Fundamental Principles and System Description
Engineering Information
SINAMICS Engineering Manual – November 2015
Ó Siemens AG
177/528
2.2. Determine the permissible short-time current during phase T
2
:
With a base load current I
H
= 514 A and a derating factor k
D
= 0.96, the permissible short-time current is
AAIkI
HDShortTime
74051496.05.15.1 =××=××=
.
This value is significantly higher than the required maximum motor current of 400 A during steady-state, periodic
press operation during phase T
2
and is therefore permissible.
2.3. Determine the I
2
t value of the motor current during phase T
2
:
According to the time characteristic of the motor current as shown in diagram 1, the I
2
t value is
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×
×
+
=
22
2
21
2
2221
2
1504001
T
kI
A
T
kI
A
TT
valuetI
DRatedDRated
• 100 %
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û
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+×
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×
+
= s
A
A
s
A
A
ss
valuetI 10
552
150
10
552
400
1010
1
22
2
• 100 %
[ ]
%30%100
20
99.5
%100738.025.5
20
1
2
=×=×+×=
s
s
ss
s
valuetI
.
The calculated I
2
t value of 30 % is well below 100 % and is therefore permissible.
Steady-state, periodic press operation during phase T
2
is thus permissible both in terms of the short-time current and
in terms of the I
2
t value.
3. Analyze the startup and shutdown behavior of the flywheel:
As a general rule, the flywheel is rarely started up and stopped while a press is in production. It is therefore easy to
imagine that these processes are rare events which have no relevance with regard to the lifetime of the IGBTs in the
power unit.
In practice however, long phases of operation during which the flywheel is periodically started up and stopped over
periods lasting hours or even days may be necessary during setup mode after a tool change or with tryout presses in
which new tools are tried out. This example application may therefore involve periodic operating conditions as shown
in diagram 2 which can lead to substantial temperature swings ΔT
Chip
in the IGBT and therefore significantly reduce
the IGBT lifetime.
T
0
: Standstill of the flywheel
T
1
: Startup (acceleration) of the flywheel
T
2
: Brief press operation with a few strokes
T
3
: Stop (braking) of the flywheel
Diagram 2: Absolute value of motor current |I(t)| and motor frequency f(t)
With periodic startup and shutdown of the flywheel
If it has to be assumed that these periodic operating conditions occur for more than around 1 to 2 % of the total
operating time of the press (which is true in the case of many flywheel presses), it must be categorized as a periodic
load duty cycle. The current derating factor k
IGBT
must therefore be applied when the drive is configured in order to
prevent premature failure of IGBTs.
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