Study on mechanical proportion and anti-demagnetization
characteristic of 15 kW, 150,000 r/min PM motor
Kohei Fujita1 and Toshihiko Noguchi2
1Student, IEEJ, Shizuoka, Japan, 2Professor, IEEE, Shizuoka, Japan
E-mail: 1fujita.kohei.14@shizuoka.ac.jp, 2noguchi.toshihiko@shizuoka.ac.jp
Abstract. This paper describes a relationship and compatibility between the mechanicalproportion and anti-demagnetization characteristic of an ultra-high-speed permanent magnet(PM) motor of which rated output power is 15 kW and rated speed is 150,000 r/min. The ultrahigh-speed motor must be designed to achieve outstanding electromagnetic characteristics undervarious server mechanical constraints. In the paper, three motor models having different
characteristics of 15 kW, 150,000 r/min are investigated from the viewpoints of electromagneticand mechanical compatibility. all the motor models designed here achieve high efficiency over
95 %, but it is found that anti-demagnetization characteristics are different among the three. In
addition, rotor shaft material is discussed for anti-demagnetization. As a result, stainless steel isfound to be a better material than carbon steel for anti-demagnetization.

1. Introduction
Recently, electrification of mechanically systems is widely and intensively spread to various industry
sectors and it is required that motor in the center of the trend has not only high-power but also highpower density and high efficiency. Permanent magnet synchronous motors (PMSMs) using rare earth
PM is intensively developed and spread to various industry sectors from the viewpoint of the above[1][2]. In order to enhance the motor output power, it is definitely necessary to deliver as highelectromagnetic torque as possible and to rotate at a higher speed as possible under some electrical andmechanical design constraints. But, in general, the former tends to cause an increase in motor volume.
So, the action of ultra-high-speed (ultra-high-frequency) of motor is valid for decreasing motor volume.
But, in the case of ultra-high-speed motor, many various problems such as electrical restriction (e.g.
voltage and current), mechanical restriction (e.g. axis vibration and centrifugal force) exist. In the paper,
three motor models have different characteristics of 15 kW, 150,000 r/min is investigated. As a result,
it is found that anti-demagnetization is very important on ultra-high-speed PM motor. So, rotor shaft
material is discussed for anti-demagnetization. As a result, stainless steel is better than carbon steel foranti-demagnetization.
2. Design specifications and design characteristics of 15 kW, 150,000 r/min PM motor
2.1. Design specifications and Simulation methods
Table 1 indicates design specifications of 15 kW, 150,000 r/min PM motor. Figure 1 indicatesmechanical constraints of 15 kW, 150,000 r/min PM motor. About design, specifications are shown inTable 1, compatible design with restricted DC bus voltage and rated current is theoretically impossible.

And so, while one of the two restrictions is kept, some models were designed in restricted conditions.
Then three motor models were picked up from them. The major specifications of those motor models
are shown in Table 2. In addition, in the ultra-high-speed motor, the rotor rotating speed comes up to
the sound of speed when the rotor diameter is 37.3 mm. So, motor models are selected in this restriction.
In order to investigate losses and demagnetizing characteristics, JMAG-Designer 17.0TM,
electromagnetic simulation software is used.
2.2. Fundamental design policy
Figure 2 indicates cross-section of 15 kW, 150,000 r/min PM motor. All motors are adopted surface
permanent magnet synchronous motor (SPMSM). The reason is SPMSM doesn’t need field magnetizing
current like induction motor and reluctance motor. And it is difficult that interior permanent magnet
synchronous motor (IPMSM) is adopted from restrictions of the rotor diameter and shaft diameter, soSPMSM which has simple rotor structure is adopted. Rotor pole pair is two poles so that decrease
operating frequency. The permanent magnet which has BHmax = 350 kJ/m3 is used. This enables wide air
gap structure and decreases permeance drifting and synchronous reluctance. And so, the more
permeance coefficient is small, losses are lower in ultra-high-speed motor. So, permeance coefficient is
integrated 1.1. In regard to stator, concentrated winding is adopted so that leakage flux is decreased andmagnet eddy-current that causes heat demagnetization is decreased. Solid copper wires are used statorwinding, stator brim is equipped 3 mm on the tip of the Stator teeth.
Table 1. Design specifications of 15 kW, 150,000 r/min PM motor
Rated output power
Rated speed
Rated torqueRated efficiency
DC bus voltage
Rated current
Maximum current
Rotor diameter
Demagnetization rate

15 kW
150000 r/min
0.955 Nm
Over 92 %Less than 259.2 V
75A (Overload 200%)
150 A
Less than 37.3 mm
Within 1 %Figure 1. Mechanical constrains of 15 kW, Figure 2. Cross-section of 15 kW, 150,000 r/min
150,000 r/min 150,000 PM motor.
PM motor
3. Electromagnetic simulation conditions and comparison of simulation results
The electrical characteristics of studied three motor models are compared. When a motor is evaluated,losses and demagnetizing characteristics are important. In regard to the former, it is important not only
total power losses are low but also motor efficiency is maximum at the rated operating point. Regarding
the latter, it is desired that the demagnetizing rate is low.
Table 2. Major specifications of ultra-high-speed motor models.

#1

#2

#3358.8 V

258.4 V

243.4 V

Rated current

75 A

107.2 A

107.2 A

Rotor diameter

35.1 mm34.2 mm37.2 mmStack length25.2 mm23.0 mm30.3 mm

Windings

372921

Shaft diameter

21 mm

21 mm

21 mm

DC voltage

From those backgrounds, in the paper, simulation results are compared from the viewpoint of losses,
operating point in maximum efficiency and demagnetizing characteristics. It is assumed thattemperatures of magnet and stator winding are 70 °C about losses. On the other hand, the maximum
efficiency can be obtained when the equation below is satisfied :Wi + Wmag = Wc

(1)

Where :
Wi: Stator iron core loss
Wmag : Magnet eddy-current loss, andWc:
Stator winding copper loss.
From this equation, It is found that the maximum efficiency can be obtained if the iron losses and thecopper loss is almost the same. The maximum efficiency operating point is evaluated from the viewpointof this condition. About demagnetizing conditions, it is assumed when magnet temperature is 200 °C
and maximum current flows negative direction of d-axis, and the magnet before demagnetization iscompared with the one after demagnetization. Figure 3 shows comparison of total power losses. Figure
4 shows comparison of iron loss and copper loss rate. Figure 5 shows a comparison of the demagnetizing
characteristic. In regard to losses, #1 has the best characteristic. The reasons are # 1 needs smallest acurrent of three motor models and an area of rotor side face is smaller than #3. #1 has smallest total loss
for those reasons. #2 is the model that operates the point of nearest to the maximum efficiency operating
point. It is because #2 is the most copper machine of three motor models. Regarding, #3 has the best
demagnetizing characteristic. It is because the magnet of #3 is thickest and stator windings are smallestof three motor models. However, as shown in Figure 5, because the smallest demagnetizing rate is
17.2 %, it is found that it is important demagnetizing characteristics is improved.
4. Study on anti-demagnetization
4.1. Study on anti-demagnetization
Above described above, three motor models of 15 kW, 150,00 r/min are studied. As a result, it is foundthat anti-demagnetization characteristic is the most important problem. Although #3 has the best
demagnetizing characteristic, the demagnetizing rate is 17.2 %. The value doesn’t possibly come close
to under 1 % described in design specification. The method to improve a demagnetizing characteristic
are indicated below.
1. Increasing permeance coefficient
2. Decreasing negative magnetic field to a magnet.
3. Changing stator slot combination
It is difficult that 1 is adapted to the motor because rotor diameter and rotor shaft diameter has restriction
and magnet eddy-current is expected. In regard to 3, It is expected the number of slots is decreased,magnet eddy-current is increased, if not because of wide air gap, the flux that flows between stator toothincreases. In the paper, from the above, 2 is adopted.

Figure 3. Comparison of total power losses.

Figure 4. Comparison of iron and copper loss
rate

Figure 5. Comparison of demagnetization characteristics.
4.2. Investigating a material of the shaft.
Figure 6 indicates a route of Armature flux by Shaft permeability. Permeability of Rotor shaft is deeply
concerned with an amount of negative magnetic field to a magnet. As shown in Figure 6, because ofchanging permeability of rotor shaft cause changing magnetic resistance of rotor. As a result, the amountof magnetic field to magnet is changed. Thus, because of changing permeability of rotor shaft, magnetic
circuit route of rotor is changed. The above influences a demagnetizing characteristic.
In the paper, demagnetization characteristics of #A that is used S45C and #B that is used SUS304 onrotor shaft are compared within the mechanical restriction. Table 3 indicates Major specifications andsimulation conditions of motor models. The former is about 880 times as much as the latter. On the otherhand, because SUS304 has lower rigidity than S45C, mechanical restriction gets too strict in the case ofSUS304.
From the above, under the mechanical constraints of 15 kW, 150,000 r/min PM motor in the case
SUS304 is used on Shaft. shown in Figure 7, # A and # B are designed. The next topic is the simulation
conditions. In both # A and # B, output power is 15 kW and Rotor dimension is same. In regard to stator,

stator teeth width and coil space factor are not changed, stator diameter and number of windings areonly changed. Figure 8 indicates a comparison of the demagnetization rate. Figure 9Table 3. Major specifications and simulation conditions of motor models

Motor model
Shaft material
Relative permeability
Young’s modulus
Rated output power
Phase current amplitude
Rotor outer diameter
Rotor length
Magnet thickness
Airgap length
Permeance coefficient
Stator diameter
Stator stack length
Number of Winding turns
Winding space factor#As45c1846.2202000 MPa
15 kW
150 A
37.2 mm25.5 mm
8.1 mm10.7 mm1.1
112.3 mm25.5 mm2554.6 %#BSUS304
2.1
192000 Mpa
15 kW
150 A
37.2 mm25.5 mm
8.1 mm10.7 mm
1.1
113.8 mm25.5 mm2854.6 %Figure 6. Route of Armature flux by Shaft permeability.

Figure 7. Mechanical constraints of 15 kW, Figure 8. Comparison of demagnetization rate
150,000 r/min PM motor in the case SUS304 isused on Shaft.

Figure 9. Rotor flux density distribution of the motor model.
indicates rotor flux density distribution of the motor model. As shown in Figure 8, demagnetization rateof # A and # B are 19.3 % and 11.8 %. It is found that the demagnetization rate when SUS304 is usedon rotor shaft is lower than that when S45C is used. Thus, as shown in Figure 9, because it is found that
# A has more magnetic field near Shaft than # B, magnetic field of #A is gathered intensively. Thatcauses the deterioration of demagnetizing characteristics. From the above, SUS304 is adopted on therotor shaft.
5. Conclusion
In the paper, proportions of 15 kW, 150,000 r/min PM motor, three motor models have different
characteristics are investigated from the viewpoint of loss characteristics, the operating point of
maximum efficiency operating point, demagnetizing characteristics. From the simulation result, three
motor models have relationships of trade-off, and it is difficult that all design specifications are achieved.
Thus, it is the most important investigated problem in ultra-high-speed motor. Therefore, rotor shaft
material is discussed for anti-demagnetization. As a result, stainless steel is better than carbon steel for
anti- demagnetization. However, the demagnetization rate is about 11.8 %. For future work, it is required
to reduce the demagnetization rate and a real machine is created.
6. References[1] S. Morimoto, Y. Asano, T. Kosaka, and Y. Enomoto, (2014), “Recent Technical Trends inPMSM”, in Proc. IPEC-2014, pp.1997-2003.
[2] Kenta Yamano, Shigeo Morimoto, Masayuki Sanada, and Yukinori Inoue", 2017, Design ofSurface Permanent Magnet Synchronous Motor Using Design Assist System for PMSM, "IEEJ J.
Industry Applications, vol. 6, no. 6, pp. 409-415.