Environmental Engineering Reference
In-Depth Information
hybrid performance levels [15]. They constructed two 42 V ISG machines, identical
in all respects, except for stator turn number. One of the pair had 4t per coil and the
other 5t per coil. A third machine, with identical laminations and stack length, was
wound for 300 V operation and had 12t per coil. On the surface it would appear that
the sensitivity of machine power capability to voltage should be a second order effect
with differences due only to the slot fill factor decrease at lower voltage due to larger
diameter wires. However, there are other practical implications in a system's context
that continue to work against the low voltage ISG. Leonardi and Degner [15] list
these additional factors as follows: (1) connections in the power harness between the
battery terminals and ISG stator are not ideal and contribute resistance effects that do
not scale with cable size, (2) power electronic devices suitable for low voltage must
switch 700 A and are generally specialty products. Power MOSFETs most suitable
for low voltage operation are also majority carrier devices and have conduction
voltage drops that are a linear function of conduction current. High currents require
large MOSFET die areas to contain the voltage drops to acceptable levels, and
(3) power batteries for 42 V operation are also not in production at the present time
other than for limited application on the Toyota Crown THS-M, for example. This
means the battery is not optimized for ISG application.
When the ISG discussed in Reference 15 was tested, it was found, not sur-
prisingly, that the 5t stator produced more torque/ampere than its 4t counterpart.
Furthermore, when both of these machines were compared to the 300 V machine in
terms of battery power necessary to deliver torque, it was found that all performed
essentially the same up to about 150 Nm. Beyond this torque level the 300 V ISG
continued to deliver torque up to about 180 Nm. The losses at these high drive
levels were exorbitant, but torque was delivered. The obvious conclusion from
Figure 2.13 is that a higher system voltage has the capacity to deliver much higher
power levels to the ISG, thus providing overdrive far in excess of its thermal rating.
Cranking mode
50 rpm
16,000
14,000
12,000
10,000
8,000
6,000
4,000
2,000
0
ISG #1 (4 T)
ISG #2 (5 T)
ISG #3 (12 T @300 V)
0
50
100
150
200
Torque
Figure 2.13 Comparison of 42 and 300 V mild hybrid ISGs under engine cranking
loading at 50 rpm (Ford Motor Co.)
