Flashing in WR Motors 3

[RalphChristie]

See also thread237-53974

We are experiencing heavy overflashing problems on our 500kW slipring motors (WRIM) in our plant. The plant have a lot of conveyors, the drives on the conveyors varies between 1 to three drives per conveyor. There is no spesific drive or conveyor that gives no problems, it occur sporadicaly on all the drives. Sometimes some of the motors have no problems for a period of 5 years, other times it occurs every 2 months. All the motors have vapourmatic (liquid) starters with three steps, the last step shorting the rotor.

All the overflash faults is between the sliprings in the area where the brushes are situated.

My opinion:

1. Overflash is due to a high voltage

High voltages occur during:

a) Start-up
b) Open rotor condition during running condition (short-rotor contactor open)
c) Can a high rotor voltage occur while the motor is electrically stopped, but the are rotor still turning?
(contactors open on stator side and on rotor side, but motor is still turning)

We have found no evidence for (b), the contactors are controlled and monitored by a PLC.


2. High Humidity

Not an option in our case, it is warm and dry.


3. Dust

Dust can be a main course (Plant is very dusty) but we don't experience this problem on the smaller motors (200kW) and sometimes the 500kW motors work for 5 years.


4.Isolation material between sliprings

Is discussed with manufacturer, but it is been manufactured for 2x rotor Volts + 1000V and been tested. The Rotor voltage is 855V

5. Brushes?

We've asked the manufacturer to increase the distance between the rings for a larger gap and to clean carbondust easier - not implemented yet

Any other ideas?

Ralph

 

[edison123]

I have seen brush holder flashovers, most of them at starting when the rotor sees the rated name plate voltage. As a rewinder, I have solved such problems by rotating the middle phase brush holders by 90 to 120 deg away from the other two phases. This way, the clearance between brush holders increases quite comfortably without any phase barriesr. All these motors (with rotor volts from 400 to 1500 V) are working well for the past 5 to 15 years without any problem and I have had repeat orders for such works from many clients. Rotating the middle phase brush holdres may involve fixing additonal brush holder arms/rods.

 

[jbartos]

Comments on the original posting marked ///\\
1. Overflash is due to a high voltage
High voltages occur during:
a) Start-up
///Yes, they may\\b) Open rotor condition during running condition (short-rotor contactor open)
///Yes, they may\\c) Can a high rotor voltage occur while the motor is electrically stopped, but the are rotor still turning?
(contactors open on stator side and on rotor side, but motor is still turning)
///No, the voltage level is diminishing.\\We have found no evidence for (b), the contactors are controlled and monitored by a PLC.
2. High Humidity
Not an option in our case, it is warm and dry.
3. Dust
Dust can be a main course (Plant is very dusty) but we don't experience this problem on the smaller motors (200kW) and sometimes the 500kW motors work for 5 years.
///When it comes to dust, the motor enclosure counts most. The kW do not necessarily.\\4.Isolation material between sliprings
Is discussed with manufacturer, but it is been manufactured for 2x rotor Volts + 1000V and been tested. The Rotor voltage is 855V
///Surface resistance or impedance can deteriorate in time. Therefore, proper cleaning of surfaces between the slip rings is recommended. A suitable insulation spray or paint will help to increase the surface resistance/impedance.\\5. Brushes?
We've asked the manufacturer to increase the distance between the rings for a larger gap and to clean carbondust easier - not implemented yet
///Good idea.\\Any other ideas?
///Check the motor power supply quality for voltage spikes, swells, harmonics, etc.\\\

 

[electricpete]

For completeness, the original post mentioned dust in context of a dusty plant. Conducting brush dust is of course another possible contributor.

I vote edison a star for the idea of rotating brushholder. That is great info that I will remember.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[RalphChristie]

Thanx for the piece of info edison123, I vote you a star.
Pete and jb, thanx for the replies.

I'll keep the forum up to date on what we decide and do.

Ralph

 

[edison123]

Thx Ralph and sorry about my horrendous spelling, which is quite embarassing, now that I read it.

I once suggested to the forum administrator to have the option (like in reliability-magazine.com forum) wherein members can correct all sorts of mistakes (linguistic as well as "Oh, I wish I hadn't posted that one" mistakes) in their posts but was told to use the preview option before posting.

 

[rmw]

I'm giving Ralph a star for posting a complete question, loaded with symptoms and information that the responders can use in offering assistance.

Edison, I don't care how many times you review it, once posted, you will often still find those glitches.

rmw

 

[jraef]

Off original topic...

I used to copie my reasponces to MSWourd for spel/grammmar czek. but after a wile I figgured that cince everywon has the same baysic prolem here we can lern too live with it out of comon curtecy.

"Venditori de oleum-vipera non vigere excordis populi"

 

[jbartos]

Comment: Some of those items mentioned in the original posting:
1. Dust
2. High Humidity
3. Power Supply quality
4. Etc.
are related to the motor maintenance. A good and regular maintenance would minimize flashing and other potential problems.

 

[edison123]

rmw,

that is exactly why I recommend the option of editing one's own posts (as many times as one wants).

jraef:

shouln"t it be " of oreegynal topick ".

 

[jraef]

Oops! My slip was showing...