Low Current Protection; Signal Flow Of The Low Current Protection Function - ABB UMC100.3 Manual

C O N F I G U R I N G T H E M OTO R P R OT E C T I O N F U N C T I O N S

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:
I<I
lcwl
I/I [%]
e
I<I
lctl
After Start-up
—

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
After the undercurrent warning delay time t
the undercurrent trip level I
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.
I/I [%]
e
I
SE
I
e
I
lcwl
I
lctl
T1
Low Current Warning
---------------------------------------/===========================================================
Low Current Trip
-------------------------------------------------------------------------------/===================
—
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
& T
lcwd
& T
lctd
an undercurrent warning is set. Then at T3 the current also falls under
ucwd
but only for a period shorter than the specified undercurrent trip delay time t
uctl
T3
T2
t
lcwd
• Undercurrent Warning Level
• Undercurrent Warning Delay
Low Current
Warning
Low Current
Trip
.
ucwl
T4
t
t
lctd
lctd
.
uctd
I : Low current trip level
lctl
I : Low current warning level
lcwl
t : Low current trip delay
lctd
t : Low current warning delay
lcwd
I : Nominal current
e
I : Current defining the end of
SE
the start-up period
t
43