Woodward
Manual MRM3-2 GB
16
TD_MRM3-2_09.08_GB_Rev.New
4.1.1 Criteria for Blocking the Start
Number of monitored starts :
The MRM3-2 is equipped with a flexible supervision element which can limit the sequence of poss-
ible starts.
A start should be prevented if it is obvious that it is likely to be interrupted due to overload so that in
total the down-time can be curtailed. If a start is not recommendable at a certain time (with the mo-
tor switched off), the MRM3-2 activates an allocated output relay until the waiting time has elapsed.
Irrespectively of the adjustment of this element, the thermal image is always activated and shuts
the motor down as soon as the thermal overload threshold is reached (due to a start or overload).
The protective element can either be tied to the thermal image or be manually defined by the num-
ber of starts and cycle duration.
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 relay 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 )un-
til the 20 minutes 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