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t
I/I
e
I
lctl
I
lcwl
I
e
T1
I
SE
---------------------------------------/===========================================================
-------------------------------------------------------------------------------/===================
T2
T3
t
lcwd
t
lctd
t
lctd
T4
&
I/I
e
[%]
I<I
lcwl
T
lcwd
Low Current
Warning
After Start-up
I<I
lctl
&
T
lctd
Low Current
Trip
Low Current Protection
The low current function is triggered when the motor current falls below a desired level. The function detects loss of suction for
pumps, broken belt for conveyors, loss of airflow for fans, broken tools for machines etc. Such states do not harm the motor but
early diagnosis helps to minimise the extent of damage to the mechanical installation and subsequent loss of production. The mo-
tors on underload draw mainly the magnetising current and a small load current to overcome frictional losses. Therefore, the other
reason to isolate the motors on underload is to reduce the reactive load on the power system network. The low current function
signals a warning when the motor current falls below the warning level for a configurable period of time, after motor start-up. The
low current function creates a trip when the motor current falls below the trip level for a configurable period of time, after motor
startup.
Block diagram:
—
Signal flow of the low current protection function.
Example:
The diagram above shows an underload situation of the motor after the startup time is over (T1).
At T2 the motor current falls below the undercurrent warning limit I
ucwl
.
After the undercurrent warning delay time t
ucwd
an undercurrent warning is set. Then at T3 the current also falls under
the undercurrent trip level I
uctl
but only for a period shorter than the specified undercurrent trip delay time t
uctd
.
Therefore no trip happens.
At T4 the current falls again under the low current level and stays there. Therefore a trip is released and the motor stopped.
—
Trend diagram of the motor current over time
It shows the function of the low current protection function with the related parameters.
Relevant Parameters
• Undercurrent Trip Level
• Undercurrent Trip Delay
• Undercurrent Warning Level
• Undercurrent Warning Delay
Low Current Warning
Low Current Trip
I
lctl
: Low current trip level
I
lcwl
: Low current warning level
t
lctd
: Low current trip delay
t
lcwd
: Low current warning delay
I
e
:
Nominal current
I
SE
: Current defining the end of
the start-up period
43
CO N FI G U R I N G T H E M OTO R P R OT EC T I O N FU N C T I O N S
Summary of Contents for UMC100.3
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