IEEE Electrification Magazine - December 2014 - 19

very sensitive to air-gap size. Therefore, these machines
present challenges for successful integration with foil or
magnetic bearings.

KC 10: High-Speed Capability
High-speed capability is a composite characteristic resulting from several other parameters, such as rotor mechanical limitations, rotor losses, windage losses, rotor thermal
limitations, and machine complexity. The PMM wins primarily because of high rotor stiffness and low sensitivity
to the air gap.

KC 11: Short-Circuit Behavior
Short-circuit behavior is related to the ability of an EM to
disable excessive currents in case of failures. All PMMs
are rated very low because flux is created by the permanent magnet and cannot be disengaged electrically.
Hence, an electromotive force voltage is generated until
the rotor stops turning. As a result,
an excessive short-circuit current
may be fed to the point of failure at a
broad speed range from 5 to 100%.
Some PMMs are rated highly because
of the ability to create a high-reactance PMM (HRPMM), which significantly mitigates this issue. This type
of machine is designed with a much
higher reactance compared to conventional machines, resulting in a
short-circuit current comparable to
the operating current. The SRM and
IM receive the highest scores since
their construction does not use permanent-magnet materials. In case of
failure after shutting down the
inverter, the failure current duration
is within the electrical time constant. The magnitude may
not exceed the operating current substantially.

tooth-type PMM because it has the best efficiency and,
therefore, lowest losses. The toothless design is worse
because of the stray flux losses contained within the
outer stator ring and the inability to cool the main stator
ring directly. The current density of the IM is lower
because of additional rotor losses compared to the PMM
and overall worse efficiency.

KC 14: Power Density
The power density of a machine is a composite characteristic
dependent on other characteristics and can be applied either
to weight or volume. Based on many practical implementations, the ultimate winner is the tooth-type PMM, followed
by the toothless PMM. The SRM is next due to its ability to
operate at very high speeds. Next is the IM because of rotor
losses and rotor speed limitations.
The last row in Table 1 contains the total score of all six
evaluated machines. The winners are the PMMs, with
small differences exhibited between
different types. Within the PMM family, multipole designs are slightly better
due to more efficient utilization of the
stator core iron. Toothless designs are
slightly better compared to tooth-type
designs due to lower windage losses,
better cooling options, overall better
mechanical performance, and lack of
cogging torque. The IM and SRM are
equally rated and about 20% worse
compared with the PMM, due mainly
to much higher losses in the rotor. The
IM has a definite advantage in windage losses and torque pulsation compared with the SRM. The SRM has
advantages in rotor thermal limitation, rotor mechanical limitation,
high-speed capability, and machine complexity.

The EM
improvements as a
part of an EPGS
must be considered
in conjunction with
their integration into
electrical and
mechanical systems.

Types of Permanent-Magnet Machines
KC 12: Machine Complexity
Machine complexity has a direct impact on reliability. The
winner in this category is the SRM because of its simple
rotor construction. Next is the PMM due to its simple,
robust rotor design with a metal or a composite sleeve.
The toothless PMM is slightly worse compared with the
tooth-type design due to the additional metal ring
required to wrap around the stator to contain stray fluxes.
The IM stator is very similar to the PMM tooth-type stator.
However, the rotor complexity of the IM is worse. Consequently, the IM is placed slightly lower than the PMM.

KC 13: Current Density
Current density represents the ability of an EM to be
loaded with certain ampere turns per unit surface of the
outer diameter. This characteristic is directly related to
the ability of a machine to be cooled. The winner is the

The analysis of EMs for different KCs reveals that the PMM is
the ultimate winner for most of the potential applications.
For a better understanding of the various characteristics
applicable to different constructions, these machines are
reviewed in greater detail.
Figure 2 shows the cross sections of two different types
of toothless PMMs with two-pole arrangement. In
Figure 2(a), the copper utilization is not that great since the
outer portion of the winding does not create useful flux.
Furthermore, the stator requires an additional ring to contain the stator flux. However, the outer copper is nicely
exposed, giving an opportunity for implementing very
efficient cooling. On the other hand, the main stator ring is
difficult to cool since it is fully contained within the stator
windings. In Figure 2(b), the machine does not need an
outer ring and does not have outer copper, resulting in better material utilization and ease in cooling steel. The
IEEE Electrific ation Magazine / d ec em be r 2 0 1 4

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