80
VLT
®
3500 HVAC
MG.35.B1.02 – VLT is a registered Danfoss trademark
4.5 Special conditions
Earth leakage current
Earth leakage current is primarily caused by the
capacitance between the conductor and the motor
cable screen. When an RFI filter is used, this
contributes additional leakage current, as
the filter circuit is connected to earth through
capacitors.
The size of the leakage current to the ground
depends on the following factors:
•
Length of motor cable
•
Switching frequency
•
RFI filter used or not
•
Motor grounded on site or not
•
Motor cable w/ or w/o screen
The leakage current is of importance regarding safety
during handling/operation of the frequency converter
if an earth connection has not been established.
■
NB!:
NB!:
NB!:
NB!:
NB!:
Never operate the frequency converter without
an effective earth connection complying with
local regulations for high leakage current (>3.5
mA). Never use HFI relays. That is not allowed
because of the rectifier load.
If FI relays are used, they must be
•
Suitable for protecting equipment with direct
current in the discharge current (3-phase bridge
rectifier)
•
Suitable for power-up with pulsing, short dis-
charge
•
Suitable for a high discharge current
■
The control unit attempts to correct the ramp if
possible.
The inverter turns off to protect the transistors and
the intermediate circuit capacitors when a certain
voltage level is reached.
Mains drop-out
During a mains drop-out, VLT 3500 HVAC continues
until the intermediate circuit voltage drops below the
minimum stop level, which is typically 15% below VLT
3500 HVAC's lowest rated supply voltage.
The time before the inverter stops depends on the
mains voltage before the drop-out and on the motor
load. Ride-through and/or flying start can be pro-
grammed.
Static overload
When VLT 3500 HVAC is overloaded (the current limit
I
LIM
is reached), the controls will reduce the output
frequency in an attempt to reduce the load. If the
reduction of the output frequency does not reduce
the load, a trip is activated when the output frequency
has fallen below 0.5 Hz.
Operation in the current limit can be limited in time (0-
60 s) by adjusting parameter 310.
Extreme running conditions
Short circuit
VLT 3500 HVAC is protected against short circuit by
means of current measurement in each of the three
motor phases. A short circuit between two output
phases will cause an overcurrent in the inverter.
However, each transistor of the inverter will be turned
off individually when the short circuit current exceeds
the permitted value.
After 5-10 seconds the driver card turns off the
inverter and the frequency converter will display a
fault code.
Earth fault
In case of an earth fault in a motor phase, the inverter
is turned off within 5-10 ms.
Switching on the output
Switching on the output between the motor and the
frequency converter is fully permitted. It is not
possible to damage VLT 3500 HVAC in any way by
switching on the output. However, fault messages
may appear.
Motor-generated overvoltage
The voltage in the intermediate circuit is increased
when the motor acts as a generator. This occurs in
two cases:
1. The load drives the motor (at constant output
frequency from the frequency converter), i.e. the
load generates energy.
2. During deceleration ("ramp-down") if the moment
of inertia is high, the load is low and/or the ramp-
down time is short.
