Very strange unisaw electrical problem and solution

[Rick Christopherson]

And Matt is absolutely correct that the run capacitor is often (usually) sufficient to start a capacitor start induction motor, if there's no load on it. The starting torque will be less than if both capacitors are used, but it will be enough to get an unloaded motor spinning. And once it starts spinning, it will go to full speed.

So are you suggesting that we all throw away those useless start capacitors from our motors? Are you being argumentative for the sake of being argumentative, or do you actually believe this?

Do you have any idea how many woodworking motors have been destroyed when the operators let them slowly ramp up to speed with excessive start times due to failed start capacitors? Your postings last week were irresponsible, but this takes the cake.

Your EE degree carries some innate respect, but over the last couple of weeks your postings have been far enough off the wall that you have lost a significant amount of my respect.

And since you want to side with the original poster's stance, then I put it to your engineering knowledge to explain how his capacitor allegedly moved back and forth to create the alleged short circuit that cause his slow start times, but then moved away after the power was removed.

Pick your battles, but this is not the smartest one I have seen you go after.

I just went back and re-read his original posting.

• 15 seconds later, it turned off.....
• I know the saw is 3hp,......
• Certainly not enough to blow a breaker rated at 30A!

He sent 30 amps through the fine windings of a 3 hp motor for 15 second, and the two of you think this is fine? He heated up the power cord leading to the saw, and that cord was WAY, WAY thicker than the winding wire in the motor, but you two think this is just fine?

[Mike Henderson]

So are you suggesting that we all throw away those useless start capacitors from our motors? Are you being argumentative for the sake of being argumentative, or do you actually believe this?

Do you have any idea how many woodworking motors have been destroyed when the operators let them slowly ramp up to speed with excessive start times due to failed start capacitors? Your postings last week were irresponsible, but this takes the cake.

Your EE degree carries some innate respect, but over the last couple of weeks your postings have been far enough off the wall that you have lost a significant amount of my respect.

And since you want to side with the original poster's stance, then I put it to your engineering knowledge to explain how his capacitor allegedly moved back and forth to create the alleged short circuit that cause his slow start times, but then moved away after the power was removed.

Pick your battles, but this is not the smartest one I have seen you go after.

Okay, let me discuss this issue in more depth.

Let's say you have a normal functioning single phase, capacitor start, induction motor and the starting capacitor is 30 microfarad (value is just for this example). Everything works fine.

Now, let's say you want to know how small of a starting cap you can use and still start the motor WITH NO EXTERNAL LOAD. You have a 29 microfarad capacitor and you install it and power the motor on. Will it start?

You have a whole series of capacitors that are 1 microfarad apart, so you next try a 28 microfarad capacitor. Will the motor start?

When will the motor not start? It will not start when the "load torque" is greater than the starting torque. And since I specified that the motor has no external load, the only torque the starting circuit has to overcome is the friction and mass inertia of the motor. So an unloaded motor will start up with a fairly small capacitor in the start circuit. And the run capacitor is usually enough to provide that starting torque.

I can't tell you how far down you can go with the size of the capacitor in the starting circuit, but it's a long ways before the motor will not start up in an unloaded condition.

So to answer you implied question: "Why do motors have starting capacitors? Why not just use the running capacitor to start the motor?" The answer is that the use of the starting capacitor provides a greater starting torque than if you just used the run capacitor. And many motors are used in applications where there's a load on the system before the motor starts.

And, Rick, I don't care if you respect me at all, either personally or for my technical background. Degrees and experience have little meaning. What does have meaning is whether someone can explain the operation in a logical fashion that makes sense to other people, both lay people and those technically trained. There are plenty of EEs who participate in this forum and I'm confident they'll jump all over me if I mis-state the facts.

Mike

[Also, when someone posts their observations, we have to take what they say as accurate since they were there and we weren't. If they report things that are improbable, the most we can do is ask them if they're sure of what they observed. If they're sure and we cannot offer an explanation, all we can do is state that we have no answer for their situation.]

[Rick Christopherson]

What does have meaning is whether someone can explain the operation in a logical fashion that makes sense to other people, both lay people and those technically trained. There are plenty of EEs who participate in this forum and I'm confident they'll jump all over me if I mis-state the facts.

So what you are saying is that as long as you can get enough people to believe what you say, then it becomes correct? That mentality is why Snopes.com exists!!

You are absolutely right that there are other engineers here that will call you out on the carpet when you mis-state information...and I just did. I don't go toe-to-toe with another engineer lightly, so when I do, you should make sure you have your ducks in a row, because right now, they are not. I make my share of mistakes, but if I haven't backed down by the 3rd reply, you can bet good money that I know the topic well enough to take it to a successful end. This is not the battle you should have chosen to champion, and it is not one you can win.

[Mike Henderson]

So what you are saying is that as long as you can get enough people to believe what you say, then it becomes correct? That mentality is why Snopes.com exists!!

You are absolutely right that there are other engineers here that will call you out on the carpet when you mis-state information...and I just did. I don't go toe-to-toe with another engineer lightly, so when I do, you should make sure you have your ducks in a row, because right now, they are not. I make my share of mistakes, but if I haven't backed down by the 3rd reply, you can bet good money that I know the topic well enough to take it to a successful end. This is not the battle you should have chosen to champion, and it is not one you can win.

All you have to do is point out to me in technical terms (that stand up to scrutiny) why I'm wrong. This was standard practice while I was practicing engineering and I participated in *many* sessions where we analyzed problems or developed new ideas. I await your explanation of why my posting about the size of the capacitors is wrong.

Also, I'm not in any "battle" and I don't think of the outcome of this discussion of winning or losing. I look at it as a search for the correct answer. So if you look at it as a fight, it's a fight only on your side. My discussion of technical issues is not a personal "attack" on you.

Mike

[Matt Armstrong]

If you really did hear the centrifugal switch click when the motor was spinning down, I'd have to go with your analysis.

The start cap and the run cap (when starting) are in parallel. If the start cap is bad, the run cap may have enough capacitance to provide the phase shifting needed to start the motor, but the startup would be slower than with both caps. Also, if the start cap was shorting and drawing too much current, the voltage could have been low, contributing to the slow starting.

The start cap could have been providing a relatively high resistance to ground, something that would exist even after the centrifugal switch had opened (after the motor got up to a decent speed). The way to test for that is to measure the current in each leg of the 240V circuit when you turned the motor on. They should be balanced but if your hypothesis is correct, one of them would have a lot more current than the other, because the cap was shorting to ground. If you had a GFCI (and your hypothesis was correct) the GFCI would have tripped because of the unbalanced currents.

The problem with this analysis is what Rick pointed out. When you have a short, it's generally a low resistance short and you get sparks.

Since you didn't test the current in the two legs, I think the analysis hinges on the question: Are you sure you heard the centrifugal switch click in as the motor was slowing down? If so, you're probably right. If not, Rick's analysis may be correct.

Mike

100% sure I heard the click. I started and stopped the thing (in the course of diagnostics) over 20 times. A stuck centrifugal switch seemed like an obvious answer, so it was my first guess.

[Matt Armstrong]

The real lesson you should take away from this experience is how much damage you may have caused to your motor by letting it run underspeed long enough for a circuit breaker to trip. I don't know if you still had this on a 30 amp circuit, or for how long the motor operated with excessive current, but when you heat up the motor windings like this, you damage the fragile insulation on them, and that shortens the life of the motor. This is a cumulative effect, where each overheating cycle degrades the motor a little bit more. Once you get a short in one portion of the winding, you have effectively reduced the size of the winding, and then you end up with a cascading failure.

The motor windings weren't shorting out... they didn't have any additional current surging through them. Again, the motor spun up, it just took slightly longer than usual. I don't know what part of my story you missed but you're making faulty assumptions.

Not only is this not true, but your own experience even contradicts this. If the sole purpose of the capacitor was to dictate direction, then once the motor started to rotate, then it should have shot up to full speed, right?

Right, and it did. Please re-read the original post.

Wrong. If that were true, then there would not be a need for both run and start capacitors--everything could be accomplished with just a run capacitor, at least according to your rationale.

This is true. Induction motors do not NEED a start capacitor to spin in the right direction. You can google all the various designs of induction motors that can start and spin in the correct direction using a single phase design without a start capacitor and centrifugal switch.

Before we critique your analytical skills, let us first step back to your previous analytical assertion and bring that back to light.This was the most obvious telltale sign that you were stepping beyond your knowledge-base during your analysis.

See previous comments

Just what, praytell, was this electromechanical mechanism that propelled the capacitor forward when the current was present, and sent it back when the current was released? Well gee, a capacitor is round and a solenoid is round, so maybe they do the same thing, right? Afterall, they are both round.

As Thomas Magnum would say, "I know what you're thinking..." and if you are about to say that the current through a capacitor plate will set up a magnetic field, you would be right....but then you would have forgotten that the other plate right next to it has the opposite polarity and its magnetic field would cancel the first. Oops, shot that idea down.

So with that out of the way, let's step back to your Ohmic analysis where you were sure that you had a short to ground even though the circuit breaker didn't trip. So when you measured the continuity to ground, did you use a simple ohm meter, or did you use a dielectric withstand test? The low voltage of an Ohm meter will read even 0.001" of paint as an open circuit, but when you put 120 volts across this paint, you create a low-resistance plasma field across it, and your open circuit suddenly becomes a short circuit.

As I already told you, the marks and discoloration you observed was due to arc flash. If a short circuit was significant enough to leave discoloration, it would have tripped the breaker instantly. If you don't understand the concept of plasma, then either watch an old Frankenstein movie, or look up Jacob's ladder on the internet.

lol... ok, now I know you're just out for an argument or to get a rise out of me. won't work.

[Matt Armstrong]

I really thought the internet warriors were less common on this forum than on others. In the end, ultimately I was just trying to make a post on

a) a list of things to check (there were a lot of possible problem areas, after all... and many of them are likely to present problems to some people at one time or another)

b) an anecdote about how all my available testing methodology suggested a component (the start cap) was fine, but only the process of elimination was able to reveal it as the problem area. Maybe the cap wasn't perfectly shorted to ground... maybe thermal expansion was causing it. Maybe the way the saw was jostled around caused the capacitor lead to contact the metal casing. I was more excited to resolve the problem than to troubleshoot a known issue....


I don't expect many people to run into this exact same issue. But someone probably will run into something similar. , . And when they do, rather than shell out good hard-earned money for a new motor, I'd hope they read my story about how I set out to fix it, the things I tried, and maybe they'll be encouraged that they can fix it, too. Motors are not cheap and I just figured the internet helps us fellow woodworkers save money and resolve problems, among other things, right?

Rick - I'm not sure if you're out to prove something but let's just move on. Thanks to Mike and James for being the voice of reason here

[Matt Armstrong]

I also checked out Rick's profile here on SMC. It says

Friends:
"Rick Christopherson has not made any friends yet"


I wonder how it knows that!?

[Rick Christopherson]

The motor windings weren't shorting out... they didn't have any additional current surging through them. Again, the motor spun up, it just took slightly longer than usual. I don't know what part of my story you missed but you're making faulty assumptions.

I never suggested the windings were shorted out, but if you knew anything about induction motors, you would know that the current in the windings was inversely related to the rpm of the motor, so the longer the motor stayed below its rated rpm, the higher/longer the excessive inrush current would be.

You just don't get it, do you? You let this motor run with over 30 amps of current flowing through probably 18 gauge (or smaller) windings for up to 15 seconds! The heat you felt in the power cord was nothing compared to the heat that these windings were experiencing. If you were feeling heat in the 12 gauge power cord, just what do you think was happening in the smaller motor windings?

This motor took so long to get up to speed that you tripped a 30 amp breaker for a motor that was rated for approximately 12 amps maximum!! You let this run at twice its rated amperage for 15 seconds. What part of this potential damage is not sinking in?

This is true. Induction motors do not NEED a start capacitor to spin in the right direction. You can google all the various designs of induction motors that can start and spin in the correct direction using a single phase design without a start capacitor and centrifugal switch.

There are motors that don't need start capacitors......BUT YOURS IS NOT ONE OF THEM!

Both you and Mike are relying on what you read from the Internet, but not realizing that what you are reading is describing motors that are designed to operate as either having a start cap or not. It isn't something you can choose after the fact--it is how the motor was designed. If your motor was designed to need a start cap, then you need a start cap!

lol... ok, now I know you're just out for an argument or to get a rise out of me. won't work.

NO! You brought this topic up as your explanation. Why are you sidestepping it now? Either stand behind your assertions or retract them.

You indicated that your slow start times were related to the capacitor "mechanically" moving into a short circuit/open circuit condition with ground. I challenged both you and Mike to explain this because I know that you cannot. Like the rest of your information, you made this up...and I am calling you on it! Either stand behind it, or retract it!!!

If you retract this, then the rest of your story also falls to the BS pile, and that is why neither you nor Mike will address this.

[Rick Christopherson]

I also checked out Rick's profile here on SMC.

Matt, that is a personal attack, but I do not want that to be a reason for this thread to get locked. The same thing comes up for your own profile, so that doesn't hold much water.

Yes, this has been a heated discussion, but I won't go down that road, nor will I hold it against you. I would prefer that this discussion remains open.

Since you are new to this forum, if you want to know more about me, I suggest you do a Google search for my name. I am not a race car driver, a priest, and I have never made any statements about BB guns; but the rest of the thousands of hits you find are about me. I am not unknown.

[Mike Henderson]

I suspect that no amount of facts or logic will work with Rick. But if anyone else has any questions I'll be happy to go into detail about why Matt's analysis makes more sense than Rick's.

Mike

[Rick Christopherson]

... I'll be happy to go into detail about why Matt's analysis makes more sense than Rick's.

If you are so willing to explain things, then why have you still not addressed my questions.

1. What electromechanical mechanism caused his capacitor to allegedly move back and forth causing the short circuit that he claims tripped the 30 amp breaker, yet was not touching when he examined it with the power off?
2. If the magical-moving capacitor didn't really move, then why did a 30 amp thermal magnetic breaker trip on a motor designed to carry a maximum load current of 12 to 15 amps?
3. Follow-up to #2 (or in place of #2): If a start capacitor is "optional" on a capacitor-start motor (regardless whether it is capacitor-run or induction-run) then why is the motor start-up time so elongated that it trips a thermal magnetic circuit breaker which is significantly larger than the required circuit breaker size for the motor (by both the manufacturer's instructions and by the overly conservative NEC)?

So far, all you have said is that everything is rosy-red and copacetic, and that a start capacitor is not required, but you have completely failed to substantiate anything you have said beyond these words. It is in your postings that the "fact and logic" are lacking. If you doubt that, then why don't you pose this scenario over at Mike Holt's electrical forum where people with real electrical knowledge are present? I think we both know that your question would last all of 5 minutes before getting slapped down a lot harder than I have done so far.

[Mike Henderson]

Rick - If you think you've "slapped me down", you're probably the only person who thinks so. I'm quite sure that no matter how well I explain things, you'll stick to your own interpretation (erroneous interpretation, I might add). I doubt if others are interested in this discussion, but if anyone else does express an interest, I'll be happy to go into really excessive detail about why Matt's theory makes more sense than yours.

Mike

[Bill Keehn]

Matt,
While I am reluctant to put my two cents into what has become an unusually unplesant exchange, I experienced a very similar motor failure two years ago on my bandsaw.

I would turn on the power and the motor would start up slowly, get to what seemed like a good running speed, and then trip the 20A breaker. I didn't feel the power cord but I can tell you the motor was definitely much hotter than normal. Visual inspection of the capacitors revealed no problems. Trying to run the motor with the belt off had the same results.

Ten minutes on the phone with WMH and the tech didn't need to hear anymore. Startup capacitor, end of story. I think in that respect, Rick was probably right. Regardless of any information you acquired in your troubleshooting, the problem description fits that of a failed startup capacitor, which as you noted was in fact the problem.

Capacitors have several common modes of failure, and a "blown capacitor" is just one. Normally this happens when voltage rating is exceeded and the electrolyte begins to decompose. The resulting gas can cause the package to vent or even explode. Your capacitor more than likely vented. Depending on the capacitor type and the resulting physical damage, you may indeed see a damaged capacitor that seems to have capacitance, but only at low voltages. Rick was wrong if I understood him to mean that failed capacitors never have any capacitance at all.

I suppose it's plausible that the gasses being evolved from overheating the remains of the capacitor could cause "piston-like" movement of the package, but as an explanation of why it would work at first and then suddenly fail, it seems implausible. Did you actually see it move? More importantly, I don't think its needed to explain the symptoms. I think Rick's explanation of arc flash is more plausible in sufficiently explaing the marks.

Rick is also correct about the cumulative damage of dumping 30A into motor and overheating it. Heat kills motors. It's hard to say just how fast that damage accumulates, but I wouldn't want to find out. Unfortunately, if you lack proper test equipment, there is little you can do to diagnose the problem without turning the motor on and letting the circuit trip. You might think about moving it to a 20A breaker next time.

As Mike properly notes, we weren't there, and only have your observations to go on. What you have described sounds improbable, but I can't say it's impossible. However, if you want it to be believed in the presence of simpler explanations, you need a little more evidence. If you can prove it, then your original post will have the usefulness you intended.

Rick, it's unfortunate that this thread had to digress into a p*ss*ng match. The tone of your posts were very confrontational and a little insulting. A large number of us are engineers and have strong egos that thrive on argument and confrontation. For many of us, woodworking is a break from that. Polish wood, not your ego.

[Phil Thien]

I don't know if you still had this on a 30 amp circuit, or for how long the motor operated with excessive current, but when you heat up the motor windings like this, you damage the fragile insulation on them, and that shortens the life of the motor. This is a cumulative effect, where each overheating cycle degrades the motor a little bit more. Once you get a short in one portion of the winding, you have effectively reduced the size of the winding, and then you end up with a cascading failure.

Don't these motors have built-in overload protection? I'm surprised the windings aren't protected by something on the motor itself?