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LOSSES OF HIGH-SPEED INDUCTION MOTORS

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621.313.13(004.162)<br />

Tapani JOKINEN<br />

<strong>LOSSES</strong> <strong>OF</strong> <strong>HIGH</strong>-<strong>SPEED</strong> <strong>INDUCTION</strong><br />

<strong>MOTORS</strong><br />

ABSTRACT The paper deals with the losses, loss distribution,<br />

reducing the losses, and the construction of high-speed induction<br />

motors. The loss distribution in high-speed motors and conventional<br />

standard motors differs from each other. Friction losses are the most<br />

important loss component in high-speed motors. Losses in the<br />

windings reduce with increasing rotor speed. Iron losses remain<br />

almost constant. The motor construction has to be such that the air in<br />

the air gap changes effectively.<br />

1. INTRODUCTION<br />

Compressors, pumps and machine tools can often achieve better<br />

performance if their rotational speed is high. E.g., the energy efficiency of a<br />

pump or a compressor improves at high speeds. The rate of material removal in<br />

a spindle tool increases with the increasing speed. The size and weight of an<br />

electric motor is approximately inversely proportional to the speed. This means<br />

material saving with increasing speed of the motor.<br />

Tapani JOKINEN<br />

Helsinki University of Technology<br />

Laboratory of Electromechanics<br />

P.O.Box 3000, FI-02015 TKK, FINLAND<br />

e-mail:Tapani.Jokinen@tkk.fi<br />

PROCEEDINGS <strong>OF</strong> ELECTROTECHNICAL INSTITUTE, Issue 223, 2005


72<br />

T. Jokinen<br />

A smaller size means that the cooling surface of the machine decreases<br />

that rise the questions: How high are the losses and how difficult it is to cool the<br />

motor?<br />

This paper deals with the losses, loss distribution, reducing the losses,<br />

and the construction of high-speed induction motors.<br />

2. <strong>LOSSES</strong><br />

2.1. Losses versus speed<br />

The efficiency of a motor depends on the size of the motor. The<br />

efficiency of small motors is low, round 50 %, as the efficiency of large motors<br />

can be 97 %. On the other hand, the efficiency seems to independent of the<br />

speed. Figure 1 shows as an example the efficiency of 55 kW standard low<br />

voltage 50 Hz induction motors with speed [1]. The number of pole pairs<br />

changes according to the speed.<br />

100<br />

Efficiency / %<br />

95<br />

90<br />

85<br />

80<br />

0 500 1000 1500 2000 2500 3000 3500<br />

Speed / rpm<br />

Fig. 1. The efficiency of 55 kW standard induction motors<br />

We can see from the Fig. 1 that the efficiency is almost constant,<br />

independent of the speed. It continues to be constant also at high speeds. The<br />

efficiency of a 55 kW 60000-rpm induction motor was 94,2 % according to the<br />

measurements reported in [2]. The efficiency is the same as the efficiency of the<br />

motors in the speed range of 500...3000 rpm (Fig. 1). Same efficiency means<br />

that the total losses of high-speed and low-speed motors are the same. As the


Losses of high-speed induction motors 73<br />

size of a high-speed motor is smaller than the size of a low-speed motor, the<br />

loss density in a high-speed motor is greater than in a low-speed motor. This<br />

leads to the problem of cooling of high-speed motors.<br />

2.1. Loss distribution in electrical motor<br />

The losses of an electrical motor consist of three parts:<br />

P = P + P + P<br />

(1)<br />

H<br />

Cu<br />

Fe<br />

rho<br />

where: P Cu is stator and rotor I 2 R-losses, P Fe iron loss, and P rho friction loss.<br />

P Cu is proportional to the volume of the winding copper if the current<br />

density is constant. It is independent of the speed.<br />

P Fe consists of two parts, namely of the hysteresis loss (P h ) and eddy<br />

current loss (P e ). The hysteresis loss is proportional to the frequency, in other<br />

words to the speed (n) and the eddy current loss is proportional to the square<br />

of the frequency (speed):<br />

Ph = ph<br />

n<br />

(2)<br />

2<br />

Pe = pe<br />

n<br />

(3)<br />

where: p h and p e are the proportionality factors, which are proportional to the<br />

volume of the motor assuming that the flux density in iron do not change as the<br />

speed is changing.<br />

The friction loss P rho is proportional to the cube of the speed:<br />

P<br />

rho =<br />

p<br />

rho<br />

3<br />

n<br />

(4)<br />

where: p rho is the proportionality factor, which is proportional to the volume<br />

of the motor.<br />

The output power of an electric motor is proportional to the rotor volume<br />

and speed:<br />

P = CV n cn<br />

(5)<br />

out rt =<br />

where: C is the utilisation factor, V rt the rotor volume, and constant c = CV rt .


74<br />

T. Jokinen<br />

The efficiency of the motor depends on the quotient<br />

P<br />

P<br />

H<br />

out<br />

2<br />

3<br />

( P + p n + p n + p n )<br />

Cu h e rho PCu<br />

ph<br />

pen<br />

prhon<br />

= = + + +<br />

(6)<br />

( cn) ( cn) c c c<br />

2<br />

As P Cu , p h , p e , p rho , and c are proportional to the volume of the motor we<br />

can make the following conclusions. In high-speed machines<br />

• the portion of I 2 R-losses of the total machine losses decreases with the<br />

increasing speed,<br />

• the portion of the hysteresis loss remains constant,<br />

• the portion of the eddy current loss increases proportional to the speed,<br />

and<br />

• the portion of the friction loss increases with the square of the speed.<br />

An example of the difference in loss distribution between a conventional<br />

37 kW 1500-rpm induction motor and a high-speed 37 kW 50000-rpm induction<br />

motor is presented in Fig. 2 [3]. The electrical losses have been computed by<br />

finite element method taking into account that frequency converters are<br />

supplying the motors. The friction, cooling and bearing losses have been<br />

obtained by measurements. The 1500-rpm motor is a standard, totally<br />

enclosed, laminated cage induction motor with ball bearings. The 50000-rpm<br />

motor has a solid, copper-coated rotor, active magnetic bearings and a forced<br />

open circuit air-cooling.<br />

We can see from Fig. 2 that mechanical losses (friction, cooling and<br />

bearing losses) are the dominant losses in the high-speed motor and very much<br />

higher than in the 1500-rpm motor. The core losses in the rotor have increased<br />

a little with the speed. They have been minimized by using in the high-speed<br />

motor an increased radial air-gap. The core losses in the stator should increase<br />

according to Eqs (2) and (3). In reality, they have decreased by using highfrequency<br />

electrical steel sheets and low flux density in iron. There are less<br />

resistive stator losses in the high-speed motor because of lower winding volume<br />

and resistance.<br />

We can conclude that in high-speed motors the stator current density can<br />

be higher than in low-speed motors. To reduce iron losses, a low loss, thin<br />

electrical sheet has to be used and the flux density has to be lower than in lowspeed<br />

motors. A special attention has to be given to the friction loss. All rotating<br />

parts must have a very smooth surface.<br />

From the point of cooling, the above mentioned phenomenon is very<br />

favourable because the dominant losses are friction losses and they can easily


Losses of high-speed induction motors 75<br />

extracted from the machine without rising the winding temperature. This means<br />

that although the size of a high-speed motor is small and the cooling surfaces<br />

are also small, it is possible to find a good cooling solution for the motor.<br />

Power [kW]<br />

4,0<br />

3,5<br />

3,0<br />

2,5<br />

2,0<br />

1,5<br />

1,0<br />

0,5<br />

0,0<br />

1500 rpm 50000 rpm<br />

Friction, cooling and<br />

bearing losses<br />

Core losses in the rotor<br />

Resistive rotor losses<br />

Core losses in the stator<br />

Resistive stator losses<br />

Fig. 2. Power losses of a standard 37 kW and high-speed induction motor [3]<br />

3. COOLING AND REDUCING <strong>LOSSES</strong><br />

3.1. Cooling solution used in the motor developed<br />

at Helsinki University of Technology<br />

The Laboratory of Electromechanics of Helsinki University of Technology<br />

has developed high-speed drives in co-operation with Lappeenranta University<br />

of Technology. High Speed Tech Ltd. and Sundyne Corporation utilize the<br />

research results today. The research has concentrated mainly on induction<br />

motors.<br />

A simplified drawing of the high-speed motor developed by the research<br />

group is presented in Fig. 3. The solid steel rotor is coated with a copper tube.<br />

The end-ring region has a thicker coating than the core region. The coating<br />

method and the solid core permit the use of high rotor speeds, up to 550 m/s.<br />

The motor is air-cooled. The cooling gas is blown through the cooling duct in the<br />

stator stack and then through the air-gap. The coolant blowing out from the airgap<br />

cools the end-winding spaces partly and partly the spaces are cooled


76<br />

T. Jokinen<br />

directly. The bearings are magnetic bearings. The cooling system takes<br />

effectively the friction losses out of the motor.<br />

Cooling air<br />

Stator core<br />

Solid steel rotor<br />

Copper coating End ring<br />

Fig. 3. The construction of the high-speed induction motor developed at Helsinki<br />

University of Technology<br />

3.2. Reducing losses<br />

From the finite-element study we learned that the methods used for<br />

reducing stator-slot harmonics in conventional electrical machines also work<br />

well with the high-speed solid-rotor machines. The air gap has to be relatively<br />

large [4], the number of stator slots should be as large as the constructional<br />

factors allow, and the use of magnetic slot wedges is worth of considering.<br />

A proper chording of the stator winding reduces the lower space harmonics in<br />

the air-gap flux.<br />

The finite-element simulations also showed that a solid rotor coated with<br />

a thin layer of well-conducting material is more resistant to the effects of higher<br />

harmonics than a plain solid steel rotor. Tests, done on a 100000 rpm, 65 kW<br />

motor [5], verified that the time stepping, finite-element analysis is a valuable<br />

tool in the design of high-speed machines.<br />

High-speed motors are supplied in practice by a frequency converter.<br />

The output voltage waveform of the converter influences the motor losses. Also<br />

the converter losses depend on the converter type.<br />

In the study reported in [2] and [6], a 60 kW 60000-rpm motor was tested<br />

with four different converter types. One was a pole amplitude modulated (PAM)


Losses of high-speed induction motors 77<br />

converter and others pulse width modulated converters with 3, 9, and 15 pulses<br />

per half a cycle. The measured motor losses, converter losses, and their sum<br />

(drive losses) are presented in Figs 4...6. The lowest losses gave the pole<br />

amplitude modulation. The motor losses increase and the converter losses<br />

decrease with increasing pulse number in pulse width modulation, so that the<br />

drive losses depend only very little on the pulse number.<br />

Fig. 4. Measured power losses of a 60 kW 60000-rpm high-speed motor with different<br />

converter types<br />

Fig. 5. Measured power losses of the frequency converters


78<br />

T. Jokinen<br />

Fig. 6. Measured total losses of the drives<br />

4. CONCLUSION<br />

The loss distribution of high-speed motors and conventional standard<br />

motors differs from each other. The portion of I 2 R-losses of the total motor<br />

losses decreases with the increasing speed. The portion of iron losses remains<br />

constant or decreases if a high quality electrical sheet is used. The portion of<br />

mechanical losses increases with the square of the speed and it is the largest<br />

loss component in the high-speed motor. All rotating parts must have a smooth<br />

surface. In solid rotor motors, special attention has also to be paid to reducing<br />

harmonics in the air-gap flux. This is important, because rotor losses depend<br />

strongly on air-gap harmonics.<br />

The converter type influences also the losses of the drive. The pole<br />

amplitude modulation creates the smallest losses both in the motor and<br />

converter. The motor losses decrease and the converter losses increase with<br />

increasing pulse number with the pulse width modulation.<br />

Because of high friction losses in the air gap, the motor construction has<br />

to be such that the air in the air gap changes effectively. This leads to a large air<br />

gap, which also decreases rotor losses as the harmonics in the air-gap flux<br />

decrease.<br />

PROCEEDINGS <strong>OF</strong> ELECTROTECHNICAL INSTITUTE, Issue 223, 2005


Losses of high-speed induction motors 79<br />

LITERATURE<br />

1. ABB Catalogue BU/M2BA FI 99-03, 1999.<br />

2. Lähteenmäki, J.: Design and voltage supply of high-speed induction machines. Acta<br />

Polytechnica Scandinavica, Electrical Engineering Series No. 108, Dissertation Helsinki<br />

University of Technology, Espoo 2002, 140 p.<br />

3. Saari, J.: Thermal modelling of high-speed induction machines. Acta Polytechnica<br />

Scandinavica, Electrical Engineering Series No. 82. Helsinki 1995. 82 p.<br />

4. Patent US 5473211. Asynchronous electric machine and rotor and stator for use in<br />

association therewith. High Speed Tech Oy Ltd, Tampere, Finland Appl. No 86880,<br />

7.7.1992, 9 p.<br />

5. Saari, J.: Thermal analysis of high-speed induction machines. Acta Polytechnica<br />

Scandinavica, Electrical Engineering Series No. 90. Dissertation Helsinki University of<br />

Technology, Helsinki 1998. 73 p.<br />

6. Jokinen, T., Lähteenmäki, J.: Voltage supply of high-speed induction motor. Proceedings<br />

of XL International Symposium on Electrical Machines SME’2004, 15-18 June 2004,<br />

Hajnowka, Poland, pp. 265-269.<br />

Manuscript submitted 01.07.2005<br />

STRATY W SILNIKACH<br />

O DUŻEJ PRĘDKOŚCI OBROTOWEJ<br />

Tapani JOKINEN<br />

STRESZCZENIE W artykule omówiono straty, rozdział<br />

strat, zagadnienie zmniejszenia strat oraz konstrukcje silników indukcyjnych<br />

o dużej prędkości obrotowej. W silnikach o dużej prędkości<br />

obrotowej i standardowych silnikach rozdział strat jest inny. W silnikach<br />

o dużej prędkości obrotowej straty tarcia mają największe<br />

znaczenie, straty w uzwojeniu zmniejszają się wraz ze zwiększaniem<br />

prędkości obrotowej, straty w rdzeniu są praktycznie stałe. Konstrukcja<br />

silnika powinna zapewniać skuteczny przepływ powietrza<br />

w szczelinie powietrznej.

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