2.12.1 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 2.27
Illustration 2.27 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 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.
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).
Illustration 2.28
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
Illustration 2.29
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
Set 1-93 Thermistor Source to [6] Digital Input 33
Introduction
VLT
®
Refrigeration Drive Design Guide
34 MG16G102 - VLT
®
is a registered Danfoss trademark
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