Investigation on High-Power-Density Design of Small PM
Motors Focusing on Air Gap Configuration
Kazuaki Usami, Toshihiko Noguchi (Shizuoka University,Japan)
Abstract. This paper describes a case study of high-power-density design of small permanent
magnet (PM) motors focusing on their air gap configurations. Two types of the PM motors, i.e.,a radial air gap type and an axial air gap type, are comparatively discussed. Comparison between
the two types has been conducted from the viewpoints of I-T characteristics, N-T characteristics,
efficiency maps, torque ripple characteristics, and so forth. It has been clarified that the axial air
gap type has significant advantages to achieve the high-power-density under some limited
conditions.Start

1. Introduction
Electrification of various equipment such as automotives, industrial machines, home appliances, etc. isintensively promoted to improve the energy conversion efficiency and the control performance.
Electrical motors play an important role for the electrification, and improvement of their efficiency andpower density is an unavoidable issue to expand their application fields. Permanent magnet synchronousmotors (PMSMs) have recently been employed instead of induction motors because of their higher
efficiency and higher power density. In the case of pan-cake shaped motors, axial air gap PM motors
have significant advantages for the improvement of the power density, compared with radial air gap PMmotors. In general, when the motors are designed to have a flat pan-cake proportion, higher-powerdensity design can be achieved because the rotor surface area generating the electromagnetic torque ofthe axial air gap PM motors is much larger than that of the radial air gap PM motors. In addition, theaxial air gap PM motor can easily reduce the copper loss of the windings because of their higher spacefactor of the stator slots, which is another advantage over the radial air gap PM motor. In the paper, acomparative case study is conducted between a newly developed axial air gap PM motor and an existing
benchmark radial air gap PM motor, where both motors are designed to have almost same current-totorque (I-T) characteristics. A finite element method (FEM) based three-dimensional electromagnetic
analyses are carried out to investigate the effectiveness of the axial air gap configuration for thedownsizing of the motor.
2. Motor design conditions and three-dimensional electromagnetic analysis
2.1. Motor Design Conditions and Constraints
The following conditions are given in designing the motors:(1) using same material PM and same volume PM,(2) setting almost same winding resistance per phase,(3) assuming the same space factor of the windings, and(4) using the magnetic material of which iron loss is almost same.
An electromagnetic analysis is conducted with JMAG-Designer 17.1TM to investigate how high thepower density of the axial air gap PM motor can be achieved, compared with the radial air gap PMmotor.

2.2. Overview of Designed Motors
Figure 1 shows overviews of the designed PM motors and their major dimensions. The radial air gap
PM motor is a surface PM (SPM) type of which stator has concentrated windings. Supposing this radial
air gap SPM motor as a benchmark, the axial air gap PM motor is designed so that the I-T characteristicsof the both motors are identical as described later. The diameter of the axial air gap PM motor is adjusted
to make its I-T characteristic equal to that of the radial air gap PM motor.

33.8 mm

70 mm

120 mm

100 mm50 mm

Axial gap motor

Radial gap motor

Fig. 1. Overview of designed motors.

2.3. Motor Specifications and Materials Used in Motors
Table 1. Materials used in designed motors.
Stator core
Rotor core
Magnet
Shaft

Radial gap motor
35JN440
35JN440
MNF12E
S45C

Axial gap motor
Somaloy 1000 SPSUS304
MNF12E
S45C

Table 2. Specifications of designed motors.
Motor type
Outer diameterAir gap distanceSlot combination
Coil space factor
Number of turns
Conductor diameter
Phase resistance
Coil connection
Current density(10Arms/phase)
LdLq

Radial gap motor
SPM motor
Φ 100 mm0.637 mm
12s8p
40 %30 T/slot
Φ 1.02 mm
0.364 Ω/phase
Star connection

Axial gap motor
SPM motor
Φ 120 mm
0.750 mm24s16p40 %26 T/slot
Φ 1.03 mm
0.353 Ω/phase
Star connection

6.14 Arms/mm2

6.02 Arms/mm22.01 mH
2.05 mH

0.567 mH
0.585 mH

Table 3. Motor dimensions.
Outer diameter
Iron core axial
lengthAxial length
Including coil ends
Iron core volume
Volumeincluding coil ends

Radial gap motor
Φ 100 mmAxial gap motor
Φ 120 mm

50 mm

33.8 mm

70 mm

33.8 mm

3392 cm

382 cm3

3382 cm3

550 cmTable 4. Estimated weights of components and whole motors.
Stator core
Rotor core
Permanent magnet
Shaft
Windings
Total

Radial gap motor
1117 g
697 g
223 g
122 g
394 g
2554 gAxial gap motor
868 g
130 g
223 g
80 g
469 g
1771 gTable 1 indicates major specifications of the two motors. The air gap of the axial type is slightly wider
than that of the radial type because it is rather difficult to maintain the air gap precisely in themanufacturing process and the attractive axial force in the air gap is remarkably strong. The slot
combinations of the both motors are 2:3 based and the number of winding turns of the axial type ischosen 26 to achieve equal I-T characteristic to the radial type. Because the space factors of the windings
are identical, the winding resistance of each phase can be almost same by using the same diameterconductors. Therefore, the current densities of the windings are also approximately 6 A for both motors.
The inductances are calculated from the electromagnetic analysis results, and it can be found that theinductances of the axial type are approximately quarter of those of the radial type because of the lowwinding-turns and the wide air gap.
Table 2 shows materials used in each motor. Soft magnetic composite (SMC) (Somaloy 1000 SP:220kW/m3) is employed as a stator core material in the axial air gap PM motor because its iron loss at
400 Hz and 1 T is relatively close to that of the electromagnetic steel plate used in the radial air gap PM

motor (35JN440: 178 kW/m3). The rotor cores of the two motors are composed of the electromagneticsteel plates (35JN440) in the radial type and the stainless steel (SUS304) in the axial type, respectively.
The axial air gap PM motor employs a non-magnetic material such as SUS304 in the rotor back yoke toprevent the eddy currents. However, a general metal material (S45C) is used for the shaft from amechanical strength reason.
2.4. Proportions and Dimensions of Designed Motors
Table 3 shows major dimensions of the two motors. As described above, the outer diameter of the axial
air gap PM motor is designed to be 20-% larger than that of the radial air gap PM motor in order to makethe I-T characteristics equal. The motor axial length of the axial type can be reduced to approximately
68 % of the radial type, and can be reduced down to less than 50 % if the coil ends of the radial type areincluded. As can be found in the table, the iron core volumes their selves are almost same with eachother, but the total volume of the axial type can significantly be reduced when the coil ends of the radial
type are included in the motor volume. The volume of the motor cannot always be reduced if the axial
air gap configuration is employed due to the limitation of the rotor diameter and its mechanical strength.
However, the axial air gap approach is remarkably effective to improve the power density for small
motors from 1 to 2-kW output power range.
Table 4 shows estimated weights of the components and the total of each motor. Every weight iscalculated from the component volume and the specific gravity. The rotor weight of the axial type ismuch lower than that of the radial type, which implies that higher power rate is expected. As described
previously, both of the motors have the same weight PMs on the rotor, but the total weights of the rotors
are very different because of volume difference of the rotor cores. However, the weight of the windingsin the axial type is heavier because more slots are in the stator core, which makes the total amount of
the conductors higher. The winding weight of the axial type is increased by 19 %, compared with thatof the radial type. All in all, however, overall weight of the axial type can be remarkably reduced by31 %; hence the axial air gap approach is efficient to improve the weight as well as the volume.
3. Operating conditions and operation characteristics of design motors
3.1. Operating Conditions(1) No-Load electromotive force (e.m.f.)
The motors are operated at 3000 r/min with open windings, and no-load e.m.f. is observed at the motor
terminals.
(2) I-T characteristics
Only the q-axis current is given to the motors, and phase angle advancing control is not applied to themotors when the I-T characteristic is examined. The current waveforms are sinusoidal of which
effective value is from 0 to 10 A. The rotating speed is kept constant at 3000 r/min throughout the I-T
characteristic test.
(3) Rotating speed and Torque (N-T) characteristics
The sinusoidal currents are given to the motors, where the maximum value is 10 A. No voltage drop
caused in the inverter and the wire harness is not considered. The inverter DC-bus voltage is fixed at
100 V.
(4) Efficiency maps
The efficiency maps are obtained along with the N-T characteristics described above, so the every
condition is exactly same as that of N-T characteristic test. Particularly, a conductor eddy current loss
is not taken into account.
(5)Torque waveforms
The torque waveforms are observed under the same conditions as those of the I-T characteristics. Inaddition, the cogging torque waveforms are obtained by rotating the motors at 60 r/min.
3.2. No-Load Electromotive ForceRadial gap motorAxial gap motor

500060120

180240300

360-50-100

Electrical angle (deg)Torque (Nm)

4
3.5
32.5
2
1.5
10.50

Radial gap motor
Axial gap motor

0Fig. 2. Induced voltage waveforms.
4
3.5
3
2.5
2
1.5
10.50

Torque (Nm)

Voltage (V)10024
6Phase current (Arms)

810Fig. 3. I-T characreristics.

Radial gap motor
Axial gap motor
01000

2000

3000 4000 5000
Rotation speed (r/min)

6000

7000

8000

Fig. 4. N-T characteristics.

Figure 2 shows e.m.f. waveforms of the two motors at no load condition. The e,m.f. amplitude of theradial type is approximately 60 V, while that of the axial type is 46 V. The e.m.f. constant of the axial
type is lower than that of the radial type, which contributes to the higher-speed operation. In addition,
the e.m.f. waveform of axial type is very close to sinusoidal, compared with that of the radial type.
Owing to the sinusoidal e.m.f., reduction of the torque ripple is expected by giving the sinusoidal
currents to the motor unless the space harmonics of the concentrated winding stator affect the torquegeneration.
3.3. I-T Characteristics
Figure 3 shows the I-T characteristics of both motors. As can be seen in the figure, the I-T
characteristics of the two motors are identical and are linear to the motor current, which implies thatmagnetic saturation is not occurred in the motors.
3.4. N-T characteristics
Figure 4 shows the N-T characteristics of both motors, where the axial air gap PM motor is capable torotate in the higher-speed range whereas the torque constant operation region is almost same. As
described above, the axial type is capable to operate in the higher-speed range owing to less e.m.f.
3.5. Efficiency Maps
Efficiency maps are indicated in Figs. 5 and 6. The axial type has a larger region where higher
efficiency is obtained. However, this large high-efficiency area may slightly be degraded by theconductor eddy current that has not been considered in the analysis. This conductor eddy current loss
may be caused because of the fully open slot configuration is introduced to the axial type. It should benoted that there is a highest efficiency area over 95 % in the efficiency map of the axial type.
Figures 7 and 8 shows iron-loss maps, and Figs. 9 and 10 shows copper-loss maps, respectively. Therange of the iron-loss variation is from 0 to 10 W, but the copper-losses increase to 100 W in the N-T
characteristics; hence both of the motors can be regarded as copper machines. The magnetic materials
used in the stator cores, i.e., 35JN440 and Somaloy 1000 SP, play an important role to reduce the ironlosses. In other words, the copper-losses must be suppressed to achieve higher efficiency in thesemotors. The copper-losses can be reduced by increasing the space factor of the windings, e.g., using
flat square windings, or by enlarging the slot area.

Axial gap motor

Torque (Nm)

Efficiency (%)

Efficiency (%)

Torque (Nm)Radial gap motor

Rotation speed (r/min)Rotation speed (r/min)Fig. 5. Efficiency map of radial air gap PM motor.

Fig. 6. Efficiency map of axial air gap PM motor.

Radial gap motor
Torque (Nm)Iron loss (W)Iron loss (W)Torque (Nm)Axial gap motorRotation speed (r/min)Rotation speed (r/min)Fig. 7. Iron loss map of radial air gap PM motor.

Fig. 8. Iron loss map of axial air gap PM motor.

Radial gap motor

Axial gap motor

Rotation speed (r/min)

Torque (Nm)50W

Fig. 9. Copper loss map of radial air gap PM motor.

Copper loss (W)100W

Copper loss (W)Torque (Nm)

100W

50W

Rotation speed (r/min)Fig. 10. Copper loss map of axial air gap PM motor.

Torque (Nm)

3.6. Torque Waveforms
Figure 11 shows the torque waveforms of the two motors in the steady state. Both of the motors generate
the same average torque, but the torque ripples of the axial type and the radial type are 19.5 % and14.5 %, respectively. As illustrated in Fig. 1, the double stators of the axial type are skewed, so thetorque ripple can effectively suppressed, compared with the radial type. It is considered that the torqueripples are caused by the fifth and the seventh components of the space harmonics, which is mainly
generated by the concentrated winding configuration.
43.5
32.5
2
1.510.50Radial gap motor
Axial gap motor

060120180240Electrical angle (deg)Fig. 11. Torque waveforms.

300360Figure 12 shows an enlarged one period of the cogging torque waveforms. The peak to peak values of
the cogging torque waveforms are 0.42 Nm for the radial type and 0.45 Nm for the axial type,respectively. Compared with the rated average torque, the cogging torque is small enough for manykinds of applications. The torque ripple is varied with the average torque value as shown in Fig. 13.
The torque ripple ratio is almost inverse proportional to the average torque for both motors. It isconfirmed that the axial type is slightly superior to the radial type in the higher torque region.
60
Radial gap motor
Axial gap motor

Torque (Nm)

0.2
0.1
0.0
-0.1

060

120

180

240

300

-0.2
-0.3

Ripple angle (deg)Fig. 12. Cogging torque waveforms.

360

Torque ripple rate (%)

0.3

Radial Gap Motor
Axial gap motor

50403020
10001

2Torque (Nm)34Fig. 13. Torque ripple ratio and average torque characteristics.

4. CONCLUSION
The paper has described a case study of a motor design, where the two types of the motors have been
designed, i.e., a radial air gap PM motor and an axial air gap PM motor. Under the same conditions,e.g., the same amount of the PM, the same space factor of the windings, the similar iron-loss property
material, and the same winding resistance, it has been found that the axial air gap PM motor can reducethe motor volume (including the coil ends) by 30 % and the motor weight by 30 %, respectively. From
another point of view, the axial length of the axial type can be reduced to almost half of the radial typealthough the diameter is increased by 20 %. In general, the axial air gap PM motor is very suitable forthe pan-cake design of the motor because the center space of the double stators can be utilized for themechanism such as a bearing. In the investigation described in the paper, both motors have employed
the same amount of ferrite PMs, but the motor size can further be reduced by taking advantage of Nd
bonded PM instead of the ferrite. As a result, overall performance of the axial air gap PM motor has
been found to be superior to that of the radial air gap PM motor. However, it is indispensable to enhance
the mechanical strength of the axial type rotor because the atrractive force between the stator theeth andthe rotor PM poles is significantly strong regardless of the thin rotor stracture. In addition, reduction ofthe torque ripple is another problem to be solved, which is the future work of this study.
References[1] H.Kamiyama, T.Noguchi, A.Hattori, Y.Yamada, S.Yokoyama:“Development of High-TorqueDensity Pancake Motor for Automotive Applications”,IEEJ Annual National Conference(2017)[2] K.Takishima, K.Sakai:”Starting Characteristics of Axial and Radial Type Ultra-Lightweight
Motors Based on Magnetic Resonance Coupling”,IEEJ J. Industry Applications, vol. 8, no. 3,
pp. 471-479 (2019)[3] S. Morimoto, Y. Asano, T. Kosaka, and Y. Enomoto, “Recent Technical Trands in PMSM”, in
Proc. IPEC-2014, pp.1997-2003 (2014)[4] J.A. Tapia, F. Leonardi, and T.A. Lipo: “Consequent-pole Permanent- Magnet Machine with
Extended Field-Weakening Capability”, IEEE Trans. on IA., Vol. 39, No. 39, pp. 1704-1709(2003)[5] M.Aoyama, T.Noguchi,:”Preliminary Experimental Verification of Self-Excited Wound-Field
Synchronous Reactive Power for Robust Maximum Torque Per Ampere Control”,ICIT2015(2015)