Siemens SAMMS-MV Ultimate Trip Level and Service Factor, Phase Unbalance, Stator Protection

3 Operating the SAMMS-MV Device

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-MV 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.

3.15Dual Overload Protection (SAMMS-MVX 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-MVX 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 IFLC. The jam trip time is 360 msec. SAMMS-MVX is normally shipped with jam protection enabled, with a trip level of 200% IFLC (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

3.17Loss of Load Protection/Warning (SAMMS-MVX 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-MVX is normally shipped with loss of load protection OFF (WARNING), and with default setting of 50% IFLC.

3.18Process Current Warning (SAMMS-MVX 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.

3.19 Stator Protection

By selecting the overload class slightly greater than or equal to the motor’s starting time, the motor model defines an energy I2t value. In this value, I equals 600% of the motor’s full load current and t equals the overload class selected.

If the amount of energy exerted by the motor during start, stall or under locked rotor condition exceeds the amount of energy defined by the overload class selected, the SAMMS-MV device will trip. This protects the motor’s winding against excessive heat build up. For example, a typical motor starts within 3 to 4 seconds. By selecting an overload class 4 or 5, you can safely start the motor hot or cold and without nuisance tripping. This also provides protection against stall time exceeding 4 or 5 seconds; assuming the locked rotor current equals 600% of the motor’s full-load current (See Figure 3.11).

3.20 Rotor Protection

Since medium-voltage motors are rotor limited under locked rotor conditions, and stator limited under running conditions, the SAMMS-MV device computes an accurate thermal signa-

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