Motor-generated overvoltage
The voltage in the DC link 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).
•
If the inertia moment 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.
•
Incorrect slip compensation setting may cause
higher DC-link voltage.
The control unit may attempt to correct the ramp if
possible (
parameter 2-17 Over-voltage Control
).
The frequency converter turns off to protect the transistors
and the DC link capacitors when a certain voltage level is
reached.
To select the method used for controlling the DC-link
voltage level, see
parameter 2-10 Brake Function
and
parameter 2-17 Over-voltage Control
.
Mains drop-out
During a mains drop-out, the frequency converter keeps
running until the DC-link voltage drops below the
minimum stop level, which is:
•
314 V for 3x380–480 V.
•
202 V for 3x200–240 V.
•
225 V for 1x200–240 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
parameter 4-16 Torque Limit Motor Mode
/
parameter 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 overcurrent which makes the
frequency converter cut out after approximately 5–10 s
may occur.
Operation within the torque limit is limited in time (0–60 s)
in
parameter 14-25 Trip Delay at Torque Limit
.
2.11.1 Motor Thermal Protection
To protect the application from serious damage, VLT
®
Midi
Drive FC 280 offers several dedicated features.
Torque limit
The torque limit protects the motor from being overloaded
independent of the speed. Torque limit is controlled in
parameter 4-16 Torque Limit Motor Mode
and
parameter 4-17 Torque Limit Generator Mode
.
Parameter 14-25 Trip Delay at Torque Limit
controls the time
before the torque limit warning trips.
Current limit
Parameter 4-18 Current Limit
controls the current limit, and
parameter 14-24 Trip Delay at Current Limit
controls the
time before the current limit warning trips.
Minimum speed limit
Parameter 4-12 Motor Speed Low Limit [Hz]
sets the
minimum output speed that the frequency converter can
provide.
Maximum speed limit
Parameter 4-14 Motor Speed High Limit [Hz]
or
parameter 4-19 Max Output Frequency
sets the maximum
output speed that the frequency converter can provide.
ETR (electronic thermal relay)
The frequency converter ETR function measures the actual
current, speed, and time to calculate motor temperature.
The function also protects 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 2.37 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
parameter 16-18 Motor Thermal
.
Product Overview
VLT
®
Midi Drive FC 280
44
Danfoss A/S © 03/2016 All rights reserved.
MG07B102
2
2
