Resistor Sizing For Multiple Motoring Systems - Control Techniques Unidrive Regen Installation Manual

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
then go back to step 3 (Table B-2)
Table B-2
Resistor value Power rating
Ω
W
150 53
48 148
Table B-3
Capacitor bank
Drive size Energy per drive
1
2
3
4
5
Table B-4
RMS resistor current data
Drive size RMS current
1
2
3
4
5
Table B-5
Total resistor value data
Total softstart resistor
Drive size
1
2
3
4
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 48Ω resistors can be arranged to achieve 16Ω. In this case 16Ω 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 16Ω bank (85W per resistor) = 256W, resistor bank maximum rating = 3 x 148W = 444W, so the 16Ω resistor arrangement is
suitable.
Unidrive Regen Installation Guide
Issue Number: 2
2
rms
.
Resistor data
Energy rating
J
170
1,700
J
75
200
600
1800
2500
A
0.4
0.4
0.5
0.6
0.6
value
W
75 to 150
28 to 150
9 to 48
3 to 24
2 to 24
www.controltechniques.com
x R]. If the power dissipation exceeds the rating of the resistor bank, add more resistors,
CT part number
1270-3157
1270-2483
37