IEEE Electrification Magazine - December 2014 - 18

slightly better because of their inherently larger air gaps.
SRMs do not score as high because of the complex shape
of their rotors and the potential need to introduce gas flow
into the air gaps for cooling purposes, which results in
additional windage losses. The IMs are rated much lower
than the PMMs, primarily due to the rough surfaces of
their rotors, which are made of laminated steel and
embedded copper or aluminum bus bars. Also, the IM
needs more gas flow in the air gap for rotor-cooling purposes compared with the PMM.

KC 4: Rotor Thermal Limitations
Rotor thermal limitation is a chief characteristic that
determines the ability of an EM to be integrated in a hightemperature environment such as an engine. This characteristic primarily depends on material thermal properties.
All PMMs are rated low because of the use of permanentmagnet materials, whose temperatures are limited to
about 200 °C. The winner in this category is the SRM, which uses steel
lamination material operating up to
400 °C. The IM is slightly worse than
the SRM due to additional copper or
aluminum bars. The temperature of a
machine rotor is due to self-heating
and also to external thermal sources
such as an engine. Therefore, in some
applications, the PMM, with its inherently small rotor losses, may perform
better than other machines with
higher temperature capabilities.

KC 5: Cooling Options

KC 6: Rotor Mechanical Limitations
Rotor mechanical limitations are directly related to the
high-speed capabilities of the EM and the ability to integrate the generator with a high-speed engine. The stiffness of the rotor is paramount to high-speed integration.
Again, a rigid metal or composite sleeve construction for
the PMM behaves best. The toothless PMM is slightly better due to lower dependence on the mechanical air gap
and the ability it provides for installing a thicker sleeve.
Next is the SRM, whose simple rotor design places it at a
I E E E E l e c t r i f i c ati o n M agaz ine / december 2014

KC 7: Torque-to-Inertia Ratio
Torque-to-inertia ratio is important for an application in
which fast acceleration in the motoring mode is required.
This is typical for an EPGS with self-start capabilities.
Sometimes EPGSs have a requirement for fast acceleration and deceleration (i.e., for a rapid transition from one
speed to another). In this type of application, dynamic
and steady-state operations are traded to achieve balanced performance. Traditionally, PMMs are the best in
this category. The SRMs and IMs are the worst.

KC 8: Torque Pulsation
Torque pulsation has an important role where the application is very sensitive to vibration. Torque pulsation can
excite a system that is not mechanically well damped, and
this may lead to destructive consequences. There are two main sources
for torque pulsation in an EPGS: 1)
the current pulsation in the stator
winding and 2) the cogging torque.
The SRM is probably the only one
where the operation is based on synchronous current excitation of the
phases with irregular nonsinusoidal
wave shapes. A torque pulsation
occurs with a fundamental frequency
equal to the current excitation. Higher-frequency torque harmonics exist
as well. The cogging torque is an
inherent behavior of a tooth-type stator interacting with a permanentmagnet rotor when rotated. This is
why the tooth-type PMM is rated low.
The toothless machine does not have
cogging torque, which leads to a very
smooth operation at sinusoidal stator
currents. The IM is good due to lack of
cogging torque resulting from having
no PMs in the rotor. An additional condition is to have a
sinusoidal current during power extraction.

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Cooling options are an important feature that is determined by the location
of losses. Machines with minimal losses in the rotor are easier to cool and
create fewer complications for the
cooling system. The winner is the PMM
because of minimal losses in the rotor.
The ring PMM is considered easier to
cool than the tooth-type stator PMM, since the windings are
exposed rather than buried in the stator slots. The IM and
SRM are equally rated much lower compared with the PMM
due to their substantial rotor losses.

high level. The IM with embedded heavy bus bars is sensitive to high centrifugal forces at high speed.

KC 9: Compatibility with Bearings
Compatibility with different bearing systems is important
for achieving a high-speed EPGS. The transition from conventional bearings to foil or magnetic bearings is one of
the most powerful provisions for an EPGS. Rotor stiffness
and a large air gap are the two most important parameters for accommodating this integration. The integration
of the rotor and engine should be carefully designed to
allow for accommodation of critical speeds. The ultimate
winner is the PMM. The toothless designs are slightly better than the tooth-type PMM machine because of lower
sensitivity to air-gap size. IM and SRM performances are

Table of Contents for the Digital Edition of IEEE Electrification Magazine - December 2014