Earth Leakage Current; Extreme Running Conditions - Danfoss VLT HVAC Basic Drive FC 101 Design Manual

Introduction to VLT® HVAC B...

2.10 Earth Leakage Current

WARNING
DISCHARGE TIME
Touching the electrical parts could 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 (linkage of DC
intermediate circuit), as well as the motor connection for
kinetic back-up.
Before touching any electrical parts, wait at least the
amount of time indicated in the Table 2.1.
Shorter time is allowed only if indicated on the nameplate
for the specific unit.
NOTE
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, the cable
cross section must be at least 10 mm
wires terminated separately.
Residual Current Device protection RCD
This product can cause a DC current in the protective
conductor. Where a residual current device (RCD) is used
for protection in case of direct or indirect contact, only an
RCD of Type B is allowed on the supply side of this
product. Otherwise, another protective measure shall be
applied, such as separation from the environment by
double or reinforced insulation, or isolation from the
supply system by a transformer. See also Application Note
Protection against Electrical Hazards, MN90G.
Protective earthing of the frequency converter and the use
of RCDs must always follow national and local regulations.

2.11 Extreme Running Conditions

Short Circuit (Motor Phase – Phase)
The frequency converter is protected against short circuits
by means of current measurement in each of the three
motor phases or in the DC link. A short circuit between
two output phases causes an overcurrent in the inverter.
The inverter is turned off individually when the short
circuit current exceeds the permitted value (Alarm 16 Trip
Lock).
To protect the frequency converter against a short circuit
at the load sharing and brake outputs see the design
guidelines.
Switching on the Output
Switching on the output between the motor and the
frequency converter is fully permitted. The frequency
converter is not damaged in any way by switching on the
output. However, fault messages may appear.
®
VLT HVAC Basic Drive Design Guide
Motor-generated Over-voltage
The voltage in the intermediate circuit is increased when
the motor acts as a generator. This occurs in following
cases:
The control unit may attempt to correct the ramp if
possible (2-17 Over-voltage Control.)
The inverter turns off to protect the transistors and the
intermediate circuit capacitors when a certain voltage level
is reached.
Mains Drop-out
2
or 2 rated earth
During a mains drop-out, the frequency converter keeps
running until the intermediate circuit voltage drops below
the minimum stop level, which is typically 15% below the
frequency converter's lowest rated supply voltage. The
mains voltage before the drop-out and the motor load
determines how long it takes for the inverter to coast.
2.11.1 Motor Thermal Protection
This is the way Danfoss protects the motor from being
overheated. It is an electronic feature that simulates a
bimetal relay based on internal measurements. The charac-
teristic is shown in Illustration 2.27.
2000
1000
The X-axis is showing the ratio between I
nominal. The Y-axis is showing the time in seconds before
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MG18C402 - VLT is a registered Danfoss trademark
1. The load drives the motor (at constant output
frequency from the frequency converter), that is
the load generates energy.
2. During deceleration ("ramp-down") if the moment
of inertia is high, the friction is low and the ramp-
down time is too short for the energy to be
dissipated as a loss in the frequency converter,
the motor and the installation.
3. Incorrect slip compensation setting (1-62 Slip
Compensation) may cause higher DC link voltage.
t [s]
600
500
400
300
200
100
60
50
40
30
20
10
1.0 1.2 1.4 1.6 1.8
Illustration 2.27
f OUT = 1 x f M,N (par. 1-23)
f OUT = 2 x f M,N
f OUT = 0. 2 x f M,N
I
M
I (par. 1-24)
2.0
MN
and I
motor motor
37
2 2