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110kW DC Motor flashover
3

110kW DC Motor flashover

110kW DC Motor flashover

(OP)
Hello all, please bear with me as I am a first time "poster" here.
I'm an electronic engineer and not used to service DC drives or DC Motors, I service mostly Servodrives and VFD for AC Motors.
I had a job to service a SIEI Typact TDP3 SCR Drive for a system that is driving 2 german 110kw CONZ Dc Motors in parallel on a Ship-port crane.
I serviced the Drive and another Company made an overhauling on the 2 Motors.

On the first test run one of the motors had a massive flashover.
Testrun was made with no load on the hook and gradually increasing velocity in one direction and then in the other to about 50% of the maximum joystick position.

Motor went back to the company and returned to be tested again. On the 2nd test run the same motor did had a flashover again, but this time the pattern looked as the discharge was to the frame of the motor.

I asked the motor company about what could have been wrong with the motor and they told me that on the first test run the where to much humidity in the motor.
And on the 2nd time there told me that the motor was pushed to hard and went in over-speed.

I have just a basic understanding about Motors, so I can't argue with the motor company, but I may accept that on the first test run a high humidity could have let to a flashover. But telling me that I had pushed the motor to hard on the 2nd test run is not plausible, because I have 1 Drive controlling 2 motors in parallel, each motor driving 1 cable drum each, and each cable was pulled the same distance.

I had no change to inspect the motor on the 1st time, but on the 2nd time a had a look at the brushes and I'm quit sure that they wasn't settled in on the commutator.

Also the pressure of one spring was very loose.

Could this had caused the flashover ?

I would like to thank for every feedback on this matter, and hope to learn more about DC Motors.

Kind regards, Celso

RE: 110kW DC Motor flashover

I suspect over-voltage/over-current due to a drive issue.
I knew that as "Ringing the Comm".
Years ago we were commissioning some large motor generator drives on hoist drums.
As part of the setup procedure, the factory tech had us install shorting jumpers across the brush-gear of the motors.
We had a two sets of four, 1300 HP motors geared together and two sets of 1050 HP motors geared together.
The tech made the same mistake several times.
It is an impressive sight when four 1050 HP or four 1300 HP generators flash over brush to brush at the same time.

In our case the drives used field forcing on the exciters.
That is an intentional short duration, 200%, over-voltage on the exciter field for fast response.
If field forcing is used, it may be staying in too long.

Another possibility is that the drive is producing high frequency, high voltage transients.

Third; A weak field for any reason may cause flash-over at relatively low loading.
issue or a weak field, either from an exciter issue a wiring issue or a faulty motor field.
Fourth; If the motors are coupled together mechanically, a relatively small speed mismatch will cause one motor to try to overdrive the other motor.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: 110kW DC Motor flashover

First off, thanks to the OP for posting very good photographs.

Secondly, understanding the root cause failure related to a Direct Current machine is never a one size fits all.
There's a lot to study and understand. Don't expect an answer to this problem in a days readership.

Here is what I would investigate first to make sure is correct before chasing any other guesses.
The "brush entry" or connections from the field coils to the brush rack have to be known to be correct.
Until the above is determined.... everything else is moot.

Is it known whether the machine was "load tested" before it left the repair facility?

I'll check back in on this thread later.

John

RE: 110kW DC Motor flashover

Too dark a patina indicates poorly made or incorrect grade of brushes.

Violent flashover between the brush holders or brush holder to frame when motor is on load indicate incorrect interpole connection.

Muthu
www.edison.co.in

RE: 110kW DC Motor flashover

(OP)
Hello Bill,

The system is mechanically independent, but must run synchronized to be able to lower or rise the cables at the same time.
Each motor does have a independent field controller not for field forcing but for field weakening, in normal operation field is always 100%. Field weakening is only used, and must be manual set, to compensate a difference in cable length.



On the electrical site I have double checked everything and it seems to be OK.
But if you have any suggestion to look after or test I would highly appreciate.


Hello John,

I do not understand what you mean with "The "brush entry" or connections from the field coils to the brush rack have to be known to be correct."
Are the field coils not independent ?

The motors were not tested the first time and the 2nd time was only testes for function but without load, this was the information I have got from the technician from the repair facility.

Hello Muthu,

Is there a way to know if the brush is of the wrong grade for this motor?
The dark patina couldn't it come from the brush getting settled ?

RE: 110kW DC Motor flashover

Interpoles control brush sparking.
Reversed interpoles will show greatly increased sparking at the brushes as the load is increased long before a total flash-over occurs.

Are these compound motors?
Reversed compound windings will weaken the field and lead to flash-over.
Is it possible to interchange the motors or the drives.
That is a quick way to determine if this is a motor issue or a drive issue if it may be done relatively easily.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: 110kW DC Motor flashover

"...I do not understand what you mean with 'The "brush entry" or connections from the field coils to the brush rack have to be known to be correct."
Are the field coils not independent ?

The motors were not tested the first time and the 2nd time was only testes for function but without load, this was the information I have got from the technician from the repair facility.

---------------------------------------------------------------------------------------------------------------------------------------------------------------
Celso;

Brush Entry is a term used in electrical apparatus repair, so it is understandable most outside the craft would not know its meaning.

The field pole windings, the coils of wire wound around the laminated stacked steel sheets... they have a beginning and an end lead connection.
When these lead ends accidentally get mis-connected to the brush rack (by accident) on the repair shop floor, no one knows about it until the machine is tested or placed in service.

To use waross "Bill's" words as well,
"Reversed interpoles will show greatly increased sparking at the brushes as the load is increased long before a total flash-over occurs."

If the machine is not tested before installation and its field pole windings are not connected as originally intended by the manufacturer,
the machine will not run correctly. It will make the big mess your photos clearly illustrate.

This connection mistake will not typically show up until the machine is placed in service under load.
(Stated again on purpose.)

An improperly connected Direct Current motor will run perfectly fine in many instances on a shop floor with no hint of problems.
There won't be any sparking at the brushes, there won't be any indication by reading the machine's no load current draw etc.
The machine will be given a check mark of "good to go".

Yes other causes can be investigated like brush neutral setting, the clock spacing of the brushes in relation to how they make contact
with the commutator and so on.... but the first question of "brush entry" must be answered before anything else even matters.

You should be thankful you have been given honest information from the repair technician related to the motor not being load tested.

If he indeed told you the motor was not tested, I reluctantly say that this motor must be dismantled and inspected for the connection,
and the machine must be load tested to absolutely verify it's connected properly.
And all of this won't even take place until the damage that has occurred from the present event has been corrected.

Only after verifying all of the above (said again for emphasis) one can begin to investigate other probable causes for the damage.

I wish you the very best of luck in the days ahead with this failure.

John

RE: 110kW DC Motor flashover

Note:
The field poles and the interpoles are two distinct differnt articles.
The interpoles are small poles between the main poles.
As the load on a motor increases, the flux pattern distorts or shifts.
When this happens the brush neutral position shifts away from under the brushes and sparking occurs.
The interpoles develop a counter flux to push the main flux back into position and maintain the brush neutral position under the brushes.
The interpoles or commutating poles are seldom shown on connection diagrams.
While the diagram will show A1 and A2 going to the armature, on of the leads will first go to the corresponding interpole and thence to the armature or brush-gear.
On the other hand a compound winding is a series winding on the main field pole.
A compound winding may be additive to develop more low speed torque.
A compound winding may be subtractive to give better speed regulation.
If a subtractive or differentially connected compound motor is stalled by a heavy load, the compound winding may overcome the shunt field winding with the result that the motor becomes a runaway series motor in reverse.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: 110kW DC Motor flashover

A patina that dark in such a short time is anomaly. Check the OEM brush grade and replace with the same grade. I think the present grade has too much carbon content.

There service shop should have tested the interpole polarity before sending it out to the field. It will be difficult now in the field to check or correct it.

Muthu
www.edison.co.in

RE: 110kW DC Motor flashover

Appreciate the technical clarifications Bill.

Interpoles verses field poles... they do mean something different.

Writing in a hurry, I could have said Field Frame Windings which would cover everything fastened to the works.

Somewhere in the depths of this forum I recall having to explain the difference between
a rotor, a wound rotor, an armature, and a squirrel cage rotor.

Someone kept insisting they were all "rotors". I gave up and moved along.

Also agree with as edison123 notes: "It will be difficult now in the field to check or correct it."

In the instance of this particular thread, one could insist all that needs to be done is stick one's hands into the apparatus with a wrench,
swap leads that look suspect and try the motor until it works correctly. This could be accomplished, but it would not be a very good
representation of workman like manner. Nor would it remove all the carbon from the various surfaces of the machine that now provide
multiple paths for potential to trek... including to ground. The machine would be tripping off not because it was connected wrong... but
because the thing is grounded with the assistance of carbon dust.

In studying the very good photographs again.... it's likely an optical illusion... but the commutator looks somewhat out-of-round.
I doubt it is, but the dust groove sets an appearance to me it is. I'm not saying it is... it just looks like it could be.

And it kind of appears as the mica could have been undercut a little more than what is represented.

And to nitpick a little more I would have at least wire brushed the rusty cap screws or replaced them.

It's not a perfect world though and a machine is down.

Again, good luck man.

John

RE: 110kW DC Motor flashover

(OP)
Thank you Bill, John and Muthu for all the input and high value information.

Hello Bill,

I don't know if this are compound motors, I have to ask. On the Motors nameplate I can't read a lot.



Hello John,

Thank you for the detailed explanation,I must confess that I'm an ignorant about DC motors. On most schematics the inter-pole windings are not shown and on schematics that was showing the windings I thought that it was some suppression coil near the brushes like on some drilling machines motors.

About the commutator looking somewhat out-of-round, I tried to look on other photos but there are no better photos. But I found one photo about a brush having the surface "tilted" but has nothing to do with being out-of-round.



Hello Muthu,

I will ask if there is a part list about the motor mention what kind of brushes this DC motor uses.

RE: 110kW DC Motor flashover

Do you have a connection diagram?
A compound field will be shown on the connection diagram.
The simple shunt motors are much more common.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: 110kW DC Motor flashover

(OP)
Hello Bill,

I uploaded the electrical diagram in the previous post. Connection diagram of the motor on the motor plate, like on some induction motors, I can't find.

RE: 110kW DC Motor flashover

Celco,
better electro shops must have equipment for testing DC motors under load .This should definitely be your request in future repairs . It seems that the polarity of the main and auxiliary poles is not good and this has been explained several times how with reduced AC voltage on brusches is relatively easy to check on assembled DC motor .It seems to me that additionally edges of the bars are not well machined and allow a voltage jump from bar to bar at end of commutator.
Good luck

RE: 110kW DC Motor flashover

Some useful information on DC Motors can be found here, along with the two pictures following.
https://ijyam.blogspot.com/2013/11/reversing-dc-mo...

The first picture shows how interpoles are connected to the armature circuit, This is the arrangement I would expect with a DC crane hoist, having a solid state or Ward Lenard type speed control.


This is a two pole compound motor. This arrangement provides very good speed regulation, making this arrangement less useful for crane hoisting applications.


As you can see in both motor types there are many connections, all of which must be correct.

Fred

RE: 110kW DC Motor flashover

Bill

The wiring diagram by OP doesn't show a compound wound motor.

Celco

One method to check interpole polarity. Apply 20 V AC to two adjacent brush holders and measure the output voltage between leads A1 and A2. The measured A1-A2 voltage should be less than 20 V.

Muthu
www.edison.co.in

RE: 110kW DC Motor flashover

Quote (edison123)

Bill
The wiring diagram by OP doesn't show a compound wound motor.
Agreed, Thanks for the catch Muthu.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: 110kW DC Motor flashover

Fred, That diagram seems to show the series winding wound in the same direction as the shunt winding.
That is a common connection for a compound generator.
Under load, the series winding increases the field strength and causes a voltage rise to compensate for voltage drop on long feeders.
For speed regulation the series field is differentialy connected to oppose the shunt field and increase the speed by field weakening.
If that connection is used on a motor it will boost the torque under heavy loads, but the speed regulation will be poorer.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: 110kW DC Motor flashover

Bill most of my experience with cranes was from the mechanical side of cranes having / DC NEMA resistor control, it is simple and robust, but rare for new work due to the component cost.

Lots of these (not particularly relevant to this troubleshooting exercise)


Of course The diagrams in my previous post are for the simple case of motor across the line, I put them here to possibly aid the lack of detail in the wiring diagram provided by the OP. I perhaps did not pay enough attention to the coil winding artwork.

Regards
Fred

RE: 110kW DC Motor flashover

Hi Fred.
My crane work was with single speed motors, two speed motors and wound rotor motors.
My DC work was in sawmills with Motor generator sets and "Amplidyne" control.
By the way, large DC motors are seldom started DOL.
The locked rotor current of a large DC motor would make the locked rotor current of an induction motor look like a non event.
The starting voltage is ramped up as the motor accelerates to limit the starting surge.
This may be done with an electronic drive.
Some version of a Ward Leonard Drive or, for smaller motors, with series resistors.
This is where a cumulative compound series winding may be used.
A differentially connected series winding will improve speed regulation by field weakening.
A cumulatively connected series winding will aid starting torque an reduce starting current by field strengthening.
These are no longer the issue that they once were as most DC motors are started and controlled by solid state drives of Ward Leonard systems with Amplidynes.
Starting torque and speed regulation may now be provided by the drives and compound motors have all but disappeared.
Now DC drives are all but gone, replaced by induction motors and VFDs.
Nostalgia warning:
When a DC motor was started on a bus or full voltage DC supply the operator stepped the motor up trough the starting steps manually.
Series resistors were used in the armature circuit to limit the current and where cut out in steps.
A manual DC motor starter may look like this:

I was instructing in a technical school and had some free time.
The head instructor asked for some help unpacking some new equipment that had just been delivered.
We wrestled a big box up onto the bench and removed the cardboard packing.
In side was a brand new, automatic, across the line DC motor starter.
Timers and contactors to cut out the resistor steps.
"What do you think of it, Bill?"
"It looks like a well built, quality piece of equipment, but Pete, does anyone use these anymore?"
"Yes, you're right Bill, it is obsolete.
In our defense, it was not obsolete when we ordered it.
The order apparently got lost for some years.
About six months ago we were asked by the purchasing department if we still wanted it.
Well, if we didn't take it we would get nothing so we said yes."
That was about 45 years ago.
That gives you some idea as to how long ago across the line DC motor starters went obsolete.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: 110kW DC Motor flashover

Well they may be "obsolete" but "obsolete" does not get old equipment replaced. Several cranes installed in the 1890's are still in service with the original controls. Makes getting replacement parts interesting. Making open frame crane controls OSHA compliant was a problem for a while.

RE: 110kW DC Motor flashover

That sounds like a challenging place to work.
I love a challenge.
Are your motors running off of a common source or is there a Ward Leonard type drive for each motor?

I had a tour of the control room of this old bridge before it was replaced.
The swing and wedge motors were started and reversed by open knife switches.
It was impressive but scary.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: 110kW DC Motor flashover

The last Ward Leonard drive to go was part of a 15 foot planer in the maintenance shop.
The last Ward Leonard crane hoist drive in crane service was replaced with a solid state drive in about 1995.
The Rotary Converters were scrapped and replaced with rectifiers in about 1977, after one lost it's AC feed, and the DC feed did not trip. The report was it spun fast enough to lift the windings out of the commutator. Bits of the DC feeder system are still in service but the users are gradually going away as equipment is replaced.
Yes it is an interesting place to work, but we are not supposed to advertise here so wink.

Regarding the OP's system once he gets the motor working, that looks painful. If there is no closed loop feedback, and no mechanical linkage, there must be something else to true up position at least every few cycles, or the two hoist drums will drift out of position.

Fred

RE: 110kW DC Motor flashover

(OP)
Hello Fred,

I will try to get photos of the system. The motors are mechanically independent each with it's own cable drum, but pulling the same wire.
One motor has an encoder that's connected to the SIEI Typact TPD3 Drive for feedback. To correct the speed a small adjust on one of the independent field controller has to be done. But in general the 2 motors where running syncron.


Hello John,

Now I know why you where wishing me luck. It has been a nightmare, but hopefully motor will come back next week.



Celso

RE: 110kW DC Motor flashover

I purchased one crane with the anti sway rig similar to container cranes. All 8 ropes landed on the same hoist drum. Similar to this rigging Kocks uses on their terminal server cranes, except all of the rope is on one drum.


With two drums and no mechanical linkage I would prefer two encoders, one is reference hoist drum current position, the other is on the hoist drum that is speed trimmed, connect the two encoders to a compere, and use the output error signal to trim the speed on one of the two motors.
I did not see anything like that in the wiring diagram. But I did not see the encoder you now report in the wiring diagram either.

RE: 110kW DC Motor flashover

(OP)
Hello Fred,

You are correct, on the diagram is a tacho signal as feedback, but this was changed right in the beginning to an 1200imp/rev encoder together with an overspeed sensor.
I'm not very familiar with cranes in general, I'm more used to sit on my desk and repair electronic devices.


But I must confess that is fascinating to see some bunch of electronic components driving some massive machines.
And so far, in this matter I had big luck. The where no information about SIEI Typact TDP3 SCR Drive. Mr. Mustapha Abulhuda, a person I found on E-bay selling some SIEI spareparts, gave me all documentations he had about the drive and I own him a big thank you. And also here, as my first post and getting advice on what could be wrong with the motor, I have to thank you all folks.

Celso

RE: 110kW DC Motor flashover

Celso;

Nightmares turn into redeeming experiences you will always remember for the education they gave you.

Once upon a time, Eddy Current drives differentiated the men from the boys.

Today, Direct Current motors are somewhat following the same curve.
One day they will be a "thing" of the past.

Thank you for keeping the curious crowd ... updated.

John

RE: 110kW DC Motor flashover

Hopefully you are past needing this now. SIEI seems to be a subsidiary of GEFRAN it was hiding. Some DC drive products are offered on their website.

I think this is the page with the current generation of the drive you are working on. The model numbers hint at family resemblance.
https://www.gefran.com/en/product_categories/184-d...)

I have been through the unhappy experience of working with early generation DG hoist driver after they became technologically obsolete. GE once offered to provide some really detailed information on a hoist drive so we (the customer) could engineer a replacement for a unavailable hall effect current sensor. I declined the offer, and we eventually replaced all of the drives on that crane.

Planning for component obsolescence is unfortunately just one more thing owners need to do - and often do not until something breaks that can't be repaired at a reasonable cost.

Regards
Fred

RE: 110kW DC Motor flashover

(OP)
Hello,

I have some updates ...

What is the probability to have a motor failed again. Extremely high, especially if you receive a motor that was not tested at all.
I have to close this case on my side, I don't want to blame no one, but some body is not doing the job well.

Thank you all for your help.

Celso

RE: 110kW DC Motor flashover

Been there toothumbsup2. Even good shops that test everything out the door occasionally send out a bad motor. Does not matter how good the shop is, its a warranty call.morning

Fred

RE: 110kW DC Motor flashover

@OP

So, what exactly was the problem with the motor?

Muthu
www.edison.co.in

RE: 110kW DC Motor flashover

(OP)
Hello Muthu,

There still is a problem. As the motor came back from the shop to be mounted on the crane we tested it and it only took 30 seconds to develop a flashover again.
This time we swooped the position of the 2 motors to be sure that there where no mechanical problem with the gearbox or the electrical cables from Drive/fuses/resistor/exciter to the motors.

I had several meeting with the company in charge of the repair of the motor, and tried to hand over the informations that was given me on this forum.
They always denied that the connection could be wrong and as I asked for a test report with full load, they told me that this was not necessary because the have a lot of experience and it was never necessary to carry out such a test/report. So I was quite sure that the motor would fail again and it failed.

I was monitoring the current of the armature on both motors, there was a slightly difference of the current 1-2 Amps less on the "good" motor. There was just very little sparking till the current hit 90A - 100A, after that the flashover. It was very fast and I had no time to switch off the drive fast enough, and even I don't know if it is safe to disconnect a DC Motor abruptly.

The damage is an exact 1:1 copy of the photos of the second time the motor failed.

Celso


RE: 110kW DC Motor flashover

At this point you may be better to check the connections yourself.
I would first try using a hacksaw blade as a magnetic polarity probe.
Put a low voltage DC across A1 and A2.
That is the armature circuit including the interpoles.
Use a short length of hacksaw blade and a compass to probe the polarity of the armature field and the interpole field.
Compare the good motor with the bad motor.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: 110kW DC Motor flashover

Celso

Thanks for the update. Load tests are expensive, that's why it's only a type test even for OEM's. As a rewinder, I do no-load tests only for 99% of the jobs.

As I said before, there are other ways to test the DC machine connections without doing the load test. The repair shop is either unaware of such tests or is being just pigheaded.

I am more positive that in your case, the interpole connection is wrong since the flashover is happening only on load.

Muthu
www.edison.co.in

RE: 110kW DC Motor flashover

Celso, it was an excellent idea to swap the placement of the two motors to confirm the integrity of the electrical / mechanical installations, etc.

Acknowledging that load testing a Direct Current motor is not possible in every repair facility.

Numerous other tests can be made however on a motor before re-assembly for assurance the repair is proceeding without errors.
Voltage Drop tests on the interpoles and the field windings are very important steps in knowing the condition of the apparatus.
These tests are performed during disassembly and original inspections however. Not as an afterthought during re-assembly.
And marking, tagging the all important leads to the brush rack is extremely necessary for everything discussed so far.

Having looked at the original posted photos considerably longer than I would normally do for an Internet forum,
I keep returning to the three pictures that show a lead route going and coming from a different place.
Brush Rack jumpers are not very long typically, and are normally just long enough to make their respective connections.



I think the motor is connected improperly. None of us reading this forum can prove the connections though, and there is always a chance
we are all wrong in our hypothesis because we are not in the motor's physical presence.

Repair history on the motor is becoming a little foggy. Another factor to remember is repair shops can make the mistake of unknowingly
following someone else's errors.

Again, I'm wishing you success in navigating your nightmare.

John

RE: 110kW DC Motor flashover

(OP)
Hello,

John, you nailed it.
Finally good news, after some investigation, today I discovered that there is a 3rd party evolved in this process. The 1st motor overhaul was made by the company hired by my client and this company has lack of experience with DC motors.
2nd and 3rd overhaul was made by a well know company in my country, but they failed to do it right because o the first overhaul someone did change the interpole wiring.
As I only was talking with the 1st company, for sure they ignored my information about the interpoles and didn't pass the info to the 2nd company.

Today at the 2nd repair shop a load test was made and wiring was checked to be wrong. At 60A motor was sparking and after interpole wiring change motor was running fine.

In the next few days I will give some feedback of the motors in Place and hope to have some time to take some pictures.

Again, thank you all !!!

RE: 110kW DC Motor flashover

Quote (Waross)


waross (Electrical)
8 Feb 21 23:01
Interpoles control brush sparking.
Reversed interpoles will show greatly increased sparking at the brushes as the load is increased long before a total flash-over occurs.
Got lost in the background noise.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: 110kW DC Motor flashover

(OP)
Hello Bill,

No I didn't lost you in the background noise. Everyone pointing me at the interpoles was right. And I "repointed" several times and no one in the repair shop was lisening.

Regards, Celso

RE: 110kW DC Motor flashover

I know the feeling.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: 110kW DC Motor flashover

(OP)
Hello,

Finally everything is running smooth.
Looks nice and clean after 1 week of operation.


Here are the arrangement.



I like to work more on the ground, but working in heights can give you sometimes a nice view also.




Thank you all wink

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