TB MRM3 07.01 E
11
Number of Starts/Cycle Duration
These two are defined as parameters.
Example:
The motor should be allowed to be started three times
an hour:
This means that in theory the motor can be started
every 20 minutes (= 60 min/3).
From this it can be concluded that the load generated
by the start procedure has decayed after these 20
minutes. If the motor would be successfully started
three time in quick succession, an immediate fourth
start would overload the motor. The start blocking re-
lay would be activated and the next start would only
be advisable after about 20 minutes. The protective
element ensures that the start sequence is kept within
safe intervals but that at least three starts are allowed
during the given time frame. If the intervals between
each start are long enough then even more than three
starts an hour might be possible because the motor
was able to cool down in the mean time. The delay
can be firmly defined (through start blocking time) or
be automatically ascertained (VARI ) until the 20 min-
utes given in the example are over. The state of the
thermal image has no influence on the delay.
Figure 4.3: Relation Start Period/Start Blocking Time
Figure 4.4: Relation Start Period/Start Blocking Time with
firm Start Blocking Time
Thermal Image
A start is always possible as long as there is enough
thermal reserve for a start. This start limitation is a dy-
namic one and is orientated on the data the thermal
image is parameterized with. For this the
MRM3
de-
tects the average thermal load of the latest starts. With
the motor shut down, the start blocking relay is acti-
vated for the time when there is not enough thermal re-
serve in the storage through cooling down to enable a
new start.
Figure 4.5: Start Blocking through Thermal Image