Extreme Running Conditions - Danfoss VLT HVAC Basic Drive FC 100 Series Design Manual

Introduction to VLT HVAC Ba...
NOTE
Leakage Current
The earth leakage current from the frequency converter
exceeds 3.5mA. To ensure that the earth cable has a good
mechanical connection to the earth connection, the cable
cross section must be at least 10mm
wires terminated separately.
Residual Current Device
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 MN.90.G2.02.
Protective earthing of the frequency converter and the use
of RCDs must always follow national and local regulations.

2.10 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 will cause an overcurrent in the
inverter. The inverter will be 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 please see the
design guidelines.
See certificate in the Certificates section.
Switching on the Output
Switching on the output between the motor and the
frequency converter is fully permitted. The frequency
converter will not be damaged in any way by switching on
the output. However, fault messages may appear.
Motor-generated Over-voltage
The voltage in the intermediate circuit is increased when
the motor acts as a generator. This occurs in following
cases:
1. The load drives the motor (at constant output
frequency from the frequency converter), ie. 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.
VLT HVAC Basic Drive Design Guide
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.
2
or 2 rated earth
Mains Drop-out
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.
2.10.1 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 the following figure.
The X-axis is showing the ratio between I
nominal. The Y-axis is showing the time in seconds before
the ETR cuts off and trips the drive. The curves are
showing the characteristic nominal speed at twice the
nominal speed and at 0.2x the nominal speed.
It is clear that at lower speed the ETR cuts of at lower heat
due to less cooling of the motor. In that way the motor
are protected from being over heated even at low speed.
The ETR feature is calculating the motor temperature
based on actual current and speed.
The thermistor cut-out value is > 3kΩ.
Integrate a thermistor (PTC sensor) in the motor for
winding protection.
®
MG.18.C2.02 - VLT is a registered Danfoss trademark
t [s]
2000
1000
600
500
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200
100
60
50
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10
1.0 1.2 1.4 1.6
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)
1.8 2.0
M,N
and I
motor motor
35
2 2