Siemens SINAMICS S120 - Page 377

SINAMICS S120
Engineering Information
SINAMICS Engineering Manual – November 2015
Ó Siemens AG
377/528
Dimensioning example:
A braking power as shown by the diagram below is periodically injected into the DC busbar of a system which is
supplied from a 400 V power supply network over a rectifier without regenerative feedback capability. Within a load
duty cycle duration of 100 s, the braking power reaches a peak value of 1300 kW for 10 s.
A SINAMICS S120 Motor Module suitable as a Braking Module for this application must be selected. The data of the
required braking resistors must also be calculated.
1. Select a suitable S120 Motor Module
The required peak braking power P
max-Brake
= 1300 kW must be provided by a Motor Module with the lowest possible
current. The maximum DC link voltage V
DC-max
= 774 V is therefore selected as the response threshold.
The smallest Motor Module capable of producing the required peak braking power of 1300 kW at the selected
response threshold of 774 V is chosen from the table of technical data:
· Output power 710 kW at 400V / rated output current 1260 A.
As a 3-phase Braking Module, it is capable of producing a peak braking power of 1669 kW > 1300 kW.
The next step is to check whether the mean braking power of the given load duty cycle is below the permissible
continuous braking power of the selected Motor Module. In this case, it is 1140 kW.
· P
mean-Brake
= (200 kW • 20 s + 1300 kW • 10 s + 800 kW • 30 s + 0 kW • 40 s) / 100 s = 410 kW < 1140 kW.
The selected Motor Module is therefore suitable, both in terms of peak braking power and continuousbraking power.
2. Calculate the data of the braking resistors
The minimum value R
BR-min
of the three braking resistors can be found in the table of technical data. For the selected
Motor Module, it is
· R
BR-min
(cold state) = 0.18 Ω .
The maximum value R
BR-max
of the three braking resistors is calculated from the response threshold V
DC-max
= 774 V
and the maximum braking power P
max-Brake
= 1300 kW of the given load duty cycle to be
· R
BR-max
(operating temperature) = [ 0.49 • (V
DC-max
)
2
] / P
max-Brake
= [ 0.49 • (774 V)
2
] / 1300 kW = 0.2258 Ω .
The resistance specification is therefore as follows:
· Resistance value: 0.18 Ω (cold state) < R
BR
< 0.2258 Ω (operating temperature).
· The three resistors R
BR
operating in unison must be capable of a continuous power of 410 kW and a peak
power of 1300 kW for 10 s in a 100 s cycle. Each individual resistor must therefore be dimensioned for one
third of the power values stated above.