IEEE Electrification Magazine - June 2014 - 20

(a)

(b)

Figure 10. The hairpin shape (a) before and after insertion and
(b) after twist. (Photo courtesy of GM.)

Stator
Lamination

Insulators

Conductors

(a)

(b)

(c)
Figure 11. (a) An S-shaped insulation as used in a GM 2-Mode hybrid,
(b) an improved B-shaped insulation used in the Chevrolet Spark EV, and
(c) a three-dimensional view of the B-shaped slot liner.

length is narrow to allow for small car width usage. the du
has a thermal bypass valve that bypasses the oil cooler
until extra cooling is required, which allows the oil to warm
up faster to maximize vehicle range. additionally, the du
uses the full synthetic deXron hP transmission fluid to
improve efficiency by lowering mechanical oil drag losses.
the single planetary and bearing locations allow the du to
consume fewer than 150 w in spin losses up to 100-km/h
vehicle speed, or 600-r/min axle speed at 55 °C. a 350-vdc

I E E E E l e c t r i f i c ati o n M agaz ine / j un e 2014

variable-speed oil pump was installed to optimize the oil
flow under all driving conditions. For example, the pump
energy averages less than 16 w of high-voltage electrical load
on the ePa urban and 42 w on the ePa highway drive cycles.
ev vehicles are very sensitive to noise and vibration due to
the lack of combustion engine noise masking. therefore, the
spark ev's lower em speeds, quiet planetary gear, high housing structural stiffness, and low electric oil pump speeds are
all key factors to providing extra quiet operation. table 1 lists
the key du parameters of Chevrolet spark Bev.
more than 75% of components used in the spark du
are reuse components from other gm's high-volume production programs. the reuse components improve quality,
reliability, development time, and component costs.

electric Motor
the Chevrolet spark em is a double-layer internal Pm
(iPm) motor specifically designed targeting the vehicle
requirement. some of the key motor parameters are listed
in table 2. the du is designed with a low numerical ratio
to improve the vehicle's range and acceleration performance. therefore, the maximum motor speed is limited to
4,500 r/min.
general motors adopted bar-wound construction for its
earlier programs, the two-mode rear-wheel-drive motor
and generators, as well as the Chevrolet volt em. the barwound construction was also selected for the Chevrolet
spark em. Figure 8 shows a partial view of the Chevrolet
spark stator. the bar-wound construction has several
advantages over the more conventional stranded design:
higher slot fill, shorter end-turn length, improved cooling
performance, fully automated manufacturing process, and
improved high-voltage durability.
Because of higher slot-fill and short end-turn length, the
dc resistance of the bar-wound motor is significantly lower
than an equivalent stranded design. For instance, the dc resistance of the Chevrolet spark bar-wound design is 35% lower
than an equivalent stranded design that would deliver the
same torque as the bar-wound design. however, the barwound construction, due to higher cross-sectional area of the
individual conductors, has significant skin and proximity
effects. this results in increased resistance at higher motor
speeds, referred to as the ac resistance of the stator winding.
the ac resistance of the bar-wound construction not only
depends on the motor speed and, hence, the operational frequency but also on the motor current level. this is illustrated
in Figure 9 for the stator of the Chevrolet spark as shown in
Figure 8. the ac effect is higher at a lower current level but
approaches that of free air when the stator gets saturated at
an elevated current level. Figure 9 also shows the dc resistance of the equivalent stranded design. it is evident that the
bar-wound design outperforms the stranded design for a substantial speed range of the em, up to 4,200 r/min, only when
the bar-wound ac resistance exceeds that of the stranded
design. the bar-wound construction as shown in Figure 9 also
allows very effective cooling at the end turn area, resulting in

Table of Contents for the Digital Edition of IEEE Electrification Magazine - June 2014