SIEMENS
2/11
Siemens MD 50.8 · 2017
Configuring guide
Configuring a three-phase motor
2
■
Switching frequency
A higher switching frequency means that the motor winding will
be subject to a thermal load.
The permissible switching frequency
Z
perm
has to be deter-
mined for different operating cases.
This value is influenced by the corresponding load torque, the
additional moment of inertia, the power requirement, and the
cyclic duration factor. These can be evaluated using the factors
k
M
,
k
FI
and
k
P
.
For 60 Hz operation, the calculated permissible switching
frequency
Z
perm
must be reduced by 25 %. See the technical
specifications for
brakes in chapter 6
for the permissible
switching frequency for operation with function rectifiers.
The permissible no-load switching frequency
Z
A
for motors with
brake L must be obtained from table
"No-load switching
frequency for brakes L" on page 6/35
.
Z
perm
=
Z
A
·
k
M
·
k
FI
·
k
P
The permissible no-load switching frequency
Z
0
for motors
without brakes must be obtained from the
Selection and ordering
data page 4/6
.
Z
perm
=
Z
0
·
k
M
·
k
FI
·
k
P
Fig. 2/6
Torque when accelerating
Fig. 2/7
Additional moment of inertia
J
add
=
J
x
+
J
z
Fig. 2/8
Power requirement and duty cycle
k
P
=
t
R
· (1 - (
P
S
/
P
rated
)
2
) + 0.6 · (1 -
t
R
) + 0.24 ·
t
R
Code
Description
Unit
DC
Cyclic duration factor
%
J
mot
Moment of inertia of motor and brake
kgm
2
J
Z
Additional moment of inertia of the high inertia
fan
kgm
2
J
X
Moment of inertia of the load, reduced to
motor shaft
kgm
2
J
add
Additional moment of inertia, reduced to
motor shaft
kgm
2
k
FI
Factor for taking into account the additional
moment of inertia
-
k
M
Factor for taking into account the load torque
while accelerating
-
k
P
Factor for taking into account the required
power and duty cycle
-
P
S
Actual steady-state motor power
kW
P
rated
Rated motor power
kW
T
A
Acceleration torque of the motor
Nm
T
rated
Rated motor torque
Nm
T
x
Load torque, reduced to motor shaft
Nm
t
R
Duty cycle (decimal)
Z
A
No-load switching frequency, motor with
brake
1/h
Z
0
No-load switching frequency, motor without
brake
1/h
Z
perm
Permissible switching frequency
1/h
T
X
=
M
k
1
X
A
T
A
T
T
0
M
k
0
G_M015_EN_00040c
1.0
0.8
0.6
0.4
0.2
0.9
0.7
0.5
0.3
0.1
1.0
0.8
0.6
0.4
0.2
T
0
k
FI
0
1
2
3
4
5
G_M015_EN_00041a
mot
J
J
add
k
Fl
=
mot
J
add
J
mot
J
0.2
0.4
0.6
0.8
1.0
0.1
0.3
0.5
0.7
0.9
0
k
P
DC [%]
0
20
40
60
80
100
P
/ P
= 0
G_M015_EN_00042
1
rated
0.2
0.4
0.6
0.8
1.0
0.1
0.3
0.5
0.7
0.9
0.2
0.4
0.5
0.6
0.7
1.0
1.1
1.2
0.9
0.8
© Siemens AG 2017