Environmental Engineering Reference
In-Depth Information
Stator yoke
Stator slots
Airgap
Rotor slots
Rotor yoke
Shaft
Figure 5.34 Classical induction machine cross section
Figure 5.34 shows the construction of an IM in cross section. This is a smooth
rotor design in which rotor slots are typically semi-closed or fully closed for
inverter drive, and the slots are relatively deep. Line-start IMs, on the other hand,
will have open slots that are shallow or double cage designs in order to improve
starting torque from a fixed frequency supply. With inverter supply, the frequency
of the rotor currents is controlled in response to the rotor mechanical speed so that
flux penetration into the rotor is not restricted by eddy currents as it would be for
fixed frequency starting.
To explore the IM further for application as a hybrid propulsion system starter-
alternator for a pre-transmission, parallel hybrid as discussed in Reference 27, it is
important to understand the slot design for both stator and rotor. Figure 5.35 is used
to illustrate a practice of stator design having parallel sided teeth (iron intensive)
and parallel sided rotor slots (iron intensive). The machine is designed for high
overdrive conditions to meet the vehicle driveline package constraint in both axial
and radial dimensions.
For the lamination design illustrated in Figure 5.35 and with a 60 mm stack,
the machine develops 300 Nm of torque to 1,000 rpm at the engine crankshaft.
It should be noted in this IM design that stator slots are consistent with a 3-phase,
12-pole design for which
Q s
mP
q ¼
ð # Þ
ð 5 : 25 Þ
where q ¼ 2 SPP for the parameters given. The stator winding is double layer, 5/6
pitch and has 2 turns/coil with all coils in a phase belt connected in series.
The operation is heavily in saturation under this level of overdrive.
The developed torque peaks at 300 Nm for 360 A pk of inverter drive at 1,000 rpm
(100 Hz). In this application, it was found that the rotor teeth saturate first and to
the greatest extent followed by the stator teeth second. The rotor slots should be
'coffin' shaped to enhance the rotor flux and limit rotor teeth saturation, but in this
test machine, the original design was made using copper bars for the rotor cage and,
hence, parallel sided rotor slots.
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