P&B MPC2000+ Technical Manual page 37

MPC2000+ Technical Manual
When the running current exceeds the overload setting the thermal capacity will eventually reach 100% and trip the
motor. The time taken depends on the present value of thermal capacity used and the t6x setting, which is set as the
time to trip for a motor in a cold condition when the current is at 6 times the motor rated FLC. The heating constant is
equal to t6x setting x 32 seconds.
The expected 'Time To Trip' is the time it will take to reach 100% thermal capacity at the present running current
level is calculated and can be displayed by the MPC2000+.
If a motor speed switch is installed and connected to the MPC2000+ it is then possible to decrease the thermal trip
time in the range of 20-100% of the set hot & cold curve operating times when the motor stops rotating, i.e. stalled.
After a thermal overload trip it is not possible to reset the fault and restart the motor until the amount of thermal
capacity available has reduced to 50%.
The cooling time constant of a motor, i.e. the time taken for its temperature to drop is usually much longer than the
heating constant. The Cool Time Factor setting determines the rate of cooling and is set as a ratio of the heating time
factor between 1 and 15 times the heating factor. The higher the value of this setting the longer it will take for the
thermal capacity to reduce to the 50% level required to enable a restart.
The MPC2000+ calculates the expected 'Time To Restart' and displays this value within the 'Calculated Data' page
For testing purposes the resetting of the thermal capacity register to 0% can be manually performed only if the
'Authorise Key' input is enabled, and by pressing the Reset key which will cause the display to show 'Reset Thermal
Capacity ???'. If the Reset key is pressed again whilst this message is displayed i.e. within 1 second the register will be
reset to 0%.
The settings for 'Thermal Overload', 't6x', 'Hot/Cold Ratio' and 'Cool time Factor' should be established from data
obtained from the motor manufacturer however should it not be possible then the default settings for these are typical
settings for normal induction motors and may be used at the users own risk. The t6x setting can be set to 10% above
the normal run up time, which can be established after the unit is installed by starting the motor and noting the 'Last
Start Time' available in the MPC2000+ 'Statistical Data page.
The present value of thermal capacity is stored in the EEPROM non-volatile memory during a failure or
disconnection of the auxiliary supply. Upon restoration of the supply the former value will be maintained.
Referring to the MPC2000+ Thermal Operating curves shown, the operating time at a certain multiple of motor FLC
can be calculated for a particular motor by multiplying the corresponding time taken from the vertical scales by the
setting of t6x.
P&B Engineering ISSUE 7 28/9/99 Page 33