S7 Motor Starting; Acceleration Timer; Start Inhibit - GE 469 Instruction Manual

4.8 S7 MOTOR STARTING

4.8 S7 MOTOR STARTING
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PATH: SETPOINTS S7 MOTOR STARTING
n ACCELERATION TIMER
n [ENTER] for more
The thermal model protects the motor under both starting and overload conditions. The acceleration timer trip may be used
to complement this protection. For example, if the motor always starts in 2 seconds, but the safe stall time is 8 seconds,
there is no point letting the motor remain in a stall condition for 7 or 8 seconds when the thermal model would take it off line.
Furthermore, the starting torque applied to the driven equipment for that period of time could cause severe damage.
If enabled, the Acceleration Timer functions as follows. A motor start is assumed to be occurring when the 469 measures
the transition of no motor current to some value of motor current. Typically current rises quickly to a value in excess of FLA
4
(e.g. 6 x FLA). At this point, the Acceleration Timer will be initialized with the
seconds. If the current does not fall below the overload curve pickup level before the timer expires, an acceleration trip will
occur. If the acceleration time of the motor is variable, this feature should be set just beyond the longest acceleration time.
Some motor softstarters allow current to ramp up slowly while others limit current to less than FLA throughout the start.
Since the 469 is a generic motor relay, it cannot differentiate between a motor with a slow ramp up time and one that has
completed a start and gone into overload. Therefore, if the motor current does not rise to greater than full load within 1 sec-
ond on start, the acceleration timer feature is ignored. In any case, the motor is still protected by the overload curve.
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PATH: SETPOINTS S7 MOTOR STARTING
n START INHIBIT
n [ENTER] for more
The Start Inhibit feature prevents motor tripping during start if there is insufficient thermal capacity. The largest
value from the last five successful starts is multiplied by (1 +
CAPACITY USED
. This thermal capacity margin ensures a successful motor start. If the number is greater than 100%,
STARTING CAPACITY
100% is stored as
LEARNED STARTING CAPACITY
greater than overload pickup and then, after acceleration, falls below the overload curve pickup level. If the Start Inhibit fea-
ture is enabled, the amount of thermal capacity available (100% –
each time the motor is stopped. If the thermal capacity available does not exceed the
STARTING CAPACITY
, or is not equal to 100%, the Start Inhibit Block is activated until there is sufficient thermal capacity. When a
ING CAPACITY
block occurs, the lockout time will be equal to the time required for the motor to cool to an acceptable start temperature.
This time is a function of the
If this feature is turned "Off", the
ture should be turned off if the load varies for different starts.
For example, if the
THERMAL CAPACITY USED
is 27% × 1.25 = 33.75% used. If the motor stops with 90% thermal capacity used, a start block will be
STARTING CAPACITY
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
4-48
Courtesy of NationalSwitchgear.com
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ACCELERATION TIMER

ð
ACCELERATION TIMER
ENTER
ESCAPE TRIP: Off
ASSIGN TRIP RELAYS:
ESCAPE
Trip
MESSAGE
ACCELERATION TIMER
ESCAPE
FROM START: 10.0 s
MESSAGE
Õ×

START INHIBIT

ð
START INHIBIT
ENTER
ESCAPE BLOCK: Off
TC USED
ESCAPE
MARGIN: 25%
MESSAGE
. A successful motor start is one in which phase current rises from 0 to
Õ
S5 THERMAL MODEL THERMAL MODEL
THERMAL CAPACITY USED
for the last 5 starts is 24, 23, 27, 25, and 21% respectively, the
setpoint is programmed for 30 minutes, the lockout time will be equal to:
t – τ ⁄
× ⇒
TC TC e
=
used used_start
⇒
469 Motor Management Relay
Range: Off, Latched, Unlatched
Range: Trip, Trip & Auxiliary2, Trip & Aux2 & Aux3,
Trip & Auxiliary3
Range: 1.0 to 250.0 s in steps of 0.1
ACCELERATION TIMER FROM START
Range: On, Off
Range: 0 to 25% in steps of 1
TC USED MARGIN
THERMAL CAPACITY USED
Õ×
COOL TIME CONSTANT STOPPED
must reduce to 15% before an overload lockout resets. This fea-
⁄
t – 30
×
66% 90% e
=
66
×
t = ln ----- - – 30 = 9.3 minutes
90
4 SETPOINTS
4.8.1 ACCELERATION TIMER
value in
4.8.2 START INHIBIT
THERMAL
) and stored as the
LEARNED
) is compared to the
LEARNED
LEARNED START-
setpoint.
LEARNED
(EQ 4.11)
GE Multilin