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 overvoltage
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).
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 frequency converter 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
+
mode
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 control unit 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 approximately
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 FC 360 offers several dedicated features.
Torque limit
The torque limit protects the motor from 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 trips 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 trips is controlled in
14-24 Trip Delay at Current Limit
.
Minimum speed limit
(
4-12 Motor Speed Low Limit [Hz]
) sets the minimum output
speed the frequency converter can provide.
Maximum speed limit
(
4-14 Motor Speed High Limit [Hz]
or
4-19 Max Output
Frequency
) sets the maximum 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
1.2
1.0
1.4
30
10
20
100
60
40
50
1.8
1.6
2.0
2000
500
200
400
300
1000
600
t [s]
175ZA052.12
fOUT = 2 x f M,N
fOUT = 0.2 x f M,N
fOUT = 1 x f M,N(par. 1-23)
I
MN
(par. 1-24)
I
M
Illustration 2.40 ETR
The X-axis shows the ratio between I
motor
and I
motor
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 is protected
from being overheated even at low speed. The ETR feature
calculates the motor temperature based on actual current
and speed. The calculated temperature is visible as a
readout parameter in
16-18 Motor Thermal
.
Product Overview
Design Guide
MG06B402
Danfoss A/S © 09/2014 All rights reserved.
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