Why 42V bus 09 07 2013
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The Nature and Promise of 42 V
Automotive Power: An Update
P. T. Krein
Grainger Center for Electric Machinery and Electromechanics
Department of Electrical and Computer Engineering
University of Illinois at Urbana-Champaign
Power Area and CEME Seminar, December 2002
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Outline
• Why 42 V? Safety and other reasons.• Target power levels.
• Architectures.
• Points about engineering research needs.• Major applications: power steering,
starter-alternators, etc.
• “Mild hybrid” designs based on 42 V.
• Research opportunities.
• Conclusion.
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Why 42 V?
• The “electrification” of the automobile is amajor step in its evolution.
• Electrical applications are beneficial for thesame reasons as for systems in aircraft:
– Better efficiency – More flexible control
– Ease of energy conversion
• Low-cost control and conversion of energy is
a key point.• Electric power is rising because of electric
auxiliaries as well as more features.
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Why 42 V?
• Possible new features: – Combined starter-alternator to reduce costs and
enhance performance.
– Regenerative braking.
– “Start on demand” arrangements to avoid idleengines.
– Improved, more efficient power steering and other
subsystems.
– Active suspensions.
– Electrical valves and engine elements --
ultimately the self-starting engine.
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Why 42 V?
• The conventional car is rapidly becoming more
electric.
– The total electric load is about 1500 W today,
and is increasing toward 5000 W.
– Conventional alternators cannot deliver morethan about 2000 W, and are not efficient.
– A higher voltage system supports lower
current and loss.
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Why 42 V?
• Three alternatives: – Stick with 12 V. This limits effective power levels.
– Get the voltage as high as possible (>100 V). This
requires a major overhaul of safety systems and
basic designs. – Push the voltage as high as possible before
significant safety issues come into play.
• 42 V tries to do the last: get the voltage as
high as possible while avoiding severe safetyissues.
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Safety Issues
• A car’s electrical system is typically “open.” • Complicated wiring harnesses with close
contact and hundreds of connections.
• Regulatory agencies have set a level of about
60 V dc as the maximum reasonable level in
an “open” system.
• Headroom is required to stay below this level
under all allowed conditions.
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Safety Issues
• Industry premise: stay with an open electricalsystem for the foreseeable future.
• This philosophy supports the option for
evolutionary change of automotive electric
power.
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Safety Issues
• There are also “fully regulated” and “batteryregulated” systems.
• Battery-regulated system ultimately defer to
the battery to set the voltage level.
• A battery-regulated system must allow for
– Polarity reversal
– Disconnection: momentary or continuous
– Wide voltage swings• Inductive spikes from corrosion or deliberate
disconnect are significant.
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Safety Issues
• 12 V battery systems require undamagedoperation at –12 V or from short-term spikes
up to 75 V.
• At higher battery voltages, surge suppressors
and other add-ons will be needed to limit
these extremes to present levels.
• In a battery regulated system, 36 V is about
the highest possible level (but these arecharged at 42 V) without excessive possibility
of damage and spikes much beyond 60 V.
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Safety Issues
• In a fully regulated system, there is somebuffering between the battery and the rest of
the system.
• With full regulation, the wide swings of a
battery system are not necessarilyencountered by the user.
• 48 V batteries are possible within the 60 V
limit, with such regulation.• The higher voltages also support extra
efforts, such as anti-reversing diodes.
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Safety Issues
• The term “42 V” refers to a range of choiceswith nominal battery levels in the range of
36 V to 48 V.
• While there is incomplete consensus, the
evolutionary approach would favor 36 Vbatteries (charging at 42 V).
• For comparison, we should take 42 V to
mean a tripling of present voltage, to give atleast triple the power.
• With better generation, power up to 5x is
available.
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Safety Issues
• We can also consider a “closed system,” inwhich electrical contact is more protected.
• Closed systems are used in today’s hybrid
and electric cars.
• The voltage levels there can exceed 300 V
dc.
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Power Levels
• At 42 V, a car’s electrical system rivals that ofa house.
• But, 10 kW is not enough for traction power.
Voltage Typicalpower level
Maximumpower level
12 V 1200 W 2000 W
42 V 5000 W 10 kW
300 V 30 kW 100 kW
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Architectures
• Each automotive voltage level hasadvantages for some loads.
• 12 V or less for lamps, sensors,
electronics, controls.
• 42 V for motors, pumps, and fans.
• High voltage for electric traction
power.
• Incandescent lamps, for example, are morerugged and more reliable at low voltages (but
they are disappearing).
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Architectures
• Many possible architectures are possible.• Most retain some 12 V capacity.
• They are typically divided into single-battery
and dual-battery systems.
• There is no consensus on which to select,
and we are likely to see several.
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ENGINE42V
ALTERNATOR
42V
BATTERY
42V
LOADS
12V LOADS
DC – DC
Architectures
• Single battery at 42 V:
• Problem: jump starts?
• Problem: charge balance.
www.hoppecke.com
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ENGINE42V
ALTERNATOR
BIDIRECTIONAL
DC – DC
42V
BATTERY
42V
LOADS
12V LOADS
12V
BATTERY
Architectures
• Dual battery:
• The dc-dc converter mustbe bidirectional to support
starting and reliability.
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ENGINE
42V
STARTER/
ALTERNATOR
REGULATOR
42V
LOADS
12V LOADS
12V
BATTERY
BIDIRECTIONAL
DC – DC
Architectures
• 12 V battery
• Here a starter-alternatoris shown as well.
Source: Mechanical
Engineering Magazine
online, April 2002.
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ENGINE
42V
STARTER/
ALTERNATOR
42V
LOADS
LOADS
LOCAL
DC/DC
42V
BATTERY
Architectures
• Distributed converters with 42 V battery.
• Here there are many dc-dc
converters at the various
loads.
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Architectures
• The ultimate is a true multiplexed system: – Deliver a single 42 V power bus throughout the
vehicle, with a network protocol overlaid on it.
– Local dc-dc converters provide complete local
operation and protection. – A ring bus or redundant bus structure could be
used to enhance reliability.
– What about fuses? No central point is available.
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Architectures
• Costs would seem to dictate a single-batteryarrangement.
• However, this involves either a high-power
42V to 12V converter (bidirectional) or a
troublesome 42 V battery.
• Some researchers talk about a small dc-dc
converter just for jump starts.
• Most systems are partially multiplexed (powerand network distribution rather than individual
loads).
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Issues
• “Key off” loads: sensors, alarms, clocks,remote systems. All draw down power.
• “Flat” loads draw roughly fixed power,
although the alternator output can vary.
• Connectors.
• Fusing.
• Arcs: much above 12 V, it becomes possible
to sustain an arc in close quarters.
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Connectors
• 150 A connector for 42 V (AMP, Inc.prototype).
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Points About Research Needs
• Many of the new challenges of 42 V havebeen addressed in other contexts:
– 48 V systems throughout the telephone network
(with battery regulation)
– Higher dc voltages in several aerospaceapplications (with bigger arcing problems in low-
pressure ambients)
• Methods need to be adapted to the low-cost
high-vibration automotive case.
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Points About Research Needs
• Motors are of keen interest. – Dc motors are cheap to build because of the
convenient wound-rotor structure.
– The small machine design methods for cars do not
translate well to 42 V.• At 42 V, ac motors make sense.
• But – small ac motors have been expensive
in most contexts.
• How to build cheap, small ac motors (with
electronic controls)?
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Points About Research Needs
• Fusing is critical.• Power semiconductor circuits capable of
acting as “self fuses” – active devices used as
circuit breakers based on local sensing.
• Actual fuses and circuit breakers with cost-
effective arc management suitable for
automotive environments.
• Fusing issues (among others) have sloweddown the development of 42 V systems.
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Major Applications
• Electric power steering.• Two forms: assist pump and direct electric.
• The assist pump uses an electric motor to
drive a conventional hydraulic unit.
• The direct system
uses electric motors with
the steering rack.
• In both cases, action canbe controlled independent
of the engine.Source: Delphi Corp., Saginaw Steering Systems Div.
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Major Applications
• Electric air conditioning.• Remove the air conditioning
system from engine belt drive.
• Provides much better control
and flexibility.
• Easier cycling,possible
heat pump application.
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Major Applications
• Integrated starter-alternator (ISA).• Build an electric machine into
or around the flywheel.
• Both permanent magnet and
induction types are being
studied.
Source: Mechanical Engineering
Magazine online, April 2002.
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Major Applications
• Provides on-demand starts.• Supports regenerative braking.
• The very fast dynamics of an ac machine
allows even active torque ripple cancellation.
• If ripple can be cancelled, there is promise for
much quieter engines and much lower
vibration levels.
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Major Applications
• Electromechanical engine controls.• Valves.
• Fuel.Source: FEV Engine Technology, Inc.
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Major Applications
• Active suspensions.• Use electromechanical actuators in
conjunction with mechanical suspension
members.
• With enough actuator power, road bumps
(large and small) can be cancelled with an
active suspension.
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Major Applications
• Catalyst management systems and exhausttreatment.
• Today, most automotive emissions occur in
the first few minutes of operation, when the
catalyst is too cold to be effective.
• Catalyst heaters or short-term exhaust
management systems can drastically reduce
tailpipe emissions in modern cars and trucks.• Electrostatic precipitator methods can be of
value with diesel particulate exhaust.
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Additional Applications
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Mild Hybrids
• The key limitation of 42 V is that it really doesnot support electric traction power levels.
• As the promise of electric and hybrid vehicles
becomes clearer, engineers push for higher
power levels – beyond the reach of 42 V.
• A compromise is possible: the “mild hybrid”
vehicle.
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Mild Hybrids
• A “light” hybrid or “mild” hybrid uses a smallmotor to manage
performance.
• The engine can be
shut down at stops.
• Braking energy
can be recovered.
• The car does not operate in an“all-electric” regime.
• The Honda Insight is a good example.
Source: www.familycar.com
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Mild Hybrids
• For a mild hybrid approach, about 5 kW or socan provide a useful level of “traction” power.
• The technique is accessible in a 42 V system,
although higher voltage (144 V in the Insight)
is beneficial.
• A 42 V ISA has substantial promise for fuel
economy improvements, and straddles the
boundary between a conventional car with anISA and a mild hybrid.
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Other Hybrids
• Higher-power hybrids require high voltage(240 V and up) for traction power.
• Electrical accessories are essential.
• Such cars can benefit from 42 V systems.
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Other Hybrids
• All key
accessories areelectric.
• The Toyota hybridsystem operates at
288 V, and reaches
30 kW.
Source: www.familycar.com
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Research Opportunities
• Low-cost small ac motor systems: – 42 V dc bus
– Cheap inverters
– Small ac motors that can be manufactured easily
• Engine electromechanical devices andcontrols.
• Protection and semiconductor “fusing.”
• System-level analysis.
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Conclusion
• The continuing increase in electric powerlevels in automobiles will require highervoltages.
• 42 V systems (batteries at 36 V or 48 V) are
the highest possible in an “open” electricalsystem.
• There are fuel economy improvements just atthis level, but the extension to “mild hybrids”
offers much more.• While the industry is now is a “go slow” mode
for 42 V, no one doubts its eventual use.
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The End!
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Why Not Just Big Batteries?
• Lead-acid battery energy density is onlyabout 1% of that in gasoline.
• Our test car: 600 lb battery pack
equivalent to one gallon of gas!
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Electric and Hybrid Gallery
• General Motors EV1.• 1300 lb battery pack at
312 V, 102 kW motor.
• 0-60 mph in less than 9 s.
• Volvo turbine-based
hybrid prototype.
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Electric and Hybrid Car Gallery
• This Ford Escort was the first “true practical”prototype hybrid – a complete station wagon.
• Second-gen
diesel hybrid.
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Electric and Hybrid Car Gallery
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Toyota Hybrid Specs
• Small NiMH battery set, 288 V.• 40 HP motor, ac permanent magnet type.
• Continuously-variable transmission with sun-
planet gear set for energy control.
• 0-60 mph in about 17 s.
• 1500 cc engine can hold 75 mph indefinitely.
• Atkinson cycle engine (“5-stroke”) gets better
thermal efficiency but lower output torque.
• Rated 54 mpg city, 48 highway.
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Electric and Hybrid Car Gallery
• Toyota architecture
• Honda architecture: