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70
Unidrive SPM User Guide
www.controltechniques.com Issue Number: 3
6.4 Supplying the drive with DC / DC bus
paralleling
The drive may be supplied with DC instead of 3 phase AC.
The connecting of the DC bus between several drives is typically used to:
1. Return energy from a drive which is being overhauled by the load to
a second motoring drive.
2. Allow the use of one braking resistor to dissipate regenerative
energy from several drives.
There are limitations to the combinations of drives which can be used in
this configuration.
For further information, contact the supplier of the drive for the
Application Note
DC bus paralleling
.
6.5 Resistor sizing for Unidrive SPMU
softstart
A separate soft-start must be provided for the DC bus of a Unidrive SPMD
system when a Unidrive SPMC is not used. The start-up circuit limits the
amount of current flowing into the DC bus of the drive when the supply is
first switched on. The recommended configuration is shown in Figure 6-8.
Figure 6-8 Softstart circuit configuration
K1
: Main supply contactor to drive.
K2
: Soft-start contactor
R1
: Soft-start resistor
MCB1
: Thermal / magnetic circuit breaker
K1 and K2 should be energized at the same time.
MCB1 is normally closed
Drive is not allowed to start until K1 has closed.
K1 is energized 3s after the drive OK parameter becomes active as
shown in Figure 6-9
Figure 6-9 Switching sequence for K1 and K2
6.5.1 Procedure
Selection of the resistor and contactor is an iterative process requiring
calculations based on the total DC bus capacitance, supply voltage and
knowledge of the available parts.
1. Calculate the total DC bus capacitance of the system by simply
adding the DC bus capacitances of each drive that is to be started by
the soft-start circuit.
2. Calculate the energy stored in the systems DC bus capacitance at
the maximum supply voltage using:
Where:
W
: Maximum energy stored in the DC bus (Joules)
C
: Total DC bus capacitance (Farads)
V
ll
: Nominal line-to-line supply voltage (Volts)
N
20% over-rate has been applied to allow for component tolerances and a
further 10% over-rate has been applied to allow for supply variations.
3. Calculate the minimum number of resistors required to meet this
energy value (round up to the nearest one), (Table 6-12). Then
calculate the series parallel arrangement of resistors to produce the
total resistor value in the required range (Table 6-12 and Table 6-13).
4. Calculate the peak supply current and select the MCB. Ensure that
the peak current is less than that shown in Table 6-10. If the current
is too high, then choose a series / parallel arrangement of resistors
that will give a higher resistance and therefore a lower peak current.
Where:
R
: Total resistance of the soft-start resistor network. (Ohms)
I
pk
: Peak supply current (Amps)
The MCB must have a current rating no less than 13 x less than this
peak current to avoid nuisance trips. See Figure 6-15. MCBs available
from Control Techniques can be found in Table 6-13.
L1
L2
L3
Supply
L1
L2
L3
K2
R1
SPMU
L1
L2
L3
L1
L2
L3
Enable
I
>
24V
K1
MCB1
DC+
DC-
Drive
DC+
DC-
Rectifier Status
Status
U
W
V
To
Motor
+
-
0V
K 1
OPEN
CLOSED
K 2
OPEN
CLOSED
3s
Under Voltage
Drive Healthy
3s
Under
voltage
active
Pr
10.16
W
1.45 C V
ll
2
×
×
=
NOTE
I
pk
1.56 V
ll
×
R
-------------------------
=