ETR (Electronic Thermal relay):
The
frequency
converter ETR
function measures actual current, speed and time to
calculate motor temperature and protect the motor from
being overheated (Warning or trip). An external thermistor
input is also available. ETR is an electronic feature that
simulates a bimetal relay based on internal measurements.
The characteristic is shown in Illustration 3.8
:
1.21.0 1.4
30
10
20
100
60
40
50
1.81.6 2.0
2000
500
200
400
300
1000
600
t [s]
175ZA052.12
f
OUT
= 2 x f
M,N
f
OUT
= 0.2 x f
M,N
f
OUT
= 1 x f
M,N
(par. 1-23)
I
MN
(par. 1-24)
I
M
Illustration 3.8 ETR: The X-axis shows the ratio between I
motor
and I
motor
nominal. The Y- axis shows the time in seconds before
the ETR cut of and trips the drive. The curves show the charac-
teristic nominal speed, at twice the nominal speed and at 0,2 x
the nominal speed.
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 FC 51 Micro Drive Programming Guide,
MG02CX.
Introduction to VLT® Micro ...
VLT
®
Micro Drive FC 51 Design Guide
MG02K202 - VLT
®
is a registered Danfoss trademark
25
3 3
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