Extreme Running Conditions - Danfoss VLT FC 102 Series Design Manual

Introduction to VLT® HVAC D...
NOTICE!
OVC cannot be activated when running a PM motor
(when 1-10 Motor Construction is set to [1] PM non salient
SPM).
2.12.4 Brake Resistor Cabling
EMC (twisted cables/shielding)
Twist the wires to reduce the electrical noise from the
wires between the brake resistor and the adjustable
frequency drive.
For enhanced EMC performance, use a metal shield.

2.13 Extreme Running Conditions

Short Circuit (Motor Phase – Phase)
The adjustable frequency drive is protected against short
circuits by current measurement in each of the three
motor phases or in the DC link. A short circuit between
two output phases causes an overcurrent in the inverter.
The inverter is turned off individually when the short
circuit current exceeds the permitted value
(Alarm 16 Trip Lock).
To protect the adjustable frequency drive against a short
circuit at the load sharing and brake outputs, see the
design guidelines.
Switching on the output
Switching on the output between the motor and the
adjustable frequency drive is permitted. Fault messages
may appear. Enable flying start to catch a spinning motor.
Motor-generated overvoltage
The voltage in the intermediate circuit is increased when
the motor acts as a generator. This occurs in following
cases:
•
The load drives the motor (at constant output
frequency from the adjustable frequency drive),
i.e., the load generates energy.
•
During deceleration (ramp-down) if the moment
of inertia is high, the friction is low 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.
MG11BC22
Design Guide
•
WARNING
The adjustable frequency drive must be equipped with a
brake chopper.
The control unit may attempt to correct the ramp if
possible (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 2-10 Brake Function and 2-17 Over-voltage Control to
select the method used for controlling the intermediate
circuit voltage level.
NOTICE!
OVC cannot be activated when running a PM motor
(when 1-10 Motor Construction is set to [1] PM non
salient SPM).
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 AC 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 4-16 Torque Limit Motor Mode/4-17 Torque
Limit Generator Mode is reached), the controls reduces the
output frequency to reduce the load.
If the overload is excessive, a current may occur that
makes the adjustable frequency drive cut out after approx-
imately 5-10 s.
Operation within the torque limit is limited in time (0–60 s)
in 14-25 Trip Delay at Torque Limit.
Danfoss A/S © Rev. 06/2014 All rights reserved.
Back-EMF from PM motor operation. If coasted at
high RPM, the PM motor back-EMF may
potentially exceed the maximum voltage
tolerance of the adjustable frequency drive and
cause damage. To help prevent this, the value of
4-19 Max Output Frequency is automatically
limited based on an internal calculation based on
the value of 1-40 Back EMF at 1000 RPM,
1-25 Motor Nominal Speed and 1-39 Motor Poles.
If it is possible that the motor may overspeed
(e.g., due to excessive windmilling effects),
Danfoss recommends using a brake resistor.
plus
mode
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