Siemens MCC-3298 - Manual (Page 24)

22
3 Operating the SAMMS-LV Device
of 10 guarantees that a current of 600% of I
FLC
will cause a trip
in 10 seconds or less. In the SAMMS-LV, the protection curves
are designed to cause a trip in 90% to 100% of the class time
for a current of 600% of I
FLC
.
The lowest overload class, greater than or equal to the motor’s
starting time, gives the best protection. Overload classes 2
through 23 are available in SAMMS-LV. The range of protection
curves are shown in Figures 3.6 through 3.9.
Important: In the unlikely event that the motor acceleration
time (overload class) exceeds the motor’s stall time, use a
speed switch as an input source to open the contactor if the
switch senses a locked rotor or stall condition.
Note: For clarity, the time-current characteristic curves in this
manual are shown with overload classes 2, 5, 10, 15 and 23
only. Use interpolation to derive values of other overload
classes not shown.
3.13 Ultimate Trip Level and Service Factor
The ultimate trip level is the maximum continuous current that
does not cause an overload trip. Any higher current applied
indefinitely to an unprotected motor will ultimately damage it.
The ultimate trip level for motors with a unity service factor is
110% of the full-load current setting. For motors with a service
factor of 1.15, the ultimate trip level is 120%. You can select a
service factor of 1.00 or 1.15 with HHC function F6.
3.14 Phase Unbalance
Unbalanced three-phase voltages or loss of one phase can
result in unbalanced currents being supplied to the motor.
Unbalanced currents include negative-sequence components
that produce heat in the motor winding, decreasing the effi-
ciency of the motor for a given load. The SAMMS-LV device
measures the magnitude of the current unbalance as the
maximum deviation from the average of the three-phase cur-
rents, divided by the average of the three-phase currents. In
case of loss of a phase, the unbalance is 100% in a three-wire,
ungrounded system.
With phase unbalance protection enabled, current unbalances
of between 20% and 40% cause the Phase Unbalance LED to
flash as a warning. If the unbalance increases to above 40%, the
Phase Unbalance LED illuminates solidly and overload tripping
accelerates. The effect of an unbalance of greater than 40% is
to shift the motor protection curve in the direction of faster
tripping.
You enable the phase unbalance protection using HHC function
F9.
• Enabled = ON
• Disabled = OFF
With phase unbalance protection enabled, HHC function F17
displays the percentage of current unbalance. Both functions
display OFF if phase unbalance protection is disabled.
3.15 Dual Overload Protection
(SAMMS-LVX Only)
Because the high-speed and low-speed currents differ in two-
speed motors, different full-load current settings are often
required. In such applications, use function F5 to select the full-
load current for low-speed operation.
3.16 Jam Protection (SAMMS-LVX Only)
Certain mechanical loads, especially those containing gears,
are susceptible to jamming. Jam protection can prevent dam-
age to the motor and the driven equipment by quickly tripping
the motor off-line in the event of a jam. If you have enabled jam
protection, you may adjust the jam trip level from 120-400% of
I
FLC.
The jam trip time is 360 msec. SAMMS-LVX is normally
shipped with jam protection enabled, with a trip level of 200%
I
FLC
(See Figure 3.11). Jam protection is locked out for 5 times
the overload class after a start, speed, or direction change. If the
jam limits are exceeded, the motor is tripped off-line and the
External Trip LED on the front panel solidly illuminates. You can
enable jam protection using HHC function F23.
• enabled = ON
• disabled = OFF
3.17 Loss of Load Protection/Warning
(SAMMS-LVX Only)
If the running current of the motor suddenly drops below 20 to
95% (adjustable) of its previous reading in 360 msec, a loss of
load has occurred. Such a loss of load could be due to a broken
belt or loss of back pressure in a pump. Loss of load detection
is locked out for five times the overload class time after a start,
direction, or speed change (See Figure 3.11). You can select
loss of load protection or warning using HHC function F24.
• protection = ON
• warning = OFF
With protection selected, detection of a loss of load trips the
motor off-line and illuminates the External Trip LED. With
warning selected, the motor does not trip off-line and the
External Trip LED flashes until the current returns to its former
steady-state level or higher. SAMMS-LVX is normally shipped
with loss of load protection OFF (WARNING only), and with
default setting of 50% I
FLC
.
3.18 Process Current Warning
(SAMMS-LVX Only)
It is often convenient to have an indication that the load current
is above its normal operating level so that the process can be
checked and, if necessary, adjusted. You can set the process
current warning from 0% (displayed as “OFF”) to 100% of the
full-load current with HHC function F22. As long as the motor
running current exceeds the process current warning level, the
External Trip LED flashes. Process current warning is locked out
for five times the class time after a start or a direction or speed
change.