2.12. Earth leakage current
Warning:
Touching the electrical parts may be fatal - even after the equipment has been dis-
connected from mains.
Also make sure that other voltage inputs have been disconnected, such as load-
sharing (linkage of DC intermediate circuit), as well as the motor connection for
kinetic back-up.
Before touching any electrical parts, wait at least 15 minutes.
Shorter time is allowed only if indicated on the nameplate for the specific unit.
Leakage Current
The earth leakage current from the frequency converter exceeds 3.5 mA. To ensure
that the earth cable has a good mechanical connection to the earth connection (ter-
minal 95), the cable cross section must be at least 10 mm
2
or 2 rated earth wires
terminated separately.
Residual Current Device
This product can cause a d.c. current in the protective conductor. Where a residual
current device (RCD) is used for extra protection, only an RCD of Type B (time de-
layed) shall be used on the supply side of this product. See also RCD Application
Note MN.90.GX.02.
Protective earthing of the frequency converter and the use of RCD's must always
follow national and local regulations.
2.13. Control with brake function
2.13.1. 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 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. The below figure 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)
VLT
®
HVAC Drive Design Guide
2. Introduction to VLT HVAC Drive
MG.11.B2.02 - VLT
®
is a registered Danfoss trademark
45
2
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