Delta motor behaviour on loss of phase

[herban]

Hi all,
I am trying to develop a motor protection scheme for the following scenario.
There are two 3.3kV 250kW delta connected motors close coupled to a gearbox driving a crusher.
Both have separate DOL starters and protection systems.
What happens to the motor current in a motor that loses a phase (1) via a broken winding connection and (2) via a broken supply connection.
I know what happens for a single motor in both of these situations but am not sure in a situation where the rotor of the motor is being driven by the other motor via the close coupled gearbox.

 

[jim hardy]

No suggestions yet ?

You probably have more expertise than me, so i'll only venture a qualitative guess.

What happens to the motor current in a motor that loses a phase (1) via a broken winding connection

Unbalanced voltage lowers the speed-torque curve.

So in both cases the faulted motor will slow down a little. .
And the remaining phases will draw more current.
But you knew that already.
As the system slows, the good motor's operating point moves up its torque curve so it'll draw more current because it's delivering the lion's share of the total torque.

(2) via a broken supply connection.

Wow now that motor is single-phased. It should really complain audibly...
Likewise its torque will go down and the other motor will have to make up the slack...

This article has interesting explanation of what happens to currents in a delta connected motor when a phase opens either internally or externally.
I think that was your original question...
Though the individual motor winding currents increase, the vector current addition is no longer 120 degrees.
For the internal open phase, current in the supply lines may actually go down ...
Look at the section "Thermal protection and operation in case of phase loss", starting at page 32(of document, 33 of PDF file) here:
http://www05.abb.com/global/scot/scot209.nsf/veritydisplay/5e6a1c128ae4fab1c1257b490033f301/$file/1SDC007106G0201.pdf

now to your real question:
The motors being connected by a gearbox just locks their speed. One with the fault experiences a downward shift of its speed-torque curve, the other does not. So they no longer share torque equally. So the faulted motor will behave like a faulted motor that's lightly loaded.

I'm not a relay guy. The reading i did tells me they use negative sequence to protect against phase loss, which makes perfect sense. One of the items i stumbled across was a Westinghouse instruction brochure for their offering... can't seem to find it again, but here's a GE introductory leaflet:

https://www.gedigitalenergy.com/multilin/family/motors/principles4.htm

Unbalance Protection
Unbalanced load in the case of AC motors is mainly the result of an unbalance of the power supply voltages. The negative-sequence reactance of the three-phase motor is 5 to 7 times smaller than positive-sequence reactance, and even a small unbalance in the power supply will cause high negative sequence currents. For example for an induction motor with a staring current six times the full load current, a negative sequence voltage component of 1% corresponds to a negative sequence current component of 6%. The negative-sequence current induces a field in the rotor, which rotates in the opposite direction to the mechanical direction and causes additional temperature rise. Main causes of current unbalance are: system voltage distortion and unbalance, stator turn-to-turn faults, blown fuses, loose connections, as well as faults.

any help? You already knew all the above , i'll wager.

old jim

 

[seeyouaunty]

I don't think you need to worry about the motor, all you need to know is only two phases will be carrying current in both scenarios. Most DOL overload relays have a feature called phase loss protection. The DOL overload relay should trip/activate if the 3 phases are not carrying equal current.

 

[anorlunda]

I believe that your answer was posted recently in a different thread.

"The Art and Science of Protective Relaying."

http://www.gedigitalenergy.com/multilin/notes/artsci/artsci.pdf

The link leads to a free online PDF of that book. Pages 230-237 address motor protection.