4.18
SEL-749M Relay Instruction Manual Date Code 20080918
Protection and Logic Functions
Basic Motor Protection
In certain pump applications, fluid flowing backward through the pump may
spin the pump motor for a short time after the motor is stopped. Any attempt
to start the motor during this time can be damaging. To prevent motor starts
during the backspin period, enter a time in minutes in the RESTART BLK
TIME setting. If the relay trips or the motor is stopped, the relay will generate
a trip signal and maintain it for at least this amount of time. The relay will not
issue a start during the restart block period.
The relay will maintain the trip signal until enough time passes for the motor
to be safely restarted. During the lockout period, the relay will display a
countdown time in minutes to the next allowed start.
The emergency restart function overrides all three limits, allowing the motor
to be placed back in service in an emergency.
Phase Reversal
Protection
The SEL-749M uses phase currents or phase voltages (if available) to
determine that the phase rotation of signals applied to the relay matches the
phase rotation setting, PHROT. When you set E47T equal to Y, the relay trips
0.5 seconds after incorrect phase rotation signals are applied to the relay. For
relays equipped with current inputs only, the trip will occur approximately
0.5 seconds after the motor start is initiated. When the relay is equipped with
voltage inputs, the trip will occur approximately 0.5 seconds after ac voltages
are applied to the relay (see Figure 4.9).
To ensure that phase reversal protection is enabled in case PHROT is set
wrong, make sure the setting E47T is equal to Y and Relay Word bit 47T is in
the TRIP equation.
Ta b l e 4 .1 7 P ha s e Rev e rs a l S e tt i n g
Setting Prompt Setting Range Setting Name := Factory Default
PH REV. ENABLE Y, N E47T := Y
Courtesy of NationalSwitchgear.com
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