Brake Function - Danfoss VLT FC 102 Series Design Manual

Introduction to VLT® HVAC D...
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 0.016 in
[10 mm 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 (GLCBs), are used, comply
with the following:
•
Use RCDs of type B only which are capable of
detecting AC and DC currents
•
Use RCDs with a soft-charge delay to prevent
faults due to transient ground currents
•
Dimension RCDs according to the system configu-
ration and environmental considerations
RCD with low f
Leakage current
RCD with high f
50 Hz 150 Hz
Mains 3rd harmonics
Figure 2.37 Main Contributions to Leakage Current
Leakage current [mA]
Figure 2.38 The Influence of the Cut-off Frequency of the
RCD on What Is Responded to/Measured
See also RCD Application Note, MN90G.
MG11BC22
Design Guide
2
cut-
cut-
f Frequency
sw
Cable
100 Hz
2 kHz
100 kHz
Danfoss A/S © Rev. 06/2014 All rights reserved.

2.12 Brake Function

2.12.1 Selection of Brake Resistor
In certain applications, such as in tunnels 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 free-wheeling. In
such applications, dynamic braking with a brake resistor
may be utilized. Using a brake resistor ensures that the
energy is absorbed in the resistor and not in the
adjustable frequency drive.
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 known as the intermitted duty cycle.
The resistor intermittent duty cycle is an indication of the
duty cycle at which the resistor is active. Figure 2.39 shows
a typical braking cycle.
The intermittent duty cycle for the resistor is calculated as
follows:
Duty Cycle = t / T
b
T = cycle time in seconds
t is the braking time in seconds (as part of the total cycle
b
time)
Load
Speed
ta tc to
tb
T
Figure 2.39 Intermittent Duty Cycle for the Resistor
Danfoss offers brake resistors with duty cycle of 5%, 10%
and 40% suitable for use with the VLT
adjustable frequency drive series. If a 10% duty cycle
resistor is applied, it is capable of absorbing braking
energy up to 10% of the cycle time, with the remaining
90% being used to dissipate heat from the resistor.
For further selection advice, contact Danfoss.
ta tc to ta
tb
Time
®
HVAC Drive
53
2 2