SIEMENS
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Siemens MD 50.8 · 2017
Configuring guide
Configuring a three-phase motor
2
■
Additional moments of inertia
The motor moment of inertia with standard fan is specified in the
motor selection lists. The higher moment of inertia should be
used for metal or high-inertia fans. This is also valid for mounted
brakes, backstops and encoder systems.
■
Line feeder cables
Line feeder cables must be adequately dimensioned. The num-
ber of required parallel (if applicable) feeder cables is deter-
mined by the maximum connectable conductor cross-section,
the type of cable, the cable installation, the ambient temperature
and the permissible current. In Germany, DIN VDE 0298 must be
applied when dimensioning cables.
Undervoltage
For an undervoltage condition as a result of weak line supplies,
catalog values such as motor power, torque and speed are not
reached. This is especially important when considering motor
starting.
■
Motor protection
A distinction is made between current-dependent and
temperature-dependent protective devices for motors.
Current-dependent protective devices
Fuses
are only used to protect line cables in the event of a
short-circuit. They are not suitable for protecting the motor
against overload. The motors are usually protected by thermally
delayed overload protective devices (circuit breakers for motor
protection or overload relays).
This protection is current-dependent and is particularly effective
in the case of a locked rotor. For normal operation with short
starting operations, starting currents that are not excessive and
for low numbers of starting operations, motor circuit breakers
provide adequate protection. Motor circuit breakers are not
suitable for heavy duty starting or high numbers of starting
operations. Differences in the thermal time constants for the
protective devices and the motor results in unnecessary early
tripping when the circuit breaker is set to the rated current.
Temperature-dependent protective devices
Temperature-dependent protective devices are integrated in the
motor winding and can be implemented as
temperature
sensors
and
temperature switches
.
The number of temperature-dependent protective devices
depends on the number of windings and their function.
The alarm is normally set to 10 K below the switch-off tempera-
ture. The rated response temperatures of the protective devices
depend on the thermal class of the motors.
In order to achieve full thermal protection, it is necessary to
combine a thermally delayed overcurrent release and a
PTC thermistor.
■
Coolant temperature and installation altitude
The rated power specified in the
selection tables in chapter 4
is
valid for a coolant temperature of +40 °C and an installation alti-
tude of 1 000 m above sea level.
Please contact Siemens for higher coolant temperatures.
The table with correction factors provides a rough idea of the
derating required if conditions are different.
This results in a permissible motor power of:
Factor k
HT
for different installation altitude and coolant
temperature
Code
Description
Unit
P
perm
Permissible motor power
kW
P
rated
Rated motor power
kW
k
HT
Factor for abnormal coolant temperature and
installation altitude
-
Installa-
tion alti-
tude
Coolant temperature
IA
CT
m
< +30 °C
+30 … +40 °C
+45 °C
+50 °C
+55 °C
+60 °C
1 000
1.07
1.00
0.96
0.92
0.87
0.82
1 500
1.04
0.97
0.93
0.89
0.84
0.79
2 000
1.00
0.94
0.90
0.86
0.82
0.77
2 500
0.96
0.90
0.86
0.83
0.78
0.74
3 000
0.92
0.86
0.82
0.79
0.75
0.70
3 500
0.88
0.82
0.79
0.75
0.71
0.67
4 000
0.82
0.77
0.74
0.71
0.67
0.63
P
perm
P
rated
k
HT
=
© Siemens AG 2017