Regenerative braking can reduce vehicle emissions significantly, and hence, improve energy efficiency (Clarkeet al., 2010). Such a system permits a vehicle’s kinetic energy, which is conventionally dissipated by the brake pads during brak- ing, to be recaptured and stored for later use in acceleration; it can be incorporated into both HEVs and EVs. In this section, the system configuration, mathematical models and control strategies will be presented.
Substantial efficiency gains can be realized through regenerative braking technology, whereby motive energy is recovered to recharge the battery as the vehicle brakes, instead of simply being lost to the brake discs as waste heat (Kendall, 2008). Regenerative braking is very essential for EVs and can extend the vehicle driving range by up to 12% (Rajashekara, 1994; Ching, 2005). Battery- powered EVs desire frequent regenerative braking. During braking, the motor operates as a generator to convert the kinetic energy into electrical energy while the converter must allow for reverse power flow to restore the electrical energy to the battery system (Chau et al., 1999; Ching and Chau, 2001). This energy-recovery feature is particularly attractive to battery-powered EVs (Ching, 2006). Toyota reported that the greatest factor for the improved fuel efficiency of HEVs is regenerative braking, which accounts for about 35% of the total energy efficiency improvement, as evident in the Toyota Prius. Studies showed that HEVs have remarkably improved their fuel efficiencies by 30–40% through regenerative braking (Ko et al., 2015). In the power-assist HEV, the recuperation of vehicle braking energy via regeneration through the electric drive subsystem to the battery partially offsets the operating cost of electricity generation (Miller, 2004). A recently launched PEV, the LEAF, is equipped with an ‘eco mode’ that functions to conserve energy. When selected, acceleration power is saved, the regenerative braking system becomes more aggressive and power is conserved in the auto cli- mate control system. By activating the eco mode, the driving range can be extended by approximately 10% in city driving (Tokuoka, 2010).
6.2.1 Electric machines and power electronic drives
Motor drives are the core technology for EVs and convert the on-board electrical energy to the desired mechanical motion. Meanwhile, electric machines are the key element of motor drive technology. Figure 6.6 shows the classification of electric machines for EVs (Chan and Chau, 2001) in which the bold types are those that have been applied to EVs, including the series DC, shunt DC, separately excited DC, permanent magnet (PM) DC, cage-rotor induction, PM brushless AC (BLAC), PM brushless DC (BLDC) and switched reluctance machines (SRMs) (Chau, 2015).
On-board electromagnetic energy regeneration for electric vehicles 159
Power converters play a vital role in the success of braking energy recovery.
Electric propulsion systems require that the power electronic converter performs multi-quadrant operations and allows reverse power flow to restore electrical energy to the battery system (Ching, 2009a), as shown in Figure 6.7.
In a DC motor drive, a two-quadrant (2Q) chopper is preferred as it converts battery DC voltage to variable DC voltage during the motoring mode and reverts the power flow during regenerative braking (Chauet al., 1999). A four-quadrant (4Q) DC chopper is employed for reversible and regenerative speed control of DC motors (Ching, 2005). The 2Q and 4Q DC choppers, in which regenerative currents could return to the battery system via the anti-parallel diodes of power switches, are shown in Figure 6.8.
Current-fed inverters are seldom used for EV propulsion. In fact, voltage-fed inverters are almost exclusively used because they are simple and can have power flow in either directions (Ching, 2007). A typical three-phase full-bridge voltage-fed inverter for induction and brushless motor drives is shown in Figure 6.9. The output waveform of an inverter may be rectangular or with pulse width modulation (PWM),
EV motor
Commutator
Self-
excited Separately
excited Induction Synchronous PM
brushless DC
Switched reluctance PM
hybrid
Series Shunt Field- excited PM
excited Wound- rotor Squirrel-
cage Wound- rotor PM
rotor Reluctance Commutatorless
Figure 6.6 Classification of EV motors
Forward motoring
Forward motoring
Speed Forward
regenerating (braking) Forward
regenerating (braking)
Speed
(a) (b)
Electromagnetic torque Electromagnetic torque
0
Reverse regenerating
(braking)
Reverse motoring
Figure 6.7 DC motor operations: (a) 2Q and (b) 4Q
depending on the switching strategy for different applications. For example, a PWM output waveform is for an induction motor and PM BLAC motor while a rectangular output waveform is produced for a PM BLDC motor. The regenerative currents could return to the battery system via the anti-parallel diodes of power switches.
A conventional converter for three-phase SRM drives is shown in Figure 6.10, and the corresponding conduction modes are shown in Figure 6.11. It can be found
S2
S1
S4
S1
S2
S3
VDC
VDC +
–
+
–
Regenerative current
Reverse regenerative current
Forward regenerative current (a)
(b)
DC motor
DC motor
Figure 6.8 DC choppers: (a) 2Q and (b) 4Q
On-board electromagnetic energy regeneration for electric vehicles 161
VDC
S1
S4
S3
S6 S2
S5
BLAC, BLDC or induction
motor
Regenerative current of one phase +
–
Figure 6.9 Three-phase full-bridge voltage-fed inverter
+– VDC
Sm
Ph.a Dm
Sa Sb Sc
Ph.c Ph.b
Da Db Dc
Figure 6.10 Conventional converter for three-phase SRM drives
+– VDC
Dm
Ph.a Da Ph.a Ph.a
Sa
Sm
Dm Sa
VDC +
–
(a) (b) (c)
Figure 6.11 SRM drive conduction modes for one phase: (a) powering, (b) freewheeling and (c) regenerating
that, wheneverSais turned off, the current is returned to the battery system through DmandDa. Thus, the SRM drive can readily offer regenerative braking, which is a key demand for EV application (Ching, 2009b).
6.2.2 System configuration for braking energy recovery
Quantitative analysis in this section gives an overview of energy recovery during the general braking process, and the analysis can be simplified and limited to longitudinal movements on a straight test track.
Figure 6.12 shows the power flow of regenerative braking energy in which the energy is recovered from the wheels to the battery and back to the wheels again (Spichartzet al., 2014a):
Erecẳð
PbhMhGhRhBidt (6.6)
whereErecPb,hM,hG,hRandhBiare the recuperation energy, the braking power, and the efficiencies of mechanical part, generator, rectifier and battery charging, respectively.
Similarly, the fraction of the brake power, which is usable for a subsequent acceleration, Pacc, is calculated as follows:
PaccẳPbhMhGhRhBihBohIhMohMe (6.7) where hBo, hI, hMo and hMe are the efficiencies of battery discharging, inverter, motor and mechanical transmission, respectively.
With regard to efficiency, several solutions are proposed to avoid unnecessary losses and undesired recuperations. When a driver really wants to brake, the driver must use the brake pedal; as long as no brake pedal is activated, the EV sails.
Undesired recuperation is prevented as the driver has to use the brake pedal to activate regenerative braking.
A small part of the power is dissipated through the air and through rolling resistances; however, most of the power has to be absorbed by the brakes. Assuming that all brake power is provided by the drivetrain with the objective of total recup- eration, all main drivetrain parts in a commercially available EV would be over- loaded. These components are not dimensioned for these kinds of accelerations and power values as the EV would become too heavy and, above all, too expensive.
Pacc
Pbrake
Battery Power
converter M
Figure 6.12 Power flow of energy recuperation
On-board electromagnetic energy regeneration for electric vehicles 163
Batteries have a comparatively low power density. Therefore, batteries may not be able to deliver or store high power peaks, and beyond this, current peaks would reduce the lifetime of batteries. It was proposed to use ultracapacitors (ultracaps), which have a lower energy density but a higher power density in comparison to batteries, in parallel to the battery (Rotenberget al., 2011). The structure is shown in Figure 6.13.
In order to optimize power conversion between the electric machine and the ultracap, a DC/DC converter is required to accommodate the changeability of the voltage of both the ultracap and the electric machine. A digital machine controller is usually used to control the power of the motor during acceleration, as well as to control regenerative braking power to ensure smooth braking as shown in Figure 6.14 (Clarke et al., 2010). A bidirectional power electronic converter
Torque reference
Torque reference Controller
Engine
Motor
Transmission Wheels
Power converter Ultracaps
Figure 6.13 Structure of parallel hybrid with ultracap energy storage
Input control
Motor
control Battery
Ultracaps Converter
Motor
Figure 6.14 Regenerative braking system for EVs
(Chau et al., 1998, 1999; Ching and Chau, 2001) is essential for the charging and discharging of ultracaps in this system.
6.2.3 Modelling of braking energy recovery
Experimental studies have been conducted recently to measure the recuperable energies from an EV (Spichartzet al., 2014b). While driving, a collective tractional resistance force,Fresworks against the moving direction. The EV drivetrain has to provide a driving force, Fd at the wheels with the same absolute value in the moving direction to hold the desired driving condition, as shown in Figure 6.15 (Spichartz and Sourkounis, 2013):
Fresẳ Fd (6.8)
While the energy transfer to the road is fulfilled by the four wheel contact patches, in a steady state with constant velocity, three types of tractional resistance exist, namely air resistance, Fair, climbing resistance,Fcland the resistance of the four wheels, Fwh. During a straight forward movement on a dry road surface, the latter is approximately equal to the rolling resistance since the others in Fwh, including resistances like water displacement or bearing friction, have no or only a comparatively small influence.
During acceleration processes, the force of inertia, Fa, has to additionally be considered. Consequently, the absolute values of tractional resistance and driving force are described by the following equation:
FdriveẳFesẳFairỵFwhỵFclỵFa (6.9)
During a journey with a distance,Sx, the total energy,Etot, is transformed at the wheels to overcome the tractional resistance:
Etotẳ ð
Edriveds (6.10)
The energy for overcoming air resistance,Fair, and the resistance of the four wheels,Fwh, are converted into thermal energy and are transferred to the air and to
Fair
Fd Fa
Fw
Fcl
α
α
FG
Figure 6.15 Simplified model for longitudinal driving dynamics
On-board electromagnetic energy regeneration for electric vehicles 165
the road surface, respectively. In contrast, the energy for overcoming the two other forces is stored in the vehicle in its state and driving condition. While moving uphill and overcoming climbing resistance,Fcl, potential energy, Epot, is stored in the vehicle and can be recovered during a downhill journey.
Epot ẳmgh (6.11)
During acceleration, the velocity of the vehicle,v, and the kinetic energy,Ekin, will increase. In an HEV or PEV, this energy can also be recuperated.
Ekinẳ1
2mv2 (6.12)
Consequently, the total energy can be divided into two parts: dissipating and recuperable energy (Spichartzet al., 2014a):
EtotẳErecỵEdis (6.13)
Edis ẳ ðSx
0
Fairðsị ỵFwhðsị
ds (6.14)
Erec ẳðSx
0
Fclðsị ỵFaðsị
dsẳEkinỵEpot (6.15)
The distribution between these two parts depends on the journey travelled.
During trips on highways with constant high velocities, air resistance,Fair, is the major tractional resistance force. While, for urban traffic, air resistance plays a minor part, and the force of inertia,Fa, has more influence on the total tractional resistance because of many brakings and accelerations (Spichartz and Sourkounis, 2013). Therefore, recuperation can help to reduce energy consumption and extend the driving range, especially in the city.
6.2.4 Control strategies for regenerative braking
As most of the currently available EVs are driven by one electric machine at the front or at the rear axle, a single motor-drive system is considered in this section.
In consideration of all the limitations in a drivetrain, blending methods are usually used in many EVs, i.e. the torque of the electric machine and the mechanical brakes is combined as a function of the velocity and other conditions as shown in Figure 6.16 (Spichartzet al., 2014a). Thus, the braking power is partially converted into thermal energy by the mechanical brakes and partially recuperated by regenerative braking.
There are two major blending methods available. One method is called the parallel regenerative braking system, as shown in Figure 6.17 (Spichartz et al., 2014a). For every value of torque request, the regenerative and the mechanical brakes work together. There might be a constant ratio until the regenerative brake reaches its maximum deliverable torque. This maximum deliverable torque relies on the overall sizing of the whole drivetrain and on the current rotational speed of the electric machine. When a higher brake torque is required, the ratio shifts to the
mechanical brakes. It has to be considered that the rear axle must not lock before the front axle during a brake application in order to stabilize the EV. Consequently, the brake torque for the front axle is normally set higher than the brake torque for the rear axle and is usually kept at a constant ratio.
Another major method is the series regenerative braking system, as shown in Figure 6.18 (Spichartzet al., 2014a). As long as the requested torque is lower than the available brake torque from the electric machine, the machine delivers it. The mechanical brakes are not used until the requested torque exceeds the current maximum value of the machine, provided that the other parts of the drivetrain can handle the power.
It is obvious that the series braking system is more reasonable regarding energy recovery in most circumstances. The difficulties lie in the higher complexity of implementation and compliance with safety regulations. Therefore, most of the available EVs use a parallel braking system.
Vehicle speed Conventional braking torque
Recuperation braking torque
Resultant braking torque
Braking torque
Figure 6.16 Combination of mechanical braking and electrical braking
Mechanical brake front axle
Mechanical brake front axle
Mechanical brake rear axle
Mechanical brake rear axle Regenerative brake
front axle
Regenerative brake front axle Requested
braking torque
(a) (b)
Requested braking torque Drag torque by regenerative braking
Developed braking torque Developed braking torque
Figure 6.17 Parallel regenerative braking system: (a) without drag torque and (b) with drag torque
On-board electromagnetic energy regeneration for electric vehicles 167