Extreme Running Conditions - Danfoss FC 300 Design Manual

FC 300 Design Guide

Extreme Running Conditions

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Short Circuit (Motor Phase – Phase)
The adjustable frequency drive 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 drive against a short circuit at the load sharing and brake out-
puts, please see the design guidelines.
Switching on the Output
Switching on the output between the motor and the adjustable frequency drive is fully
permitted. You cannot damage the adjustable frequency drive 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 gener-
ator. This occurs in the following cases:
The load drives the motor (at constant output frequency from the adjustable frequency
1.
drive), i.e. the load generates energy.
During deceleration ("ramp-down"), if the moment of inertia is high, the friction is low
2.
and the ramp-down time is too short for the energy to be dissipated as a loss in the
adjustable frequency drive, the motor, and the installation.
Incorrect slip compensation setting may cause higher DC link voltage.
3.
The control unit may attempt to correct the ramp if possible (par. 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.
See par. 2-10 and par. 2-17 to select the method used for controlling the intermediate circuit voltage level.
Line Drop-out
During a line drop-out, the adjustable frequency drive keeps running until the intermediate
circuit voltage drops below the minimum stop level, which is typically 15% below the
adjustable frequency drive's lowest rated supply voltage.
The line voltage before the drop-out and the motor load determine how long it takes for the inverter to coast.
Static Overload in VVC
When the adjustable frequency drive is overloaded (the torque limit in par. 4-16/4-17 is reached),
the control reduces the output frequency to reduce the load.
If the overload is excessive, a current may occur that makes the adjustable fre-
quency drive cut out after approx. 5-10 s.
Operation within the torque limit is limited in time (0-60 s) in par. 14-25.
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Introduction to FC 300
plus mode
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