An Article by Evgeni Ganev, Ph.D., “Selecting the Best Electric Machines for Electrical Power-Generation Systems,” IEEE Electrification Magazine, Dec. 2014, page 13-22, which was based on “High Performance Electric Generators for Aerospace More Electric Architectures,” by Evgeni Ganev, Ph.D., Honeywell International, Aerospace Engineering & Technology, Torrance, CA, USA, comprehensively details the key performance criteria between all electric motors and generators:

An optimal machine design process starts from a good understanding of the machine variables expressed by the output equation in formula (1). This equation represents the output power of a machine as a function of the basic geometry and electromagnetic characteristics.

P = C x BL x AC x D2 x L x N (1)

P is the electrical power generated at the EM terminals; N is the shaft speed; L is the length of the magnetically active machine; D is the rotor (or air-gap) diameter; AC is the specific electric loading, which is the armature ampere conductors per unit length of armature periphery at the air gap; BL is the machine magnetic loading, which represents the average flux density over the air gap; and C is a machine constant dependent on various factors such as rotor pole numbers and stator winding arrangements. The product D2 x L represents the machine air-gap volume and is a measure of the used active magnetic materials. It is logical that the total weight and volume of an EM is proportional to the air-gap volume.

NOTE: The table of key characteristics (KCs) from the article do not include the compounding cost, loss, and size effects of the essential electronic controller for practical application of the electric motor-generator (i.e., machine) “system.” Since equitable comparisons must be based on the same power rating and optimized packaging, the KCs are relative to “per unit of electric machine power rating.” All of the contestants are asymmetric electric machines with the asymmetry of a “passive” rotor assembly comprising rare-earth permanent magnets (RE-PM), slip-induction dependent windings, DC field windings, or reluctance saliencies.

Unlike the asymmetric electric machine systems with a “passive” rotor assembly that consumes precious motor real-estate, cost, and loss but cannot produce “working” power, the brushless symmetric multiphase wound-rotor synchronous doubly-fed electric machine system, known as SYNCHRO-SYM, has the symmetry of an “active” rotor assembly comprising another directly excited multiphase winding set, as only possible with the relatively recent enabling invention of brushless real time emulation control (BRTEC) for providing synchronous stabilization at any speed between sub-synchronous and super-synchronous speeds, including zero and synchronous speeds. As a result, SYNCHRO-SYM provides twice the constant-torque speed range (i.e., double Maximum Load RPM) as the asymmetric electric motor system with the same torque, frequency and voltage of excitation, and packaging (i.e., 7200 RPM with 1 pole-pair @ 60 Hz of excitation versus 3600 RPM). Tantamount to effectively providing double the power density, half the loss, half the cost, octuple the peak torque, and twice the armature ampere conductors per unit length of armature periphery at the air gap with similar “active” winding sets strategically positioned on each side of the airgap (and all based on “per unit of electric machine power rating” for scaling), the specific loading term, “AC,” of SYNCHRO-SYM of equation (1) is twice the specific electric loading as asymmetric electric motors and as a result, the KCs (per unit of power rating) of SYNCHRO-SYM is twice the KCs rating of the Induction motor (IM), which also has winding sets on the rotor and stator, respectively, albeit a passive and an active winding sets.  As shown in Table 1A, which is Table 1 of Dr. Evgeni Ganev’s article with the additional column for SYNCHRO-SYM’s KCs, SYNCHRO-SYM has 184 total KCs (or double the IM KCs per unit of power rating), which is nearly twice the 120 total KCs for the best case, toothless, multi-pole PMM, as theoretically expected by at least 60 years of classic electric machine study and research.

Table 1A. Electric Machine Key Characteristics (KC) [leveraged from Table 1 of the Article]

Note 1. Higher numbers are better
Note 2. Rotor thermal limitation for PMM is typically 200°C.
Note 3. KC 1-14 Descriptions can be found in the Article, scaled to per unit of power rating

Machine Type
PMM Tooth, Multi-polePMM Toothless,
PMM Toothless, Multi-pole
KC 1 Rotor losses126610101010
KC 2Stator losses168891089
KC 3Windage losses1051991010
KC 4Rotor thermal limitations168104444
KC 5Cooling options1055991010
KC 6Rotor mechanical limitations1057991010
KC 7Torque-to-inertia ratio1057991010
KC 8Torque Pulsation1893661010
KC 9Compatibility with bearings1055991010
KC 10High-speed capability1057991010
KC 11Short-circuit behavior2010104433
KC 12Machine complexity14710998 8
KC 13Current density1477101088
KC 14Power density1478101088
TOTAL KC WEIGHT1849292116117119120