［ New Technologies ］

Evaluation of a 250kW20,000min－1 High-Speed PM MotorKeywords

Takashi Okitsu,
Sho Uchiyama,
Keisuke Matsuo,
Daiki Matsuhashi

PM motor, High-speed rotation, Loss separation

Abstract

For high-speed ﬂuid machinery like fans, blowers, and compressors, highspeed rotation is required to achieve a high energy conversion efﬁciency.
Aiming at applying to such high-speed application, we have developed aprototype of 250kW-20,000min－1 Permanent Magnet synchronous motor (PMmotor) using an active magnetic bearing system. By applying the high speed PM
motor to such applications, it will be realized of replacing the conventional system using speed increasing gear with direct drive system with eliminating thegear, so that making the system size compact, improving the system efﬁciency,
and reducing the time of maintenance can be merit.
As a result of performance evaluation of the prototype, we conﬁrmed thatthe motor efﬁciency at the rated point was 96.6% and the maximum efﬁciency inthe whole motor operation range was 97.5%, and that the temperature rise ofeach part was lower than the speciﬁc value by performing heat run test.
Further, we have performed loss separation on prototype for improving thedesign accuracy toward a development of higher efﬁciency, speed, and larger
capacity PM motor.

1 Preface
According to the government policy for theClimate Change, the demand for energy conservation
has increased and the applications which incorporates
Permanent Magnet synchronous motors (PM motors)
with features of compact size and high efﬁciencies
have been prominent in various industrial ﬁelds (1).
Aiming at applying to high-speed ﬂuid machinery such as turbo-blowers, compressors, and refrigerators, we have developed a prototype of 250kW20,000min−1 high-speed PM motor using an Active
Magnetic Bearing (AMB) (2). When this motor is applied
to high-speed ﬂuid machinery, a direct driving canbe realized for a system where speed-increasing
gears were formerly used and the following advantages can be obtained:( 1) Simple and compact system conﬁguration.
(Space saving)( 2 ) High system efﬁciency due to no gear loss
(Energy conservation)( 3 ) Oil-free (Easy maintenance)( 4 ) Reduction of vibration and noise due to noncontact rotation.

On the other hand, although making the motor
size compact and increasing output power density
will be realized by high-speed rotation, it will lead tohigh loss density of the motor. Consequently, they
will be an issue to reduce the motor loss and tochoose a proper cooling system. In this paper, wepresent the characteristics of the prototype highspeed PM motor and the result of the loss and thetemperature rise evaluation (3)(4).

2 Motor Speciﬁcations
Table 1 shows speciﬁcations and Fig. 1 showsa cross sectional conﬁguration of the prototype ofthe high-speed PM motor. The major features of themotor are as described below.
( 1) The magnetic bearings are applied for assuring the stabilized ﬂoating rotation control of the rotor
at a high speed, reduction of bearing loss, and realizing lubricant-free and grease-less system.
( 2 ) The Carbon Fiber Reinforced Plastics (CFRP)
is applied as the material for the rotor sleeve tosecure sufﬁcient strength against centrifugal force
and reduce the rotor eddy-current losses.
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Table 1

Specifications of Prototype of High-Speed PMMotor

Speciﬁcations of the prototype of the high-speed PM motor areshown.
ItemSpecifications

No. of poles2

Rated output

250kW

Rated rotational speed

20,000min−1

Frequency

333.3Hz

Rated torque119.4N・m

Cooling system

Forced air cooling

Bearings

Magnetic levitation type

Stator core
Permanent magnet rotor

Axial magnetic bearing

3.1 Loss Separation by Polynominal
Approximation
The input power for driving the motor in no-loadstate is equal to the no-load loss. Since the rotorshaft of the motor is levitated by the magnetic bearings, there will not be the friction loss due to contact
between the rotor shaft and the bearings. The
no-load loss, therefore, is separated roughly into
windage loss, iron losses (eddy current loss andhysteresis loss), and copper loss.
The no-load loss, except included copper loss,
is dependent on the rotational frequency. When the
no-load loss is expressed in a cubic function of therotational frequency, the tertiary term corresponds
to windage loss (5), the secondary term corresponds
to eddy current loss, and the primary term corresponds to hysteresis loss. Accordingly, characteristics of the no-load losses (WNL-loss) for the rotationalfrequency ( f ) can be approximated into Expression( 1) below. In this case, coefﬁcients of the respective
terms are assumed to be K W, KE, and KH.
WNL-loss ＝ K W・f 3 ＋ KE・f 2 ＋ KH・f

Radial magnetic bearing

Fig. 1

Cross-Sectional Configuration of Prototype ofHigh-Speed PM Motor

A cross-sectional conﬁguration of the prototype of the highspeed motor is shown.

( 3 ) By adopting a forced air cooling system using
a cooling blower, it doesnʼt need to prepare a wateror oil cooling circulation system.

3 Separation of No-Load Loss
We investigated approaches to separate themotor loss generated under the no-load conditions
into each loss factor. For this time, we adopted two
methods for loss separation. One was the method
where the measured motor input power in no-loadstate was approximated by polynominal expression(“Separation by Polynominal Approximation”). The
other was the method where the mechanical loss
was directly measured with prepared the non-magnetized rotor (“Separation by Mechanical Loss
Measurement”).

22MEIDEN REVIEW Series No.172 2018 No.1

( 1)

During the testing, no-load loss was measured
at the intervals of 5000min−1. Based on the actual
measured values, coefﬁcients of the respective
terms can be derived from Expression ( 1), and theneach loss at each rotational frequency was determined by the derive coefﬁcients.

3.2 Loss Separation by Mechanical Loss
Measurement
In this method, we ﬁrstly measured the mechanical loss by measuring the input power for rotating
the non-magnetized rotor. Here, the mechanical loss
includes the windage loss generated in the air gap
and the iron loss generated in the magnetic bearings. And then the iron loss generated in the stator
core of the motor in no-load state was determinedby subtracting the value of measured mechanicalloss from the one of total no-load loss at each rotational speed. In addition, the iron loss of the stator
core can be separated into the eddy current loss andthe hysteresis loss by polynominal approximation.

3.3 Result of No-Load Test
Fig. 2 shows the result of the separation of noload loss assuming that total no-load loss at
20,000min−1 is 100%. Based on the separation bypolynominal approximation, we conﬁrmed that therate of the windage loss and the eddy current loss

No-load losses (%)

120

Driving side100
80

f 3 component(windageloss)

f component(eddy current loss)
2

60
40

0－1in

m
5k－1in

km
10－1

in

km
15－1inm
0k

2Separation by polynominal
approximation

Fig. 2MechanicallossesStator core
Eddy current
loss

f component(hysteresisloss)

20Regeneration side

Stator core
Hysteresisloss－1inm0k

Copper loss

2Fig. 3

Separation by mechanicalloss measurement

State of Coupling for Back to Back Test

The state of coupling is shown, where two prototype models (thesame type) are coupled.

Result of Separation of No-Load Loss

According to the result of loss separation by polynominal approximation, windage and eddy current losses account for morethan 80% of total losses at 20,000min−1. The result of loss separation by mechanical loss measurement indicates that about
68% of total losses are occupied by mechanical losses (windageloss and eddy current loss around magnetic bearings).

4 Back to Back Test
4.1 Method of Back to Back Test
Fig. 3 shows the state of coupling the twosame speciﬁcation prototypes for the back to backtest. In order to calculate the shaft output and themotor efﬁciency, terminal voltages and currents ofthe two motors and also the both power of drivingside (Win) and regenerative side (Wout) were measured respectively by two power analyzers. The relation of Win and Wout with the shaft output (Wtrg) canbe expressed by Expressions ( 2 ) and ( 3 ) below.
Win ＝ Wtrq ＋ ( loss of the drive motor )(2)

Wout ＝ Wtrq − ( loss of the regeneration motor )(3)
Since the terminal voltage and the current of
the motor on driving side are almost the same withthose on regenerative side at an output of Wtrq, each
motor loss generated on both side also can beassumed to be almost the same. On this assump-96.5

Efficiency (%)
10097.09580Load factor (%)

became larger as the rotational speed was increased and that the rate of those losses accounted
for more than 80% of the total no-load loss at
20,000min−1. Moreover, according to the separation
by mechanical loss measurement, we conﬁrmedthat the mechanical loss accounted for about 68%
of total no-load loss. Consequently, it is main factor
to reduce the mechanical loss for improving theefﬁciency of the high-speed PM motor.

100

90

97.5

96.5

97.0

60

85

96.0

80

40
94.0
2075

90.0

92.0
85.0

70

80.06505000Fig. 4

10,00015,000
Rotational speed (min－1)

20,000Efficiency Map of Prototype Motor

The result of motor efﬁciency measurement performed during
the load test is shown. This measurement was carried out foreach set of rotational speed and load factor.

tion, Value Wtrg can be derived from Expressions ( 2 )and ( 3 ) and expressed into Expression ( 4 ) below.
Wtrq ＝ (Win ＋ Wout)/2(4)4.2 Load Characteristics
Based on the method as mentioned above, wemeasured the electrical characteristics and calculated the motor efﬁciencies while changing the rotational speeds and load factors up to the rated pointof 250kW and 20,000min−1. Fig. 4 shows the efﬁciency map of the prototype motor. Motor efﬁciency
at the rated point was found to be 96.6% and themaximum efﬁciency in the whole motor operation
range was 97.5%. As the result of the measurement,
it is conﬁrmed that the motor efﬁciency tends to be
low in the high rotational speed region and it is considered that the cause of that is an inﬂuence of thelosses that increase in proportion to the square or
the cube of the rotational frequency as described in
MEIDEN REVIEW Series No.172 2018 No.123Exhaust temperature①

Coils in slot
③ ④ ⑤ ⑥

Exhaust temperature⑧②
Coil end

Coupling side⑦
Coil end

Center shaft of rotation

Oppositecoupling side(a) Temperature measuring positions(Cross section in axial direction)
80
Temperature rise (K)7060Analytical value

5040

Measured value

30

5 Postscript20
100①②③④⑤⑥⑦⑧(b) Temperature rise values

Fig. 5

performance was sufﬁcient.
According to the comparison between analytical and measured values in Fig. 5, both gradients oftemperature rise are almost equal, even though it
seems that measured values tend to be slightly
higher than the analytical values. As one possible
cause, there was a difference of the loss distribution
between the losses set in the analysis model andgenerated in actual state. In the high-speed rotation
region, in particular, the proportion of windage loss
and eddy current loss becomes high. It is, therefore,
important to accurately estimate such losses interms of estimating the temperature rise accurately.

Result of Temperature Rise Measurement inEach Part during Rated Operation

The measurement result of temperature rise is shown. Temperature rise values were obtained from the respective parts of themotor body. (a) shows the speciﬁed temperature measuring
points and (b) shows actual measured values and analytical values of temperature rise. These data suggest that sufﬁcient cooling performance is secured.

Fig. 2. Therefore, we assume that it is the future task
to investigate factors of increase in those losses
and techniques for reducing the mechanical loss
and the iron loss for improving the motor efﬁciency
at especially high-speed region.

We have developed a prototype of 250kW20,000min−1 high-speed PM motor. As the result ofan evaluation, the motor efﬁciency at the rated point
was found to be 96.6% and the maximum efﬁciency
in the whole motor operation range was 97.5%.
Further, we performed the loss separation and clariﬁed the proportion of each loss in order to improve
design accuracy.
In the future, we will work on further improving
the accuracy of the loss separation for improving
the motor efﬁciency and the accuracy of calculating
the motor temperature with the thermal ﬂuid analysis. Additionally, we will progress the developmentof higher speeds and larger capacities motors.
・All product and company names mentioned in this paper are
the trademarks and/or service marks of their respective owners.
《References》

4.3 Heat Run Test
We carried out the heat run test at the rated
point (250kW, 20,000min−1) for measuring steady
temperature rise of each part in the motor. Fig. 5 (a)shows cross section view in axial direction for indicating the points of measuring temperature andFig. 5 (b) shows the both measured and analyticalvalues of the temperature rise. In Fig. 5 (b), theanalytical values were calculated by using a general Computational Fluid Dynamics (CFD) softwareand the measured values were obtained throughthermocouples mounted at the respective measuring points.
As the result of the heat run test, we conﬁrmedthat temperature rises of each point in armature coilwere lower than the speciﬁed level and the cooling

24MEIDEN REVIEW Series No.172 2018 No.1

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