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thekingpin (Electrical) (OP)
12 Nov 04 15:43
Recently I had a blown fuse on the primary of a DYn11 high voltage transformer. The motors connected to the secondary of the transformers burnt out due to the reduced voltage. I believe that the motors did not "single Phase" in the true sense of the phrase but suffered from a reduced and unbalanced voltage on the secondary side of the transformer. Can anyone comment on what the secondary line and phase voltages would be.
dpc (Electrical)
12 Nov 04 18:40
An open fuse on the primary of a delta-wye transformer serving motor loads causes large amounts of negative sequence current to flow in both the primary and secondary windings.  The running motors will tend to generate a voltage on the opened phase.  If you have non-motor load in parallel with the motor loads, they will draw current that is being provided by the motors.  This energy has to come from somewhere and reduces the mechanical energy available to the motor shaft.  

The negative sequence currents produce flux rotating in the opposite direction from the normal positive sequence current and from the viewpoint of the rotor creates a double-frequency current in the rotor.  The resistance to this double frequency current in the rotor goes up due to skin effect, but the overall impedance is much lower than for positive sequence current, so you get a lot of heating in the rotor.

If you just measure the motor current with a scalar meter, the current doesn't appear all that high.  If you can measure the negative-sequence current, you will see very high current.  It's tough to protect motors against damage from unbalanced currents with standard motor overload relays.  
busbar (Electrical)
13 Nov 04 0:14
 
One paper is www.basler.com/downloads/practical.pdf Detecting Single-Phasing on a Three-Phase Power System [registration required]
   
Bung (Electrical)
14 Nov 04 21:30
Two phases will be at 1/2 volts, and the third at full volts, with the 1/2 volts phases in phase and 180 degrees from the full volts phase.  Hence a large neg ph seq V (=unbalance V)which drives the aforementioned neg ph seq I in th emotor and causes overheating.

Bung
Life is non-linear...

buzzp (Electrical)
15 Nov 04 15:47
I believe your motors seen the loss of a phase but as pointed out, generated the 'missing phase' itself. So your volt meter would not see this. However, if you had voltage unbalance protection you might have saved the motors (depends on the UB percent before tripping). To guarantee this from happening again, I would use a relay with current unbalance protection (make sure it looks at negative sequence currents as well). This would have definately caught it.   

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