×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Contact US

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

DC motor flashover during dynamic braking

DC motor flashover during dynamic braking

DC motor flashover during dynamic braking

(OP)
First, let me give as much background information as possible. Brace yourselves, this will be a semi lengthy explanation.

Here at work, we have a Skoda horizonal boring mill. The drive system was designed and installed in 1980, so it is dated, but we haven't had a need for a controls upgrade yet. When it was last being used, the spindle drive shut down in the middle of operation and would not run anymore. There was no power surge or anything that happened out of the ordinary to cause this. Upon inspection, it was found that the time delay fuse in series with the braking resistor had blown. It was replaced, however, when the spindle was operated again, the braking resistor would start to dissipate energy at the same time that it was accelerating up to speed, and caused this fuse to blow again. This told me that there was an issue with the SCR's within the braking circuitry, allowing current to flow through the Dynamic Brake while the spindle was running. I pulled the Field Supply assembly board (Allen Bradley board No. 12M03-00232-00), and tested all 8 SCR's. It was found that two of them were faulty, and the appropriate replacement was ordered. While waiting for the rectifiers to be delivered, I tested other components in the machine to ensure proper operation within the spindle, and found that a signal transformer within the crossover circuit (AB Board No. 12M3-00221-00) had also failed. After all of the previous components came in and were replaced, we ran the spindle again. This time, the machine would run up in speed perfectly. Great! It seemed as though the constant braking issue that we had previously had been resolved. It would maintain speed as expected, and increase speed through the rheostat as expected. However, when we attempted to slow the motor down, it would start to slow for about 2-3 seconds, then the motor would flashover. The spindle would bog and slow more for a moment before coming back up to speed set by the rheostat, and then operate at the slower speed we set it to just fine. Obviously this is not what we want, as we are trying to avoid damaging the motor during this repair. The stop pushbutton was selected, and yet again, we had another flashover in the motor. All research I have found about these flashover events are not related to dynamic braking, or the switching to the brake circuit. Only intermittent issues caused by damaged brushes. Given the initial issue, I don't see the brushes being the issue here, especially since the motor accelerates up to speed correctly, and maintains speed with no issues. Most of what I have read up on indicate that this event causes catastrophic failure in the motor, and shows extensive damage from the event. In this instance, we have none of the sort. There is little to no damage to the commutator, however there were little copper dust particles behind the brushes on the brush bar. I assume these were created from the flashover events. The front end of the commutator bars seemed to have minor chipping to them, but are far enough away from the brushes that I don't see this being the problem. If it was, I would expect to see flashover at regular points of operation, and not just when switching to the brake. I find it odd that the motor operates just fine until it is switched to the braking circuit.

Now, I am an entry level Electrical Engineer, just graduated and started here a few weeks ago. At the company that I work for, they have not had an electrical engineer for the past few years, and I am now the only electrical guy employed here, so I don't have any mentors to turn to or references to contact about such an issue. So please bear with my limited knowledge of these systems, we only utilized small DC motors in class, that were operated by a 24v output from a PLC, not a three phase system rectified into a DC signal for the motor. Everything that I know about this machine was developed while trying to troubleshoot the problem, so if I am unable to answer a specific question I apologize in advance.

If anyone has any insight, advice, places to look or inspect, it would be highly valued and greatly appreciated. I do not have access to the PLC programming software, as the previous electrical engineer here had passed away and his family had taken his personal computers which had the program on it. The PLC is an Allen Bradley PLC5/15. I have attached the label for the 50 H.P. DC motor and a video of the flashover event when stopping the spindle.
https://res.cloudinary.com/engineering-com/video/upload/v1685629960/tips/Flashover_woigsj.mov

RE: DC motor flashover during dynamic braking

Is it stopping almost instantly? If so I'd guess the dynamic brake resistor was shorted to low ohms or something else is creating a short across the resistor creating high current and the flashover during braking.

RE: DC motor flashover during dynamic braking

(OP)
Our foreman who was operating the controls said that it seemed to brake at the same rate as normal. It wasn't an instantaneous immediate halt, but it was not coasting. It is about a 1-2 second brake time interval

RE: DC motor flashover during dynamic braking

Do you keep the shunt field energized during dynamic braking?

Muthu
www.edison.co.in

RE: DC motor flashover during dynamic braking

I have heard that described as;
"Ringing the Comm"
In my experience it is a result of excess current.
If you have a current limiting circuit that would be a place to check.
Loose connections or failed components.
When I saw it happen it was generators.
We were working with a factory technician to set up some very large drives.
8 x 1300 HP motor generator sets working together to provide 10400 HP.
We had shorting jumpers across the motor commutators to set up the current limits.
The factory tech made a mistake and all 8 generators flashed over brush to brush around the commutators.

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

RE: DC motor flashover during dynamic braking

I think it's an issue related to control field power supply during braking.
Seems to be a short pause in field current.
Check field current during braking .
If true (short current missing/severe drop), need to check braking control sequence and signals to field power supply.

RE: DC motor flashover during dynamic braking

(OP)
edison123 I do believe the shunt winding is short circuited through the braking circuit. This page in the electrical manual may bring some clarity as to how it is switching.

RE: DC motor flashover during dynamic braking

(OP)
waross and iop95 thank you, I'll look into both suggestions next and update this thread with my findings as soon as I can

RE: DC motor flashover during dynamic braking

There are some schematics?
I think must be a permanent connected resistor across G1 dynamo excitation to avoid current missing when S4 open.
This resistor may be about 10-20 times excitation resistance.
Check schematics/resistor/connections.

RE: DC motor flashover during dynamic braking

Quote (op)

I do not have access to the PLC programming software, as the previous electrical engineer here had passed away and his family had taken his personal computers which had the program on it.
Perhaps use of the computer could be investigated. If it still holds the software and program it would be very helpful to gain access to the PLC code. This seems like something I would task to management or HR, as it will likely require some sort of compensation.
Eventually some troubleshooting will require access to the code. That is a really bad time to reverse engineer the code.
PLC code can not be extracted and reverse compiled to reliably result in human readable PLC program. Almost always all of the programming notes are lost.

RE: DC motor flashover during dynamic braking

Quote:

By the opening of contactor S4 excitation of dynamo G1 is interrupted and its shunt winding short-circuited with contactor S8b....
That, by itself may cause "Ringing the Comm".
With the shunt field shorted, the dynamo voltage will drop to zero while the back EMF from the motor will be close to 300 Volts if the motor was at full speed.
The impedance of the motor rotor and dynamo rotor will be very low and the back EMF of close to 300 Volts willdrive a very heavy current.
BUT
There are other things going on.

Quote:

and the "break" contact opens contactor S8b so the the short-circuit connection of the shunt-field circuit of dynamo G1 is interrupted
I am not sure what is happening after the shunt field is short-circuited, but I suspect that contactor S8b is not opening when it should.
I would check wiring and function of the components mentioned in connection with the shunt field circuit;
permanent alnico magnet unit A1, (line 26)
Relay R7
Relay R6
Contactor S8b

Then go on to check;
Relay R6
Contactor S6
Relay R5a
Contactor S8a
I would pay particular attention to the permanent alnico magnet unit.

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

RE: DC motor flashover during dynamic braking

Can you post the power and control schematics?

Muthu
www.edison.co.in

RE: DC motor flashover during dynamic braking

Too low-resistance of the rheostat used for braking causes high current to flow through the armature and may cause flashover. Yes, you will get more braking torque if the resistance value is low. However, it is important that the sizing of the braking resistor must be done taking into account the maximum armature current of the DC motor.

RE: DC motor flashover during dynamic braking

(OP)
Hey guys i'm back, things got really busy here and I have had no time to work on this in the past few months, but I am back at working on this.

I was completely wrong in my previous messages. I was told by my boss that the machine uses dynamic braking. This used to be true when the machine was first designed and installed. Since then, it has had a controls upgrade, and uses regenerative braking now... The sheet with description of the dynamo circuit is irrelevant. I am uploading the current schematics as well as the operating manual describing each boards purpose. The resistor that was getting red hot was resistor 206R, highlighted in yellow. It is a Chassis mounted power resistor. The fuse that would blow, stopping the heating of the resistor is Fuse 201F, highlighted in red. The SCR's that are highlighted are the ones that I had replaced. I know that this circuit uses Field weakening to increase speeds above base speed, however this does not explain why there is flashover upon deceleration speed control and stop initiation. Could the armature circuit stop firing the SCR's completely when trying to slow or stop? causing a large and sudden reversal in current from back EMF? Could it be any of the components that I had replaced? It seems like the issue was just the SCR's blowing out, however the flashover event stumps me. https://res.cloudinary.com/engineering-com/image/upload/v1698339306/tips/Spindle_Schematic_ffwmix.pdf
https://res.cloudinary.com/engineering-com/image/upload/v1698339348/tips/Instruction_Manual_for_50_H.P_Drive_wfoka1.pdf

RE: DC motor flashover during dynamic braking

It sounds as if the braking is ramping down too quickly.

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

RE: DC motor flashover during dynamic braking

(OP)
I agree with you that it sounds like this could be the issue, however there have been no parameter adjustments made to anything in the system. So then I ask myself the question, If it ramped down just fine before all of this, why would that change now? The only things that have changed is replacing faulty components. The parameters should still work the way they were set before, right?

RE: DC motor flashover during dynamic braking

I have had equipment malfunction when everything says it should work.
It turns out one resistor was at the far end of the tolerance.
That meant that the power supply was not able to push the full 20 mA through a 4-20 mA loop.
We almost lost a 100 foot stack when a liquid carryover combusted in the stack and the high alarm did not function.

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

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login



News


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close