Siemens MCC-3298, User Manual

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.