2.15.1 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:
Illustration 2.6: The X-axis is showing the ratio between I
motor
and I
motor
nominal. The Y- axis is showing the time in seconds before the ETR
cuts off 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
Motor Thermal
in the frequency converter.
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).
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 par. 1-90
Motor Thermal Protection
to
Thermistor Trip
[2]
Set par. 1-93
Thermistor Source
to
Digital Input 33
[6]
VLT
®
AQUA Drive Design Guide 2 Introduction to VLT AQUA Drive
MG.20.N5.02 - VLT
®
is a registered Danfoss trademark
35
2
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