Specifications

130BB955.12
a
b
Leakage current
Motor cable length
Illustration 2.35 Cable Length and Power Size Influence on
Leakage Current. Pa > Pb
130BB956.12
THVD=0%
THVD=5%
Leakage current
Illustration 2.36 Line Distortion Influences Leakage Current
NOTICE
When a filter is used, turn off 14-50 RFI Filter when
charging the filter to avoid that a high leakage current
makes the RCD switch.
EN/IEC61800-5-1 (Power Drive System Product Standard)
requires special care if the leakage current exceeds 3.5 mA.
Grounding must be reinforced in one of the following
ways:
•
Ground wire (terminal 95) of at least 10 mm
2
•
2 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 earth
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 earth currents
•
Dimension RCDs according to the system configu-
ration and environmental considerations
130BB958.12
f
sw
Cable
150 Hz
3rd harmonics
50 Hz
Mains
RCD with low f
cut-
RCD with high f
cut-
Leakage current
Frequency
Illustration 2.37 Main Contributions to Leakage Current
130BB957.11
Leakage current [mA]
100 Hz
2 kHz
100 kHz
Illustration 2.38 The Influence of the Cut-off Frequency of the
RCD on what Is Responded to/measured
See also RCD Application Note, MN90G.
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
brake resistor may be utilised. Using a brake resistor
ensures that the energy is absorbed in the resistor and not
in the frequency converter.
If the amount of kinetic energy transferred to the resistor
in each braking period is not known, the average power
can be calculated on the basis of the cycle time and
braking time also called intermitted duty cycle. The resistor
intermittent duty cycle is an indication of the duty cycle at
which the resistor is active. Illustration 2.39 shows a typical
braking cycle.
The intermittent duty cycle for the resistor is calculated as
follows:
Duty Cycle = t
b
/ T
T = cycle time in seconds
t
b
is the braking time in seconds (as part of the total cycle
time)
Introduction to VLT® HVAC D...
Design Guide
MG11BC02 Danfoss A/S © Rev. 06/2014 All rights reserved. 47
2 2