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:
Figure 2.6: The X-axis show the ratio between I
motor
and I
motor
nominal. The Y-axis shows the time in seconds before the ETR cuts off and
trips the 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 par. 16-18
Motor Thermal
in the adjustable frequency drive.
The thermistor cut-out value is > 3 k
Ω
.
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 adjustable frequency drive trips when the motor temper-
ature 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]
2 Introduction to the VLT AQUA Drive
VLT
®
AQUA Drive Design Guide
2-30
MG.20.N5.22 - VLT
®
is a registered Danfoss trademark
2
