Rockwell Automation 825 Smart Motor Manager User Manual - Manual (Page 50)

3-17 Functions
Publication 825-UM001B-EN-P January 2001
While the motor is running, the iron losses as well as losses caused by asymmetry are fed to
the simulation model. Allowance for the ambient temperature of the motor, as an option,
enhances the maximum utilization of the installation even with considerable variation of the
temperature. Without the optional inclusion of the ambient temperature of the motor, the
thermal model bases the thermal calculation on an ambient temperature of 40° C. The
different cooling conditions of a self-ventilated motor when running and at standstill are
taken into account by two different time constants. After switching off, the rapid cooling of
the winding to the iron temperature and the subsequent slow cooling of the motor as a whole
are simulated.
The two-body simulation can be represented as a capacitance-resistance network. See
Figure 3.7.
Figure 3.7 Two-Body Simulation of the Heating Up of a Motor
Adjustable Ratio of Cooling Constants
The ratio of the cooling constant when the motor is at standstill to the cooling constant when
it is running allows for the difference in cooling in these states. The cooling constant ratio is
set to 2.5 in the factory. This value is correct for the majority of self-cooled AC motors.
For separately ventilated and special motors, and those which respond very quickly or very
slowly, you may have to modify the cooling factor.
C1
C2
R1
P
Cu
P
Fe
R2 R3
S1
ϑ
amb
≈
( I
M
2 + k I
G
2 )
C1 Capacitance representing the heat capacity of the winding (adjustable)
C2 Capacitance representing the heat capacity of the iron an other masses of the machine
R1 Resistance representing resistance to heat transfer between winding and iron
R2 Resistance representing heat dissipation to the surroundings when stationary
R3 Resistance representing heat dissipation to the surroundings when running
P
Cu
Input of a current proportional to the copper losses
P
Fe
Input of a current proportional to the iron losses
S1 Changeover from stationary to running
,
M
Motor current
,
G
Opposing component caused by asymmetry
ϑ
amb Allowance for the temperature of the environment coolant (optional PT100 #7)
k Constant factor according to IEC and NEMA