Danfoss VLT Micro Drive FC 51 Design Manual page 27

Product Overview
overload setpoint. Programmable options at the overload
allow the frequency converter to stop the motor, reduce
output, or ignore the condition. Even at low speeds, the
frequency converter meets I2t Class 20 electronic motor
overload standards.
This method is called electronic thermal relay (ETR).
t [s]
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1.0 1.2 1.4 1.6 1.8
Illustration 3.8 ETR Characteristics
The X-axis shows the ratio between I
nominal. The Y axis shows the time in s before the ETR cut
of and trips the frequency converter. The curves show the
characteristic nominal speed, at twice the nominal speed
and at 0.2 x the nominal speed.
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
is calculating the motor temperature based on actual
current and speed. The calculated temperature is visible as
a readout parameter in parameter 16-18 Motor Thermal in
the product-specific Programming Guide.
A special version of the ETR is also available for EX-e
motors in ATEX areas. This function makes it possible to
enter a specific curve to protect the Ex-e motor. The
Programming Guide takes the user through the set-up.
MG02K402
Design Guide
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
M
I (par. 1-24)
2.0
MN
and I
motor motor
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