DC Shunt wound motor flashover

Thread Starter

ADSSparky

Joined Oct 26, 2023
3
Hello all, new member here so I definitely apologize if I am placing this thread in the wrong area, however this issue seems to be a power system related issue, so I thought this would be the most suitable location.

I have a lot of background context to give, so please make sure you read over everything.

I work in a machine shop where we do industrial milling and machining to create steel processing lines. I am dealing with an issue on one of our machines that had stopped working. These drives were implemented in the late 1970's. Being a new graduate, I had absolutely no knowledge of this kind of dinosaur system, so it has been quite the learning experience throughout. Please reference the provided schematic as I go through what was done.

This is our 50 H.P. Shunt wound DC motor used to operate the spindle.

The initial problem observed: As spindle run was initiated, the motor would start to accelerate. As it was accelerating, resistor 206R (Highlighted in yellow) would start to heat up, glowing red hot until fuse 201F (Highlighted in red) would blow, causing the motor to coast to a stop. So I broke out the schematics and figured the issue would be within the field supply board.

I was going to test the SCR's on the board, however my foreman had informed me that we had an issue with the machine before and that the SCR's on the armature power unit had to be checked and two were replaced. After that, the machine worked fine until this break down again. Not thinking about electrical theory, I had just went ahead and checked the SCR's that he had mentioned. Not knowing how to properly test one, I experimented and fried one of them. A replacement was ordered, the others were tested properly and turned out good. Replaced all of the components in the enclosure and tried the spindle again. No change here, as the same resistor heated up greatly as before. (Note: The resistor is a chassis mount power resistor)

I started looking through the boards for any sign of physical damage/burns/melting/etc. I found a signal transformer on the crossover circuit that had burned away some insulation around one of the contacts. I de-soldered it, and tested it, which couldn't pass a continuity test. I got a new one, replaced it, and put the board back in its slot. Now i attempted the spindle once more and we had a small change. The spindle would start to speed up, however after 2-3 seconds it would stop and start coasting. Then, it would start itself back up, as if to attempt again, but would do the same failsafe shut down after 3 seconds. On the third time attempting to speed up on its own, It tripped that initial fuse (206R highlighted in red) and energized the "phase loss reset" pushbutton. In this instance, the resistor did not get red hot.
To me, this told me two things:
1. The signal transformer that I had swapped out was preventing the Phase loss indicator light to illuminate and shut the drive down. This is why the resistor got red hot until the fuse blew and took care of the problem rather than the failsafe. (I presume. Still not 100% sure)
2. That I needed to look at the SCR's on the field supply board like I initially wanted to do.

SO, I pulled the board, and removed each SCR and tested them accordingly. To no surprise, I found two faulty SCR's, #5 and #8. We had spare firing boards available, just newer revisions of them that were very identical, so I took two of the SCR's from the spare board and installed them. I was unable to find the original SCR's as they are from the late 70's and a part number gives nothing on the web. Not so much as a spec sheet to cross reference with either.

Now, the spindle would accelerate up to speed like normal. No more energy dissipation through he resistor. Everything seemed to be great until we attempted to slow the spindle down via the rheostat on the pendulum. It would make an odd sound while trying to slow for about 2 seconds. Then BOOM we got a flashover. We weren't sure if this was just with the rheostat or not, so we ran the motor once more, didn't tough the rheostat, and attempted to stop it using the STOP pushbutton. We received the same flashover event.

This is where the story ends. It is tough to determine what exactly is causing this flashover without testing and getting the issue again. I don't want to keep ringing the comm as it is going to lead to extensive damage if this reoccurs. I have done a megger test on the motor, and all insulation seems to be intact and working great, even after 60 years (the motor is from 1964, controls were updated sometime around 1978). This drive is a Speedpak R-G (Regenerative) drive by Allen Bradley. From what I understand, a regenerative drive is to return the back EMF current to the line when reducing such line current in the armature, using the reversed generating torque from the armature to slow the motor down as you reduce the current through the SCR's. According to the schematic, It looks as if the current has no reverse direction to dissipate the energy back to, based on the layout of the SCR's (since current can only flow in one direction through these).
I was thinking that one possibility could be that the SCR's stop firing completely when initiating a slowdown or stop, either that or the firing pulses reduce in such great magnitude instantly that it is unbalanced and the back EMF is too much for the applied voltage, causing the flash.

Are there any wizards on here that may be able to provide me with insightful wisdom that could lead to a solution? From what I understand in reading the manual, the drive uses field weakening to increase the motor speed above base speed, but this does not explain why we get a flashover when reducing speed or braking. I suspect it has to do with the regen. braking method, however I am stumped to this point as to where to look next. The motor accelerates and maintains speed fine, so all feedback components should be of in working order. Could it be because of the field supply SCR's that I had changed? maybe the one SCR in the armature power unit that I had changed could be the culprit? If these are to be in question, then why would the motor accelerate without issue?

I have provided some parts of the manual that describe each boards function, as well as anything else that I thought to be relevant in understanding the basic function of the drive. If anyone has experience with this sort of braking method, it would be extremely helpful to get your input. Thank you so much in advance.

DISCLAIMER: I do not have access to the PLC. It is a PLC 5/15, and the programing software is nowhere to be found, as the previous electrical engineer who worked on these machines passed away a few years ago, and his wife came here and took all of his equipment, including the software and his computer. The software is now thousands of dollars, along with the adapters to connect to modern operating systems through DH+ communication. The shop does not want to spend the money on such software and hardware if we do not have to. This machine is getting a new controls upgrade after we finish using it for the current job that we have in the shop. It is essential that this machine runs to finish the job as it is the only one with a wide enough table to complete the job.

If you are looking for more information, just let me know and I will see what I can find and provide.
 

Attachments

MaxHeadRoom

Joined Jul 18, 2013
28,703
Some odd occurrences there?
Normally a shunt field motor, it can accelerate to destruction, if the field happens to go open whilst running, normally on large motors, a field loss detection circuit is in place to prevent it by shutting off the main power.
If the field current is high, then the motor RPM will be limited, depending on just how much current is introduced.
It sounds like you have some odd ball field supply system as these generally are the last to have a problem.
It may be worth looking for a more up to date field controller, although I imagine now with PM field motors, this may be hard to find.
What do AB say about it?
 

HasBeen

Joined Jul 28, 2023
29
Hello all, new member here so I definitely apologize if I am placing this thread in the wrong area, however this issue seems to be a power system related issue, so I thought this would be the most suitable location.

I have a lot of background context to give, so please make sure you read over everything.

I work in a machine shop where we do industrial milling and machining to create steel processing lines. I am dealing with an issue on one of our machines that had stopped working. These drives were implemented in the late 1970's. Being a new graduate, I had absolutely no knowledge of this kind of dinosaur system, so it has been quite the learning experience throughout. Please reference the provided schematic as I go through what was done.

This is our 50 H.P. Shunt wound DC motor used to operate the spindle.

The initial problem observed: As spindle run was initiated, the motor would start to accelerate. As it was accelerating, resistor 206R (Highlighted in yellow) would start to heat up, glowing red hot until fuse 201F (Highlighted in red) would blow, causing the motor to coast to a stop. So I broke out the schematics and figured the issue would be within the field supply board.

I was going to test the SCR's on the board, however my foreman had informed me that we had an issue with the machine before and that the SCR's on the armature power unit had to be checked and two were replaced. After that, the machine worked fine until this break down again. Not thinking about electrical theory, I had just went ahead and checked the SCR's that he had mentioned. Not knowing how to properly test one, I experimented and fried one of them. A replacement was ordered, the others were tested properly and turned out good. Replaced all of the components in the enclosure and tried the spindle again. No change here, as the same resistor heated up greatly as before. (Note: The resistor is a chassis mount power resistor)

I started looking through the boards for any sign of physical damage/burns/melting/etc. I found a signal transformer on the crossover circuit that had burned away some insulation around one of the contacts. I de-soldered it, and tested it, which couldn't pass a continuity test. I got a new one, replaced it, and put the board back in its slot. Now i attempted the spindle once more and we had a small change. The spindle would start to speed up, however after 2-3 seconds it would stop and start coasting. Then, it would start itself back up, as if to attempt again, but would do the same failsafe shut down after 3 seconds. On the third time attempting to speed up on its own, It tripped that initial fuse (206R highlighted in red) and energized the "phase loss reset" pushbutton. In this instance, the resistor did not get red hot.
To me, this told me two things:
1. The signal transformer that I had swapped out was preventing the Phase loss indicator light to illuminate and shut the drive down. This is why the resistor got red hot until the fuse blew and took care of the problem rather than the failsafe. (I presume. Still not 100% sure)
2. That I needed to look at the SCR's on the field supply board like I initially wanted to do.

SO, I pulled the board, and removed each SCR and tested them accordingly. To no surprise, I found two faulty SCR's, #5 and #8. We had spare firing boards available, just newer revisions of them that were very identical, so I took two of the SCR's from the spare board and installed them. I was unable to find the original SCR's as they are from the late 70's and a part number gives nothing on the web. Not so much as a spec sheet to cross reference with either.

Now, the spindle would accelerate up to speed like normal. No more energy dissipation through he resistor. Everything seemed to be great until we attempted to slow the spindle down via the rheostat on the pendulum. It would make an odd sound while trying to slow for about 2 seconds. Then BOOM we got a flashover. We weren't sure if this was just with the rheostat or not, so we ran the motor once more, didn't tough the rheostat, and attempted to stop it using the STOP pushbutton. We received the same flashover event.

This is where the story ends. It is tough to determine what exactly is causing this flashover without testing and getting the issue again. I don't want to keep ringing the comm as it is going to lead to extensive damage if this reoccurs. I have done a megger test on the motor, and all insulation seems to be intact and working great, even after 60 years (the motor is from 1964, controls were updated sometime around 1978). This drive is a Speedpak R-G (Regenerative) drive by Allen Bradley. From what I understand, a regenerative drive is to return the back EMF current to the line when reducing such line current in the armature, using the reversed generating torque from the armature to slow the motor down as you reduce the current through the SCR's. According to the schematic, It looks as if the current has no reverse direction to dissipate the energy back to, based on the layout of the SCR's (since current can only flow in one direction through these).
I was thinking that one possibility could be that the SCR's stop firing completely when initiating a slowdown or stop, either that or the firing pulses reduce in such great magnitude instantly that it is unbalanced and the back EMF is too much for the applied voltage, causing the flash.

Are there any wizards on here that may be able to provide me with insightful wisdom that could lead to a solution? From what I understand in reading the manual, the drive uses field weakening to increase the motor speed above base speed, but this does not explain why we get a flashover when reducing speed or braking. I suspect it has to do with the regen. braking method, however I am stumped to this point as to where to look next. The motor accelerates and maintains speed fine, so all feedback components should be of in working order. Could it be because of the field supply SCR's that I had changed? maybe the one SCR in the armature power unit that I had changed could be the culprit? If these are to be in question, then why would the motor accelerate without issue?

I have provided some parts of the manual that describe each boards function, as well as anything else that I thought to be relevant in understanding the basic function of the drive. If anyone has experience with this sort of braking method, it would be extremely helpful to get your input. Thank you so much in advance.

DISCLAIMER: I do not have access to the PLC. It is a PLC 5/15, and the programing software is nowhere to be found, as the previous electrical engineer who worked on these machines passed away a few years ago, and his wife came here and took all of his equipment, including the software and his computer. The software is now thousands of dollars, along with the adapters to connect to modern operating systems through DH+ communication. The shop does not want to spend the money on such software and hardware if we do not have to. This machine is getting a new controls upgrade after we finish using it for the current job that we have in the shop. It is essential that this machine runs to finish the job as it is the only one with a wide enough table to complete the job.

If you are looking for more information, just let me know and I will see what I can find and provide.
You have used the term "flashover" and made it sound like a bad thing. I can't relate to that term in the context of the problem you are having. Could you explain it / how it applies here?
 

Thread Starter

ADSSparky

Joined Oct 26, 2023
3
You have used the term "flashover" and made it sound like a bad thing. I can't relate to that term in the context of the problem you are having. Could you explain it / how it applies here?
Flash-overs occur on main generators, motors, etc. A flash-over is the dissipation of a large amount of electrical energy from the positive point to the negative or earth. This energy flashes in the form of a large arc from the positive sets of brushes to the negative sets, or the earthed iron work of the machine, through the intervening air space. When a flash-over takes place this separating air space is broken down through the carbon gas off the brushes being blown across by the windage of the commutator and forming a path of low resistance for the electric current to follow. The electric arc dissipating in the air is very rapid and the noise made is like the roar of a cannon but of a much sharper nature. A flashover can be catastrophic to electric motors, causing extensive damage to brush holders, commutator bars, and even the brush arm insulation. In higher power situations, it can destroy the motor completely. I am having trouble finding where this large amount of energy is coming from when slowing the machine down, or stopping it.
 

Thread Starter

ADSSparky

Joined Oct 26, 2023
3
Some odd occurrences there?
Normally a shunt field motor, it can accelerate to destruction, if the field happens to go open whilst running, normally on large motors, a field loss detection circuit is in place to prevent it by shutting off the main power.
If the field current is high, then the motor RPM will be limited, depending on just how much current is introduced.
It sounds like you have some odd ball field supply system as these generally are the last to have a problem.
It may be worth looking for a more up to date field controller, although I imagine now with PM field motors, this may be hard to find.
What do AB say about it?
AB no longer offers support for this 50 year old technology.
 

HasBeen

Joined Jul 28, 2023
29
Flash-overs occur on main generators, motors, etc. A flash-over is the dissipation of a large amount of electrical energy from the positive point to the negative or earth. This energy flashes in the form of a large arc from the positive sets of brushes to the negative sets, or the earthed iron work of the machine, through the intervening air space. When a flash-over takes place this separating air space is broken down through the carbon gas off the brushes being blown across by the windage of the commutator and forming a path of low resistance for the electric current to follow. The electric arc dissipating in the air is very rapid and the noise made is like the roar of a cannon but of a much sharper nature. A flashover can be catastrophic to electric motors, causing extensive damage to brush holders, commutator bars, and even the brush arm insulation. In higher power situations, it can destroy the motor completely. I am having trouble finding where this large amount of energy is coming from when slowing the machine down, or stopping it.
Excellent description / clarification! Thank you. Now, is this event across the brushes, or at some other plus-to-minus path in the system? If it is across the brushes, are the brushes in contact with the armature at the moment it occurs? That would seem to be a very low-resistance path to arc around, and the amount of required energy would be huge. If it occurs some other point, it could be anything from insulaton breakdown to ???. One more question might be, does it only occur when you are actively(electrically) slowing / stopping the motor, or does it also occur when the motor is just coasting to a stop?
 

MaxHeadRoom

Joined Jul 18, 2013
28,703
If you have an arc that appears to circle the commutator, it usually indicates a coil short circuit, often in the armature.
After some time in service, the comm may need attention, and of of the older type that has mica com insulation, may need turning and com bar insulation separators undercut .
Later versions do not need undercutting as they consist of wearable mateerial.
If an armature fault is suspect, It may need a 'Growler' test.
 

BVSM

Joined Oct 29, 2023
6
Hello all, new member here so I definitely apologize if I am placing this thread in the wrong area, however this issue seems to be a power system related issue, so I thought this would be the most suitable location.

I have a lot of background context to give, so please make sure you read over everything.

I work in a machine shop where we do industrial milling and machining to create steel processing lines. I am dealing with an issue on one of our machines that had stopped working. These drives were implemented in the late 1970's. Being a new graduate, I had absolutely no knowledge of this kind of dinosaur system, so it has been quite the learning experience throughout. Please reference the provided schematic as I go through what was done.

This is our 50 H.P. Shunt wound DC motor used to operate the spindle.

The initial problem observed: As spindle run was initiated, the motor would start to accelerate. As it was accelerating, resistor 206R (Highlighted in yellow) would start to heat up, glowing red hot until fuse 201F (Highlighted in red) would blow, causing the motor to coast to a stop. So I broke out the schematics and figured the issue would be within the field supply board.

I was going to test the SCR's on the board, however my foreman had informed me that we had an issue with the machine before and that the SCR's on the armature power unit had to be checked and two were replaced. After that, the machine worked fine until this break down again. Not thinking about electrical theory, I had just went ahead and checked the SCR's that he had mentioned. Not knowing how to properly test one, I experimented and fried one of them. A replacement was ordered, the others were tested properly and turned out good. Replaced all of the components in the enclosure and tried the spindle again. No change here, as the same resistor heated up greatly as before. (Note: The resistor is a chassis mount power resistor)

I started looking through the boards for any sign of physical damage/burns/melting/etc. I found a signal transformer on the crossover circuit that had burned away some insulation around one of the contacts. I de-soldered it, and tested it, which couldn't pass a continuity test. I got a new one, replaced it, and put the board back in its slot. Now i attempted the spindle once more and we had a small change. The spindle would start to speed up, however after 2-3 seconds it would stop and start coasting. Then, it would start itself back up, as if to attempt again, but would do the same failsafe shut down after 3 seconds. On the third time attempting to speed up on its own, It tripped that initial fuse (206R highlighted in red) and energized the "phase loss reset" pushbutton. In this instance, the resistor did not get red hot.
To me, this told me two things:
1. The signal transformer that I had swapped out was preventing the Phase loss indicator light to illuminate and shut the drive down. This is why the resistor got red hot until the fuse blew and took care of the problem rather than the failsafe. (I presume. Still not 100% sure)
2. That I needed to look at the SCR's on the field supply board like I initially wanted to do.

SO, I pulled the board, and removed each SCR and tested them accordingly. To no surprise, I found two faulty SCR's, #5 and #8. We had spare firing boards available, just newer revisions of them that were very identical, so I took two of the SCR's from the spare board and installed them. I was unable to find the original SCR's as they are from the late 70's and a part number gives nothing on the web. Not so much as a spec sheet to cross reference with either.

Now, the spindle would accelerate up to speed like normal. No more energy dissipation through he resistor. Everything seemed to be great until we attempted to slow the spindle down via the rheostat on the pendulum. It would make an odd sound while trying to slow for about 2 seconds. Then BOOM we got a flashover. We weren't sure if this was just with the rheostat or not, so we ran the motor once more, didn't tough the rheostat, and attempted to stop it using the STOP pushbutton. We received the same flashover event.

This is where the story ends. It is tough to determine what exactly is causing this flashover without testing and getting the issue again. I don't want to keep ringing the comm as it is going to lead to extensive damage if this reoccurs. I have done a megger test on the motor, and all insulation seems to be intact and working great, even after 60 years (the motor is from 1964, controls were updated sometime around 1978). This drive is a Speedpak R-G (Regenerative) drive by Allen Bradley. From what I understand, a regenerative drive is to return the back EMF current to the line when reducing such line current in the armature, using the reversed generating torque from the armature to slow the motor down as you reduce the current through the SCR's. According to the schematic, It looks as if the current has no reverse direction to dissipate the energy back to, based on the layout of the SCR's (since current can only flow in one direction through these).
I was thinking that one possibility could be that the SCR's stop firing completely when initiating a slowdown or stop, either that or the firing pulses reduce in such great magnitude instantly that it is unbalanced and the back EMF is too much for the applied voltage, causing the flash.

Are there any wizards on here that may be able to provide me with insightful wisdom that could lead to a solution? From what I understand in reading the manual, the drive uses field weakening to increase the motor speed above base speed, but this does not explain why we get a flashover when reducing speed or braking. I suspect it has to do with the regen. braking method, however I am stumped to this point as to where to look next. The motor accelerates and maintains speed fine, so all feedback components should be of in working order. Could it be because of the field supply SCR's that I had changed? maybe the one SCR in the armature power unit that I had changed could be the culprit? If these are to be in question, then why would the motor accelerate without issue?

I have provided some parts of the manual that describe each boards function, as well as anything else that I thought to be relevant in understanding the basic function of the drive. If anyone has experience with this sort of braking method, it would be extremely helpful to get your input. Thank you so much in advance.

DISCLAIMER: I do not have access to the PLC. It is a PLC 5/15, and the programing software is nowhere to be found, as the previous electrical engineer who worked on these machines passed away a few years ago, and his wife came here and took all of his equipment, including the software and his computer. The software is now thousands of dollars, along with the adapters to connect to modern operating systems through DH+ communication. The shop does not want to spend the money on such software and hardware if we do not have to. This machine is getting a new controls upgrade after we finish using it for the current job that we have in the shop. It is essential that this machine runs to finish the job as it is the only one with a wide enough table to complete the job.

If you are looking for more information, just let me know and I will see what I can find and provide.

Motor seems to be ok. When decelerating and stopping, the field and thus also the armature polarity changes and the motor supplies current through the armature power unit back to the grid. Flashover can be caused by the armature voltage becoming too high so that the thyristors cannot switch off. Therefore check that the field current is the same in both directions and does not exceed the rated current of the motor: Open the armature circuit, start, apply a small positive speed reference and then a small negative speed reference, while measuring. Tacho loss may disengage.
On the regenerative Drive control card there is potentiometer reg.C.L. to adjust brake current.
Or try to get a newer 4Q drive.
 
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