Motors/Generators for Traction/Propulsion Applications ... - IEEE Xplore

Motors/Generators for Traction/Propulsion Applications Ayman M. El-Refaie

©comstock

A Review

Digital Object Identifier 10.1109/MVT.2012.2218438 Date of publication: 26 February 2013

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T

he growing interest in electrification has led to a growing interest in hybrid/electrical traction applications. Many hybrid/electrical vehicles have been commercially introduced. Various technologies for the traction motors/generators have been developed. The requirements for motors/generators for hybrid/electrical traction applications are very demanding in terms of power density, efficiency, and cost. This article will provide a comprehensive review of the state of the art highlighting the key global trends and tradeoff of various technologies. The article will also discuss future trends and potential areas of research. The article will cover light-duty vehicles (with more focus), medium- and heavy-duty vehicles, off-highway vehicles (OHVs),

1556-6072/13/$31.00©2013ieee IEEE vehicular technology magazine | march 2013

locomotives, and ship propulsion. The goal of the article is to serve as a comprehensive reference for engineers working in the traction/propulsion area. There has been growing interest in electrification especially for hybrid/electrical traction and propulsion applications. Although the main focus has been on areas like energy storage and power electronics, there is growing recognition of the importance of traction motors and generators. There is wide recognition of the need for advanced motors and generators in order to meet the aggressive targets (in terms of power density, efficiency, and cost) of the electric drive train. This recognition led several of the main OEMs like General Motors Company, Detroit, Michigan, to decide to make large investments in developing and producing their own traction motors and generators (Figure 1) [1].

Electric Motor Innovations Enabled the Petroleum Era of Automotive Growth

• 1912 Cadillac—World’s First Self-Starting ICE Automobile • The Electric Starter Made the Gasoline-Powered Vehicle Practical • Set Up More Than a Century of ICE Vehicle Development

Figure 1 Historical importance of electric traction motors [1].

Light-Duty Vehicles and Argonne National Laboratory, Lemont, Illinois. Table 1 provides a summary of the various motors/generators used in the various vehicles, the technology used in each machine, and the machine rating [2]–[5]. Several key observations are evident from this table. ■■ Almost the entire light-duty hybrid vehicle industry has shifted to permanent magnet (PM) machines in order to meet the increasing power density and efficiency requirements.

The light-duty vehicle market has been leading the way in terms of hybrid and electric vehicles. Companies like Toyota Motor Corporation, Aichi, Japan, and Honda Motor Company Ltd., Tokyo, Japan, have been pioneers in commercializing hybrid vehicles. The machines from Toyota have been viewed for many years as the state of the art. A lot of effort has gone into trying to benchmark the motors/ generators in the various vehicles. Much of this work has been done at Oak Ridge National Laboratory, Tennessee,

Table 1 Summary of the motors/generators in the key hybrid vehicles. Motor/generator

Vehicle

Honda Insight 2000

Honda Civic 2003

Honda Accord 2005

Honda Civic 2006

Stator

Concentrated winding




Rotor

SPM

SPM

Inset PM

Inset PM

Rating

9.2 kW/83 Nm

12 kW/108 Nm

14 kW/136 Nm

15.5 kW/123 Nm

DC bus voltage

144 Vdc

144 Vdc

156 Vdc

156 Vdc

Vehicle

Toyota Camry 2007

Lexus 2005 R # 400h

Toyota Prius 1998

Toyota Prius 2004

Stator

Distributed winding

Distributed winding

Distributed winding

Distributed winding

Rotor

IPM

IPM

IPM

IPM

Rating

105 kW

123 kW

30 kW (33 for 2003)

50 kW/400 Nm

DC bus voltage

244–650 Vdc

650 Vdc

273 Vdc

200–500 Vdc

Motor/generator

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Almost the entire light-duty hybrid vehicle industry has shifted to permanent magnet (PM) machines in order to meet the increasing power density and efficiency requirements.

■■

■■

The Honda Motor Co. Ltd. vehicles (which can be classified as mild hybrid) focused on surface PM (SPM) and inset PM machines with segmented stator structures and fractional-slot concentrated windings. Segmented structures have the potential of increasing copper slot factor and reducing manufacturing costs. However, the segmented structures compromise the stator back iron rigidity. The Honda Motor Co. Ltd. designs focused on 0.5 slot/pole/phase configurations, which tend to have low winding factor of 0.866. Also, it can be seen that the Honda Motor Co. Ltd. designs utilize low dc bus voltage. The Toyota Motor Corp. designs (which can be classified as full hybrid) focused on interior PM (IPM) machines with continuous laminations and regular integral-slot distributed windings. More recently, Toyota Motor Corp. has started using fractional-slot concentrated windings in the Prius 2010 generator. Some of the Toyota Motor Corp. designs have a single layer of magnets and some have multiple layers of magnets to increase reluctance torque contribution.

Bar-Wound Stators Versus Stranded Wire Wound Stator Features

Figure 2 General Motor Co./Remy, Inc. (www.remyinc.com) IPM motors with bar-wound (hairpin) windings [1].

Also, it can be seen that the more recent designs utilize higher dc bus voltage (by using a boost converter). This reduces the flux-weakening requirements especially for higher speed machines like the Toyota Camry and Lexus. ■■ The Honda Motor Co. Ltd. designs as well as the Toyota Prius have a top speed of around 6,000 r/min. In more recent designs like the Camry and Lexus (as well as some of the General Motor Co. and Ford Motor Co., Dearborn, Michigan, designs), the machines are designed for higher top speeds of up to 14,000 r/min in order to increase the machine power density. ■■ IPM is becoming the dominant type of machines. • 30–70 kW ratings/machine for full hybrids (cars) • 1 120 kW for full-hybrid sport utility vehicles. ■■ Liquid cooling is becoming a common practice in the industry in order to meet higher performance requirements. The peak air gap shear stress for all these machines ranges from 8 up to 17 lbf/in2 depending on the aggressiveness of the cooling and duration of the transient power requirement. ■■ Higher coolant inlet temperature of up to 105 ºC (to eliminate additional cooling loop) are required. ■■ Electric machines are tightly integrated with internal combustion engine (ICE)/drive train housing. ■■ General Motors Co. and Remy, Inc. are pursuing IPM machines with bar-wound (hairpin) windings (Figure 2) [1]. This winding has higher fill factor (hence power density), but the main concern is the ac losses in the windings, especially at higher speeds. The performance of the machine will rely heavily on the vehicle duty cycle and how long the motor can thermally operate at higher speed; it will also depend on the cooling mechanism implemented. More examples and a detailed discussion of the tradeoffs will be included in the final article. In addition to the regular central traction motors, numerous wheel-hub motor concepts have been developed (some were installed in few vehicles), but no commercial vehicles have been developed using these concepts. One of the main technical challenges of wheel motors is the unsprung mass [6]. Many ideas have been developed to minimize this issue. Protean Electric, Auburn Hills, Michigan [7] developed an inside-out radial-flux PM wheel motor (54 kW/83 kWpk) in which the power electronics and controls are distributed along the motor circumference as shown in Figure 3. Mitsubishi Group, Tokyo, Japan (50 kW) and TM4 Electrodynamic Systems (http://www.tm4.com/en/home.aspx) (80 kW) also developed inside-out radial-flux PM wheel motors [8], [9] as shown in Figures 4 and 5, respectively. General Motors Co. developed a dual-rotor axial-flux PM with an epoxy-based stator [10], [11]. The motor is shown in Figure 6. In spite of all these concepts and significant

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Nitrile Seal and Labyrinth Capacitor Module Power Electronics Back Plate Bearing Bus Bars Stator Rotor

(a)

(a) Individual Microinverter/ Power Electronics Modules with Embedded Motor Control Software

(b)

Figure 3 Wheel motor from Protean Electric, Auburn Hills, Michigan [7]. progress, cost, fault tolerance, system complexity, and unsprung mass continue to be significant challenges for wheel motors.

(b)

Medium- and Heavy-Duty Vehicles

Figure 4 Wheel motor by Mitsubishi Group [8].

Even though the light-duty vehicle industry has been dominated by liquid-cooled (as previously mentioned) induction machines, this is not the case with mediumand heavy-duty vehicles, e.g., delivery trucks, buses, and class 8 (super) trucks. In these vehicles, induction machines are still considered the main workhorse. Table 2 summarizes some of the available traction

systems for medium- and heavy-duty vehicles [12]–[15]. The majority are liquid-cooled induction machines. There is also growing interest in IPM machines. UQM Technologies, Longmont, Colorado, is considered one of the leading companies that provide PM liquid-cooled

Table 2 Summary of hybrid systems for medium- and heavy-duty vehicles. Allison Transmission

BAE

Azure Dynamics

Solectria

UQM Technologies

Oshkosh

Allison EP-50/40 Motor/generator 75 kW nominal, 150 kW peak induction machine by Remy

HybriDrive propulsion systems induction machine traction motor 160 kW (215 hp) continuous 200 kW (268 hp) peak

50 kW continuous shaft power at 1,000–2,500 r/min 97 kW peak shaft power induction machine air-cooled AC90 motor with DMOC645 controller

AC55 is a single-output, 78 kW threephase ac induction motor with a nominal speed of 2,500 r/min and a maximum speed of 8,000 r/min

PM liquid cooling 200 kW peak, 115 kW continuous motor power 200 kW peak, 120 kW continuous generator power/ regenerative braking 150 kW peak, 100 kW continuous motor power 150 kW peak, 100 kW continuous generator power

The ProPulse system combines a 300 hp diesel engine with a 225 kW induction generator and ultracapacitors (1.4 MJ) to drive two 140 hp induction traction motors


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Current Rotor Flux Stator Torque

Casing Cooling System

Permanent Magnets Laminations and Windings

Magnets

(a)

Rotor 2

Rotor 1 Stator (a)

(b)

(b)

Figure 5 TM4 wheel motor [9].

Figure 6 Axial-flux PM wheel motor by General Motor Co. [10].

traction systems for medium- and heavy-duty vehicles [17]. Switched reluctance machines (SRMs) are also used in some heavy-duty vehicles [18]. In this type of vehicles, the well-known issues of SRMs, including acoustic noise as well as torque ripple and vibration, might not be of great concern. Nidec SR Drives, United Kingdom, is the leading supplier of SRMs. PM machines have higher power density and efficiency at the machine torque-speed envelope. Induction machines have advantages in terms of partial load efficiency (also there are very low drag losses if the machine is unloaded). Also, induction machines do not have the issues of fault tolerance [19] (and the braking torque produced in case of a fault) or uncontrolled generation mode as in the case of a PM machine [20].

OHVs There is growing interest in hybrid-electric drive systems in construction and OHVs to replace the existing mechanical and hydraulic systems to achieve higher performance [21]. Table 3 provides a summary of these examples. It can be seen that induction machines are the dominant type. General Electric Co., Schenectady, New York, and Siemens AG, Munich, Germany, are considered leaders in terms of providing electric drive trains for large mining trucks. Also, SRMs are being used due to their robustness and, as previously mentioned, acoustic noise and vibrations are not key issues in such applications. PM machines are also being evaluated, but there are concerns about the shock/vibration levels that the machines can tolerate due to their brittleness [22].


There is growing interest in hybrid-electric drive systems in construction and OHVs to

Table 3 Examples of OHV motors and generators [21].

replace the existing mechanical and hydraulic systems to achieve higher performance.

Application

Company

Tractor

Caterpillar (CAT D7E with electric drive train)

2 # 110-hp induction motors

Loader

Volvo (Volvo L220F hybrid wheel loader)

700 Nm and 50 kW peak induction motors

Loader

LeTourneaue Technologies (50-series for loaders and dozers, e.g., new hybrid wheel loader L-1150)

1,050 hp switched reluctance motors and an SR generator

Mining trucks

Caterpillar 795F-AC Rear axle mounted

3,400 hp wheel induction motor

Mining trucks

Hitachi EH5000AC

3,000 hp induction motor

Mining trucks

Liebherr TI 274

3,000 hp Siemens AG induction wheel motors

Mining trucks

Komatsu 860E

2,400 hp Siemens AG induction motors

Rail Vehicles Induction machines are the dominant type of traction motors in locomotives. This can be seen in Table 4, which summarizes the traction motors used in various locomotives [23]. The typical maximum speeds for these motors are in the range of 3,000–6,000 r/min. All these motors are air cooled. More recently, PM motors have been developed for high-speed rail [24], e.g., the AGV PM traction motor developed by Alstom, Levallois-Perret, France, shown in Figure 7. This motor is closed and selfventilated and has a maximum speed of 4,500 r/min. The next step in meeting the requirements for lightweight, high-speed rail is to move toward liquid cooling of both the power converter and traction motors. This is currently under investigation and development.

Machine type and rating

Table 4 Summary of the motors/generators in various rail vehicles [23]. Vehicle

Motor type and rating

Vehicle

Motor type and rating

Italian Pendolino ETR 460 tilting train generation, maximum speed 250 km/h, in operation since 1992

12 # 500-kW induction motors traction motors made by Alstom

German DB 152 electrical locomotive, maximum speed 170 km/h, in operation since 2001

4 # 1,600-kW induction machines made by Siemens AG

German DB ICE 3 high-speed train, maximum speed 330 km/h, traction motors, power 16 # 500-kW, in operation since 1999

16 # 500-kW induction motors made by Siemens AG, maximum speed of 6,000 r/min

German DB-Railion 189 electrical freight locomotive, maximum speed 140 km/h, in operation since 2003


Swiss Railways SBB FLIRT RABe 521/523, maximum speed 160 km/h, in operation since 2004

4 # 500-kW Induction motors made by TSA Traktionssysteme

Swiss SBB Re 460 electrical locomotive, maximum speed 230 km/h, in operation since 1992

4 # 1,525-kW induction machines made by ABB

Spanish Talgo 350 high-speed train, maximum speed 350 km/h, traction motors, in operation since 2005

8 # 1,000-kW induction motors made by Siemens AG

Indian three-phase electric freight locomotive WAG-9, maximum speed 100 km/h, in operation since 1996

6 # 850-kW induction motors made by ABB

Korean KTX high-speed train, operational speed 300 km/h, in operation since 2004

12 # 1,130-kW self-commutated synchronous machines (replaced by induction machines in KTX-II) made by Rotem, power

United States, Houston Texas, MetroRail, Avanto light-rail vehicle, maximum speed 105 km/h, in operation since 2001

4 # 200-kW at 2,500 r/min induction machines made by Siemens AG

Austrian ÖBB Taurus electrical locomotive 1016/1116, maximum speed 230 km/h, in operation since 2000


Czech Republic, Prague 15T low-floor tramway, maximum speed 60 km/h

16 # 60-kW induction motors made by Škoda Electric

Chinese Railways DJ4 electrical locomotive, maximum speed 120 km/h, in operation since 2006

8 # 1,200-kW induction machines made by Siemens AG and Zhuzhou Electric Locomotives Works

Austria, Graz, Cityrunner, low-floor tramway, maximum speed 70 km/h, in operation since 2001

8 # 50-kW induction motors made by Škoda Electric


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AGV—Technical Innovative Highlights—PM Motors 1

2

3

4 First Very High Speed Railway Application for High-Power (760 kW) PM Traction Motor (More Than 1 kW/kg) Closed and Self-Ventilated & MaintenanceFree Motor Intensive Test Already Performed (World Record, Test Benches, Prototype Train, etc.)

significant increase in power density by replacing conventional machines with high-temperature superconducting machines by American superconductor. Table 5 summarizes some of the key motors used in podded ship propulsion. As can be seen in the figure, there is a mix between wound field synchronous machines, induction machines, and PM machines. Also, several rim-driven machines have been proposed [33]. A few examples are shown in Figures 12–14 [34]–[36].

How Does the Future Look?

Historically, dc and synchronous machines have been the dominant types in ship propulsion [25]. Figures 8 and 9 show examples of such machines by LDW Lloyd Dynamowerke, Germany (www.ldw.de). More recently, PM machines have been developed for ship propulsion [26]. Figure 10 shows an example of a shipboard propulsion high-speed 36.5-MW PM motor developed by DRS Technologies (www.drs.com). Some companies are also working on superconducting machines for marine propulsion applications [27]. Figure 11 shows the potential

This section will discuss not only how the future looks but also ongoing and potential areas of research. The main focus will be on light-duty vehicles. In the near future, PM machines will continue to dominate the light-duty vehicle market. Also, they have started to extend into medium- and heavy-duty markets as well as high-speed rail, OHVs, and ship propulsion as mentioned in the previous sections. In terms of PM machines, there are several research areas of interest. These include developing concepts for integrated motor and drive. This can have a significant impact on reducing system cost as well as electromagnetic interference issues. A couple of examples are shown in Figures 15 and 16 [37], [38]. Another area of research is developing traction motors with 105 °C coolant inlet temperature as previously mentioned. This can have a significant impact on eliminating a separate cooling loop for the motor and inverter, which can cause significant reduction in system cost.

Figure 8 A 21,000-kW variable-speed synchronous propulsion motor for cruise liners [24].

Figure 9 A 950-kW dc motor, delivered by LDW, as main drive for the research vessel “Solea.”

INNOTRANS–Berlin September 2008–P 19

Figure 7 AGV high-speed PM traction motor by Alstom, Levallois-Perret, France.

Ship Propulsion


Copper Motor 21 MW, 150 r/min, 4 kV, 183 ton

HTS Motor 36.5 MW, 120 r/min, 6.6 kV,