Flash overs in 5000kW (6700hp) Wound Rotor Induction Motor During Start Up with Liquid Rheostat

[h0v]

I am looking for help/advice in identifying what the fault may be in the following scenario:

A large wound rotor induction motor is experiencing flash overs during start up. The company that winds and constructs the motor has stated that it has passed all insulation and safety testing, and are insinuating that the fault lies in the methods of start up/use. It starts using a Uni-serve SSE Series liquid rheostat to limit in-rush current, I have very little experience with liquid rheostats so I am hoping that the pool of engineering knowledge contained in this forum can help me out. Have any of you had similar issues in starting Wound rotor induction motors with liquid rheostats? Could the Rheostat be operating too slowly, limiting current and causing voltage levels to increase to maintain power? My first reaction was that it was being caused by brush ware/accumulation of carbon dust but I am told this is not the issue.

Any and all advice you are able to provide is greatly appreciated! If my explanation was insufficient or you have any questions please ask and I will provide additional information. I am an Engineer in Training working under a Professional Engineer and he has delegated this work to me, so I would really like to get to the bottom of this.

I am new to this website and offer my sincerest apologies if I have posted to the wrong forum or broken any other of your guidelines. This was not my intention.

THANK YOU!

 

[Parchie]

Can't really think of possible causes with the very limited information you gave.
1. Which part of the motor did the flashover occur?
2. Was the start up its first run or it failed after a series of failed starts?
3. Was the electrolyte concentration properly set?
A simple narrative of the events that transpired could help you zero-in on the problem/s. Dump the items that you verified as unrelated and dig more on the factors that you can't find answers on.

 

[edison123]

Where did the flashover occur? Across sliprings? Brush holders? Windings?

Could you post some pictures?

The rotor voltage cannot exceed the rated and it happens only at zero speed. As the speed increases, the rotor voltage drops rapidly.

Muthu

http://www.edison.co.in

 

[waross]

If the rheostat is flashing over it may be poor circulation or incorrect formulation of the liquid. A bad connection to one of the electrodes of the rheostat may cause problems.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Skogsgurra]

As other posters indicate, an open secondary (rotor) circuit during start will cause a strong inductive overvoltage (L*di/dt), which can easily be above rated voltage for the rotor winding and terminals.

The reason, if there are no loose connections in the circuit, is usually that the water boils away too quickly (steam bubbles form at the electrodes) and interrupts the current. The reason for that is probably that the electrolyte has too low a conductivity. Often because not enough salt has been added to the water.

Make sure that the electrolyte has the right composition and concentration.

Also, check that the electrodes do not move too quickly and that the liquid level is within limits.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[h0v]

Thank you all for your answers! I will post additional information as soon as I am provided with it.

Thanks again!!

 

[h0v]

Update: After many discussions with their shop foreman & engineering team the root of the problem has been identified as contamination. The motor was very dirty and maintenance happens very seldom as the motor needs to run almost constantly and downtime is very costly. This information was unbeknownst to me at the time of my original post. This has created a new challenge for me that I am hoping this online community may be able to assist me in: In a nutshell I was told that increasing maintenance just doesn't make economic sense based on the role of the motor, and what they want me to do is suggest some form of protection that will prevent flashovers or at least diminish the damage caused by them despite the the motor being filthy most of the time. I know this is an unorthodox approach to protection and could be described as down right irresponsible in terms of motor lifespan but please remember I am an Engineer in Training trying to make a good impression and follow orders. Any and all suggestions are beyond appreciated, and I would like to once again thank everyone who took the time to read and respond to my post.

 

[Skogsgurra]

Quote from OP (1 Mar 16 20:12): "The company that winds and constructs the motor has stated that it has passed all insulation and safety testing, and are insinuating that the fault lies in the methods of start up/use."

If you now find that dirt is the main reason for the flash-overs - I think that you should have a serious talk to the maintenance people. Or, better, with the bean counters. Tell them that they will probably lose all the production quite soon if they do not accept periodic maintenance stops. Or are they willing to invest in a spare motor to minimize downtime?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[ScottyUK]

Where is the dirt coming from - the brushgear, or from an external source? If it is external then look at methods to stop it getting in: filtration, air purge, etc.

 

[waross]

I know that it is difficult to get a logical idea accepted BUT:
The problem occurs during start-up. No need to stop. The machine is already stopped. Clean the slip ring supports before starting!

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

As other posters indicate, an open secondary (rotor) circuit during start will cause a strong inductive overvoltage (L*di/dt), which can easily be above rated voltage for the rotor winding and terminals.

A wound rotor motor will only produce rated secondary voltage with an open secondary. It also produces rated secondary voltage during a start when full stator voltage has been applied but it's not yet rotating. You could think of a wound rotor motor as electrically being similar to a transformer with the secondary voltage inversely proportional to the motor speed.

You may be thinking synchronous motor which will produce a very high voltage on the field if the field is left open circuit during a start.

 

[Skogsgurra]

No, LH. I mean sudden disruption of a high starting current in an inductive circuit and the resulting inductive kick. My comments says so, very clearly.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[LRUSA]

If it's brush wear causing the contamination, consider installing a brush lifting system in the slip ring compartment.