THERMAL CHARACTERISTICS OF TRACTION MOTORS WITH REGENERATIVE BRAKING
S.L. Ho
H.C. Wong* and S.K. Poon*
Department of Electrical Engineering, Hong Kong Polytechnic University
* Industrial Centre, Hong Kong Polytechnic University
Abstract
This paper describes a computer program developed to simulate the temperature distribution of a
traction motor for a journey between two railway stations in Hong Kong. In particular, the authors
have used the software to study the temperature distribution of the traction motor running with
regenerative scheme along an entire railway network. Special attention are given to evaluate the
temperature rise on the winding surface and at specific points where the insulation were reported to
be damaged easily. Overall, the traction motor thermal model developed is useful for traction design
engineers and timetablers who are required to check whether there are overheating inside the traction
motor for a given duty.
1.
INTRODUCTION
Over the past 20 years, the passenger and freight traffic
of the Kowloon-Canton Railway Corporation (KCRC)
is steadily increasing. Saturation is being approached
on the cross-border traffic and, together the massive
new town developments at Shatin, Tai Po, Fang Lang
and Sheung Shui, a new demand for mass
transportation is appearing rapidly. A modernization
programme in the railway network was thus initiated in
the early 1980's.
In this modernization programme, the twelve-coach
passenger trains which were trailed by diesel-electric
locomotives were replaced by a fleet of Electrical
Multiple Units (EMUs) running on an electrified rail.
The whole nature of this railway has been transformed,
from the principally rural, slow frequency service to
become a modern, fast inner and outer suburban
system. Electrification was at 25 kV AC and the track
was doubled throughout the entire route length. The
KCRC becomes the provider of the most intensive
railway service in the world.
In 1988, due to the advent of power electronics, a new
fleet of EMU using thyristor-controlled traction system
were being brought into service to run alongside with
the original tap-changer type of EMUs.
Unlike other typical underground mass transit systems,
the grading of the track to save energy cannot be used
in the KCRC system because of the geographical
constraints in Hong Kong. However, the use of
electrical braking is possible. Apart from saving
energy, the use of electrical braking could also reduce
the wear upon the mechanical brake discs, hence
prolonging the working life of the discs and reducing
the amount of asbestos dusts escaping into open air.
One of the major problems of introducing the electrical
braking system in the KCRC system is that regeneration
is very difficult and costly. In addition, the regenerated
energy after being inverted to ac may have unavoidable
harmonic contents which might contaminate the
electricity supply of the power company, unless a bulky
and expensive harmonic filter is installed. These are
thus the main reasons why an electrical braking system
has not been installed in the EMUs at the design stage.
In this paper, the authors are proposing to feed the
braking energy into the 110 V 165 Ah nickel cadmium
emergency batteries which are always available in the
trains in Hong Kong. Note that with electrical braking,
the train retardation is increased and one must control
the rate of regeneration to alleviate any discomfort
which may appear to the passengers. During the
electrical barking period, the coasting speed as well as
the cooling effect is reduced. The additional heat
generated by the traction motor during regeneration
might cause overheating in traction motors. Indeed
there are evidence in Hong Kong that regenerative
operation might overheat the traction motors to the
extent that the service life of the motors were drastically
reduced. Hence one must take all these factors into
consideration when regeneration is introduced.
2.
LITERATURE SURVEY
Although DC series motors have been used traditionally
in traction applications, regenerative braking has not
been used very often in conventional traction
equipment. The main reason for this is because the
operation of a series generator becomes rather unstable
when connected to a fixed voltage supply. A separate
excitation is commonly required for realistic and stable
operation. Such an arrangement of series motor,
moreover, is very sensitive to supply voltage
fluctuations and a fast dynamic response is required in
order to provide an adequate brake control. The use of
a DC chopper facilitates the regenerative braking of
DC series motors due to its fast dynamic responses.
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200 km on 1 kWh of electricity. It is worth pointing out
that the greatest energy losses in the whole journey are
incurred whilst accelerating the trains up to their
maximum allowable speed. The use of regenerative
braking will normally recover a substantially amount of
the kinetic energy. Experiences indicate that the energy
consumption on level routes with regenerative
equipment would be about 10 % lower than that in
trains without regeneration.
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Fig. 1 GTO Regenerative Braking Chopper Circuit
for PSPICE Simulation
In a DC chopper controlled series motor with
regenerative braking, the braking power P in a constant
power braking region can be expressed as a function of
 multiplied with a polynomial of Ia. The order of the
polynomial is dependent on the accuracy of the traction
motor model and it is normally over 6. In order to
determine the relationship of Ia = f() for a particular P,
an iterative solution using Newton-Raphson method
was used in the simulation programme.
The speed-time curve for a run between two station
stops consists of periods of acceleration, free running,
coasting and retardation. The speed-time profiles for all
inter station journeys were determined jointly by the
timetable schedule and the maximum service
retardation.
An efficient use of energy is an important factor in the
achievement of an operating profit, as energy typically
represents 20 % of the operating cost in the local
railway. Recent developments with semi-conductor
devices have made it possible for regenerative braking
to be installed in rapid transit trains having frequent
stops, thus maximizing the braking energy with a small
weight penalty [1]. For modern, efficient mass transit
trains, a typical energy consumption of 24 Wh/t km
translates approximately to carrying a passenger for
TL(s) + Tf(s)
-
+
1
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Ia(s)
Thyristor
W(S)
1
+
2 Km Ia
gain
Rm(S e + 1)
Td(s)
B(S m + 1)
-
Eg(s)
+
Km W
+
Km Ia
V (kph)
area A
V1
max. braking rate
'
(slope) = 5 kph
V2'
T = t1 + t2' + t3'
area B
area A = area B
t (S)
0
t1
t2'
t3'
Fig.2
Speed-time Curve of a EMU with Electrical
Braking
The shapes of the various parts of large electrical
machine are too complicated for exact analysis of the
heat flow in different parts of the machine. This
situation has led to the use of the water models by
Limbora [2], because there are serious difficulties in air
models when measuring windage losses and the relative
gas velocity over rotor cooling surfaces. Water model
are preferred to air model in measuring (i) static
pressures and (ii) pressure differences, (iii) hydraulic
resistance, (iv) flow rate in duct or in the modelling of
hollow conductor, (v) flow velocity vector fields over
heat transfer surface, and (vi) windage losses. Water
model represents, to scale, all parts of the machine
which could have an essential influence upon the flow
of the cooling gas.
For the mixed flow case, the strength of rotation is
identified by the ratio of tangential velocity on the
surface of the rotating shaft to the mean axial velocity.
The ratio, ( = V/Vm) is called "the rotation ratio". It
was found that at a fixed Reynolds number, the Nusselt
number increases with an increase in the rotation ratio,
. Moreover, the effect of  on Nu is particularly strong
at low Reynolds numbers and it becomes negligible
above Re = 50,000 for the highest rotation. A physical
parameter that correlates the mixed-mode friction
coefficient and Nusselt number data by using the
normal pure axial flow relation can be defined as the
rotation parameter. Moreover, it was common to
assume that the rotation of inner cylinder does not
significantly affect the Nusselt number until the rotation
ratio reaches a value about 0.8.
The speed of rotation covers the range of the Taylor
number up to about 106, and the range of the Reynolds
number based on the axial velocity components and the
gap distance can be assumed to be valid up to 7000. It
was found that rotation does not affect the Nusselt
number at low Taylor Numbers and the heat transfer is
determined by the axial Reynolds number [3]. For a
smooth rotor the Taylor number for the onset of vortex
flow increase firstly with increasing axial flow, and,
after reaching a maximum, the Taylor number appears
to decrease slightly with further increases in axial flow.
This tendency towards decreasing critical Taylor
number is very observable for slotted rotors.
For Pr > 0.6 and Re > r x 103, the thermal boundary
layer is thin compared with the transverse dimension of
the stream. In closed ducts this means that the main
thermal resistance is localized in the region of the
viscous and buffer layers. Thus, local imperfections
and gaps (e.g. at corners) in the walls of a duct cause a
limited amount of deformation of these layers and have
comparatively little effect on the thermal resistance of
the stream. In practice, it is possible to calculate, to an
accuracy of about 10 %, the heat transfer in noncircular ducts with Pr > 0.6 and Re > 7000, by using the
value of the so-called equivalent hydraulic diameter
(Dh = 4 x internal cross sectional area of a
conduit/wetted perimeter of a conduit).
The earliest attempts at predicting the temperature rises
in machines by relating temperature rise to losses, as a
thermal network of lumped conductances and
capacitances, was due to Tustin [4]. He had assumed
that lumped-resistance networks may be found to
reproduce the relationships between the inputs heat
power and the mean temperature rises over the various
parts of the machine, although the thermal resistivities
are distributed.
The theory of the analogy is summarized by Simonson
[5]. The purpose of his paper is (a) to outline some
previous work to show the contribution that can be
made by using electrical analogies; (b) to summarize
the theory of such analysis, and (c) to show how the
technique may be applied in the thermal analysis of a
traction motor armature. The finite difference approach
to a heat transfer problem consists of defining a mesh
or grid to cover the temperature field and determining
the temperature at the grid point. Each grid point
corresponds to, and is situated at the centre of, a certain
area of the field, and the thermal resistances between
grid points are equal to the resistances between the
centres of the appropriate area. The electrical analogue
technique is based on using an equivalent electrical
circuit to correspond to the thermal network of the
finite difference grid.
A resistance network analogue representation of the
stator core and winding has been derived by Roberts
[6] and is based on the finite difference approximation
to the heat flow equations. A solution of the analogue is
achieved by matrix methods, on a digital computer.
The input data necessary before an analogue solution
can be obtained are: (a) the internodal resistances
within the packet; (b) the loss inputs at the nodal
points; (c) the thermal capacities of the various air
streams determined from air flow and thermal
properties of the air, (d) the air temperature which
forms part of the input data although corrections to the
assumed values can be made during the course of the
solution.
3.1 Heat generated by traction motor
Heat is generated in traction motors due to the electrical
losses in different parts of the machine, primarily
because of the copper loss in the conductor and the iron
loss in iron. The heat generated by mechanical means,
such as frictional losses at the motor bearings and at the
interface between the carbon brushes are relatively
small compared with that of the electrical losses. The
electrical heat generated causes the temperature of the
traction motor to rise and the heat generated is
dissipated to the ambient air by a combination of
conduction, radiation and essentially forced convection.
The electric current supplied to the motor varies with
the travelling speed. The copper loss equals to I2R and
is therefore directly proportional to the square of the
current magnitude with the assumption that the
electrical resistance of the conductors is constant.
The iron loss composed of two components, namely,
the eddy current loss and the hysteresis loss. The iron
loss is only a minor source of the total heat generated
when compared with that generated by the copper loss.
It is acceptable to assume that the iron loss is equivalent
to a fraction of the copper loss. The copper loss is
dependent on the speed of the train and the magnitude
of the traction input current, and the iron loss is of
similar nature.
3.2 Heat transfer inside traction motor
The cooling method adopted by the traction motor is
called the self-circulation type in which the circulation
of the cooling air is created by means of a fan mounted
directly on the armature shaft of the motor. When the
armature rotates, the fan will drive the cooling air
through the air gap. The heat generated is moved
essentially by forced convection. Radiation will also
take place at the machine surface but will not be
regarded as significant at the level of temperature
encountered by the motor.
From the Newton's Law of cooling, the rate of heat
transfer by heat convection (q) between a heated
surface (with surface area A) can be determined from q
= hA(w -f). The heat transfer coefficient h is a very
complicated function which depends on the fluid flow
condition,
fluid
properties
and geometrical
configuration of the heat surface. For forced
convection, the heat transfer coefficient may be
determined by the non-dimensional expression, i.e. Nu
= f(Re, Pr). Different boundary surfaces will have
different value of heat transfer coefficients. However,
due to geometrical similarity, two heat transfer
coefficients can be reasonably assumed. The first one is
required on the surface of the armature and the
laminated iron, whereas the second one is required for
the surfaces of the field coil.
It is assumed in this simulation that convective heat
transfer on these surface occurs because of the air
flowing pass through a concentric annulus with the
inner cylinder rotating.
3.3 Heat transfer at the field coil surfaces
Geometry around the field coil surfaces is very
complicated and cannot be simulated exactly by a
single shape. Thus the following three configurations
have been considered in order to arrive at a better
approximation. These are, namely, (a) flow over a flat
plate; (b) flow through a parallel plate and (c) flow
through a conduit with defined cross section. The air
flow through a triangular duct is considered as a better
approximation of the geometry under investigation.
Effect of the duct length and the cross-sectional
dimension of the duct on the heat transfer coefficients
have been taken into account in the heat transfer
analysis by the authors.
It is worth pointing out that turbulent flow will occur
inside the airgaps because of the relatively high fluid
velocity. In fact, there is no suitable non-dimensional
expression to predict the heat transfer in a triangular
duct in which turbulent flow exists. Noting that the
Prandtl number of air is about 0.697 at 350 K and is
greater than 0.6, the concept of hydraulic diameter can
be applied.
irregular geometries and complicated boundary
conditions. Finite element method is not as efficient as
the finite difference method in manipulating the
iteration process and in formulating the model. Since
the main heat generation and heat dissipation are on the
coil surface which is a relatively plane surface and not a
complicated irregular 3-dimensional object, the finite
difference method is very suitable to handle such
geometry. In addition, the finite difference method is
more stable when sets of simultaneous equations are
solved by sequential iteration compared with the finite
element method. Therefore finite difference method has
been employed by the authors for the development of
numerical models.
4.1 Assumptions of the numerical model
The following assumptions have been adopted in
developing the numerical model:
i) There is no heat transfer in the axial direction of
the motor. It is because the core is laminated and
therefore the heat transferred between laminated plates
is assumed to be negligible. The axial conduction is
also ignored. Since the gap between the two cylinders is
very small when compared to their diameters, the
developing region is negligible when compared to the
well-developed region. Variation in temperature and
velocity profiles of the fluid boundary layer along the
axial direction can be assumed negligible and heat
convection is assumed to be a 2-dimensional process in
the radial direction. The heat transfer model can
therefore be represented simply by a 2-D model.
ii) The fluctuation in the thermal properties i.e. , k,
Cp and , caused by changes in the fluid temperature is
negligible.
iii) Heat generation is due to the winding loss and
core loss only.
During the whole journey, the internal energy of the
motor will be increased each time after every motoring
and braking cycle.
4.
COMPUTER
SIMULATION
OF
TEMPERATURE OF TRACTION MOTOR
Many models could be developed for predicting the
temperature distribution inside the traction motor. Both
finite element models and finite difference models have
been considered by the authors.
iv) The system of equation shall be expressed in the
matrix form of [A][x] = [B]. After the first iteration, the
error C1 = [A][X] - [B] is calculated. The iterations will
be continued until the accumulative error (C12 + C22 +
.... ) is less than 0.01 and the temperature solutions will
then be assumed to have converged.
5.
The principal advantages of the finite element approach
for convective heat transfer is that it can handle
SIMULATION RESULTS
As the existing KCRC fleet of trains do not have
regenerative features, one must validate the simulation
results by considering the normal mode of operation
with the traction motors operated without regenerative
braking as shown in Fig. 4.
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From Fig. 4 it could be seen that the simulated results
and the temperature measured at the main pole of the
traction motor are in good agreement. Other parts of
the motors being studied also have more or less the
same order of agreements as well.
To compare the temperature distribution of the traction
motor with and without electrical braking, the
temperature of the traction motor along the journey
from Kowloon to Lo Wu are continuously monitored
and analyzed. A typical result for the temperature
profile of the interpole for one journey without
regenerative braking is shown in Fig. 5.
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Traction Motor Grid Line and Nodal Point
Fig. 3 Traction Motor Temperature Distribution
Fig. 4
Temperature profile of traction at the main
pole
electrical braking in a journey can be typically
explained in three stages as:
State 1
Fig. 5
Temperature profile of traction motor at the
interpole
The temperature simulation program is used to simulate
the temperature distribution of the traction motor
during the journey. It requires the input of the speedtime profiles and current-speed profiles for the journey.
As we have seen from the previous discussion, the
speed-time profile and current-speed profile of the
traction motor with regeneration were different from
the original case without electrical braking. This will
give different temperature-time profiles for the traction
motor being studied. The authors can also use the
software to evaluate whether there are hot-spots inside
the machine being studied.
5.1 Discussion on simulation results
The macroscopic thermal behaviour of a traction motor
is the temperature rise of the machine resulting from a
change in train speed and motor current. The formation
of the temperature-rime profile for a EMU without
Temperature of the motor rises quickly when the train
starts to accelerate from a station. The temperature rise
occurring during this stage is due to the very high
current input of 650 A into the motor. The heat
generation rate is higher than the rate of heat dissipation
from the motor by convection because the rotational
speed of the shaft fan is relatively low during this
period and hence the corresponding forced convective
heat transfer is relatively ineffective. During this period,
a rapid temperature rise of the motor occurs and a
considerable amount of heat is stored in the motor coil.
Stage 2
When the train speed runs in excess of about 45 kph,
the rate of heat dissipation from the motor by forced
convection increases rapidly because of increases in
both the rotational speed of the motor and the cooling
air velocity through the annulus gap of the motor. When
the speed of the train reaches about 45 kph and above,
the weak field control starts to function and the input
current is reduced from 650 A to 350 A, and hence a
lower heat generation rate results immediately. The
lower rate of heat generation plus the rate of heat
conduction from the coil centre to the coil surface of the
motor due to the heat storage during the previous stage
could be dissipated to the convective air rapidly by the
enhanced forced convection. Temperature of the motor
starts to decrease and the cooling of the motor is
ensued. It also led to the removal of part of the heat
stored up in the motor during stage 1.
positive slope profile. For short distance journeys, the
slope of the profile became negative.
Stage 3
6.
A second rise in temperature (positive slope) or fall in
temperature with smaller slope (negative slope) is
observed when the motoring command is retreated and
a braking command is called upon before the train
approaches a station. This temperature profile during
the coasting period is dependent on the following four
factors:
From the simulation results it was found that there was
no hot spots inside the traction motor. Subsequent
investigation also revealed that the motor failures were
indeed not caused by overheating. The studies as
presented in this paper also suggests that it is feasible to
feed the regenerative energy back to the emergency
batteries without the requirement of complicated
controllers.
Firstly, there was a significant overall temperature rise
of the traction motor after every trip which indicates
that the rate of heat generation is as a whole greater
than the rate of heat dissipation, especially when the
cooling period of the motor during the trip is not long
enough. Note that the copper field coils of the motor
are wrapped by insulation materials and reinforced by
varnish, resulting in a very low thermal conductivity
and thus the heat dissipation from the coil centre
towards the surface is carried out at a relatively low
rate. The heat generated is therefore accumulated in the
coil and a great temperature gradient is created between
the internal region and the surface of the motor.
7.
1.
2.
3.
Secondly, the motor is very close to the braking
equipment such as the brake disc and the brake pad.
The braking system employed by the train is a simple
frictional braking system which converts all the kinetic
energy of the train into heat at the brake discs and pads.
The local ambient temperature around this area
increases suddenly because of such heat dissipation.
4.
Thirdly, as the train speed decelerates quickly to zero,
the convective heat transfer coefficient decreases
rapidly to result in a rapidly decreasing convective heat
transfer. The convection is further suppressed by the
high ambient temperature which reduces the
temperature differential between the motor surface and
the ambient air.
6.
Finally, the under frame equipment is partially enclosed
by the platform of station which would give further
reduction in the heat dissipation from the motor.
The sign of slope (positive of negative) of the profile at
braking period therefore depends on the journey
distances between stations. For long distance journeys,
the maximum travelling speed is much higher and the
time taken for coasting is longer, therefore the effect
stated in stage 3 would be more significant. Hence the
amount of heat generation is higher to result in a
5.
CONCLUSIONS
REFERENCES
Wilkinson,
D.T., Electric Braking Performance of Multiple
Unit Trains, Proc. IMechE, 1985, Vol 199, No.D4.
Limbora,
K.,
Water Models for the Investigation of Cooling Gas
Flow in Large Rotating Electrical Machines, Proc.
IMechE, 1969-70, Vol. 184, Pt. 3E.
Lee,
K.Y.,
Effects of Harmonics on Thermal Performance of
Traction Motors, Mhil. Thesis, H.K. Polytechnic
University, 1992.
Bates, J.J and
Tustin, A., Temperature Rises in Electrical
Machines as Related to the Properties of Thermal
Networks, Proc. IEE, 1956, No. 2026U.
Simonson, J.R.,
The Use of Electrical Models in Cooling Studies of
Electrical Machines, Proc. IMechE., 1969-70, Vol
184 Pt. 3E.
Roberts, T.J.,
The Solution of the Heat Flow Equations in Large
Electrical Machines, Proc. IMechE., 1969-70, Vol.
184 Pt. 3E.
THERMAL CHARACTERISTICS OF TRACTION MOTORS
WITH REGENERATIVE BRAKING
S.L. Ho
H.C. Wong* and S.K. Poon*
Department of Electrical Engineering, Hong Kong Polytechnic University
* Industrial Centre, Hong Kong Polytechnic University
In this modernization programme, the twelve-coach passenger trains
which were trailed by diesel-electric locomotives were replaced by a
fleet of Electrical Multiple Units (EMUs) running on an electrified rail.
Electrification was at 25 kV AC and the track was doubled throughout
the entire route length. The KCRC becomes the provider of the most
intensive railway service in the world.
Apart from saving energy, the use of electrical braking could also
reduce the wear upon the mechanical brake discs, hence prolonging the
working life of the discs and reducing the amount of asbestos dusts
escaping into open air.
In this paper, the authors are proposing to feed the braking energy into
the 110 V 165 Ah nickel cadmium emergency batteries which are
always available in the trains in Hong Kong.
During the electrical barking period, the coasting speed as well as the
cooling effect is reduced. The additional heat generated by the traction
motor during regeneration might cause overheating in traction motors.
Indeed there are evidence in Hong Kong that regenerative operation
might overheat the traction motors to the extent that the service life of
the motors were drastically reduced. Hence one must take all these
factors into consideration when regeneration is introduced.
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5
3
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L
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0.136
7
6
4.7u
L
m
82
4
1.64184H
Cs2
Rs2
2
R
m
0.044
4.7u
2100u
Cs1
1
T
9
C
e
1
8
10
10MEG
E g
82
0V
+
Vg1
(V ) g
Rg
V
Rs1
T
0
GTO Regenerative Braking Chopper Circuit for PSPICE
Simulation
110V
V
s
V (kph)
area A
V1
max. braking rate
'
(slope) = 5 kph
V2'
T = t1 + t2' + t3'
area B
area A = area B
t (S)
0
t1
t2'
t3'
Speed-time Curve of a EMU with Electrical Braking
An efficient use of energy is an important factor in the achievement of
an operating profit, as energy typically represents 20 % of the operating
cost in the local railway.
Thus it is important to regenerate the braking energy back to the system.
In the KCRC system it will be rather difficult to regenerate the energy
direct into the 25 kV system. There could be instability problems indeed
when one is trying to feed the regenerative energy using dc series
motors. However it is possible to regenerate the energy into the
emergency battery systems.
From the Newton's Law of cooling, the rate of heat transfer by heat
convection (q) between a heated surface (with surface area A) can be
determined from q = hA(w -f). The heat transfer coefficient h is a
very complicated function which depends on the fluid flow condition,
fluid properties and geometrical configuration of the heat surface. For
forced convection, the heat transfer coefficient may be determined by
the non-dimensional expression, i.e. Nu = f(Re, Pr). Different boundary
surfaces will have different value of heat transfer coefficients.
A rm
a tu
re
67
83
68
69
84
85
70
86
100
71
87
101
88
72
102
115
89
103
73
116
129
1
104
117
90
74
130
2
3
4
118
131
5
132
6
105
91
106
119
75
92
76
7
133
8
15
14
16
17
22
13
9
10
20
21
120
134
23
18
24
19
25
26
107
93
77
12
11
28
27
29
121
135
30
108
122
136
94
78
109
95
123
79
110
96
80
81
31
32
33
34
35
36
37
38
39
40
41
42
124
137
111
97
82
98
138
125
112
99
43
44
45
46
47
48
49
50
51
52
53
145
113
126
139
54
148
114
141
144
127
150
147
140
128
142
143
146
151
149
55
56
57
58
59
60
61
62
63
64
65
66
152
159
153
160
161
162
163
164
165
166
167
168
169
170
171
172
154
173
155
174
156
175
158
157
176
180
177
178
179
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
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219
220
221
237
222
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226
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228
229
230
231
232
233
234
235
236
238
Traction Motor Grid Line and Nodal Point
Traction Motor Temperature Distribution
Temperature profile of traction at the main pole
Temperature profile of traction motor at the interpole
The macroscopic thermal behaviour of a traction motor is the temperature
rise of the machine resulting from a change in train speed and motor current.
The temperature-time profile for a EMU without electrical braking in a
journey can be typically explained in three stages as:
State 1
Temperature of the motor rises quickly when the train starts to accelerate
from a station. The temperature rise occurring during this stage is due to the
very high current input of 650 A into the motor. The heat generation rate is
higher than the rate of heat dissipation from the motor by convection because
the rotational speed of the shaft fan is relatively low during this period and
hence the corresponding forced convective heat transfer is relatively
ineffective. During this period, a rapid temperature rise of the motor occurs
and a considerable amount of heat is stored in the motor coil.
Stage 2
When the train speed runs in excess of about 45 kph, the rate of heat
dissipation from the motor by forced convection increases rapidly because of
increases in both the rotational speed of the motor and the cooling air
velocity through the annulus gap of the motor. When the speed of the train
reaches about 45 kph and above, the weak field control starts to function and
the input current is reduced from 650 A to 350 A, and hence a lower heat
generation rate results immediately. The lower rate of heat generation plus
the rate of heat conduction from the coil centre to the coil surface of the
motor due to the heat storage during the previous stage could be dissipated to
the convective air rapidly by the enhanced forced convection. Temperature of
the motor starts to decrease and the cooling of the motor is ensued. It also led
to the removal of part of the heat stored up in the motor during stage 1.
Stage 3
A second rise in temperature (positive slope) or fall in temperature with
smaller slope (negative slope) is observed when the motoring command is
retreated and a braking command is called upon before the train approaches a
station. This temperature profile during the coasting period is dependent on
the following four factors:
Firstly, there was a significant overall temperature rise of the traction motor
after every trip which indicates that the rate of heat generation is as a whole
greater than the rate of heat dissipation, especially when the cooling period of
the motor during the trip is not long enough. Note that the copper field coils
of the motor are wrapped by insulation materials and reinforced by varnish,
resulting in a very low thermal conductivity and thus the heat dissipation
from the coil centre towards the surface is carried out at a relatively low rate.
The heat generated is therefore accumulated in the coil and a great
temperature gradient is created between the internal region and the surface of
the motor.
Secondly, the motor is very close to the braking equipment such as the brake
disc and the brake pad. The braking system employed by the train is a simple
frictional braking system which converts all the kinetic energy of the train
into heat at the brake discs and pads. The local ambient temperature around
this area increases suddenly because of such heat dissipation.
Thirdly, as the train speed decelerates quickly to zero, the convective heat
transfer coefficient decreases rapidly to result in a rapidly decreasing
convective heat transfer. The convection is further suppressed by the high
ambient temperature which reduces the temperature differential between the
motor surface and the ambient air.
Finally, the under frame equipment is partially enclosed by the platform of
station which would give further reduction in the heat dissipation from the
motor.
When electrical braking is introduced, the temperature-time profile will be
slightly changed because motor current will flow in the windings during the
regenerative braking period. Hence the rise in temperature at stage 3 is
higher.
The overall temperature-time profile with and without regenerative braking
was however roughly the same.
CONCLUSIONS
From the simulation results it was found that there were no hot
spots inside the traction motor. Subsequent investigation also
revealed that the motor failures were indeed not caused by
overheating. The studies as presented in this paper also suggests
that it is feasible to feed the regenerative energy back to the
emergency batteries without the requirement of complicated
controllers.