26 Planning – basics
Defining the energy absorbing capacity of the common DC link
Use formula 1 below to examine if the common DC link capacitors can absorb the surplus
energy of the common DC system. If the condition is true, the energy absorbing capacity is
sufficient and you do not need a brake chopper and a resistor to handle the surplus
energy.
W
dc
> E
gen
(1)
C
dca
Capacitance value of drive a
C
dcb
Capacitance value of drive b
E
gen
Generating energy of the common DC link during one duty cycle
P
g1
Generating power of the common DC link during time t
g1
P
g2
Generating power of the common DC link during time t
g2
P
gen
(t) Generating power of the common DC link as a function of time over one duty cycle.
T Duty cycle time
t
g1
Duration of generating power P
g1
t
g2
Duration of generating power P
g2
U
ac
AC power line voltage
U
dc
Nominal DC link voltage. See section DC voltage limits of the drive on page 45.
U
dc,lim
Maximum DC link voltage. See section Defining the maximum DC link voltage on page 27.
W
dc
Energy absorbing capacity of the common DC link
(C
dca
+ C
dcb
+ C
dcc
… + C
dcn
)
× (U
dc,lim
2
- U
dc
2
)W
dc
=
2
U
dc,lim
=
2
× U
ac
E
gen
= ʃP
gen
(t) ×dt = Σ((P
g1
× t
1
) + … + (P
gn
× t
n
))
0
T
t
Drive a
Common DC
M M M
P
duty cycle
t
duty cycle
t
Drive b
duty cycle
t
Drive c
duty cycle
P
g1
P
g2
t
g2
T
t
g1
P
P
P
a
b
c
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