Rockwell Automation 825-P Modular Protection System for Motors User Manual - Manual (Page 73)

Rockwell Automation Publication 825-UM004D-EN-P - November 2012 73
Configuring Protection & Logic Functions Chapter 6
The Acceleration Factor setting reduces or extends the allowed accelerating time
under locked rotor conditions. You can always safely set this value equal to 1.00.
If you know that the driven load always accelerates in less than the rated locked
rotor time, you could use an acceleration factor less than 1.00 to provide a faster
trip in locked rotor conditions. Do not, however, set the value greater than 1.00,
except to allow a start with a longer than normal accelerating time (e.g., high
inertia motor application, emergency condition).
Acceleration Factor Setting Calculation:
When the motor thermal capacity used exceeds the Overload Warning Level
setting, the relay issues a warning. The early warning could allow you to correct
the load problem before a thermal trip occurs.
The motor tripping and starting functions include supervision to help prevent a
thermal trip on a normal start. The relay prevents motor starting until the thermal
element has enough available thermal capacity to allow a motor start without
tripping. Set START INH. LEVEL to a value equal to the incremental increase in
percent thermal capacity utilized for one start plus additional margin. The 825-P
allows a new start when the percent thermal capacity utilized value is below 100 -
START INH. LEVEL. This feature can be disabled by setting START INH.
LEVEL equal to Off.
A stopped motor could take longer to cool than a running motor because of
reduced airflow or loss of forced coolant. The equation used is:
Round up the result to the next integer. The value k = 1.0 yields stop cool time
equal to run cool time.
The factory default settings assume that the motor stopped cooling time is
approximately the same as the motor running cooling time. Calculate the
STOP COOL TIME setting for your application using the value that you
want for k > 1.0.
EXAMPLE In a particular application, a motor with a 10 second hot-locked rotor time always
starts in 5 seconds.
Setting the acceleration factor equal to 0.75 causes the relay to trip in 7.5 seconds
under locked rotor conditions. This setting allows ample time for the motor to start,
but does not subject the motor to the full 10 seconds of locked rotor current if a locked
rotor start attempt takes place.
COOLTIME = 0.03 • k • LRTHOTn • (LRAn) 2 minutes
where:
n = 1 or 2 (select the one that gives higher COOLTIME)
k > 1.0