Danfoss VLT FC 102 Series Design Manual page 58

Introduction to VLT® HVAC D...
2.13.1 Motor Thermal Protection
2 2
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 characteristic is shown in Figure 2.40
t [s]
2000
1000
600
500
400
300
200
100
60
50
40
30
20
10
1.0
Figure 2.40 The X-axis show the ratio between I
nominal. The Y-axis is showing the time in seconds before the
ETR cuts off and trips the adjustable frequency drive. The
curves show the characteristic nominal speed at twice the
nominal speed and at 0.2x the nominal speed.
At lower speeds, the ETR cuts off at lower levels due to
reduced 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 calculated temperature is
visible as a readout parameter in 16-18 Motor Thermal in
the adjustable frequency drive.
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).
56
1.2 1.4 1.6 1.8 2.0
Danfoss A/S © Rev. 06/2014 All rights reserved.
Design Guide
fOUT = 1 x f M,N
Figure 2.41 The Thermistor Cut-out
fOUT = 2 x f M,N
fOUT = 0.2 x f M,N
Using a digital input and 24 V as power supply:
Example: The adjustable frequency drive trips when the
motor temperature is too high.
IM
Parameter set-up:
IMN
Set 1-90 Motor Thermal Protection to [2] Thermistor Trip
and I
motor motor
Set 1-93 Thermistor Source to [6] Digital Input 33
12 13 18
PTC / Thermistor
Figure 2.42 Using a Digital Input and 24 V as Power Supply
Using a digital input and 10 V as power supply:
Example: The adjustable frequency drive 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
OFF
19 27 29 32 33 20 37
ON
<6.6 k Ω >10.8 k Ω R
MG11BC22