Motor Thermal Protection (Etr); Thermistor Inputs - Danfoss VLT HVAC Basic Drive FC 101 Design Manual

Product Overview
•
Incorrect slip compensation setting
(parameter 1-62 Slip Compensation) may cause
higher DC-link voltage.
The control unit may attempt to correct the ramp if
parameter 2-17 Over-voltage Control is enabled.
3 3
The frequency converter turns off to protect the transistors
and the DC-link capacitors when a certain voltage level is
reached.
Mains drop-out
During a mains dropout, the frequency converter keeps
running until the DC-link voltage drops below the
minimum stop level, which is typically 15% below the
frequency converter's lowest rated supply voltage. The
mains voltage before the drop-out and the motor load
determines how long it takes for the frequency converter
to coast.

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 Illustration 3.56.
t [s]
2000
1000
600
500
400
300
200
100
60
50
40
30
20
10
1.0
Illustration 3.56 Motor Thermal Protection Characteristic
The X-axis shows the ratio between I
nominal. The Y-axis shows the time in seconds before the
ETR cuts off 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 off 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.
44
f OUT = 1 x f M,N (par. 1-23)
f OUT = 2 x f M,N
f OUT = 0. 2 x f M,N
I
1.2 1.4 1.6 1.8 2.0
and I
motor
Danfoss A/S © 04/2016 All rights reserved.
®
VLT HVAC Basic Drive FC 101

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:
•
•
•
R
(Ω)
4000
3000
1330
550
250
-20 °C
Illustration 3.57 Trip due to High Motor Temperature
Example with digital input and 10 V power supply
The frequency converter trips when the motor temperature
I
M
is too high.
(par. 1-24)
MN
Parameter set-up:
Set parameter 1-90 Motor Thermal Protection to [2]
Thermistor Trip.
Set parameter 1-93 Thermistor Source to [6] Digital Input 29.
motor
PTC sensor in motor windings.
Mechanical thermal switch (Klixon type).
Electronic thermal relay (ETR).
 nominal -5 °C
 nominal
 [°C]
 nominal +5 °C
MG18C702