Ground Leakage Current; Control With Brake Function - Danfoss VLT AQUA Drive Design Manual

2. Introduction to the VLT AQUA Drive

2.12. Ground leakage current

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2.13. Control with brake function

2.13.1. Selection of Brake Resistor
In certain applications, for instance, in centrifuges, 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.
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. The figure below shows a typical braking cycle.
The intermittent duty cycle for the resistor is calculated as follows:
Duty Cycle = t
T = cycle time in seconds
t is the braking time in seconds (as part of the total cycle time)
b
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Warning:
Touching the electrical parts may be fatal - even after the equipment has been dis-
connected from line power.
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 backup.
Before touching any electrical parts, wait at least 15 minutes.
A shorter time is allowed only if indicated on the nameplate for the specific unit.
Leakage Current
The ground leakage current from the adjustable frequency drive exceeds 3.5 mA.
To ensure that the ground cable has a good mechanical connection to the ground
connection (terminal 95), the cable cross-section must be at least 0.016 in.
2
mm ] or have 2 rated ground wires terminated separately.
Residual Current Device
This product can cause DC 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.yy.
Protective grounding of the adjustable frequency drive and the use of RCDs must
always follow national and local regulations.
/T
b
MG.20.N2.22 - VLT ® is a registered Danfoss trademark.
VLT ® AQUA Drive Design Guide
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