Siemens SINAMICS G130 Engineering Manual page 101

Calculation example:
A variable-speed drive must perform a heavy duty start at intervals of 60 min on a 690 V supply system. The diagram
below shows the motor current I(t) characteristic as a funtion of time. The maximum ambient temperature in the
converter room is specified as 45° C and the installation altitude is 400 m.
Motor current I(t) over time during starting
A SINAMICS G150 in degree of protection IP20 is selected as the drive converter. Its rated data are V = 690 V and
I = 575 A. It has, according to the catalog D11, a base load current I
rated
I of 1.5 I = 771 A. We shall now use a calculation to check whether the selected converter operating on the
ShortTime * H
factory-set pulse frequency is the right unit for the required load duty cycle under the specified conditions:
1. Determine the current derating factor:
With the following derating factors
•
k = 0.95 (ambient temperature 45 °C, installation altitude < 2000 m, degree of protection IP20),
Temp
•
k = 1.0 (factory-set pulse frequency)
Puls
•
k = 1.0 (no parallel connection of S120 Motor Modules)
Parallel
•
= 1.0 (duty cycle duration T ≥ 60 s)
k
ShortDutyCycle
the current derating factor k
= ⋅
k k k
D Temp Puls
2. Determine the permissible short-time current:
With a base load current I
H
= ⋅
I 1 5 . I
ShortTime
This value practically corresponds to the motor current of 730 A required at the beginning of the starting process and
is therefore just within the permissible limit.
2
3. Determine the I t value of the motor current:
For the purpose of simplifying the calculation, the actual time characteristic of the motor current I(t) during starting is
approximated by three time phases, i.e. T
must be selected such that the total of phases T
duration of T=300 s. The calculation for the I
⎡
1
= ⋅ ⎢
2
I t value
⎢
T
⎣
1
=
2
I t value
300
1
=
2
I t value
300
2
The I t value of 97 % is lower than the maximum permissible value of 100 % and is thus acceptable within the limits
of accuracy of the approximations used to calculate the current characteristic over time.
Fundamental Principles and System Description
is
D
⋅ ⋅ =
k k
Parallel ShortDutyC ycle
= 514 A and a derating factor k
⋅ = ⋅ ⋅
k 1 5 . 514 A . 0 95
H D
to T , each with a constant current I
1 3
+T
1
2
t value is therefore
2
⎛ ⎞ ⎛
I
⎜ ⎜ ⎟ ⎟ ⎜ ⎜
⋅ +
1
T
⋅
1
⎝ ⎠ ⎝
I k I
Rated D Rated
⎡
2
⎛ ⎞ ⎛
730 A 500
⋅ ⋅ +
⎜ ⎟ ⎜
⎢
120 s
⎝ ⎠ ⎝
s 546 A 546
⎣
[ ]
⋅ + + ⋅
215 s 50 s 25 s 100
s
of 514 A and therefore a short-time current
H
⋅ ⋅ ⋅ =
, 0 95 1 0 , 1 0 , 1 0 , , 0 95
= 0.95, the permissible short-time current is
D
=
732 A
.
+T does not exceed the maximum permissible load duty cycle
2 3
2
⎞ ⎛
I I
⎟ ⎟ ⎜ ⎜
⋅ +
2 n
T
⋅ ⋅
2
⎠ ⎝
k I k
D Rated D
2 2
⎞ ⎛ ⎞
A 250 A
⋅ +
⎟ ⎜ ⎟
60 s
⎠ ⎝ ⎠
A 546 A
290 s
= ⋅ =
% 100 % 97
300 s
SINAMICS Engineering Manual - May 2008
Engineering Information
.
to I ; in this case, the last phase T
1 3
⎤
2
⎞
⎟ ⎟
⋅ ⎥
•
T
100 %
3
⎥
⎠
⎦
⎤
⋅
⎥
•
120 s
100 %
⎦
%
© Siemens AG
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