# External Shunt Resistor



## MDShunk (Jan 7, 2007)

jimlectro said:


> 08-08-2010
> My background is in construction electricity, But with the construction market being what it is these days, I have taken work in a factory as a Maintenance mechanic/ Electrician. Today I had to troubleshoot a line that ran intermittently because an external shunt resistor was tripping the temperature sensor associated with it. when I jumped out the sensor the motor ran, but the shunt resistor heated up to the point of discoloring the paint on the machine. So I shut it down and locked out the machine. My question is firstly what is likely to have caused this failure? and secondly what purpose does this shunt resistor serve?


A shunt on a temp sensor is a little odd. What was the process?


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## TheRick (Apr 13, 2008)

MDShunk said:


> A shunt on a temp sensor is a little odd. What was the process?


I think he has a temperature sensor on a shunt, which makes more sense.

Is it a current sensing shunt? If so is the resistance rating of the shunt correct for the application, or maybe the current is just high?


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## jimlectro (Aug 9, 2010)

The shunt is not on the sensor it's on a VFD the process is a rotating cutting blade on a Rosenthal sheeter stacker


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## TheRick (Apr 13, 2008)

So the temperature sensor is there to tell you the shunt is seeing too much current......thus getting HOT.

Was the shunt recently replaced........possibly with an incorrect resistance value?

Is the circuit seeing a higher than normal current flow?


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## Big John (May 23, 2010)

jimlectro said:


> The shunt is not on the sensor it's on a VFD the process is a rotating cutting blade on a Rosenthal sheeter stacker


 This is a dynamic-braking resistor on the VFD, I take it? So, the motor this VFD is controlling is running, and while it's running, the dynamic brake is also engaged? I'll bet it's overheating! There's something wrong with the brake control circuit.

-John


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## jimlectro (Aug 9, 2010)

This is what I don't understand. is the brake built into the motor? and how does this resistor work? the resistor is fairly large and I checked the tempprature with an infrared temp gauge when I saw it was at 500+ degrees I shut it down in a hurry I've installed temperature controls in new buildings but process control is a bit of a leap. is it likely the motor or the resistor that is failing perhaps I should replace both.


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## Big John (May 23, 2010)

jimlectro said:


> This is what I don't understand. is the brake built into the motor? and how does this resistor work? the resistor is fairly large and I checked the tempprature with an infrared temp gauge when I saw it was at 500+ degrees I shut it down in a hurry I've installed temperature controls in new buildings but process control is a bit of a leap. is it likely the motor or the resistor that is failing perhaps I should replace both.


 When you stop a motor driven by a VFD, the motor can act like a generator and put voltage back on the DC bus of the drive. If there's nothing to dissipate the voltage it can increase to the point where it can damage the drive.

A dynamic braking resistor gets engaged when the motor is stopping, it takes the power generated by the stopping motor and applies it across the resistor which dissipates it as heat. 

This resistor should not be receiving any current when your motor is running. If it is engaged, it means it's trying to dissipate the voltage that the VFD is producing to turn the motor. You can bet the resistor is gonna overheat in a hurry.

Unfortunately, I don't know why it would be engaged, so someone else will have to chime in there.

-John


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## jimlectro (Aug 9, 2010)

Thanks John.


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## micromind (Aug 11, 2007)

Here are a few things to look for;

1) Has any parameter in the VFD been changed recently?

2) Has the incoming voltage to the service been changed, or any transformer taps changed? (If the incoming voltage is too high, it can trigger the braking resistor)

3) To test, measure current (it'll be DC) or voltage across the resistor when the motor is at a standstill, as well as running at speed. It should be zero. If not, and nothing has been changed with either VFD parameters or incoming voltage, you've got a shorted SCR inside the VFD, or the contactor that controls the resistor has a welded contact (if there is one).

The only time the resistor has any current on it is when the motor is stopping, and stopping quicker than coasting. 

Rob


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## jimlectro (Aug 9, 2010)

micromind said:


> Here are a few things to look for;
> 
> 1) Has any parameter in the VFD been changed recently?
> 
> ...


The problem turned out to be the cutting blade.It became misaligned and would bind just slightly in it's rotation cycle. As product runs through the machine it's cut at precise intervals. this slight binding of the blade caused the motor amperage to go up to get passed the bind then the VFD applied the brake I think to maintain the cutting interval, that's 2 blades 180 degrees apart on the same axle each binding once every second (two amperage spikes and two brake applications every second) No wonder we had heat issues on the motor ,the drive and the shunt resistor. Thank you all for your input you definitely helped me solve the problem. and I learned something too.


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## Big John (May 23, 2010)

jimlectro said:


> The problem turned out to be the cutting blade.It became misaligned and would bind just slightly in it's rotation cycle. As product runs through the machine it's cut at precise intervals. this slight binding of the blade caused the motor amperage to go up to get passed the bind then the VFD applied the brake I think to maintain the cutting interval, that's 2 blades 180 degrees apart on the same axle each binding once every second (two amperage spikes and two brake applications every second) No wonder we had heat issues on the motor ,the drive and the shunt resistor. Thank you all for your input you definitely helped me solve the problem. and I learned something too.


 Glad you found it. Thanks for posting the solution, it's always neat to hear how these things turn out.

-John


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## John Valdes (May 17, 2007)

jimlectro said:


> The problem turned out to be the cutting blade.It became misaligned and would bind just slightly in it's rotation cycle. As product runs through the machine it's cut at precise intervals. this slight binding of the blade caused the motor amperage to go up to get passed the bind then the VFD applied the brake I think to maintain the cutting interval, that's 2 blades 180 degrees apart on the same axle each binding once every second (two amperage spikes and two brake applications every second) No wonder we had heat issues on the motor ,the drive and the shunt resistor. Thank you all for your input you definitely helped me solve the problem. and I learned something too.


There is no way the blade caused the resistor bank controlled by the VFD to turn on. The resistor bank and transistor that cycles the resistor bank is only energized when the VFD is is stop mode. (closing the stop input). This is called electronic braking.
The only thing the blade could do was increase current. This would not increase bus voltage. Without increased bus voltage, the transistor cannot or should not send any excess voltage to the resistor bank. 

The bus voltage is fairly constant (directly related to power input voltage) until an input is present to stop the motor. As the motor is stopping in deceleration mode the motor pumps the bus voltage up. (like a generator) When the bus voltage reaches the over voltage threshold, a switch (transistor) closes, shedding the excess current to the resistor bank to be dissipated as heat. This transistor switches on and off to keep the heat from destroying the resistor bank. 

Since your problem is no longer evident, I would go over exactly what happened and find out why your resistor bank was getting power when it should not have. The blade and subsequent overload cannot induce firing of the braking transistor. Call your drive distributor and see if he can assist you. Something is not right here.

Note: It is very possible you have a burned out braking transistor or resistor bank. Either of these will not effect operation with the exception of electronic braking. Put simply. You may have one or both devices bad and not know it.


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## Big John (May 23, 2010)

John Valdes said:


> There is no way the blade caused the resistor bank controlled by the VFD to turn on....


 Maybe I interpreted it wrong, but the way I read it was that the brake was actually being applied properly during the stopping cycle of the blade-motor. 

I [assumed that the binding blade was causing the VFD to apply more power to the blade motor to maintain motor speed. If there are very short cycles between running and braking applications, I could see a slight overspeed on each cycle because of this, and the brake would have to deal with added voltage of each overspeed, causing overheating.

This is a guess, but what say you?

-John


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## John Valdes (May 17, 2007)

Big John said:


> Maybe I interpreted it wrong, but the way I read it was that the brake was actually being applied properly during the stopping cycle of the blade-motor.
> 
> I [assumed that the binding blade was causing the VFD to apply more power to the blade motor to maintain motor speed. If there are very short cycles between running and braking applications, I could see a slight overspeed on each cycle because of this, and the brake would have to deal with added voltage of each overspeed, causing overheating.
> 
> ...


John you are correct. That's what I get for reading through the post fast enough to miss the critical statement the OP made regarding the brake cycle.
My concern would be in an application such as this requiring braking, running then braking again is not well suited for electronic braking. A direct shaft brake (brake motor) or another sure quick stop brake device would be better suited. He does say every second. That's asking alot of an electronic brake. More than I would ask of the VFD.

OP. Does this motor stop, make the cut and restart for the next cut? Asking, does the motor have to start and stop for each cut in one second? If this statement is *not* true then I stand by my first post.


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## jimlectro (Aug 9, 2010)

John Valdes said:


> John you are correct. That's what I get for reading through the post fast enough to miss the critical statement the OP made regarding the brake cycle.
> My concern would be in an application such as this requiring braking, running then braking again is not well suited for electronic braking. A direct shaft brake (brake motor) or another sure quick stop brake device would be better suited. He does say every second. That's asking alot of an electronic brake. More than I would ask of the VFD.
> 
> OP. Does this motor stop, make the cut and restart for the next cut? Asking, does the motor have to start and stop for each cut in one second? If this statement is *not* true then I stand by my first post.


I timed the cycle and it is actually approximately 2 seconds for a complete rotation of the blade. We sent them out to have them sharpened. as our problem did not completely go away from readjustment alone but then that's what the resister is for isn't it? In the end I think we became too acutely aware that the resistor was still warm. I now realize that it will never be perfect, so the resistor will go on protecting the drive. (which the company wants replaced just in case) The blade does not come to a complete stop but I think the smart drive sensed the over speed as a result of getting past the binding point and applied the brake to maintain the timing. When the new drive comes in we will send the old drive out to be rebuilt. I'll post again if I find out there was anything wrong with it. I am always open minded just because the problem hasn't manifested itself in a while doesn't necessarily mean it's gone Again I Thank you all 

Sincerely Jimlectro


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## John Valdes (May 17, 2007)

jimlectro said:


> I timed the cycle and it is actually approximately 2 seconds for a complete rotation of the blade. We sent them out to have them sharpened. as our problem did not completely go away from readjustment alone but then that's what the resister is for isn't it? In the end I think we became too acutely aware that the resistor was still warm. I now realize that it will never be perfect, so the resistor will go on protecting the drive. (which the company wants replaced just in case) The blade does not come to a complete stop but I think the smart drive sensed the over speed as a result of getting past the binding point and applied the brake to maintain the timing. When the new drive comes in we will send the old drive out to be rebuilt. I'll post again if I find out there was anything wrong with it. I am always open minded just because the problem hasn't manifested itself in a while doesn't necessarily mean it's gone Again I Thank you all
> 
> Sincerely Jimlectro


It is very difficult to diagnose or troubleshoot these types of issues online. I would be picking this machines manufacturers reps about how this is supposed to work. After all, they are the people. Also, the drive manufacturer could have been involved with the application. What brand is the drive and whats the model number? Also, the same information on the braking resistor bank would be somewhat helpful.
I cannot even picture the application. Will they let you take a few pictures? That would be real cool to see and could very well be helpful. :thumbsup:


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## Big John (May 23, 2010)

John Valdes said:


> ...Will they let you take a few pictures? That would be real cool to see and could very well be helpful. :thumbsup:


 Agreed. All these words are hard on me. I've always liked picture books more. :thumbup:

-John


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## jimlectro (Aug 9, 2010)

I'll see about the pics and get the drive info if there's time tomorrow. Sorry though they will will most likely be low def phone pics. Below is a link to Rosenthal's website the sheeter stacker you see is similar to the ones we have at work. Some have pneumatic cutters both lengthwise and width wise, but the one I'm describing has 2 rotating blades mounted 180 degrees apart on a rectangular frame mounted to a shaft the blades are 60" long they cut with scissor action against a fixed anvil.(if that's the correct description) So in order to maintain uniform width and length blade rotation speed is very important as well as the material feed speed. the cutting process happens just ahead of what you see in the video. I hope this helps you understand it better. (I really love a good challenge).
http://www.rosenthalmfg.com/video/vacuum_stacker.html


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