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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
%
100
80
60
40
20
0
First start-up
Running 5 min
11 s at 6.2 x I θ
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,
%
100
80
60
40
20
0
Overload leading
Cooling at standstill.
to a thermal trip.
Time scale in minutes.
Time in minutes
Fig. 4. The thermal behaviour of an overload trip and cooling, followed by a motor restart.
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.
Second start-up
Running continuously
11 s at 6.2 x I θ
at 0.9 x I θ
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
, e.g. 4 ....8. When the used thermal ca-
c
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.
Restart inhibit
removed.
Restart at I = Is
Motor running.
for time ts
Time in minutes.
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