Specifications
Altivar
®
31 Adjustable Speed AC Drives
Dynamic Braking Resistor Kits
15
4/2005 © 2004–2005 Schneider Electric All Rights Reserved
DB resistor requirements:
Peak braking power required to develop braking torque (T
b
) when decelerating from a given speed:
P
i
= (T
b
x N
base
)/7.04 = (1.97 x 1740)/7.04 = 487 W
The braking power that must be absorbed for a time (t
d
) during a stop or speed change operation:
P
d
= 0.5 x P
i
= 0.5 x 487 = 243 W for a period of t
d
seconds
The average braking power that must be dissipated during a machine cycle:
P
a
= P
d
x t
d
/t
c
= 243 x 5/30 = 40.5 W
Capability of VW3A66711 DB resistor assembly for ATV31HU40N4 controller:
Peak braking power that can be developed with the VW3A66711 DB resistor assembly with the
controller configured for 460 Vac input line operation: P
i
= (V
db
)
2
/R
db
= (850 V)
2
/120 Ω = 6020 W
The braking power that can be absorbed for t
d
(based on the DB resistor hot state current-time
characteristic curve shown below):
P
d
= R
db
x [(Multiple of I
r
at t
d
) x Ir]
2
= 120 Ω x (3.5 x 1.0)
2
= 1470 W
Since R
db
limits the peak current that can be drawn from the drive controller DC bus, the value of
[(Multiple of I
r
) x I
r
] must be limited to no greater than (√ P
i
/R
db
).
The average braking power that can be dissipated continuously:
P
a
= R
db
x (I
r
)
2
= 120 Ω x (1)
2
= 120 W
For this example, the VW3A66711 DB resistor assembly will work as intended for the application.
Current/time characteristics for DB resistor assemblies
The figure below shows the allowable trip times
1
as a function of current setting multiples with the
dynamic braking resistor assembly located in a 40 °C (104 °F) ambient temperature environment. See
“Calculating resistor size” on page 14 for an example of how to calculate resistor size.
1. Of the GV2 manual starter; see page 16.
Time (s)
10000
1000
100
10
1
0.1
0.01
0.001
1
1.5
10
100
Multiples of Current Setting (lr)
Cold State
Hot State
DynBrkCurves.eps
Courtesy of Steven Engineering, Inc.-230 Ryan Way, South San Francisco, CA 94080-6370-Main Office: (650) 588-9200-Outside Local Area: (800) 258-9200-www.stevenengineering.com