Ground Leakage Current And Rcd Relays; Extreme Operating Conditions - Danfoss VLT DMS 300 Manual

Ground leakage current and RCD relays

Ground leakage current is primarily caused by the
capacitance between motor phases and motor cable
shield. When an RFI filter is used, this contributes
additional leakage current, as the filter circuit is
connected to ground through capacitors.
The size of the ground leakage current depends on
the following factors, in order of priority:
1. Length of the motor cable
2. Motor cable with/without shield
3. High switching frequency
4. RFI filter used or not
5. Motor grounded on site or not
The leakage current is of importance to safety
during handling/operation of the variable frequency
drive if (by mistake) the variable frequency drive
has not been grounded.
NOTE
Since the leakage current is > 3.5 mA, reinforced
grounding must be established, as this is
required to ensure compliance with EN 50178.
Never use ELCB relays (type A) that are not suitable
for DC fault currents from 3-phase rectifier loads.
If ELCB relays Type B are used, they must be:
- Suitable for protecting equipment with a DC content
in the faulty current (3-phase bridge rectifier)
- Suitable for a pulse-shaped, brief discharge
on power-up
- Suitable for a high leakage current (300 mA)

Extreme operating conditions

Short circuit
The variable frequency drive is protected against
short circuits on motor terminals U, V, W (96, 97,
98). A short circuit between two motor terminals
would lead to an overcurrent in the IGBT module,
which means that all transistors in the IGBT module
would independently cut out.
The inverter turns off after 5-10 µs and the variable
frequency drive displays a fault code, although this
depends on impedance and motor frequency.
180
Decentral Solutions
Ground fault
The IGBT module cuts out within 100 µs in case
of a ground fault on one of the motor terminals
U, V, W (96, 97, 98), although depending on
impedance and motor frequency.
Output connection
Motor terminals U, V, W (96, 97, 98) for the motor
can be connected/disconnected as often as required.
There is no way that the variable frequency drive can
be destroyed by connecting/disconnecting the motor
terminals. However, fault messages may appear.
Motor-generated overvoltage
The voltage in the intermediate circuit is increased when
the motor acts as a generator. To protect the variable
frequency driver the IGBT module is disconnected
when a specific voltage level is reached.
Motor-generated overvoltage can occur in
two instances:
1. The load drives the motor, i.e. the load
generates energy.
2. During deceleration (ramp-down) if the moment of
inertia is high, the load is low and the ramp-down
time too short for the energy to be dissipated
as a loss in the variable frequency drive, the
motor and the unit. The control unit attempts
to correct the ramp if possible.
The fault can be removed by connecting a brake
resistor, if the variable frequency drive has an integrated
brake module. If the variable frequency drive does not
have an integrated brake module an AC brake can
be used, see parameter 400 Brake function.
See the section entitled Brake resistors.
Static overloading
When the variable frequency drive is overloaded
(current limit in parameter 221 Current limit I
reached), the control reduces the output frequency in
an attempt to reduce the load. If the overloading is
extreme, there might be an output current that causes
the variable frequency drive to trip after approx. 1.5
sec. See parameter 409 Trip delay overcurrent, I
An extreme overload will cause the switching
frequency to be derated to 3000 Hz.
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