48V Traction Drive for Electric Vehicles:
How to Combine Low Cost,
Large Driving Range and High Power
Dieter Gerling
Universität der Bundeswehr München
48V Traction Drive for Electric Vehicles
 requirements to electrical traction drives / traction systems

low costs

large driving range (i.e. high efficiency, especially at partial load)

high power (example Tesla Model S: 600Nm @ 5300min-1, i.e. 330kW)

low voltage (UDC ≤ 60V, UAC,rms ≤ 25V)

modularity / principle of building blocks

high reliability, high availability

ease of integration of electrical machine and power electronics

good resources efficiency, good recyclability
2
48V Traction Drive for Electric Vehicles
 standard system up to now
=
M
3̴
ca. 375V
ca. 150V
3
48V Traction Drive for Electric Vehicles
 stator „winding“ evolution
Quelle:
derkonstrukteur.de
4
48V Traction Drive for Electric Vehicles
 possible system architecture with low voltage DC circuit
⁞
⁞
M
battery
electrical machine
24V … 48V
DC electrical system
5
power electronics
48V Traction Drive for Electric Vehicles
 electrical machine: concept
The m-phase stator winding supplied with multiphase inverter
MMF Characteristics (18-slots per pole pair)
Stator cage winding
1.5
Conv. Winding
New Winding
MMF [ p.u. ]
1
0.5
0
-0.5
-1
-1.5
0
6
Conv. Winding
New Winding
1
MMF [ p.u. ]
Conv. distributed winding
0.8
0.6
0.4
0.2
1
2
3
4
theta [rad. degree]
5
6
0
0
5
10
15
Space Harmonics
20
48V Traction Drive for Electric Vehicles
 electrical machine: cooling
7
48V Traction Drive for Electric Vehicles
 electrical machine: benchmark with Tesla Model S
ISCAD ASM
Tesla ASM
375 V
Comparison of results
* Aluminum material for the ISCAD-ASM and Copper material for the Tesla-ASM
** Total Joule losses for the stator cage winding including skin and proximity effect
8
48V Traction Drive for Electric Vehicles
 electrical machine: temperature distribution for T=600Nm, n=5300 rpm
conventional stator frame cooling with hc = 2000W/(Km²)
Tesla ASM
ISCAD ASM
stator slot
700
T [ °C ]
600
500
400
ISCAD-asm
300
tesla-asm
200
100
0
0
40
80
Loss
density:
*
pLosses
,Tesla
p
*
Losses , ISCAD
T [ °C ]
PLosses , ISCAD
2
200
240
ISCAD-asm
500
Losses:
160
rotor slot
600
PLosses ,Tesla
120
time [s]
tesla-asm
400
300
200
4
100
0
0
9
40
80
120
time [s]
160
200
240
48V Traction Drive for Electric Vehicles
 power electronics: concept

requirements from the ISCAD motor with 60 slots:
•
•
•
DC supply: 24V
AC phase voltage (rms): 8.5V
AC phase current (rms): 620A
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48V Traction Drive for Electric Vehicles
 power electronics: exemplary power switches

Example MOSFET – StrongIRFETTM IRFS7430-7PPbF
•
•
𝑉𝐷𝑆𝑆 = 40 V
𝑅𝐷𝑆 𝑜𝑛 = 0.75 mΩ (max. )
•
𝐼𝐷 = 522 A

Estimation of inverter losses via conduction and switches losses: 𝑃𝐿 = 𝑃𝐶 + 𝑃𝑆𝑊

Design: equal losses at maximum power
•
•

3 phases * 2 switches * 16 IGBTs in parallel
60 phases * 2 switches * 3 MOSFETs in parallel
= 96 IGBTs
= 360 MOSFETs
Comparison of cost for power switches
•
•
•

Tesla:
ISCAD:
Higher number of devices for ISCAD inverter
Higher cost per devices for Tesla inverter
40 % decreased cost for extra-low voltage solution
Pricing on basis of medium quantity @ electronics distributors
11
48V Traction Drive for Electric Vehicles
 power electronics: losses at partial load


advantages of MOSFETs in relevant driving cycles
lower forward voltage at partial load (𝑅𝐷𝑆,(𝑜𝑛) 𝐼1 instead 𝑉𝐹 )
𝑃𝐿 ↓
 further optimization potential for MOSFET inverter


optimization of cost
optimization of efficiency
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48V Traction Drive for Electric Vehicles
 power electronics: gate drivers

assumptions
•
•
3 MOSFETs IRFS7430-7PPbF in parallel for every switch
total gate charge 3𝑄𝑔 = 1380 nC
•
•
switching frequency 𝑓𝑆𝑊 = 20 kHz
gate voltage 𝑉𝐺𝑆 = 10 V

required channel power 𝑃𝐷𝑟𝑖𝑣𝑒𝑟 ≈ 0,3 W

very basic solution: IR21844
•
•
•
•
•
half-bridge driver (high- and low-side)
two input signals: PWM, Enable
low amount of external components
programmable dead time
very cheap
13
48V Traction Drive for Electric Vehicles
 first functional prototype
14
48V Traction Drive for Electric Vehicles
 power electronics: concept
Aluminum plate
MOSFETs
Driver board
Capacitor pack
15
48V Traction Drive for Electric Vehicles
 power electronics: cooling concept
16
48V Traction Drive for Electric Vehicles
 battery and DC electrical system: concept
17
48V Traction Drive for Electric Vehicles
 losses and voltage drop of DC-bus
ca. 2W
ca. 0.04V
18
48V Traction Drive for Electric Vehicles
 battery and DC electrical system: benchmark with Tesla Model S (battery set-up)

design constraint:
TESLA:
7104 cells (type 18650) organized in 16 modules
each module: 6 cells in series, 74 cells in parallel
in total: 96 cells in series (voltage span: 278V to 403V)
ISCAD:
using the same voltage span per cell of 2.9V to 4.2V and
realizing a DC-link voltage of 24V gives 8 cells in series: s  24V 2.9V  8
the same battery capacity leads to 888 cells in parallel
p  7104 s  888
the battery losses are independent from the configuration
(the current per cell is identical)

battery management system (BMS):
much more simple in low-voltage version due to insulation, voltage monitoring, balancing

current interrupt device (CID): much more simple at low voltage
19
48V Traction Drive for Electric Vehicles
 EMC: magnetic field

field produced by pure DC currents for sandwich structure (well below limit 500mT)

field exposure due to passing automobile (200km/h, 20cm distance → well below limits)
20
48V Traction Drive for Electric Vehicles
 EMC: magnetic field

field due to AC currents on the DC bus: is to be investigated;
main differences to HV-system:
•
•
•
•
•
no coaxial cable in HV-system
higher switching frequency
higher number of phases
different leakage capacitances
different voltage transients (MOSFET)
 EMC: electric field


steeper voltage slope (MOSFET) than in HV-system, but even much lower voltage level
less electric field interference is estimated
21
48V Traction Drive for Electric Vehicles
 changing the number of pole pairs during operation (schematic)
22
48V Traction Drive for Electric Vehicles
 efficiency (for optimized design higher potential likely)
component
Tesla
ISCAD
electrical machine
90%
95%
•
•
at maximum power
at partial load (cycle) greater difference
power electronics
96%
96%
•
•
at maximum power identical (design)
at partial load (cycle) greater difference
battery
>90%
>90%
•
•
identical (at same electrical power)
advantages for same operating point of car
•
•
•
worst case for maximum power
smaller difference for larger cross section
for smaller maximum power or partial load
(i.e. driving cycle) hardly different
•
partial load is decisive for the driving range
•
changing the AC/DC-converter (this efficiency
is not decisive for the driving range)
DC electrical system
99%
97%
entire sytem in
typical driving cycles
ca. 60%
ca. 80%
95%
94%
on-board charger
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remarks
48V Traction Drive for Electric Vehicles
 cost estimation (for optimized design higher potential likely)
component
Tesla
electrical machine
(incl. housing)
100%
power electronics
(switches, driver,
capacitors)
100%
ISCAD
remarks
50%
•
•
•
stator and rotor die-casted
no copper, but aluminum
smaller cooling effort
85%
•
•
large number of identical modules
low voltage
•
smaller energy for same driving range
(efficiency in cycle-relevant operating
points)
battery
100%
80%
DC electrical
system
100%
95%
•
•
•
Board net for very high current
chassis as one DC conductor
voltages less than 60VDC
charging (standard)
100%
95%
•
changing the AC/DC-converter
entire system
100%
ca. 80%
•
battery dominates the costs
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48V Traction Drive for Electric Vehicles
 additional advantages

12V-supply can be realized simply and very cost-efficiently (low voltage DC/DC-converter)
(improved efficiency, lower costs)

drop out of single battery cells does not result in large power reduction

drop out of single half-bridges (power electronics) does not result in large power reduction

power electronics and electrical machine are perfectly integratable

improved EMI characteristics

scalability of the system is very simple

effort for high-voltage training of employees, … is drastically reduced

…
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48V Traction Drive for Electric Vehicles
 conclusion


low costs

large driving range (i.e. high efficiency, especially at partial load)

high power (example Tesla Model S: 600Nm @ 5300min-1, i.e. 330kW)

low voltage (UDC ≤ 60V, UAC,rms ≤ 25V)


modularity / principle of building blocks


high reliability, high availability

ease of integration of electrical machine and power electronics

good resources efficiency, good recyclability


26


