Environmental Engineering Reference
In-Depth Information
For the conditions noted above and where
N
c
= number of interconnects we
find that starter motor current is 556 A maximum. In today's automobile, the starter
motor is internally geared with a planetary set having a ratio
G
gr
= 3.6:1 and
externally geared at the pinion to ring gear at the crankshaft of
G
rg
= 14:1 or slightly
higher, depending on the engine. In fact, the same starter motor is used for several
different engine displacements by changing the ring gear number of teeth (gear
ratio). Now using the fact that this starter motor has a torque constant,
k
t
= 0.011
Nm/A, we can calculate the torque delivered to the engine crankshaft as
T
crank
¼
N
gr
N
rg
k
t
I
starter
ð
Nm
Þ
ð
4
:
34
Þ
For the conditions given, and for the approximations made, we find that this
permanent magnet, geared, starter motor is capable of delivering 308 Nm of torque
to the engine crankshaft. If we now assume the temperature is at
30
C, we can
recalculate the starter motor maximum current by first noting that although
the cable, termination and starter motor armature resistances will decrease in
proportion to temperature, the battery internal resistance will actually increase
due to slower ion transport dynamics and increased polarization. We will make
the approximation that cable and armature resistance, since these are copper
wire based, will have the same resistance change. Terminations, on the other
hand, will be assumed to have negligible change. The battery internal re-
sistance for this example is approximated according to the following expression
(
a
= 0.0003,
b
= 2):
b
R
int
ð
T
Þ¼
R
int
ð
T
0
Þ½
1
það
T
T
0
Þ
ðWÞ
ð
4
:
35
Þ
With this modification to (4.33) the starter motor current at
30
C actually
drops to 504 A, yielding a cranking torque of 280 Nm. In reality the starter motor
brush voltage drop will also increase, further reducing the available torque. Typi-
cally, starter motor torque capability, when at cold temperature, is in excess of the
torque necessary to break over and crank a cold engine. Cranking speeds at cold
temperature are also slower due to higher friction in the engine so that crank speeds
of 100 rpm or less are typical.
One further point to make regarding brushed dc motors and cold conditions is
that if improperly designed the cold inrush current, assuming a fresh battery, may
be higher than the room temperature design point, resulting in demagnetization of
the ceramic magnets. Unlike rare earth magnets used in starter-alternator and other
high performance electric machines, a starter motor and most dc motors in the car
will rely on ceramic 7 or ceramic 8 magnets, which have less coercive force at cold
temperatures and so could conceivably be partially demagnetized.
4.5.2 Inverter bus bars
An important and easily overlooked component of the hybrid propulsion system
make-up is the bus bar structure present in all power electronic converters and
inverters. The power electronic component resides in the main electric power flow
