4-54
469 Motor Management Relay
GE Power Management
4.8 S7 MOTOR STARTING
4 SETPOINT PROGRAMMING
4
4.8.2 START INHIBIT
a) FUNCTION
The Start Inhibit feature helps to prevent tripping of the motor during start if there is insufficient thermal capac-
ity. The largest value of
THERMAL CAPACITY USED
from the last five successful starts is multiplied by (1 +
TC
USED MARGIN
) and stored as
A3 LEARNED DATA\MOTOR STARTING\LEARNED STARTING CAPACITY
. This thermal
capacity margin ensures that a motor start will be successful. If the number is greater than 100%,
100%
is
stored as
LEARNED STARTING CAPACITY
. A successful motor start is one in which phase current rises from 0 to
greater than overload pickup and then, after acceleration, falls below the overload curve pickup level. If the
Start Inhibit feature is enabled, each time the motor is stopped, the amount of thermal capacity available
(100% –
THERMAL CAPACITY USED
) is compared to the
LEARNED STARTING CAPACITY
. If the thermal capacity avail-
able does not exceed the
LEARNED STARTING CAPACITY
, or is not equal to 100%, the Start Inhibit Block will
become active until there is sufficient thermal capacity. When a block occurs, the lockout time will be equal to
the time required for the motor to cool to an acceptable temperature for a start. This time will be a function of
the
S5 THERMAL MODEL\THERMAL MODEL\COOL TIME CONSTANT STOPPED
setpoint.
If this feature is turned
"Off"
, the
THERMAL CAPACITY USED
must reduce to 15% before an overload lockout
resets. This feature should be turned off if the load varies for different starts.
b) EXAMPLE
If the
THERMAL CAPACITY USED
for the last 5 starts is 24, 23, 27, 25, and 21% respectively, the
LEARNED START-
ING CAPACITY
is 27%
×
1.25 = 33.75% used. If the motor stops with 90% thermal capacity used, a start block
will be issued.
When the motor has cooled and the level of thermal capacity used has fallen to 66%, a start will be permitted.
If the
COOL TIME CONSTANT STOPPED
setpoint is programmed for 30 minutes, the lockout time will be equal to:
y
START INHIBIT
y
[ENTER] for more
START INHIBIT
BLOCK: Off
Range: On, Off
TC USED
MARGIN: 25%
Range: 0 to 25%, step: 1
ENTER
ESCAPE
ð
ð
MESSAGE
ESCAPE
TC
used
TC
used_start
e
t
–
τ
⁄
×
=
66%
90%
e
t
–
30
⁄
×
=
t
66
90
------
ln
30
–
×
=
9.3 minutes
=
