8
The behaviour of the thermal model under a
couple of typical drive conditions is shown in
Fig. 3 and 4. Figure 3 shows the thermal behav-
iour during a dual start-up sequence from a fully
cold state. During the first start-up, the start
current is heating up the motor during the start
time and a total of about 65% of the thermal
capacity is used. After the start, the motor is
left running for a few minutes with a normal
load of about 90% of the full load current. As
soon as the motor leaves the start-up condition,
the hot spots start levelling out and the used
thermal capacity decreases rapidly to a level de-
termined by the long term thermal loading of
the motor. The second start brings the thermal
capacity used up to a level close to tripping but
still allows the motor to run up. After the sec-
ond start the motor is left running for a long
time with a normal load and we can see the ther-
mal capacity curve level out, first by eliminat-
ing the hot spots and then by successive cool-
ing to a steady state where about 37% of the
thermal capacity is used.
0
20
40
100
60
80
%
First start-up
11 s at 6.2 x I
θ
Second start-up
11 s at 6.2 x I
θ
Running 5 min
at 0.9 x I
θ
Running continuously
at 0.9 x I
θ
Fig. 3. The thermal behaviour for two cold starts followed by a motor running at normal load.
Figure 4 shows a situation where the motor has
been running for a long period, whereafter it is
overloaded until the relay carries out a trip. The
restart enable relay is reset as the thermal level
is higher than the set level, which in this case is
40%. The motor begins to cool down, first by
levelling out hot spots and then by slowly re-
ducing the thermal curve now mainly consist-
ing of used thermal capacity due to a long term
thermal overload. As the motor is at standstill,
the cooling rate is reduced as the cooling fan on
the motor shaft is not running. The reduced
cooling is also taken into account by the relay
by a suitable setting of the cooling time multi-
plier k
c
, e.g. 4 ....8. When the used thermal ca-
pacity has fallen below the set restart inhibit level
40%, the restart enable relay of the motor pro-
tection is activated and the motor can again be
started.
0
20
40
100
60
80
%
Overload leading
to a thermal trip.
Time in minutes
Cooling at standstill.
Time scale in minutes.
Restart inhibit
removed.
Restart at I = Is
for time ts
Motor running.
Time in minutes.
Fig. 4. The thermal behaviour of an overload trip and cooling, followed by a motor restart.
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