Sizing the drive system - Propulsion Systems for Hybrid Vehicles - page 167

power electronics more so than for the machine. The following definitions
emphasize the limitations of such capability curves:

Continuous rating : The ac drive can be operated within its continuous rated
region indefinitely, provided (1) the motor thermal management system is
operated at or below its cooling medium maximum inlet temperature condi-
tions for the coolant used (air or liquid), (2) the power inverter thermal man-
agement system is within its maximum inlet temperature of coolant (air or
liquid) and (3) the power electronics electrical parameters are within nominal
stress levels of 50%. For example, the operating voltage of power switching
devices should be at 50% of the device rated breakdown voltage.

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Intermittent overload operation is permitted for short durations ( < 30 s) to
contain low energy transients such as responding to fast gear changes or clutch
actuation intervals when the M/G may be called upon to furnish additional
torque and power.

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Peak overload operation is within the capability of the electric machine but
outside the capability of the power electronics. There have been attempts to
redefine this peak condition to contain fast transients having low energy, but
very high power - for example, an ISA mild hybrid in which engine cranking is
desired under cold conditions. Some specifications may call for peak overdrive
torque for < 50 ms in order to overcome engine crankshaft striction. The
application of a very high torque impulse is necessary to break away the engine
and permit sustained cranking at the ac drive system intermittent rating. A
issue here is the need for the power electronics to sustain over currents at the
peak overdrive condition. Such requirements generally do not pan out because
the electronics must still be sized for the peak operating envelope.

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Thermal management systems for passenger car hybrids consist of auxiliary
coolant reservoirs, pumps and fans, along with a small radiator. The M/G
will have a separate coolant supply from either the vehicle's engine coolant
( < 115 C) or transmission oil cooler ( < 120 C). The power electronic coolant
is restricted to glycol-water mixtures having a maximum inlet temperature of
65 C. With this thermal boundary condition on the power electronic internal
cold plate, the temperature fluctuations at the semiconductor junctions
can be held to < 40 C temperature rise and thereby achieve high durability
( > 6,000 h life).

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In Figure 4.15 the continuous operating boundary contains efficiency contours.
Hybrid propulsion simulations are best performed with the M/G torque-speed
capability inserted into the driveline definition. Driveline loss mapping utilizes the
efficiency contours (map) to extrapolate loss components at each operating point.
The M/G for a hybrid propulsion system is designed differently from either an M/G
for a BEV or an industrial application. For a BEV the M/G is designed to have the
peak efficiency island trend towards zero and be as broad as possible through the
constant torque region and out into constant power. An industrial electric machine
may have its peak efficiency plateau situated near the capability curve corner point
so that operation at rated conditions is most efficient. A hybrid, on the other hand,