Greater than LRA on starting due to unbalance
Greater than LRA on starting due to unbalance
(OP)
Is it possible to draw greater than LRA on starting due to a voltage unbalance caused by a bad connection?
This weekend a 1000HP 4160volt motor with a normal 12 second acceleration time tripped two of its 3 mechanical overloads in 5 seconds during a start. The installation was checked and a bad connection was found on one of the R rated fuses causing it to be severally discolored. After the problem was fixed the motor started normally.
This weekend a 1000HP 4160volt motor with a normal 12 second acceleration time tripped two of its 3 mechanical overloads in 5 seconds during a start. The installation was checked and a bad connection was found on one of the R rated fuses causing it to be severally discolored. After the problem was fixed the motor started normally.





RE: Greater than LRA on starting due to unbalance
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RE: Greater than LRA on starting due to unbalance
Actually some motor overload relays had some ingeneous machanical arrangements that made them respond to unbalanced currents at less than rated balanced currents.
I can still vividly recall the mechanical arrangement of one type, but alas, I cannot recal the manufacturer.
Respectfully
RE: Greater than LRA on starting due to unbalance
The problem I have is that I can't figure out how the overloads could trip in 5 seconds if they routinely see a 12 second inrush.
RE: Greater than LRA on starting due to unbalance
RE: Greater than LRA on starting due to unbalance
RE: Greater than LRA on starting due to unbalance
RE: Greater than LRA on starting due to unbalance
The sad part is that the extra current (negative sequence) generates a braking torque.
RE: Greater than LRA on starting due to unbalance
If the blower was spinning slowly from process pressure before the motor was started could we draw more than LRA?
RE: Greater than LRA on starting due to unbalance
RE: Greater than LRA on starting due to unbalance
The other problem with a back spinning motor is that sometimes a little residual magnetism in the rotor can have the motor acting as a generator when you first energize it, but the field of that generator is not in synch with the line, so you get a massive spike of current as the line pulls that generator into itself.
RE: Greater than LRA on starting due to unbalance
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