3. System Design
MV3DB Series DB Units
ALSPA MV3000e Dynamic Braking Units
(07/06)
Page 3-2
3.4 Alternative
Braking
Resistor Selection
If the braking duty required is less than that given by the standard resistor (shown in Figures 2-
2 to 2-4), an alternative resistor may be selected. The following points should be noted:
1.
The energy regenerated into the ac drive from the motor(s) and its load is limited to a
rate based on the drive's capability. The drive overload current limit applies when
motoring or regenerating.
2.
The resistor value in ohms is usually determined by the DB unit current capability and
the maximum voltage at which it operates.
3.5
Calculation for Alternative Resistors
Where it is required to use a braking resistor other than a standard
Converteam
resistor as
shown in Table 2-1, the following procedure should be used. Two methods are given, one
based on a known power and the other upon a known inertia.
3.5.1
Calculation Based on Power
If the power being returned to the drive is known (e.g. overhauling load), the resistor may be
calculated as follows:
Resistance,
Ω
=
p
DB
P
V
R
2
where
DB
V
= DB operating voltage (see Table 2-2).
p
P
= Peak
power during braking.
Alternatively, a resistor whose resistance is lower than this calculated value may be used, in
which case the DB unit will cycle on and off. However, the resistance value must not be less
than that given in Table 2-1.
The resistor should have a peak power capability equal to the power during braking and be
able to withstand this power for the braking time
t
1
.
It should also have an average power capability given by:
Long term average power = Average power during braking
3
1
t
t
×
where
1
t
= braking time
and
3
t
= total on/off cycle time.
This is illustrated in Figure 3-2.
