Lightweight Electric Hybrid Vehicle Design Episode 6 docx

Fourth, DC series field windings make much better use of the field window than high voltage shunt windings where much of the window is occupied by insulation.. The series field winding i

Typical machine specification for 60 kW, 10 000 rpm (surface mounted) would be: stator OD

10 in, rotor diameter 7 in, active length 3 in; operating point 0.7 tesla at 666 Hz, 8 poles, 380 V,

103 A, efficiency 0.97, power factor 1; winding resistance 0.015 Ω L/L, winding inductance 300

mH L/L; iron loss 1.5 kW at 666 Hz, core Transil 270 0.35 mm non-orientated; load torque 57

Nm, peak torque 150 Nm, vector control current 100 amps for 0.7 tesla

3.6.6 IRONLESS PM SYNCHRONOUS MOTOR

This machine has been developed by UNIQ (USA) for hub mounted motors for use in electric vehicles It consists of a machine with both an internal and external rotor which are mechanically linked and a thin stator winding which is usually fabricated using printed circuit techniques The result is a lightweight machine with a very high power density and low winding inductance since there is no stator iron Performance is largely determined by the quality of permanent magnet

used The d-axis reluctance is high due to the double air gap so that the currents needed for vector

control can be large compared with a conventional PM machine Such machines have been built

up to 40 kW rating at 7500 rpm with epicyclic speed reducers that are wheel-mounted

At present such machines are costly to manufacture because of the large amount of PM material involved, which has to be of the cobalt/neodynium variety to achieve good performance Losses are all due to stator copper which is generally operated at extremely high current density to give a very thin stator

3.6.7 WOUND ROTOR SYNCHRONOUS MACHINE WITH BRUSHLESS EXCITATION

This machine is sometimes used for inverter drives in addition to the well-known use as an electricity generator The presence of the exciter/rectifier means that this solution is applied at higher powers The rotor can be salient pole or of surface slot construction at high speed Whichever solution is chosen, the full field thermal loss in the motor is significant and a particular problem if the machine

is to be run slowly at high load torques This type of machine is used in traction drives using thyristor-based converters

Fig 3.13 Wound rotor synchronous machine with brushless exciter.

STATOR SALIENT POLE ROTOR

ROTATING TRANSFORMER

EXCITATION (HIGH FREQUENCY) ROTATING

RECTIFIER SHAFT

POLYPHASE STATOR

DAMPER WINDINGS

IN POLEFACE

SHAFT WOUND ROTOR

(SALIENT POLE TYPE)

Electric motor and drive-controller design 73

Fig 3.14 Motor characteristics.

3.7 Innovative drive scheme for DC series motors

Many DC brushed motor drive schemes for EVs use a DC shunt motor and it has been suggested that such a solution is the most appropriate5 This section investigates an alternative solution There are many railway locomotives which successfully use series wound motors and we hope to establish that indeed this is the best solution for electric vehicles

3.7.1 MOTOR DRIVES: WHY CHANGE THE SYSTEM?

Because the system is already subject to change brought about by new requirements and developments First, we have the introduction of sealed battery systems These will permit much higher peak powers than hitherto possible and consequently will run at high voltages 216

V DC is a common standard working with 600 V power semiconductors Second, we have the introduction of hybrid vehicles This will result in the need for drives and motors to operate for long sustained periods – previously batteries did not store enough energy Third, the DC series motor has the right shape of torque–speed curve for traction, constant power over a wide speed range Fourth, DC series field windings make much better use of the field window than high voltage shunt windings where much of the window is occupied by insulation The series field winding is a splendid inductor for use in battery charging mode Losses in series mode are significantly reduced

B A

TORQUE

344 Nm

45 kW

86 Nm

SPEED Torque Speed Curve

An example specification is typified by the Nelco N200, Fig 3.15(a), which compares with a

240 mm stack, Fig 3.15(b):

Hot resistance 7 Ω Hot resistance 0.014 Ω

Watts 700 at 10 A Watts 500 at 189 A

So why hasn’t somebody attempted to use series motors in EVs before? They have for single quadrant low voltage systems but not on multi-quadrant, high voltage schemes This account proposes a new control concept akin to vector control for AC machines We will show how it is

possible to achieve independent control of field current If and armature Ia, with very fast response,

using a transistor bridge

3.7.2 VEHICLE DYNAMICS AND MOTOR DESIGN

A vehicle represents a large inertia load with certain elements of resistance some of which increase with speed; see Chapter 8 For a small family car, mass = 1250 kg at 60 mph (26.8 m/sec) typical cruising speed Windage accounts for 6 kW, rolling resistance 2 kW and brake drag 2 kW, a total

of 10 kW in steady state conditions Windage varies as the 3rd power of vehicle relative velocity with respect to the wind

Kinetic Energy = 1/2 MV2, where M = mass = 1250 kg and V = velocity in metres/sec So we have:

What this illustrates is that recovered energy below 20 mph is small, consequently regeneration only matters at high speed It also illustrates that the inertia load, not the static resistance, is the main absorber of power during acceleration

3.7.3 MOTOR CHARACTERISTICS

These are shown in the following table:

Rated power 45 kW, 1250–5000 rpm

Frame D 200 M- 4 pole with interpoles

Fig 3.15 Field windings: (a) shunt field machine; (b) 3 state strategy for series field machine.

A

A

Electric motor and drive-controller design 75

Cooling air forced, separate fan

Winding, series field 245 A/216 V full load

Efficiency at full load 85%

Field Resistance 10 milliohm, inductance 1.2 mH

Armature Resistance 30 milliohm, inductance 260 mH

inc brushgear interpoles

Dimensions A = 490 mm, B = A + shaft, C = 335 mm, D = 350 mm; see Fig 7.14 This illustrates that when the field current is strengthened in the constant power region, the armature voltage can be made to exceed the battery voltage and regenerative braking will take place Below 1250 rpm plug braking must be used; however, the energy stored at this speed is small

3.7.4 SWITCHING STRATEGY (SINGLE QUADRANT), FIG 3.15

Figure 3.15(a) shows the arrangement for a 216 V, 45 kW shunt field machine with separate choppers for field and armature There are some disadvantages with this scheme: (a) field is

energized when not needed; (b) forcing factor of field is small – for a 45 kW shunt field, R = 7 ohm, I = 10 A nominal, L = 1.2 henries, t = 0.17 seconds; (c) when extended to multi-quadrant

design two bridge chopper systems are needed if contactor switching is to be avoided; (d) extensive modifications are needed to provide for high power sine wave battery charging; (e) field power losses are significant (3 kW at max field)

Figure 3.15(b) illustrates the proposed new circuit which has a single 3 state switch: state (1) open-circuit; state (2) armature + series field; state (3) armature So as an example, consider the following situation:

Full load torque at standstill

Field voltage for 245 A = 2 V

Armature voltage for 245 A = 16 V

Fig 3.16 Three state circuit expanded to 4 quadrant operation.

1 2

4 3

D 1

D 2

S 1 D 5

D 6

S 2

D7

D8

S 3

S 4

D 3

D 4

A

D 9

If

Ea

I a

Ef

FORWARD BRAKING FORWARD MOTORING

REVERSE BRAKING REVERSE MOTORING

E f

If

E f

I f

Ef

If

Ef

If

D 1

D2

D3

D4

C1

S1

S2

D 5

D6

SERIES FIELD COIL

S 3

S 4

D 7

D 8

C 2

BATTERY

220/240

V AC MAINS

so with 216 V battery:

D = 2/216 in state 2

D = 16/216 in state 3

The balance of the time will be off (D = duty cycle ratio for chopper).

It can be seen that by manipulating the relative times spent in each of the states, separate control

of field and armature currents may be exercised

When the speed of the motor exceeds the base speed (1250 rpm) the back-EMF is equal to the battery voltage and the switch henceforth operates only in states (2) and (3)

Let D = duty cycle for single quadrant chopper, then Vout/Vin = D, hence

D2 (VB−5−KAωI f − IaR a − LadIa/dt) = If Rf + Lf dIf/dt and

VB− 5 = (KAωIf + IaRa + La dIa/dt) × (D2+ D3) where

ω = motor speed, rads/sec

VB = battery voltage

KA = armature back-EMF constant V/amp/rad/sec (D2 + D3)

D2 = duty cycle state 2

D3 = duty cycle state 3

Other symbols are self-explanatory

3.7.5 MULTI-QUADRANT STRATEGY

Figure 3.16 illustrates the 3 state circuit when expanded to 4 quadrant operation: state 1 is all switches off; state 2 either Sl/S4 or S2/S3 on and state 3 is either Sl/S2 or S3/S4 on As is clear, the third state is produced by having a controlled shoot-through of the transistor bridge It may be considered that with two transistors and two diodes in series, voltage drops in the power switching

path make the circuit inefficient In fact with the latest devices: Vce sat for switches = 1.5 V at 300

A; Vf for diodes = 0.85 V at 300 A, giving a total drop = 4.7 V So (4.7/216) × 100 = 2.3% power loss

When the motor loses 15% this is a small deficiency It represents 1.2 kW at full power As the table illustrates in Fig 3.16, all states of motoring and braking can be accommodated The outstanding feature of this scheme is that the full power of the armature controller can be used to force the field, giving very fast response From Fig 3.16, it will be seen that the 4 quadrant circuit

Fig 3.17 4 quadrant circuit.

Electric motor and drive-controller design 77

consists of a diode bridge Dl–D4 and a transistor bridge Sl–S4 (D5–D8) D9 acts as a freewheel diode when the transistor bridge is operated in shoot-through mode Bridge Dl/D4 is required because the direction of armature current changes between motoring and braking Control in braking mode

is a two-stage process At high speed the armature voltage exceeds the battery voltage and the battery absorbs the kinetic energy of the vehicle At low speed the field current is reversed and plug braking of the armature to standstill is achieved via D9

3.7.6 DEVICE PROTECTION IN A MOTOR CONTROLLER

Switches S1–S4 form a bridge converter and the devices require protection against overvoltage spikes from circuit inductances The main factors are: (1) minimize circuit inductances by careful layout The key element is the position of D9 and associated decoupling capacitor relative to

Dl–D4; (2) fit 1 mF of ceramic capacitors across the DC bridge S1 /S4 plus varistor overvoltage protection

Dl–D4 can be normal rectification grade components but D9 must be a fast diode with soft recovery D5–D8 are built into the transistor blocks

3.7.7 SINE WAVE BATTERY CHARGER OPERATION

With little modification the new circuit, Fig 3.17, can be used as a high power (fast charge) battery charger with sine wave supply currents The circuit exploits the series field as an energy storage inductor Sl and D6 are used as a series chopper with a modulation index fixed to give 90% of battery volts This creates a circulating current in the storage inductor Switch S4 and diode D7 function as a boost chopper operating in constant current mode and transfer the energy

of the storage inductor into the battery Charging in this manner is theoretically possible up to

250 amps but will be limited by: (a) main supply available and (b) thermal management of the battery

Fig 3.18 Full circuit diagram of combined chopper/battery charger.

K 4

A

K 5

D 9

Rd

100 µ F

D 10

C 1

100 µ F

D 1

D 2

Rd

D 3

D 4

C 2

0.5 µ F

S 2

S 1 D 5

D 6

D 7

C 4

VDR1 VDR2

0.5 µ F

D 8

D 11

S 3

S 4

Rd 100 C3µ F

K 3

K 2

K 1

DV/DT

FILTER

ARMATURE

OF MOTOR

BATTERY

(NORMAL RUNNING)

MAINS

VIA RF FILTER DURING BATTERY CHARGING

SERIES FIELD

OF MOTOR

C

BATTERY

(BATTERY CHARGING)

– D +

CONTACTOR K1 K2 K3 K4 K5

BATTERY CHARGER C 0 C 0 C

Experience shows that charging at 30 amps is possible on a 220 V, 30 A, USA-style house air conditioning supply Charging at greater currents will require special arrangements for power supply and cooling One advantage of the scheme presented is that it may be used on any supply from 90 V to 270 V

It is also possible to adopt the circuit for 3 phase supplies in one of two ways: (1) add an additional diode arm – this would produce a square wave current shape on the supply; (2) fit a

3 phase transistor bridge on the supply – this would permit a sine wave current in each line at a much increased cost

3.7.8 POWER DIAGRAM FOR MOTORING AND CHARGING

Figure 3.18 presents the combined circuit diagram for motoring and battery charging Reservoir capacitors and mode contactors have been added The capacitors function as snubbers when running

in motoring mode As drawn, to adapt to battery charging, the battery plug is moved to outlet D and the mains inserted into plug B, alternatively contactors could be used to do the job Battery safety precautions comprise: (1) the battery is connected via a circuit breaker capable of interrupting the full short-circuit current of a charged battery; (2) this circuit breaker is to contain a trip to disconnect battery by mechanical means only; (3) battery/motor/controller are each to contain

‘firewire’ to disconnect the circuit breaker; (4) circuit breaker is to be tripped by ‘G’ switch when 6G is exceeded in any axis

3.7.9 CONTROL CIRCUIT IN MOTORING MODE

Figure 3.19 shows the block diagram of the controller for motoring mode The heart of the system

is a memory map which stores the field and armature currents for the machine under all conditions

Fig 3.19 Control system in motoring mode.

TD1

TD2

TD3

ACC

BRAKE

TfB

TORQUE

TORQUE ERROR M

MEMORY ROM MAP

MDAC

MDAC INVERT

M D

M D MOTOR DIRECTION DEMAND

DEMAND

If FEEDBACK

FIELD

PWM CLOCK

V FIELD <20 V

FIELD

DEMAND

BATTERY VOLTAGE FEEDBACK

B 1

0.05 g BIAS (ROLLS OFF BELOW 10 mph)

MOTOR SPEED

B 2

B3

B 4

Ia ARMATURE

DEMAND

TORQUE/FORCE DEMAND

ROLLS OFF

BELOW 10 mph

PEDAL POSITION

ACCELERA

TOR

DEM AND

BRAKE DEMAND

BRAKING ON ACCELERATOR

TO SIMULATE NATURAL ENGINE BRAKING

-1

Electric motor and drive-controller design 79

Fig 3.20 Block diagram of battery charging controller.

of operation These demands for If and Ia are then compensated for in accordance with the battery voltage before conversion into analogue form, to be passed to operational amplifier loops which drive the modulators Current feedback is provided by Hall effect CTs The torque loop has input from two pedals and a feedback from a torque arm attached to the motor Above the base speed there is no open circuit condition and the armature loop error is used to control the field

3.7.10 CONTROL CIRCUIT IN BATTERY CHARGING MODE

The control circuit for battery charging is shown in Fig 3.20 When the battery is below 2.1 V per cell and 40°C it is charged at the maximum current obtainable from the supply Above 2.1 V/cell the battery is operated at reduced charging up to 2.35 V per cell, compensated at −4 mV/°C for battery temperature This data assumes lead–acid cells

As can be seen from the block diagram there are two separate loops for the buck and shunt choppers The fast current loops stabilize the transfer function for changes in battery impedance The current limit function must be user-set in accordance will supply capabilities

References

1 Hodkinson, R., Operating characteristics of a 45 kW brushless DC machine, EVS 12, Aneheim,

1995

2 Hodkinson, R., Towards 4 dollars per kilowatt, EVS 13, Osaka, 1996

3 Al’Akayshee et al., Design and finite element analysis of a 150 kW brushless PM machine, Electric Power Transactions, IEEE, 1998

4 Hodkinson, R., The characteristics of high frequency machines, Drives and Controls Conference, 1993

5 Hodkinson, R., A new drive scheme for DC series machines, ISATA 24, Aachen, 1994

6 Jardin and Hajdu, Voltage Source Inverter with Direct Torque Control, IEE PEPSA, 1987

Further reading

Alternative transportation problems, SAE, 1996

The future of the electric vehicle, Financial Times Management Report, 1995

Battery electric and hybrid vehicles, IMechE, 1992

Electric vehicle technology seminar report, MIRA, 1992

Electric vehicles for Europe conference report, EVA, 1991

V BATT

V MAINS

A B DIVIDER A /B

PWM CARRIER

BUCK CHOPPER

SHUNT CHOPPER

TRIG

TRIG

I f PWM

If

PEAK OVERLOAD COMPARATOR

PWM SLOW

VOLTAGE LOOP

SLOW OVERLOAD LOOP

If

Process engineering and control of fuel cells, prospects for EV packages

4.1 Introduction

The first three sections of this chapter will give a history of fuel cells; describe the main types

of fuel cells, their characteristics and development status; discuss the thermodynamics; and look at the process engineering aspects of fuel-cell systems It is based on a series of lectures given by Roger Booth to undergraduates at the Department of Engineering Science at the University of Oxford, under the Royal Academy of Engineering Visiting Professor Scheme in

1999 The assistance of Dr Gary Acres of Johnson Matthey in preparing this chapter is greatly appreciated

The remaining sections deal with the control systems for fuel cells that turn them into ‘fuel-cell engines’ and considers the problems of package layout for all EVs as an introduction to the package design case studies reviewed in the following two chapters

4.1.1 WHAT IS A FUEL CELL?

The easiest way to describe a fuel cell is that it is the opposite of electrolysis In its simplest form it is the electrochemical conversion of hydrogen and oxygen to water, as shown in Fig 4.1 Hydrogen dissociates at the anode to form hydrogen ions and electrons The electrons flow through the external circuit to the cathode and the hydrogen ions pass through the electrolyte to the cathode and react with the oxygen and electrons to form water The theoretical electromotive force or potential of a hydrogen–oxygen cell operating at standard conditions of 1 atm and 25oC

is 1.23 V, but at practical current densities and operating conditions the typical voltage of a single cell is between 0.7 and 0.8 V Commercial fuel cells therefore consist of a number of cells

in series

4.1.2 TYPES OF FUEL CELL

Fuel cells are described by their electrolyte:

Alkaline – AFC

Phosphoric acid – PAFC

Solid Polymer – SPFC (also referred to as proton exchange membrane – PEMFC)

Molten carbonate – MCFC

Solid oxide – SOFC

Process engineering and control of fuel cells, prospects for EV packages 81

Anode Hydrogen

Unreacted

Hydrogen

H2 2H+ + 2e

-2e - + 2H+ + 1/2 O2 H2O

Cathode

Water (O 2 N 2 )

Oxygen (air)

Single cell %1V

2

c-2H+ 2H+ Electrolyte

The reaction shown in Fig 4.1, with hydrogen ion transfer through the electrolyte, is only applicable to fuel cells with acid electrolytes and solid polymer fuel cells The reactions in each of the fuel cell types currently under development1 are:

AFC H2 + 2OH - ——> 2H2O + 2e - O2 + 2H2O + 4e - ——> 4OH

-PAFC H2 ——> 2H + + 2e - 4e - + 4H + + O2 ——> 2H2O

SPFC H2 ——> 2H + + 2e - 4e - + 4H + + O2——>2H2O

MCFC H2 + CO3 ——> H2O + CO2 + 2e - O2 + 2CO2 + 4e - ——> 2CO3

CO + CO3——> 2CO2 + 2e -SOFC H2+ O = ——> H2O + 2e - O2+ 4e - ——>2O =

CO + O = ——> CO2 + 2e

-CH4 + 4O = ——> CO2 + 2H2O + 8e

-4.1.3 HISTORY

The concept of the fuel cell was first published in 1839 by Sir William Grove when he was working on electrolysis in a sulphuric acid cell He noted a passage of current when one platinum electrode was in contact with hydrogen and the other in contact with oxygen In 1842 he described experiments with a stack of 50 cells, each with one quarter of an inch wide platinized platinum electrodes and he noted the need for a ‘notable surface of action’ between the gases, electrolyte and electrodes Over the next 90 years a number of workers published papers on both acid and alkali fuel cells, including the development of three dimensional electrodes by Mond and Langer

in 1889 But it was not until 1933, when F T (Tom) Bacon (an engineer with the turbine manufacturers C A Parsons & Co Ltd.) started work with potassium hydroxide as the electrolyte and operating at 200°C and 45 atm, that significant progress was made The main thrust for development of fuel cells was the space programme of the early 1960s, when NASA placed over

200 contracts to study and develop fuel cells The first major application was the use of solid polymer fuel cells developed by General Electric for on-board power in the Gemini programme

By 1960 Bacon had transferred to the Pratt and Whitney Division of United Aircraft Corporation (now United Technologies Corporation) in the USA, and led the development of the on-board power system for the Apollo lunar missions Ninety-two systems were delivered and 54 had been

Fig 4.1 Basic chemical reactions in a fuel cell.