Motor Thermal Protection - Danfoss VLT FC 360 Design Manual

Product Overview
the short circuit current exceeds the permitted value
(Alarm 16 Trip Lock).
To protect the frequency converter against a short circuit
2 2
at the load sharing and brake outputs, see the design
guidelines.
Switching on the output
Switching on the output between the motor and the
frequency converter is fully permitted, and does not
damage the frequency converter. 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. If the moment of inertia is high during
deceleration (ramp-down), 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 320 V. The mains voltage
before the drop-out and the motor load determines how
long it takes for the inverter to coast.
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
s.
Operation within the torque limit is limited in time (0-60 s.)
in 14-25 Trip Delay at Torque Limit.

2.11.1 Motor Thermal Protection

To protect the application from serious damage VLT
AutomationDrive offers several dedicated features.
40
®
VLT AutomationDrive FC 360 Design Guide
plus
mode
MG06B202 - VLT
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
(4-12 Motor Speed Low Limit [Hz]) limit the operating speed
range to for instance between 30 and 50/60 Hz.
Max speed limit
(4-14 Motor Speed High Limit [Hz] or 4-19 Max Output
Frequency) limit the max output speed the frequency
converter 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
Illustration 2.28
t [s]
2000
1000
600
500
400
300
200
100
60
50
40
30
20
10
1.0 1.2
Illustration 2.28 ETR
The X-axis shows the ratio between I
nominal. The Y-axis shows the time in seconds before the
ETR cuts off and trips the frequency converter. The curves
show the characteristic nominal speed, at twice the
nominal speed and at 0.2 x the nominal speed.
At lower speed, the ETR cuts off at lower heat due to less
cooling of the motor. In that way the motor are protected
from being over heated even at low speed. The ETR
feature is calculating the motor temperature based on
® actual current and speed. The calculated temperature is
visible as a read out parameter in 16-18 Motor Thermal.
®
is a registered Danfoss trademark
f OUT = 1 x f M,N (par. 1-23)
f OUT = 2 x f M,N
f OUT = 0. 2 x f M,N
I
I (par. 1-24)
1.4 1.6 1.8 2.0
MN
and I
motor motor
M