Motor Starting Time Supervision (ANSI 48)
215
7UM62 Manual
C53000-G1176-C149-3
2.32.2 Setting Hints
General
The motor starting time supervision is only effective and accessible if address
has been set to
Enabled
during the configuration of the protective
functions. Set
Disabled
if the function is not required. Address
is used to switch the function
ON
or
OFF
, or to block only the trip command
(
Block Relay
).
Pickup Values
The device is informed of the startup current values under normal conditions at
address
, and of the startup time at address
. This ensures timely tripping if the value of
I
2
t calculated by the relay is
exceeded.
If the starting time exceeds the permissible locked rotor time, the definite-time tripping
characteristic can be started by an external r.p.m. counter via a binary input (“
”). A locked rotor leads to a loss of ventilation and therefore to a reduced
thermal load capacity of the machine. Therefore, the starting time supervision must
issue a trip command before the thermal characteristic for normal operation is
reached.
A current above the threshold
(address
) is interpreted as a
motor startup. Consequently, this value must be chosen such that it is reliably attained
by the actual starting current under any load or voltage conditions during motor
startup, but not during a permissible short-time overload.
Example: Motor with the following data:
The setting for address
is calculated as follows:
For reduced voltage, the start-up current is also reduced almost linearly. At 80 %
nominal voltage, the start-up current in this example is reduced 0.8
⋅
I
STARTUP
=
2.5
⋅
I
N CT sec
.
The threshold for detection of a motor startup must lie above the maximum load
current and below the minimum start-up current. If no other influencing factors are
present (peak loads), the value (
set at address
) may be a
median value:
The tripping time of the starting time monitoring is calculated as follows:
Nominal voltage
U
N
= 6600 V
Nominal current
I
Mot.nom
= 126 A
Startup current
I
StartCurr.
= 624 A
Permissible continuous
stator current:
I
max
= 135 A
Starting time at I
StartCurr.
T
Startmax
= 8.5 s
CT ratio I
N CT prim
/I
N CT sec
200 A/1 A
I
StartCurr, sec
Start-up current
Nominal CT current
---------------------------------------------------- I
N CT sec
⋅
624 A
200 A
--------------- I
N CT sec
⋅
3.12 A
=
=
=
135 A
200 A
---------------
0.68 I
N CT sec
⋅
=
Based on the long-term current rating:
I
MotorStart
2.5
I
N
0.68
I
N
+
2
-----------------------------------------
1.6
I
N CT sec
⋅
≈
=
= 1.6 A
