Specifications

the DC link voltage increases. This is done by increasing
the output frequency to limit the voltage from the DC link.
NOTICE
OVC cannot be activated when running a PM motor
(when P-20 Motor Construction is set to [1] PM non
salient SPM).
2.12.4 Brake Resistor Cabling
EMC (twisted cables/shielding)
Twist the wires to reduces noise between the brake
resistor and the frequency converter.
Use a metal screen for enhanced EMC performance
2.13 Extreme Running Conditions
Short Circuit (Motor Phase – Phase)
The frequency converter is protected against short circuits
by current measurement in each of the three motor phases
or in the DC link. A short circuit between 2 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).
See certificate in 2.6.3 Approvals & Certificates.
Output Switching
Output switching between the motor and the frequency
converter is permitted. Output switching does not damage
the frequency converter in any way but may cause fault
messages.
Motor-generated Over-voltage
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 frequency converter),
generating 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 frequency converter,
the motor and the installation.
•
Incorrect slip compensation setting may cause
higher DC link voltage.
•
Back-EMF (electromotive force) from PM motor
operation. If coasted at high rpm, the PM motor
back-EMF may potentially exceed the maximum
voltage tolerance of the frequency converter and
cause damage. To help prevent this, the value of
F-03 Max Output Frequency 1 is automatically
limited based on an internal calculation based on
the value of 1-40 Back EMF at 1000 RPM, P-06 Base
Speed and P-39 Motor Poles.
If it is possible that the motor may overspeed, a
brake resistor is recommended.
WARNING
The frequency converter must be equipped
with a break chopper.
The control unit may attempt to correct the ramp if
possible (B-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 parameters B-10 Brake Function and B-17 Over-voltage
Control to select the method used for controlling the
intermediate circuit voltage level.
NOTICE
OVC can not be activated when running a PM motor
(when P-20 Motor Construction is set to [1] PM non salient
SPM.
Mains Drop-out
During a mains drop-out, the frequency converter keeps
running until the intermediate circuit voltage drops below
the minimum stop level, which is typically 15% below the
lowest rated supply voltage. The mains voltage before the
drop-out and the motor load determine how long it takes
for the inverter to coast.
Static Overload in VVC
plus
mode
When the frequency converter is over loaded (the torque
limit in F-40 Torque Limiter (Driving)/F-41 Torque Limiter
(Braking) is reached), the controls reduce the output
frequency to reduce the load.
If the overload is excessive, a current may occur that
makes the frequency converter cut out after approx. 5-10
s.
Operation within the torque limit is limited in time (0-60 s.)
in SP-25 Trip Delay at Torque Limit.
2.13.1 Motor Thermal Protection
Motor thermal protection prevents the motor from
overheating. It is an electronic feature that simulates a
bimetal relay based on internal measurements. The charac-
teristic is shown in Illustration 2.34
In Illustration 2.34, the X-axis is showing the ratio between
I
motor
and I
motor
nominal. The Y-axis is showing the time in
seconds before the ETR cuts off and trips the frequency
converter. The curves are showing the characteristic
Introduction VLT® HVAC Drive FC 102 Design Guide
MG16C102 - Rev. 2013-08-20 47
2
2