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)
5.2.3 Extreme Operating Conditions
Short circuit
The frequency converter 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 s and the frequency converter displays a fault code, although this depends on impedance and motor frequency.
Earth fault
The IGBT module cuts out within 100 s in case of an earth 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 frequency converter
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 frequency converter the IGBT module is discon-
nected 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 frequency converter, 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 frequency converter has an integrated brake module. If the frequency converter 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 frequency converter is overloaded (current limit in parameter 221
Current limit I
LIM
is 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 frequency converter to trip after approx.
1.5 sec. See parameter 409
Trip delay overcurrent, I
LIM
.
An extreme overload will cause the switching frequency to be derated to 3000 Hz.
5.2.4 dU/dt on Motor
When a transistor in the inverter is opened, the voltage across the motor terminals will rise by a voltage/time ratio (dU/dt) determined by:
-
the motor cable (type, cross-section, induction, capacity, length and screened/armoured/unscreened/unarmoured)
-
the mains voltage
Self-induction in the motor cable leads to an overswing U
PEAK
of the output voltage each time a transistor in the inverter is opened. After U
PEAK
the output
voltage will stabilise at a level determined by the voltage in the intermediate circuit. U
PEAK
and dU/dt influence the lifetime of the motor, especially motors
without phase insulation paper in the coils. If the motor cable is short (a few metres), the overshoot U
PEAK
is low, while the dU/dt is high. If the motor
cable length is increased, U
PEAK
is increased, while dU/dt decreases.
5.2.5 Switching on the Input
The waiting time between switching the mains voltage on terminals 91, 92 and 93 must be min. 30 sec. Start up time appr. 2.3 sec.
VLT
®
Decentral FCD 300 Design Guide
5 All About FCD 300
MG.90.S1.02 - VLT
®
is a registered Danfoss trademark
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