Unidrive Regen Installation Guide
37
Issue Number: 2
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B.2
Resistor sizing for multiple motoring systems
The softstart resistor must be recalculated for the multiple motoring system due to the increased inrush current and therefore increased power
dissipation.
For applications where the total DC bus capacitance of the motoring drives is greater than that of the Regen drive (one large drive supplying several
smaller drives). The following procedure and tables should be used to recalculate the resistor(s) required:
B.2.1
Procedure
1.
Calculate the total capacitor bank energy rating of the system (Table B-3).
2.
Calculate the minimum number of resistors required to meet this energy value (round up to the nearest one), (Table B-2).
3.
Calculate the series parallel arrangement of resistors to produce the total resistor value in the required range (Table B-5 and Table B-2).
4.
Calculate the total rms resistor current (Table B-4).
5.
Calculate the power dissipation in the resistor bank [I
rms
2
x R]. If the power dissipation exceeds the rating of the resistor bank, add more resistors,
then go back to step 3 (Table B-2)
.
Table B-2
Table B-3
Table B-4
Table B-5
Example:
Unidrive size 4 Regen with (3 x Unidrive size 3) and (5 x Unidrive size 1) motoring drives
1.
Capacitor bank energy = 1800 + (3 x 600) + (5 x 95) = 4075J
2.
Using resistors 1270-2483 (4,075 / 1,700) you will need at least 2.39 resistors = 3
3.
Three 48
Ω
resistors can be arranged to achieve 16
Ω
. In this case 16
Ω
is satisfactory for a Unidrive size 4, so parallel connection can be used.
4.
Total rms current needed = 0.6 + (3 x 0.5) + (5 x 0.4) = 4.1A
5.
Power loss in 16
Ω
bank (85W per resistor) = 256W, resistor bank maximum rating = 3 x 148W = 444W, so the 16
Ω
resistor arrangement is
suitable.
Resistor data
Resistor value
Ω
Power rating
W
Energy rating
J
CT part number
150
53
170
1270-3157
48
148
1,700
1270-2483
Capacitor bank
Drive size
Energy per drive
J
1
75
2
200
3
600
4
1800
5
2500
RMS resistor current data
Drive size
RMS current
A
1
0.4
2
0.4
3
0.5
4
0.6
5
0.6
Total resistor value data
Drive size
Total softstart resistor
value
W
1
75 to 150
2
28 to 150
3
9 to 48
4
3 to 24
5
2 to 24
