FC101 Design Guide

3.7.1 Motor Thermal Protection (ETR)
Danfoss uses ETR to protect 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 3.56.
1.21.0 1.4
30
10
20
100
60
40
50
1.81.6 2.0
2000
500
200
400
300
1000
600
t [s]
175ZA052.11
fOUT = 0.2 x f M,N
fOUT = 2 x f M,N
fOUT = 1 x f M,N
IMN
IM
Figure 3.56 Motor Thermal Protection Characteristic
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 o and trips the frequency converter. The curves
show 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 o 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.
3.7.2 Thermistor Inputs
The thermistor cutout 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).
•
Electronic thermal relay (ETR).
Figure 3.57 Trip due to High Motor Temperature
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HVAC Basic Drive FC 101
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