150HP DC motor - interpoles connected with wrong polarity 1

[JPetersen]

Hello,

We have a 150HP DC motor where the interpoles after overhaul were connected with wrong polarity. Half an hour after start the motor drive tripped and the amature suffered catastrophic overheating in the area just after the commutator, I believe it is where the windings are being connected with each other (lap winding I assume). The commutator and the brushes also showed signs of sparking (black track) but not severe overheating.

Question is if the wrongly connected interpoles itself could result in such severe over heating of the armature? Anyone experienced similar incident?

Brgds
Petersen

 

[edison123]

Wrong interpole polarity will lead to severe sparking under brushes and even brush holder to brush holder arcing. You need to reverse the interpole polarity. If the interpole lead is long enough, shift the lead from one brush holder (say +ve) to the next immediate brush holder (-ve).

Muthu

http://www.edison.co.in

 

[JPetersen]

Thanks edison123,

The amature is destroyed and needs complete renewal. I am looking for information if this polarity mix-up related to the interpoles could actually cause such severe damage to the amature. The commutator and the brushes show for sure signs of sparking but they are not destroyed in the same way as the armature. The polarity mix-up was caused by brush holder position shifted 90 deg (4 pole machine) and therefore amature current was reversed while interpole current direction remained.

Would the equalizer windings be overloaded by interpole polarity mistake? The excessive heating of the amature seem to be located in the area of the equalizer.

 

[Skogsgurra]

Yes. I have had two cases where the winder shop's "best guy" had reconnected interpoles with wrong polarity. Armature and collector completely destroyed in the first case. They had learned, but not well enough, so next time they were more careful when running the machine first time. Still wrong. They were not even in a hurry.

Gunnar Englund

http://www.gke.org

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

 

[dArsonval]

"Brush Entry" connections to interpoles is.../slash/has been a common mistake in reconnecting Direct Current motors since the very beginning of their invention.

Many 'a repair shop owners have stories to tell about how one day... all HELL broke loose... because a D.C. motor was incorrectly connected despite unknowingly declaring it ready for service.

The circumstance pulls people out of bed at 3:30 AM, and during holiday breaks when no one is around to help diagnose the situation. It upsets production managers and a whole host of others.

The beginning of the mistake takes place when the motor is initially dismantled.

[Or]

During the reassembly sequence of the apparatus.

A motor mechanic may be called away from the job and an unsuspecting "fill-in" picks up where the other person left off. (To name one of any numerous scenarios).

If the connection for Brush Entry is not specifically drawn out, documented, confirmed, during
the very early stages of repair... the job is potentially destined to become a nightmare... for someone.

The solution to this headache is something called a "Load Test".
Before the motor exits the repair shop, the apparatus must be called upon to drive its nameplate rated load.

A load test reveals incorrect brush entry connections at a 186,000 miles per second.
The person testing the motor will know immediately something is wrong if the motor is not connected properly.

Not all repair shops have the resource of a load test.

Always specify a load test of a D.C. apparatus at its rated output before accepting it as "Repaired".
[Even on motors as small as 1 HP output.]

The label "Brush Entry" is somewhat of a inside industry term or description of how connections are made to the brush holder, interpoles, etc.
It also clarifies the brush holder's physical position mounted to the D.C. motor's end bracket.
All of it... necessary to know... in reassembling a Direct Current Motor.

John

 

[JPetersen]

Thanks dArsonval,
Your lines describing such situation are here probably even more accurate than you know .

Sorry for still picking on this, but I cannot figure out why amature and collector are completely destroyed - significant brush sparking for sure happend but the commutator, brushes and brushholder can be put in operation again after minor repair and cleaning. The huge damage is on armature itself - why?? The workshop is not helpfull here, even telling me that brush holder 90 deg wrong position would just cause opposite direction of rotation....

 

[panter]

Let some picture for help us to see the problem. If there are 4 brush holders and they rotate by 90 degrees, this can be the good reason for changing the direction of rotation and wrong polarity of inter poles.
I think that Skogsgurra has here several times given a simple test for determining polarity of inter poles with a small alternating voltage on brushes and it works very reliably and well .
Good luck !

 

[dArsonval]

To say, "I Think", means (I don't know

I'll hazard a guess that not a-LOT of research has been concluded on anything related to incorrectly connecting... or re-connecting a Direct Current Motor.
The causes would obviously point back to either the manufacturer or the repair center. And many among the crowd know neither of those entities are coming forward with enlightening report highlights.

One could work out the theory electrically, yet it would still be minus the real world environment of what actually happens when [real world] power is applied to an apparatus.

This is an interesting topic... because in this particular conversation thread, we all know the reason for a failure before seeing the failure displayed visually with pictures.
(Kind of a Colombo Episode if you will ; )

It's likely too late now... but fascinating indeed it would have been... to see photos of the failure;
read all the possible guesses as to "why" the motor failed... only to be told later... the Interpoles were incorrectly connected.

Pursuing a complete Root Cause Analysis answer as to "WhY" this D.C. motor failed knowing what we all now know... will always lead back to an answer of Human Error.
Someone, or a chain of decisons made by a few... made a mistake. They certainly didn't mean to.

John

 

[waross]

Why interpoles?
The current through an armature tends to distort or skew the magnetic field across the armature.
The greater the load the greater the current and the greater the skewing of the magnetic field.
This moves the null point of the commutator away from the center of the brushes and sparking results.
When the brushes are at the null point of the commutator they are crossing from one segment to another at zero volts.
When the brushes are no longer crossing segments at the null or zero voltage point sparking is the result.
The function of the Interpoles or Commutating poles is to push the magnetic field to keep the null point of the commutator under the brushes under changing load conditions.
Reversing a DC motor.
There are several ways to reverse a DC motor.
1. Reverse the current through the field.
2. Reverse the current through both the armature and the interpoles as a unit.
3. On a motor without interpoles such as a universal motor reversing may be done by brush shifting. This technique is used to reverse some drill motors.
However, when a DC motor without interpoles is reversed by brush shifting, the interpoles are now shifting the magnetic field in the wrong direction and the brush sparking is much worse than if there were no interpoles.

The workshop is not helpfull here, even telling me that brush holder 90 deg wrong position would just cause opposite direction of rotation....

Is it feasible to find another shop or are you locked into the shop that you are presently using?

Bill
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"Why not the best?"
Jimmy Carter