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Phase Failure


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Let us assume the following situation:

- a three-phase motor is running with two phases only (one of the phases was opened somehow after it was successfully started) and its speed is approx rated (no starting issues),

- its stator current is: Line1 = 100% of FLA, Line2 = 100% of FLA, Line3 = 0% of FLA,

- no thermal and/or overload protection installed for this motor.


How do You think: will this motor be overheated in this mode? Does it depends on stator internal connection circuit: star or delta? Is the outcome common for all (3-phase squirrell cage induction) motors or some motors tend to burn in this mode while others don't?

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You can only assume such situation otherwise during phase loss, live coils draw current > 100%. The rotating magnetic field don’t find the “support” under 120 degree of disconnected coil and as a result, remaining two coils draw current > FLA and motor burnt. I did not see a single case that motor did not burn in such situation.




"Don't assume any thing, always check/ask and clear yourself".

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Research the term "negative sequence current" that occurs when you have a current imbalance. In essence, the negative sequence current sets up a counter torque in the motor, making it fight itself and it heats up at a rate disproportionate to the current draw. So even though you are not pulling more than FLC in the other phases, the motor is still overheating. That is why you want either Phase Loss protection, or if you have a Solid State Overload Relay, it has Phase Imbalance biasing of the OL curve.
"He's not dead, he's just pinin' for the fjords!"
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