respectively. The latest quantum jump occurred in early 1980s,when Neodymium Boron Iron (NdBFe) magnets with (BH) max. at 14 MG Oe. became commercially available. Now a days NdBFe magnets with (BH) max. at 58 MG Oe. are routinely manufactured and marketed by the Japanese manufactures like Neomax Co, Japanmagnets Inc., Aichi Steels Co, TDK Co., etc .The critical properties of permanent magnets for IPM motors are very high coercive force Hc, high residual magnetic flux density Br and highest (BH) max. energy product. All PM materials except NdBFe magnets are found not quite suitable for high efficiency IPM motor drives. Merrill introduced an earlier IPM motor using Alnico-5 in 1955[2]. Binn, Barnard, Jabbar presented a series of flux focused IPM motor using ferrite PM materials in 1978[3]. Rahman designed and built the first large 45 kW high efficiency IPM motor utilizing NdBFe magnets in 1982 [4,6]. Rahman, Little and Slemon provided analytical models for IPM in 1985[7-8]. Jahns incorporated the flux-weakening regime in 1987[11,13]. Sebastian and Slemon presented inverter driven IPM drives in 1987[15]. Fratta, Vagati and Villata provided design criteria of IPM for field weakening operation in 1990 [20]. Zhou and Rahman presented the finite element analysis of IPM motor incorporating field and circuit coupling in 1994[28]. Sustained and extensive research, development, analysis, control and application of IPM motors are progressing in leaps and bounds for the past two decades [12-47], perhaps even exceeding Merrill’s dream [2] and Alger’s expectation.

III.DESIGN REQUIREMENTS

The key requirements of IPM motors and generators for

traction applications in hybrid electric vehicles are: Large torque and higher power density, High torque at low speeds for starting and uphill climb High power at high cruising speeds Maximum efficiency over wide speed and torque ranges Wide speed range with constant power mode, exceeding

2- 4 times the base speed Optimum compromise between motor peak torque and

inverter volt-ampere ratings Short term overload capability, typically twice the rated

torque over short duration Low cogging torque, low ripple and low acoustic noise Optimum stator winding design New rotor design with magnets orientation for maximum

variation of d-q inductances Reduction of magnetic saturation due to cross-coupling

Limits to open circuit voltage and total harmonic contents Low copper and iron losses at high speeds High reliability for all operating conditions Minimum weight and smallest size Low fuel consumption rate (litre/km), Clean and environmentally benign Quiet, smooth and comfortable ride Better battery power and self-charging Smart sensors and interfaces Least magnet flux leakage Magnet demagnetization withstand with respect to

armature reaction Temperature and surface corrosion constraints of magnets Minimum gear and more direct drive Regenerative braking and short charging cycle No plug-in and hybrid transmission Plug-in in off peak periods Solar panel body and hybrid transmission Seamless transfer between engine and electric traction Minimum maintenance and high efficiency

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Lowest initial and operating cost

IV. MOTOR TORQUE

The developed torque Td for an IPM motor can also be expressed for synchronously revolving d-q axis reference frame as [6];

[ ]qddqqmd i)iL(Liλ2

3pT −+= (3)

Where, mλ is flux linkage due to permanent magnet excitation, Ld and Lq are d-q axis inductances, respectively; id and iq are d-q axis currents, respectively and p is number of pole pairs. It is also to be noted that the torque equation (3) is quite non-linear, because mλ , Ld, Lq, id and iq are not usually constants. All these five quantities vary during dynamic operating conditions.

It is to be noted that the first term of equation (3) is identical to the separately excited dc motor. It is important for indirect vector control of an IPM motor. The second term is the reluctance torque. Efficient utilization of this reluctance torque component of equation (3) is most critical for intensive flux weakening operations and efficiency improvements in hybrid electric vehicles (HEV) and electric traction drives [45].

Finite element (FE) analysis is a requirement for fine-tuning the parameters determination of the IPM motors for optimum efficiency in high-speed operation using smart inverter and control systems. Figure 2 shows the finite element based d-q axis magnetic flux distribution due to flux focusing arrangements of rotor permanent magnets [26]. Design optimization of the IPM motor drive system can also be carried out by various methods.

Fig. 2: Variation of d-q axis fluxes

V. STEADY STATE OPERATING MODES

The operation of a synchronous motor is conventionally explained by using the Thevenin’s per phase equivalent circuit model. The applied phase voltage Vp and the excitation voltage Eo at the airgap due to dc field current in the rotor of the motor is connected by series reactance X, neglecting stator resistance drop. The phasor voltage triangle is governed by the Kirchoff’s voltage law. For the sake of better insight of dc field current supplied in its rotor, the Thevenin’s equivalent can be replaced by its dual Norton’s equivalent circuit model. Fig.3 shows the per phase Norton’s equivalent circuit of an IPM motor. The phasor current triangle of the Norton’s equivalent circuit of a synchronous motor is governed by the Kirchoff’s current law of If + Is = Im. Note that Is is the stator current per phase, Im is the magnetising current per phase and If is the phasor current arising out of the rotor permanent magnet excitation. It is quite well known that a conventional synchronous motor can be operated at variable power factor modes by regulating its dc field current Ifdc. It is well known that the dc excitation current If is varied by controlling the rotor field current Ifdc to operate the motor at unity, leading and lagging power factor modes of operation. It is not possible for IPM synchronous motors.

If

Im

Is

VpIfXm

Fig.3: Norton’s Equivalent Circuit of IPM Motor

Fig.4: Current Phasor Diagram of IPM Motor

Unlike conventional wire-wound synchronous machines, the rotor of an IPM motor is permanently excited. The rotor permanent magnet can be modeled by equivalent current source, as indicated by If in Fig. 3. The excitation current If due to rotor permanent magnets for IPM synchronous motor is constant.

However, the IPM motor can be operated by controlling the angle β between the magnetizing current Im and constant excitation current If. This is explained by means of the current phasor triangle. An IPM motor can be operated in leading, unity and lagging power factor modes of operation by varying the angle β, as shown in Fig.4. This eliminates another constraint for its wide spread applications in industry as the singly fed permanently excited variable power factor IPM synchronous motor. Fig 5 shows an alternate method of varying power factor of an IPM synchronous motor. The direct and quadrature axis (d-q) components of the stator current may be controlled by the

Fig.5 : D-Q Currents Vector Diagram of IPM Motor

vector control (field weakening) technique, in which the d-axis current is varied to operate the IPM motor in leading, lagging and unity power factor modes of operation.

VI. ROTOR DESIGN FOR LINE START IPM The earlier design of the rotors for IPM motors using Ferrite

magnets was geared to increase the air gap flux by arranging the magnets and their orientation. Different old topologies had been tried by Binns [3-4]. Modern NdBFe magnets having high Br and very large Hc lead the recent trend for new rotor designs.

Fig.6: IPM Rotors for Line Start Motor.

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Fig. 7: Magnet flux distribution (a) FE grids (b) magnetic flux contours

Fig.6 shows the experimental rotor for an IPM motor with line start provision [41]. The left hand picture depicts the IPM rotor, and the right hand figure provides details of dimensions and permanent magnet orientation over one quadrant of 4-pole IPM rotor [41]. Figure 7 shows the finite element grids and magnet flux density contours for one quadrant of the IPM rotor of Fig.6.

The design data for an experimental 3-phase, 4-pole, Y-connected, 200V, 1 hp IPM motor with NdBFe magnets (Neomax –32) are given as; stator : OD= 128mm, ID = 77mm, stack length = 70 mm, number of slots = 24, conductors/slot = 56. rotor: OD = 76.2 mm, core length = 70 mm, inertia = 0.0015 kgm2 and load inertia = 0.0263 kgm2.. The rotor consists of 2 cages of aluminum bars with lower cage of 7.5 mm depth [41].

Table-1 shows the comparative test performance results of the IPM synchronous motor and induction motor (IM). Both the motors were run at 140V (L-L) and 200V (L-L) voltages. It is quite evident from the test results of Table-1 that the IPM motor outperformed in every category of performances.

Table-1: Performance Results of IPM and Induction Motors Quantity IPM Rotor IM Rotor

Input voltage: V (V) 140 200 Input current: I (A) 2.91 3.43 Input power: W (W) 696 818 Rotor speed: n (rpm) 1500 1434 Torque: T (Nm) 3.82 4.00 Efficiency: η (%) 86.2 73.3 Power factor: pf (%) 98.6 68.8 Output power: P (W) 600 600 Off. X pf product (%) 85.0 50.4 Max output: Pmax. (W) 1115 1240

The significant conclusion is that the efficiency and power factor as well as their product of the IPM motor is over 35% better than that for an identical rated induction motor. The energy efficiency aspect is a key factor for wide spread applications of high performance IPM motor drives.

VIII. APPLICATIONS

IPM motors with intelligent power module (IPM) are now widely used for heavy duty cycle loads, which include ventilation fans, blowers, air conditioner, heat pumps, compressors, cranes, elevators/escalators, blood pumps, ship propellers, locomotive traction drives, electric and hybrid electric vehicles (HEV). The ratings span from few watts to few megawatts range. Double IPM motors are now increasingly used for energy saving applications in hybrid electric vehicles. The key requirements of IPM propulsion motors for HEV applications include the following [25,30,32,38]: high torque and power density, high torque at low speeds for staring and uphill climb but high power at high cruising speeds, maximum efficiency over wide speed and torque ranges including at low torques, wide speed range with constant power mode, exceeding 2-4 times the base speed, optimum compromise between motor

peak torque and inverter volt-ampere ratings, short term overload capability, typically twice the rated torque over short duration, low acoustic noise, low cogging torques, low torque ripples, optimized stator distributed winding with minimum total harmonic distortion factor, innovative rotor design topology with magnets orientation for maximum variation of d-q axis inductances, reduction of cross-coupling magnetic saturation, least magnet flux leakage, magnet demagnetization withstand with respect to armature reaction, temperature and surface corrosion constraints, excessive open circuit back–emf, load and no load stator iron loss at high speeds, high reliability and robustness for various operating conditions, minimum weight and smallest size, low fuel consumption rate (litre/km), clean, quiet, smooth, powerful, efficient and low cost. It is obvious that many of the above mentioned design requirements are complex, some times conflicting and interlinked for specific HEV applications. Furthermore, these design criteria cannot be isolated from their control strategy including power electronic converter and battery. Figure 8 shows the per unit torque/power and efficiency over wide speeds for hybrid electric vehicles [30].

Fig. 8: Torque/power and efficiency requirements for HEV

Fig.9: IPM Rotors for Toyota Hybrid Electric Vehicles [45]

Figure 9 shows the IPM rotors for Toyota Prius 2000 model and sports utility vehicles (SUV2005) model of hybrid electric vehicles [45]. Table-2 shows the utilization of IPM motors for Toyota Prius and sports utility vehicles (SUV) to create variation of d-q inductances of the rotor magnets topology for reluctance torque. However, the V type arrangements are preferred for hybrid electric vehicle applications, where the reluctance torque component is critical for high-speed operation in flux weakening regime.

Table-2: d-q axis inductances (mH) [45] Axis Straight IPM (Prius) V- IPM (SUV)

d-axis Ld 1.06 0.86 q-axis Lq 2.26 2.23 Lq –Ld 1.20 1.37 Lq/Ld 2.13 2.59

For SUV 2005 Toyota models the reluctance torque component is about 63% of the total driving torque at a speed of 12,400rpm. The torque to weight ratio significantly improves by operating the IPM motor at 650 Vdc from smart dc-dc converter. The light load stator iron loss also decreased primarily by employing high-grade silicon steel for IPM motors.

Figure 10 provides an illustration of a 123kW IPM motor/generator set for the Toyota hybrid electric car. The back wheel IPM motor is rated at 50 kW for the 4-wheel drive model. The sophisticated and intelligent control in a hybrid electric

(a) (b)

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vehicle forms the key to successful utilization of IPM motors. Smart power electronic modules as well as new nickel metal hydride battery are the enabling technology for the popular Toyota ‘Prius’ sedans and 4-wheel SUV models. The design of Toyota hybrid system (THS) includes gasoline engine, new transmission system, IPM traction motor, IPM generator, converter/inverter module, battery and control units. Figure 11 shows the complete transmission layout of the Toyota hybrid system (THS) for its popular hybrid electric vehicle models. This innovative THS transmission is geared to achieve maximum fuel efficiency and a high degree of driving comfort.PM traction drive motors are crucial for fulfilling the power characteristics required for high performance automobiles.

Fig 10: IPM Motor / Generator Set for Toyota Hybrid Car Fig. 11: Toyota Hybrid System (THS) Transmission

Fig. 12: Layout for Motor/Generator, dc/dc converter

Figure 12 shows the layout of IPM traction motor, IPM generator, dc/dc converter and batter systems. Fig.13 graphically shows the contributions of electric torque due to permanent magnet and the reluctance torque produced within the IPM rotor for the 2000 Prius and 2005 SUV models. This confirms the better choice of IPM motors technology for HEVs. Perhaps it ends the debate of using either the induction motors or reluctance motors for efficient traction drives for mass transportations. Fig. 14 shows the efficiency contours of the Toyota SUV 2005 models at extended speeds of operation, respectively. It is to be noted that Table-3 contains the uses of IPM motor drives in Japanese hybrid electric vehicles.

VIII. CONCLUSIONS This paper gives a brief introduction to the emergence of high efficiency interior permanent magnet (IPM) synchronous motors. A list of references provides a state of the art survey of significant as well as few incremental contributions in chronological order of appearance over the past 50 years. Simple expressions for developed power and torque are given.

Fig. 13: Ratio of Magnet and Reluctance Torques

Fig. 14: Efficiency Contour for SUV 2005

Table-3: Application of IPM Motor drives in Japanese Hybrids

Rotor design features for specific applications are briefly covered. On line and soft starting provisions are included. Operation of IPM motors at variable power factor is illustrated with the help of Norton’s equivalent circuit and phasor diagrams. Comparative performances of IPM and induction motors are summarized. An example of successful traction application is given. Highlight of IPM motor drives includes its wide spread application in Japanese hybrid electric vehicles, which are just one of many items of ac motor drives in passenger automobiles to save precious energy. The paper opens up the debate on plug-in, solar and auto-charged smart hybrid electric vehicles [48-49]. It concludes by hinting on the economic, environmental and social impacts in poor countries.

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Year Company Brand Vehicle type Power Voltage km/liter1997 Toyota Prius Sedan 30kW 274V 22 2000 Toyota Prius-1 Sedan 33kW 288V 22.5 2004 Toyota Prius-2 Sedan 50kW 500V 25.0 2005 Toyota Camry Sedan 60kW 650V 25.6 2005 Toyota Kluger* V6-SUV 123kW 650V 17.8 2005 Toyota Estima* V6-Van 123kW 650V 18.6 2005 Toyota Harrier* V6-SUV 123kW 650V 17.8 2007 Toyota Lexus Sedan 147kW 650V 20.0

* Japanese 4WD, front motor/generator, 123 kW, 12400 rpm, rear motor, 50 kW, nickel metal hydride battery

IPM Generator

Front IPM Motor

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