Low Voltage Dc Supply Types; External Softstart Resistor - Control Techniques Unidrive Installation Manual

4.3

Low Voltage DC supply types

The DC supply may be connected to ground or left floating. In the event of a contactor
sticking, high voltage would be present at the negative terminal of the drive.
The instructions below ensure user safety in both cases.
4.3.1 Systems with an isolated DC supply
The supply can be floating with respect to ground, although it may have a high
impedance ground to drain leakage currents.
In the event of a fault where AC and DC supplies are connected at the same time a high
current would have no fault path to ground.
In this case:
• The 48V, +DC, -DC terminals of the drive and DC terminals of the Low Voltage DC
source must be protected from user contact.
• The Low Voltage DC supply must be able to withstand mains potential with respect
to ground and be suitable for use in an industrial environment (category 2 supply).
• Cables rated for the voltage of the rectified 3-phase AC supply must be used to
connect the drive to the Low Voltage DC supply.
In the event of a fault, the 48V, +DC, -DC terminals of the drive and DC terminals of the
Low Voltage DC source (including any wiring in between) could be at a potentially lethal
voltage.
WARNING
4.3.2 Systems with a grounded DC supply
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If the I t of the ground connection is not greater than that of the fuses used, then the 48V/
DC terminal and associated wiring could be at a potentially lethal voltage in the event of
a fault.
WARNING
The ground connection for the supply must be a high current connection with an I
rating greater than the fuses F2a & F2b (see section 5.1 Fusing on page 30).
This is so that in the event of a fault where AC and DC supplies are connected at the
same time a high current will flow to ground and blow the fuses in the Low Voltage DC
path.
• The wiring from the drive to fuses F2a, F2b & F2c must be protected to a voltage
rating equal to or exceeding the rectified 3-phase AC supply voltage (see section
5.1 Fusing on page 30).
• The wiring from the fuse to the supply must be rated correctly for the supply.
4.4

External softstart resistor

When Unidrive SP size 1 to 3 operates from AC or DC there is a built in soft start
resistor to limit the inrush current. However when Unidrive SP size 4 and larger operate
from Low Voltage DC there is no inbuilt soft start resistor and therefore an external soft
start resistor is required between the Low Voltage DC supply and the drive.
Failure to fit a soft start resistor may damage the drive or/and external components.
NOTE
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Unidrive SP Low Voltage DC Installation Guide
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