54
Relationship:
Armature control range: The armature and the power at constant current increase
proportionally to the speed. The rated motor voltage is reached when the rated speed is at
nominal flux.
Field control range: To increase the speed beyond the rated speed and to prevent the
voltage from increasing beyond the rated value, the motor field must be weakened. A
constant current and constant voltage therefore results in a constant power and a torque that
is reduced inversely proportional to the speed.
Field weakening at constant power is possible up until the maximum field weakening speed
n
Fmax
is reached. At even higher speeds, the current must be reduced while taking account of
the commutation (brush sparking!). This is done in the SINAMICS DCM by setting the speed-
dependent current limitation, see Section 10.21 of the SINAMICS DCM Operating
Instructions. Due to the necessary current reduction in this range, an additional power
reduction and as a result an even greater torque reduction will result at constant voltage.
Protection of the motor winding from overtemperature:
A thermal motor protection can be approximated with the I
2
*t monitoring implemented in
SINAMICS DCM. The resulting winding temperature is calculated with the help of a thermal
model of the motor and a warning and fault message are output depending on that result. A
thermal equivalent time constant is entered as the required setting value in parameter
p50114. The curve depicted in Section 10.19 of the Operating Instructions for p50114 applies
to all Siemens direct current motors of type 1G.5/6/7 1H.5/6/7. Please contact the motor
manufacturer if non-Siemens motors are to be used.
It is important to remember that this I
2
*t monitoring does not provide full motor protection. A
temperature sensor is required for that.
The 6RA80 can evaluate the following temperature sensors: PTC thermistor Warning, or
switch off, no temperature display possible. NTC, PT100, PT1000, KTY84 temperature
display possible, temperature threshholds for warning and switch-off can be set.
(see parameters p50490 to p50494 of the 6RA80).
The temperature thresholds for warning and switch-off (required for NTC, KTY; PT100) are
motor specific, for which a request for quotation is required, please give the entire order
number and factory number of the motor.
n
n
n
mech
n
Fmax
Current
Torque M
Voltage Ud
Power P
Armature control
Field control range
Speed
n
n
: Rated speed
n
Fmax
: Maximum field weakening speed
where P
n
= constant
n
mech
: Highest permissible operating speed
