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Earth leakage current
Earth leakage current is primarily caused by the
capacitance between motor phases 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.
See drawing on the following page.
The size of the leakage current to the ground depends
on the following factors, in order of priority:
1. Length of motor cable
2. Motor cable with or without screen
3. 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 frequency converter if (by
mistake) the frequency converter has not been earthed.
Extreme running conditions
Short circuit
VLT 7000 Booster is protected against short circuits
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 a few microseconds the driver card turns
off the inverter and the frequency converter will
display a fault code, although depending on
impedance and motor frequency.
Earth fault
The inverter cuts out within a few micorseconds in
case of an earth fault on a motor phase, although
depending on impedance and motor frequency.
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 7000 Booster in any way by switching
on the output. However, fault messages may appear.
Motor-generated overvoltage
118
VLT® 7000 Booster
NB!:
Since the leakage current is > 3.5 mA,
rein-forced earthing must be established, which
is required if EN 50178 is to be complied with.
Never use ELCB relays (type A) that are not suitable for
DC fault currents from three-phase rectifier loads.
If ELCB relays are used, they must be:
- Suitable for protecting equipment with a
direct current content (DC) in the fault current
(3-phase bridge rectifier)
- Suitable for power-up with short pulse-shaped
charging current to earth
- Suitable for a high leakage current (300 mA).
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 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.
MG.70.A1.02 - VLT is a registered Danfoss trademark
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