Earth Leakage Current; Selection Of Brake Resistor - Danfoss FC 300 Design Manual

FC 300 Design Guide

Earth Leakage Current

Warning:
Touching the electrical parts may be fatal - even after the equipment has been disconnected
from mains.
Also make sure that other voltage inputs have been disconnected, such as load sharing (link-
age of DC intermediate circuit), as well as the motor connection for kinetic back-up.
Using VLT AutomationDrive FC 300: 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 FC 300 exceeds 3.5 mA. To ensure that the earth cable
has a good mechanical connection to the earth connection (terminal 95), the cable cross sec-
tion must be at least 10 mm
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 delayed) 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 na-
tional and local regulations.

Selection of Brake Resistor

To handle higher demands by generatoric braking a brake resistor is necessary. Using a brake resistor
ensures that the energy is absorbed in the brake 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 aver-
age 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.
NB!
Motor suppliers often use S5 when stating the permissible load which is an expression of in-
termittent duty 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 (of the cycle time)
b
Danfoss offers brake resistors with duty cycle of 5%, 10% and 40%. If a 10% duty cycle is applied, the
brake resistors are able to absorb brake power for 10% of the cycle time. The remaining 90% of the cycle
time will be used on deflecting excess heat.
40
Introduction to FC 300
2 or 2 rated earth wires terminated separately.
MG.33.B7.02 - VLT is a registered Danfoss trademark