9
05.01
9.12 Operation at power failure
9-299
 Siemens AG 2001 All rights reserved
SIMODRIVE 611 Planning Guide (PJU) – 05.01 Edition
9.12.3 DC link buffering
The energy, available in the DC link of the drive units is calculated as follows at
power failure:
E = 1/2 * C * (V
DC
link
2
– V
min
2
)
In this case, E= energy in Watt seconds [Ws]
C= total capacitance of the DC link in Farad [F]
V
DC
link
= DC link voltage (response limit, line supply failure)
V
min
= lower limit for safe reliable operation
(taking into account the motor–specific EMF, in any case,
above the switch–off threshold of e.g.350V)
Example:
For C= 6000µF (refer to Table 9-3, 1. line) – 20% = 4800 µF
V
DC
link
= 550V
V
min
= 350V
is obtained as follows:
E = 1/2 * 4800F *((550V)
2
– (350V)
2
) = 432Ws
This energy, under load conditions is available for a time of
t
min
= E / P
max
* 
in order to initiate emergency retraction.
In this case, t
min
= buffer time in milliseconds [ms]
P
max
= power in Kilowatt [kW]
 = efficiency of the drive unit
For the example above, with:
E = 432Ws
P
max
= 16kW (refer to Table 9-3, 1. line)
 = 0.90
is obtained as follows:
t
min
= 432Ws / 16kW * 0.9 = 24.3ms
as minimum achievable buffer duration for emergency retraction.
9 Important Circuit Information
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