International Journal of Mechanical And Production Engineering, ISSN: 2320-2092,
Volume- 3, Issue-4, April-2015
HEAT TRANSFER IN ELECTRICAL MACHINE: A CASE STUDY
1
CHINABABU ANIMA, 2RAVI KATUKAM
Team Lead, Cynergy Team, Cyient Limited, Hyderabad, india
Asst Project Manager, Cynergy Team, Cyient Limited, Hyderabad, India
Email: [email protected]
Abstract: Future design of Aircraft and Automotive needs innovation in propulsion mechanism which are based on
electrical mode of operation. More Electric mobility needs a compact high performance electric motor which will assure
long operating life with maximum efficiency. Electric motor design needs a multidisciplinary approach including efficient
mechanism for heat transfer. The present study reports a Combined Computational fluid dynamics and network approach
(CCFDN), applied to the problem of cooling a 1200kW traction alternator. In this combined approach, analysis is first
performed using equivalent, lumped thermal network with a simplified circuit aimed at delivering fast, design class results.
CFD calculations are next performed to estimate thermal resistances, which are used as input to the thermal networks.
Iterative procedure is adopted for solving the network. Results are presented in terms of temperatures at different locations of
the device for three cases: one without forced cooling (fan), and second with a fan. The maximum temperature values
obtained in the second case is found to be 2.1 times lower than the values obtained in the first case. Results are compared
with pure network approach using empirical correlations and with pure conjugate analysis using CFD. The present CCFDN
approach avoids using empirical relations, yet much faster than the full conjugate CFD analysis.
inside electric motors [6]. This paper presents a
thermal circuit model of an electric motor aiming to
simulate its thermal behaviour during the transient
duty towards steady-state operation at rated load. The
model is applied to a specific induction electric motor
and the predictions are compared to measurements.
The purpose of the model is to become an integral
part of the motor design process aiming to optimize
components and manufacturing processes. In view of
the following emerging trends in the industry thermal
network analogy will find its extensive application as
design decision support platform.
 More Electric Aircrafts & Automotives
 Heat in Electrical Motors
 Miniaturization
 Technology
 High performance
 Low power consumption
I. INTRODUCTION
The prediction of the temperature distribution inside
an operating electric motor is one of the most
important issues during its design. This prediction
allows the engineer to evaluate if the machine will
reach the thermal class for which it is being designed,
establishing the bearing lubrication intervals as well
as checking if the supplied air flow of the cooling
system is sufficient for ensuring normal motor
operation at rated conditions The high power
densities and the varying operating conditions make
the cooling of heavy duty rotating electrical machines
a challenging task. The loss distribution of a rotating
electric machine changes when its rotational speed is
varied. Accordingly, the cooling requirement also
changes with speed. The designs of the cooling fan
and coolant passages have to be carried out with care
by considering the wide range of rotational speeds.
The thermal models of the rotating electrical
machines ought, therefore, to include the losses and
convection heat-transfer coefficients which both vary
with the rotational speed of electrical machines and
that of the fan supply the coolant flow. The objective
of the present work is to develop a thermal model for
finding the temperature distribution of different
components of the rotating electrical machine.
Several works have been done for the electric motor
modelling. Most of these works are based on thermal
circuit modelling [7]. These works allow predicting
the overall motor temperature variation and
distribution can be used for determining the effects of
different designs, duties and cooling mechanisms of
the motor temperature during its operation. Detailed
analysis of fluid flow and heat transfer with finite
volumes and finite elements are also available [4].
Some works also couple the temperature field
determination with the electric and magnetic fields
II. PROBLEM IDENTIFICATION
The rotating electrical machines convert electrical
energy into mechanical energy vice versa using
magnetic
forces.
An
alternator
is
an
electromechanical device that converts mechanical
energy to alternating current electrical energy. The
main mechanisms of heat generation in alternator
/induction electric motors are generally divided in
four groups, related mainly to the places where they
occur. These are Joule losses, iron losses, stray load
losses and mechanical losses. Each one of these kinds
of energy conversion from electric to thermal energy
is detailed below.
Joule Losses: This mechanism corresponds to the
conversion of electric energy into thermal energy in
electrical conducting media. This type of losses is
directly related to the electric resistance of the
conductor and changes proportionally to the square of
Heat Transfer In Electrical Machine: A Case Study
72
International Journal of Mechanical And Production Engineering, ISSN: 2320-2092,
the current, i.e., Pj = R*I². Energy conversion by
Joule effect in squirrel cage induction electric motors
occurs in the stator (copper windings) and in the
squirrel cage (aluminium bars). Construction details
of generic electrical machine is shown in fig.1.
Volume- 3, Issue-4, April-2015
Iron Losses: These losses are due to the conversion of
electric energy into thermal energy in the iron. They
are divided in hysteresis and Foucault (eddy currents)
losses. The eddy-current losses are Joule losses that
occur in the iron due to the flow of an induced
electric current. The hysteresis losses are due to the
energy expended to align the iron magnetic poles to
the applied magnetic field
III. METHODOLOGY
The thermal modelling of the machine is investigated
using a Combined CFD and Thermal network
approach. This CCFD approach enables us to
evaluate the thermal behaviour of the machine in
normal operating mode.
Physical model of rotating machine considered for
the thermal analysis is shown in Fig.3.Thermal
analysis is carried out to find temperature distribution
of different components of rotating machine using
thermal network approach. The lumped resistance
network was developed based heat flow paths for the
machine. The thermal resistance network for
alternator is shown Fig 4 Thermal conduction and
convective resistances were calculated from the
literature.
Temperature
distributions
were
investigated using computer program MAT Lab.
Thermal analysis is also carried out for the machine
using commercial CFD package FLUENT.
Segregated, steady, implicit solution method RNG k model used for the analysis. Extracts the heat
transfer coefficient from CFD analysis, incorporated
in CCFDN approach, the temperature distribution in
alternator is calculated.
Fig .1 Main parts of typical alternator and motor
Electrical machine efficiency significantly depends
on various losses that need to be minimized by design
and selection of right materials. Among the other
losses heat loss forms a major part which can be
minimized by computational fluid dynamics approach
while designing.
Component wise losses are
indicated in fig 2.
Fig .3 Physical model of rotating machine
R8,AMB
AM B
R8,9
R8,10
R1,14
R1 ,16
R2,1
1
R4,8
R3,8
R1,8
8
R9,AM B
9
R14,9
14
R3,14
3
R2,3
2
R2,4
4
R 4,16
16
R10,16
10
R 10,AM B
AM B
R2,15
AM B
5
R5,16
R6,5
R5,14
R10,12
R13,16
15
R1 5,16
R5,15
R15,14
R6,11
11
6
R6,13
13
R7,11
R7,13
7
HEAT GENERATION
Fig .4 Thermal resistance network
Fig .2 Power losses in alternator and motor
Heat Transfer In Electrical Machine: A Case Study
73
R13,12
12
R7,12
R11,AMB
R6,7
AM B
R 12,AM B
AM B
International Journal of Mechanical And Production Engineering, ISSN: 2320-2092,
Volume- 3, Issue-4, April-2015
combined CFD and network approach is one of the
new techniques to find the temperature distribution
alternator. Fig. 7 to Fig. 8 shows the variation of
temperatures of stator and end winding. Using the
CFD approach, network approach, and a combined
CFD network approach for different loading
conditions at the 0.58kg/s mass flow rate. From the
figures the combined network approach is more
efficient than an analytical approach. The cost wise
the combined CFD and network approach is less
compare to CFD approach, so we can say that this
combined CFD network approach is an optimized
solution for cost and accuracy.
IV. RESULTS AND DISCUSSION
The present work main objective of to
determine temperature distribution of all the
components of present model for varies loading
conditions. It is evident from the Fig. 5 that high
temperature occur at the stator core and end winding
due to iron losses are completely concentrated at
stator core. The high temperature at stator has to be
cooled by some external agency i.e. like fan etc. this
fan is simulated in present work using the pressure
jump boundary condition. The maximum temperature
is around 407.6 K when there is a pressure jump of
3800 Pa, the minimum pressure jump is given based
on the calculation of pressure drop using CFD.
Results are presented in terms of temperatures at
different locations of the device for three cases: one
without forced cooling (fan), and second with a fan.
The maximum temperature values obtained in the
second case is found to be 2.1 times lower than the
values obtained in the first case. Results are
compared with pure network approach using
empirical correlations and with pure conjugate
analysis using CFD.
CFD
CombinedCFD
Correlations
445
470
445
Temparature(k)
Temparature(k)
420
395
370
345
320
200
420
CFD
CombinedCFD
Correlations
395
370
345
400
600
800
1000
Power(kw)
1200
1400
320
200
400
600
800
1000
1200
1400
Power(kw)
Fig .7 Temperature of stator for different powers
Fig .8 Temperature of end winding for different powers
CONCLUSION
In the light of developments in more electric Aircraft
and Automotive especially the autonomous cars
design of compact high performance electric motors
is need of the hour Heat transfer in the electric motors
is a crucial necessity high density high speed motors.
Thermal network approach will enable designers’
gain an insight on proposed design paving a way for
Innovation Thermal network model helps to design
cooling mechanism using suitable mode of heat
transfer.
the present study a combined CFD and network
approach is proposed to find the temperature
distribution in an alternator. This approach is found to
be effective in simulation of temperature distribution
in alternator. The solution of thermal network with
thermal resistance computed from CFD resulted in
the values of temperatures of different components in
alternator like stator, rotor, bearing and end winding.
The maximum temperatures obtained in the free
convection model is 948K,in forced convection is
448K and in forced convection with fan is 408K from
the maximum load condition. There is a 31% raise in
the maximum temperature from minimum load
condition to maximum load condition.
The system maximum temperature is
reduced to a 9.15% when the fan is simulated by a
pressure jump of 3800 Pa, which shows the
importance of providing fan. A lumped-circuit
approach using correlations from available literature
and the conjugate CFD analysis are also performed
and the results are compared with the CCFDN
approach. It is concluded that the combined CFD
Fig .5 Temperature contour with pressure jump condition
Fig .6 Temperature contours with stand still air condition
Fig 6 shows the temperature distribution of the
alternator when there is stand still air condition. The
stand still air is simulated using a Natural convection
boundary condition. Maximum temperature is
938.84K in this condition .The forced convection
simulation is carried out for different loads like full
load, minimum load, and 60% load condition. The
maximum temperature in
each
simulation
respectively 448.4 K (maximum load), 340.2 K
(minimum load), 395.2 K (60% load condition) at
0.58kg/s mass flow condition.
The temperature distribution of an alternator can be
found using CFD approach or network method. A
Heat Transfer In Electrical Machine: A Case Study
74
International Journal of Mechanical And Production Engineering, ISSN: 2320-2092,
network is found to be an optimized way of finding
the distribution of temperatures.
[5]
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