6.8.6 Application
Systems with critical motor applications provide a backup power source that can
be switched in when the primary power source becomes faulty. When the power to
a motor is suddenly lost, the motor terminal voltage does not immediately fall to
zero because the rotating motor now acts as a generator producing its own voltage.
Remanent voltage decays as the motor comes to a stop at a rate depending on the
motor and load. Connecting the backup power source while the remanent voltage is
at a high level can result in damage to the motor shaft and windings.
MSVPR is used to monitor the motor remanent voltage over a decaying frequency
after power supply is lost, before allowing connection of backup power. It provides a
permissive signal indicating when the backup power can be safely connected.
This application example describes the re-energization process. MSVPR is used to
supervise the Bus1 remanent voltage. Control logic is used to detect the loss of
primary power source and initiate the transfer to backup power. Normally, bus tie
breaker CB5 is open and motor voltage power is supplied by incoming Feeder1 (see
Figure 766
).
M
Incoming Feeder1 (primary source) Incoming Feeder2 (backup source)
Outgoing Feeder1
Bus1 Bus2
CB5
CB1 CB2
CB3
CB4
MSVPR
Control logic
Measurement
Measurement
Figure 766: Example of remanent voltage supervision before fault
CB1 is tripped when a fault occurs on incoming Feeder1. MSVPR gives permission to
control logic when the remanent voltage of the motor reaches a safe level on Bus1.
Control logic closes the tie breaker CB5 if closing conditions are met. Finally, the
motor is re-energized by incoming Feeder2 (see
Figure 767
).
1MRS759142 F
Supervision functions
REX640
Technical Manual
1349
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