LOSSES OF HIGH-SPEED INDUCTION MOTORS
LOSSES OF HIGH-SPEED INDUCTION MOTORS
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.