Danfoss VLT HVAC Basic Drive FC 101 Design Manual page 45

Product Overview
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 adjustable frequency
drive, the motor and the installation.
3. Incorrect slip compensation setting (1-62 Slip
Compensation) 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.
Line drop-out
During a line drop-out, the adjustable frequency drive
keeps running until the intermediate circuit voltage drops
below the minimum stop level, which is typically 15%
below the adjustable frequency drive's lowest rated supply
voltage. The AC line voltage before the drop-out and the
motor load determine how long it takes for the inverter to
coast.
2.11.1 Motor Thermal Protection
This is the way Danfoss protects the motor from being
overheated. It is an electronic feature that simulates a
bimetal relay based on internal measurements. The charac-
teristic is shown in Figure 2.27.
t [s]
2000
1000
600
500
400
300
200
100
60
50
40
30
20
10
1.0 1.2 1.4 1.6
Figure 2.27 Motor Thermal Protection Characteristic
MG18C522
Design Guide
fOUT = 1 x f M,N
fOUT = 2 x f M,N
fOUT = 0.2 x f M,N
IM
1.8 2.0 IMN
Danfoss A/S © Rev. 2014-01-14 All rights reserved.
The X-axis show the ratio between I
The Y-axis is showing the time in seconds before the ETR
cuts off and trips the adjustable frequency drive. The
curves are showing the characteristic nominal speed at
twice the nominal speed and at 0.2x the nominal speed.
It is clear that at lower speed the ETR cuts off at lower
heat due to less cooling of the motor. In that way, the
motors are protected from being overheated even at low
speeds. The ETR feature calculates the motor temperature
based on the actual current and speed.
The thermistor cut-out value is >3 kΩ.
Integrate a thermistor (PTC sensor) in the motor for
winding protection.
Motor protection can be implemented using a range of
techniques: PTC sensor in motor windings; mechanical
thermal switch (Klixon type); or Electronic Thermal Relay
(ETR).
Figure 2.28 Trip due to High Motor Temperature
and I nominal.
motor motor
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