Environmental Engineering Reference
In-Depth Information
8.1.2 Copper losses and skin effects
Copper losses are calculated from the total conductor length per phase belt and
account for gauge size, parallel paths if any, and wire pull tension. This latter
consideration is due to the manufacturing process wire tension during coil forming
and must be taken into account as it may result up to 5% diameter reduction.
Second order effects such as wire roundness or ovaling are not generally considered
because the effects are difficult to predict. The procedure to calculate copper loss is
to determine the total effective conductor length based on the machine design and
to correct for diameter shrink resulting from manufacturing. Operating temperature
corrections are added to the calculation of winding resistance. Copper losses are
then based on operating current levels of the machine.
General purpose M/Gs, when inverter driven, tend to have shortened life due to
voltage transients (i.e. line reflections when the M/G is removed several metres
from the inverter) and insulation breakdown due to corona. The highest incidence
of insulation breakdown occurs in random wound machines as turn-turn or phase-
phase failures [5]. In response to this new failure mechanism of conventional
motors that are inverter driven, the magnet wire industry has developed wire
insulation systems that have no increase in overall thickness, are machine windable
and have much higher resistance to electrical stress, particularly
dV
/
dt
.
Wire insulation system testing is now done using a pulse endurance tester that
subjects the wire in the form of a twisted pair to simultaneous temperature and
electrical stress. Test conditions of the pulse endurance test are stated in Table 8.4.
Table 8.4 Wire pulse endurance test conditions
Specification
Voltage (V
pk-pk
)
1,000-5,000
Frequency (Hz)
60-20,000
Pulse rise time
<
100 kV/
m
s
Duty cycle (%)
10-50
Temperature (
C)
<
180
Wire preparation
Twisted pair
In an inverter driven M/G, the voltage seen at the machine terminals is cor-
rupted by transmission line effects excited by the high
dV
/
dt
of the inverter
switching. The lead inductance tends to ring with the motor winding capacitance at
a frequency usually in the range 0.5 MHz
<
f
ring
<
4 MHz. Voltage overshoots of
>
50% can occur for lead lengths as short as 5 m (e.g. the length of cable from
under-hood to the rear axle) and higher, in fact two to three times the bus voltage,
for longer lead lengths. This is problematic since NEMA standards require that
electric motors designed for 600 V or lower must be designed to withstand
1,600 V
pk
. It is clear that in the push for higher bus potentials in hybrid and fuel cell
vehicles, M/G terminal voltages are on the order of 600 V
rms
line-to-line so that
