Danfoss VLT HVAC Drive FC 100 Series Design Manual page 46

2. Introduction to VLT HVAC Drive
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 characteristic is shown in the following figure:
2
Illustration 2.7: The X-axis is showing the ratio between I
and trips the 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 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 par. 16-18 in the frequency converter.
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and I nominal. The Y- axis is showing the time in seconds before the ETR cuts off
motor motor
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).
MG.11.B7.02 - VLT ® is a registered Danfoss trademark
®
VLT HVAC Drive Design Guide