IEEE Electrification Magazine - December 2014 - 17

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Reliability directly affects mission success and perfor-

mance, maintenance, repair, and dispatchability.
xx
Efficiency becomes a major driver for fuel savings in
an environment in which mechanical energy is converted to electrical energy in large amounts. In that
regard, the energy-optimized aircraft (EOA) initiative is
gaining momentum.
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Cost determines the affordability of a new platform.

require additional cooling provisions and capacity and
may create additional machine losses, such as windage.
The ultimate winners in this category are the PMMs,
where most of the losses are in the stator. Minimal eddy
current losses are present in the magnet and in the rotor
sleeve when a metallic material is used. In the case of the
composite sleeve, even fewer losses are experienced. IMs
and SRMs experience much worse rotor losses.

Electric Machine Key Characteristics

KC 2: Stator Losses

A KC is a parameter whose value and variation is a key to
an MR. Identifying a parameter as a KC levies a requirement
that the supplier measures and applies the statistics for that
parameter. For a high-performance EPGS, the KCs are
addressing high-speed capability, rotor thermal limitations,
torque-to-inertia ratio, windage losses, and power density.
This section summarizes trade study results of the
more popular EMs for advanced EPGSs characterized in
the classification section. The quantitative results are displayed in Table 1. A comprehensive description for each
KC is included as an aid for better understanding.

All machines have relatively similar ratings for stator losses because stator construction is generally similar. The
toothless PMM is considered to have more stator losses
compared to tooth-type machines due to flux in the containment ring. The multipole PMM has fewer stator losses
due to better back-iron utilization.

KC 1: Rotor Losses
Rotor losses are an important characteristic related to providing efficient cooling for the EM. It is much easier to cool
a stator than it is to cool a rotor. The removal of heat from
a fast-rotating object is much more difficult and may

KC 3: Windage Losses
Windage loss occurs in the mechanical air gap of the
machine. This loss is a function of the size of the air gap,
the tip speed of the rotor, the rotor and stator surface
quality, and the medium in the air gap. If additional gas
flow is introduced in the air gap for cooling or other
purposes, the loss may be increased. The winners in this
category are all PMMs due to the smooth cylindrical shape
of their rotors and the capability they present for building
machines with large air gaps. Toothless machines are

Table 1. electric machines rated by Key characteristics.
Rating1
Machine-Type KCs*

SRM

PMM Tooth,
Two-Pole

PMM Tooth,
Multipole

PMM Toothless,
Two-Pole

PMM Toothless,
Multipole

KC 1

Rotor losses

KC 2

Stator losses

KC 3

Windage losses

KC 4

Rotor thermal limitations

KC 5

Cooling options

KC 6

Rotor mechanical
limitations

KC 7

Torque-to-inertia ratio

KC 8

Torque pulsation

KC 9

Compatibility with
bearings

KC 10

High-speed capability

KC 11

Short-circuit behavior

KC 12

Machine complexity

KC 13

Current density

KC 14

Power density

Total

116

117

119

120

*Descriptions of the KC numbers are provided in the section "Electric Machine Key Characteristics"
1 10=best, and 1=worst.
The rotor thermal limitation for PMM is typically 200 °C.

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