Environmental Engineering Reference
In-Depth Information
speed. Corner point speed in Figure 5.33 is 4,000 rpm and the torque is 205 Nm at
stall for the double layer rotor. Maximum speed is 10,000 rpm.
5.4 Asynchronous machines
The most convenient definition of an asynchronous machine is that of a singly fed
ac machine in which the rotor currents are also alternating [4]. All synchronous
machines have dc rotor currents from either field windings or permanent magnets.
Recall that a permanent magnet may be modelled as an equivalent current sheet
that produces the intrinsic coercive force exhibited by the magnet. In this section,
the various types of asynchronous machines that are considered for hybrid pro-
pulsion systems are evaluated. It should be emphasized that in hybrid propulsion
the need persists for machines having wide CPSR. Traditionally, asynchronous
machines are capable of operating over a range of 2.5-3:1 in CPSR. This is due in
some respects to the specification based on thermal constraints for peak to con-
tinuous rating of line-start applications and in some respects to the fact that inverter
driven asynchronous machines are limited by the resolution of currents injected
into the
d
-axis of the machine at high speeds. Magnetizing current requirements
are low at high speed, and regulating a 10 A
d
-axis current in the presence of 350 A
q
-axis current is constrained by the sensor resolution, A/D word length and
microcontroller limitations.
This section starts with a brief overview of the classical IM having a cast rotor
and then elaborates more on the research activities directed at improving the
operating speed envelope of IMs in general. The wound rotor and other doubly fed
asynchronous machines are noted but are generally not of high interest in hybrid
propulsion systems.
5.4.1 Classical induction
The cage rotor IM is durable, low cost and relatively easy to control for fast
dynamic response under vector control. In hybrid propulsion systems, the avail-
ability of such a rugged electric machine is very beneficial to designs in which the
M/G is located inside the transmission or on the vehicle axle in the case of electric
four wheel drive.
There exists voluminous literature on IM design, modelling and control [26].
Our interest here will be on those attributes of IMs that make them attractive for
hybrid propulsion and how this machine compares with other types. It has already
been noted in section 5.3.2 that the IM does not possess the torque density of a
permanent magnet design, and that is quite true because the IM must receive its
excitation from the stator leading to higher VA requirements on both the stator
windings and inverter drive to deliver this excitation. The IM itself is low cost for this
reason, and all excitation costs are passed on to the user in the form of reactive kVA
requirements. In the permanent magnet design, the machine excitation is provided
during manufacturing and therefore represents a first cost to the manufacturer.
