Extreme Running Conditions; Motor Thermal Protection - Danfoss VLT AutomationDrive Design Manual

Introduction to FC 300

3.11 Extreme Running Conditions

Short Circuit (Motor Phase – Phase)
The frequency converter 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 frequency converter against a short circuit
at the load sharing and brake outputs please see the
design guidelines.
See certificate in 3.9 Certificates.
Switching on the Output
Switching on the output between the motor and the
frequency converter is fully permitted. You cannot damage
the frequency converter in any way by switching on the
output. However, fault messages may appear.
Motor-generated Over-voltage
The voltage in the intermediate circuit is increased when
the motor acts as a generator. This occurs in following
cases:
1. The load drives the motor (at constant output
frequency from the frequency converter), ie. the
load generates energy.
2. 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.
3. Incorrect slip compensation setting may cause
higher DC link voltage.
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.
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
frequency converter's lowest rated supply voltage. The
mains voltage before the drop-out and the motor load
determines how long it takes for the inverter to coast.
FC 300 Design Guide
Static Overload in VVC
When the frequency converter 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 frequency converter cut out after approx. 5-10
sec.
Operation within the torque limit is limited in time (0-60
sec.) in 14-25 Trip Delay at Torque Limit.

3.11.1 Motor Thermal Protection

To protect the application from serious damages VLT
AutomationDrive offers several dedicated features
Torque Limit: The Torque limit feature the motor is
protected for being overloaded independent of the speed.
Torque limit is controlled in 4-16 Torque Limit Motor Mode
and or 4-17 Torque Limit Generator Mode and the time
before the torque limit warning shall trip is controlled in
14-25 Trip Delay at Torque Limit.
Current Limit: The current limit is controlled in 4-18 Current
Limit and the time before the current limit warning shall
trip is controlled in 14-24 Trip Delay at Current Limit.
Min Speed Limit: (4-11 Motor Speed Low Limit [RPM] or
4-12 Motor Speed Low Limit [Hz]) limit the operating speed
range to for instance between 30 and 50/60Hz. Max Speed
Limit: (4-13 Motor Speed High Limit [RPM] or 4-19 Max
Output Frequency) limit the max output speed the drive
can provide
ETR (Electronic Thermal relay): The frequency converter ETR
function measures actual current, speed and time to
calculate motor temperature and protect the motor from
being overheated (Warning or trip). An external thermistor
input is also available. ETR is an electronic feature that
simulates a bimetal relay based on internal measurements.
The characteristic is shown in the following figure:
®
MG.33.BD.02 - VLT is a registered Danfoss trademark
plus
mode
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