Decaying Motor Voltage
Decaying Motor Voltage
(OP)
Does anyone have a good read on the subject of when we have a Main Bus A -Tie-Main Bus B system where one of the main “B” transformer failed and need to close my tie breaker to avoid all
the motors shutdown on bus “B”.
Is there a time cycle where I can catch motor before I will do damage to the shaft? Looking at 10 - 16 milliseconds for system to re-energize.
Motors are 800 to 200HP, 3600RPM, 4kV, - application are screw compressors.
thankyou
the motors shutdown on bus “B”.
Is there a time cycle where I can catch motor before I will do damage to the shaft? Looking at 10 - 16 milliseconds for system to re-energize.
Motors are 800 to 200HP, 3600RPM, 4kV, - application are screw compressors.
thankyou





RE: Decaying Motor Voltage
I also wouldn't be confident I would be able to restore voltage to the stator in under one cycle post fault/failure.
The tie breaker should be interlocked with the upstream transformer protection so that it does not close on the live bus. Therefore, lets do some quick math:
a. Transformer protection of any sort (87, 50/51, 63, etc.) initiates and sends trip signal to breaker = ~1 cycle
b. Transformer main breaker opens = ~3 cycle minimum
c. Tie breaker gets close signal from PLC and closes on dead bus = ~1 cycle minimum.
A minimum of 5 cycles have passed before you can restore voltage.
Whether or not this will have an adverse effect on the shaft depends on the time constant of the load.
RE: Decaying Motor Voltage
thanks for the comments. Even though SCIM are 3600RPM (low LRT, IE 60 to 70%), concern is if motors re-energize at a different phase angle,
that the motor may go into breakdown torque (BDT of 200%) and cause some shaft damage - motor or coupling or screw compressor shafting.
RE: Decaying Motor Voltage
RE: Decaying Motor Voltage
xnuke
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RE: Decaying Motor Voltage
These schemes usually initiate closing of the oncoming source breaker before the offgoing breaker has fully cleared to minimise the time off supply. If the offgoing breaker fails to open then the results will probably be very disruptive to plant operation, especially if the new supply closes into an uncleared fault and subsequently trips.
RE: Decaying Motor Voltage
EE
RE: Decaying Motor Voltage
Bill
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