Danfoss FC 102 Design Manual page 52

Introduction to VLT® HVAC D...
Mains drop-out
During a mains drop-out, the frequency converter keeps
running until the intermediate circuit voltage drops below
2 2
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.
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.
2.13.1 Motor Thermal Protection
This is the way Danfoss is protecting 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 Illustration 2.40
t [s]
2000
1000
600
500
400
300
200
100
60
50
40
30
20
10
1.0
Illustration 2.40 The X-axis is showing the ratio between I
and I nominal. The Y-axis is showing the time in seconds
motor
before the ETR cuts off and trips the frequency converter. 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 of 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 in the frequency converter.
50
plus
mode
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
1.2 1.4 1.6 1.8 2.0
Danfoss A/S © Rev. 06/2014 All rights reserved.
Design Guide
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).
R
(Ω)
4000
3000
1330
550
250
-20°C
Illustration 2.41 The Thermistor Cut-out
Using a digital input and 24 V as power supply:
Example: The frequency converter trips when the motor
temperature is too high.
Parameter set-up:
Set 1-90 Motor Thermal Protection to [2] Thermistor Trip
Set 1-93 Thermistor Source to [6] Digital Input 33
I
M
I (par. 1-24)
MN
12 13 18
motor
PTC / Thermistor
Illustration 2.42 Using a Digital Input and 24 V as Power
Supply
Using a digital input and 10 V as power supply:
Example: The frequency converter trips when the motor
temperature is too high.
Parameter set-up:
Set 1-90 Motor Thermal Protection to [2] Thermistor Trip
 nominel -5°C
 nominel
OFF
19 27 29 32 33 20 37
ON
<6.6 k Ω >10.8 k Ω
 [°C]
 nominel +5°C
R
MG11BC02