Effect of Blown Primary Fuse on secondary voltage
Effect of Blown Primary Fuse on secondary voltage
(OP)
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.






RE: Effect of Blown Primary Fuse on secondary voltage
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.
RE: Effect of Blown Primary Fuse on secondary voltage
One paper is www.basler.com/downloads/practical.pdf Detecting Single-Phasing on a Three-Phase Power System [registration required]
RE: Effect of Blown Primary Fuse on secondary voltage
Bung
Life is non-linear...
RE: Effect of Blown Primary Fuse on secondary voltage