The ground leakage current is made up from several
contributions and depends on various system configu-
rations including RFI filtering, shielded motor cables and
adjustable frequency drive power.
a
b
130BB955.10
Cable length [m]
Leakage current [mA]
Figure 2.33 Cable Length and Power Size Influence on Leakage
Current. Pa > Pb.
130BB956.10
Leakage current [mA]
THVD=0%
THVD=5%
Figure 2.34 Line Distortion Influences Leakage Current.
NOTE!
When a filter is used, turn off
14-50 RFI Filter
when
charging the filter, to avoid a high leakage current making
the RCD switch.
EN/IEC61800-5-1 (Power Drive System Product Standard)
requires special care if the leakage current exceeds 3.5mA.
Grounding must be reinforced in one of the following
ways:
•
Ground wire (terminal 95) of at least 0.016 in
2
[10mm
2
]
•
Two separate ground wires both complying with
the dimensioning rules
See EN/IEC61800-5-1 and EN50178 for further information.
Using RCDs
Where residual current devices (RCDs), also known as
ground leakage circuit breakers (ELCBs), are used, comply
with the following:
Use RCDs of type B only which are capable of
detecting AC and DC currents
Use RCDs with an inrush delay to prevent faults
due to transient ground currents
Dimension RCDs according to the system configu-
ration and environmental considerations
L
leakage
[mA]
f [Hz]
f
sw
Cable
f
s
150 Hz
3rd harmonics
50 Hz
Mains
130BB958.10
RCD with low f
cut-off
RCD with high f
cut-off
Figure 2.35 Main Contributions to Leakage Current.
Leakage current [mA]
100 Hz
130BB957.10
2 kHz
100 kHz
Figure 2.36 The influence of the cut-off frequency of the RCD on
what is responded to/measured.
See also RCD Application Note, MN.90.GX.02.
2.12 Brake Function
2.12.1 Selection of Brake Resistor
In certain applications, for instance in tunnel or
underground railway station ventilation systems, it is
desirable to bring the motor to a stop more rapidly than
can be achieved through controlling via ramp-down or by
free-wheeling. In such applications, dynamic braking with a
braking resistor may be utilized. Using a braking resistor
ensures that the energy is absorbed in the resistor and not
in the Adjustable frequency drive.
Introduction to VLT® HVAC D...
VLT
®
HVAC Drive Design Guide
2-40
MG11BB22 - VLT
®
is a registered Danfoss trademark
2
2
