ABB UMC100-FBP - Page 44

- 44 -UMC100-FBP
Universal Motor Controller
UMC100-FBP
Technical Description
FieldBusPlug / Issue: 03.2012
Low Current
Warning
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 ma-
chines etc. Such states do not harm the motor but early diagnosis helps to minimise the extent of dam-
age to the mechanical installation and subsequent loss of production.
The motors on underload draw mainly the magnetising current and a small load current to overcome fric-
tional 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/Ie[%]
I<Ilcwl
Tlcwd
Low Current
Warning
After Start-up
I<Ilctl
& Tlctd
Low Current
Trip
 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.
[%]
t
I/I
e
I
lctl
I
lcwl
I
e
T1
I
SE
---------------------------------------/===========================================================
-------------------------------------------------------------------------------/===================
T2
T3
t
lcwd
t
lctd
t
lctd
T4
 Trend diagram of the motor current over time showing 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
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