Environmental Engineering Reference
In-Depth Information
easily 10 if not 100s of times faster than the mechanical system, the torque change
is viewed as occurring nearly instantaneously.
Now, if the machine were operating in motoring mode at 2.5 krpm and
+100 Nm of torque, which is basically in boosting mode for, say, passing, and the
driver aborts the manoeuvre and slows to re-enter traffic, the M/G may be com-
manded to switch to generating at
100 Nm of torque, for example, but at a lower
speed, say 1.5 krpm. Since the M/G was operating well into field weakening
(according to the chart in Figure 4.16) initially and the new operating point is
basically on the constant torque boundary (full field), the controller must boost the
field to maintain the commanded generating level. This process is slower than
simply changing the torque at constant speed. The flux in the machine must be
readjusted to its new and higher level, and this occurs at the electrical time constant
of field control in the machine (depends on machine size/rating and ranges from 30
to
>
100 ms for hybrid traction motors). Although torque control responds in sub-
millisecond times, field control takes much longer. However, this is still about ten
times faster than the mechanical system. The same process occurs, going from
CCW motoring to CW generating except that the speed is now determined by the
mechanical system at its much slower time constant. The M/G power controller
easily tracks the speed changes of normal operation. We will see later that some
manoeuvres can be more demanding on the M/G response.
The four major classes of electric machines suitable for hybrid propulsion
applications are highlighted in the taxonomy of electric machines in Figure 4.17.
There are only two major classes of electric machines, those that are synchronous
with applied excitation and those that are asynchronous to it. When excitation of
the electric machine rotor is direct current, dc, via field windings or permanent
magnets, the machine is a synchronous type. When excitation of the electric
machine rotor is alternating current, ac, then operation is asynchronous. The defi-
nition gets vague when inside out motors are used, such as brushed dc motors with
stationary permanent magnets. However,
the distinction persists in how the
machine excitation is applied, be it dc or ac.
Electric machines
for hybrid vehicle ac drives
Synchronous
Asynchronous
Induction
cage rotor
Induction
wound rotor
Induction
doubly fed
Permanent
magnet
Variable
reluctance
Unipolar
Switched
reluctance
Doubly fed
reluctance
Brushed
DC
Surface PM
Inset PM
Interior PM
IM
IPM
SPM
VRM
Figure 4.17 Taxonomy of electric machines
