4–18
MM200 MOTOR MANAGEMENT SYSTEM – INSTRUCTION MANUAL
PROTECTION ELEMENTS
CHAPTER 4: SETPOINTS
Cooling rate
The model causes the thermal capacity used to decrease exponentially when the steady-
state thermal capacity used value is less than the actual thermal capacity used. This
simulates motor cooling. As a stopped motor normally cools significantly slower than a
running motor, the relay has two cooling time constant setpoints, one used when the
motor is off (stopped, tripped, locked out, pre-contactor, etc.), the other used when the
motor is on (starting, running, stopping). In each case, the time constant is time in minutes
for the motor temperature to cool by 63% of the difference between the initial
temperature and ambient temperature.
Figure 4-6: Thermal model cooling following a trip at
t
= 0
Thermal protection
reset
Thermal model operation is a serious event, and therefore results in a lockout that can not
be reset until the motor has cooled, except with a level 2 or level 3 security login. A setpoint
is available that controls whether lockout persists until the motor has cooled until the
thermal capacity used reaches 15% (approximately twice the cool time stopped setpoint),
or until the relay estimates based on learned thermal capacity used on start that the
motor has cooled sufficiently for a successful restart. For the latter, a 2% safety margin is
included. While in lockout, the motor can not be started via the MM200.
If the motor is re-started it may re-trip quickly. Should process interruption concerns
outweigh the probable damage to the motor that early starting would incur, an external
circuit should be added that bypasses the relay to directly close the motor contactor.
A second setpoint controls whether once the motor has cooled as described above, the
lockout is replaced with a trip that can be manually reset without security login, or
alternatively the condition is fully reset allowing immediate restart.
0%
25%
50%
75%
100%
0
30
60
90
120
150
180
Time (minutes)
Cool time constant = 30 minutes
63%
Thermal
capacity
used
888732A1.CDR
