Capacitor Explodes on Bandsaw

      A long story about failure and repair of a capacitor (note: capacitors can kill you with live electricity even when the machine is turned off or unplugged). March 28, 2012

Anyone ever experience a small explosion from an electric motor? I went to turn on my 5hp bandsaw and there was a wicked flash and a boom like a 12 gauge. The bandsaw was still running. I was on the feed side, thankfully. So I did not actually see flames blowing out the end of the motor. And from what I can tell the end is sealed. I am even unsure if it could have been electrical, whether power cord arcing or dust collection releasing static electricity.

The 4" flex hose, SJO power cord, and a trailer receiver all lay amongst each other on the concrete floor. This was a mean blast. I'm sure everyone in the neighborhood heard it. Like I said, the bandsaw never stopped running. I powered down the whole shop at the panel, not wanting to even touch the saw at that moment.

I looked all over for signs of electrical grounding out. The power cord was all in fine repair. I opened the capacitor block and there seemed no explosion from there, though it should have blown the box all to *%#@.

Power to the bandsaw was good, so I powered up and everything sounded fine. Started good. Made a cut but felt uneasy, so I pulled the plug.

Could there have been an arc between the ungrounded DC hose and the bandsaw insulated power cord? Static electricity igniting dust that settled on the hose? Wish I had a better view of what happened when it did. Any thoughts where to start?

Forum Responses
(Solid Wood Machining Forum)
From contributor J:
You say this has capacitors? Leading me to think it is single phase. Can you confirm this? If so, there are usually two capacitors (not always). When you removed the covers, did it have a bad odor? I can't describe it. It's just real pungent.

From the original questioner:
Single phase, correct. A smell for sure. Not electrical burning, but chemically. Could not really smell anything outside the capacitor housing, but when I opened the box it was right there. I found on another site that there could be a discharge of...? Anyone know if static electricity can arc off a hose? The loud bang was definitely electrical.

From contributor J:
Okay. There should be no smell whatsoever.

In a 1~ motor there is a switch that is attached to the arbor that opens/closes at a given RPM. This is the click you hear on some motors when you start and stop them - the switch is opening and closing. This switch engages a capacitor. Which capacitor is determined on the type of motor. Generally there are 3 types of capacitors. Start, Run, Start/Run.

Let's assume you have a simple two capacity motor. When the motor is at rest, the start capacitor will be activated when power is applied to the motor. This capacitor sets up a false lag in the electrical phase and causes the motor to rotate. Without this lag, the motor does not know to rotate clockwise or counter-clockwise. Once the motor reaches a given rpm the click you hear is a centrifugal switch changing states.

Sometimes the run capacitor is on full time; sometimes it is called for via the centrifugal switch. The run capacitor adds an extra boost of power to the motor when placed under load. It is possible to damage the run capacitor and not the start capacitor. You will notice that the motor will perform like it is weak when placed under heavy load. This is a very brief description of what happens in a 1~ motor.

To your other question, Yes. Static can jump from a dust collection hose to ground. If it passed into your power cord I would be very surprised. If this happened, most likely it would be to something metal that is grounded. Regardless, you should have all your dust collection grounded, whether it is metal or plastic. Wood dust will explode, not just burn. Have you ever noticed dust particles clinging to the dust hose?

From the original questioner:
Thank you for the motor power description. This makes good sense. So from what you have said it could be the run capacitor. But from the sound of the blast and the amount of visual light from the flash, I would think the plastic box the capacitors reside in would have been blown apart.

On the grounded DC, yeah I know…

Has anyone seen a flash of electricity grounding itself out from a hose to a metal part on the floor? Could the trailer hitch on the floor be considered a ground?

Like I said, there were no telltale signs of what occurred. No black spots, not soot, cindered parts, etc. The bandsaw was a Minimax MM16, about 6 years old with moderate heavy use. Don't know if the European motors are different in design than American.

From contributor J:
I doubt that trailer hitch would have anything to do with it unless it was buried or directly touching something that was grounded. I don't think that merely sitting on the floor would matter. I would agree with you that you should see some signs of damage to the capacitor housing if it failed, but electricity is funny that way. I have seen edgebander glue stations fail in a ball of fire and show absolutely no signs of anything.

So, the DC is certainly a strong possibility, but I would still lean towards something in the motor. But I am a technician - I look at stuff with that prejudice.

Your understanding of the American/European motors is correct. For what we are discussing here, there is little difference. Principles and basic concepts are the same or very close.

From contributor M:
One time I was using an edgebander with a 3 phase converter and a capacitor blew up. It was loud and lit up the inside of the converter box. It smelled badly as well. I expected to see some carnage when we took the converter apart, but there was no sign of damage at all.

From the original questioner:
Good responses. Contributor M, thanks for lending your story. Sounds like what happened to me and reaffirms contributor J's thoughts.

So Minimax is down for the weekend. The machine still turns on. The local winding shop said they could test/replace capacitors. Thoughts on how to proceed?

I would like to think the capacitors could be removed, then taken for a bench test at the local electrical shop. I have heard they can store juice and I should be wary about handling them.

I'd hate to have to remove the whole motor. Why would a capacitor fail? Should I be looking for a cause other than part failure? Any thoughts to cost?

From contributor J:
Caution: they can knock the crap out of you!

After opening the cover, place a multimeter across the capacitor terminals. Is there any voltage? If there is, you should see it dissipating the longer you keep the meter attached. It should dissipate quickly.

Finally, short the two terminals with a screwdriver. This will neutralize the capacitor. Then remove them. It should not be necessary to remove the motor unless it restricts the access. If both test as functional, it could be the centrifugal switch. It is usually found underneath the fan, but sometimes it is inside of the motor housing.

From the original questioner:
Going to take a picture. Unsure of what you meant by putting the probe across both contacts. At once? Do I go to ground with the other probe like when testing for current?

The motor seems fully enclosed with a fan attached to the end of the motor shaft protruding through the end cap. The fan then has the metal housing to protect. The cap box is mounted on the side of the motor and easy access.

I'd love to hear what MM has to say... I had ordered parts for an F1 and it took all of seven months to get it online again. Don't think they will cover it anyway.

From contributor M:
If you are unsure about what you are doing, capacitors can kill you! They hold electricity even after you unplug your machine.

From contributor J:
That is absolutely correct. Capacitors carry the same power as the machine runs on. And they can kill you.

From the original questioner:
Here are some pictures inside the cap housing. I could only see the labeling on one, but it shows the block and the feed wires. Definitely smelly in there. Smells like a bad battery.

I have a bit of electrical experience. Mostly wiring residential circuits, can do panels, and have wired two shops and a couple three phase converters. But I had great support. I am no engineer. So, is it possible to figure out from these pics how to drain the juice from the caps so I could get them to the rewind shop on Monday? Or I could probably find electrical parts online too. What are the causes of cap failure?

Click here for higher quality, full size image

Click here for higher quality, full size image

Click here for higher quality, full size image

Curious… The one cap that shows the windings - does it look like it blew its guts out? Wasn't sure why it looked that way when I first saw it. Definitely the odd ball out.

From the original questioner:
I have found a lot of info online about discharging caps. Most say to touch a screw driver across the terminals of the cap. With all necessary safety precautions observed first. Also with a lamp. I kind of like the sounds of discharging with a lamp, then double checking with a screw driver. Since they are all connected to the terminal block, can you just put the screwdriver across terminal there? I really do not see any reason why I shouldn't be able to remove them from their chassis and discharge them on a bench. There are no worries about just touching the shell and handling them, are there?

From contributor C:
The casing shouldn't be dangerous... The metal bits are the ones with teeth. And yes, discharging across the terminal block should be fine, providing there are no electronics on the machine that could be fried.

Additionally, there is no need to remove both. Just replace the one that has obviously exploded.

From the original questioner:
Thanks. I guess I will not short out across the terminals I just replaced the starter switch a few months back... Hate to buy another. The controls are kind of delicate. So , assuming I'm dense, and that I have never seen a cap, I'm guessing the one with the exposed windings is the faulty one.

I am unsure if I need an exact replacement - one from Italy sold as the Inco Syntex brand - or brand X with matching specs, i.e. 250v 160uf 50/60 hz. I guess I will discharge via 120v lamp.

From the original questioner:
Here are pictures of the blown cap. The caps read zero with the multimeter. I still tried to drain the juice with a lamp, then finally tried the screwdriver. I laid on a rubber mat with rubber flip flops on while wearing six condoms. Still alive to tell the tale.

Interesting as the contacts are labeled prev. and prst. Previous and present? This is to get the motor going in the right direction? This must be a start cap right? I labeled them of course. Hopefully the replacement will have obvious marks for reconnecting.

Thanks for all that have helped thus far. Still unsure of a good avenue for replacement. Cost, etc. I know local winding shops will be list price or more. Anyone think it matters if I get an Italian make to replace?

Click here for higher quality, full size image

Click here for higher quality, full size image

Click here for higher quality, full size image

From contributor C:
Yuppers... It is the one with the exposed cap. Any capacitor with those specs would work. The only question would be, will it fit in the same spot, or will it need to be fitted Frankenstein-style in another location?

From the original questioner:
Thanks. So to be sure of what I'm fishing for on the www, this would be a 60uf, but is it 450v or 400v? There's the RP-2, is this a model #?

From contributor C:
The "uf" is the capitance of the capacitor… How much electricity it can store, measured in milli-farads. The voltage specs need to be over your incoming voltage to the machine, i.e. a capacitor rated at 110v will not be sufficient, whereas anything rated over 220v will be, whether it is 250v, 450v, 600v, etc.

Another factor to consider if getting an aftermarket replacement is the temperature rating, which is only a factor if you are mounting it in the original enclosure (not a Frankenstein mount in some other location where it can have air circulation).

Incidentally, you should be able to pick up an equivalent at a local motor shop, so there is probably no need to spend much time on a web search.

From the original questioner:
Many thanks. I certainly hope I get it locally tomorrow. That would be sweet! The guys at Minimax group forums said I might as well replace both while going through this anyway. I did see that some capacitors were rated for so many hours at a given temp. Hopefully will find an appropriate replacement here.

From contributor J:
Contributor C is correct. If you cannot one locally, try Grainger. I have purchased a few from them in the past. Don't expect too much help from their will call desk, due to the nature of what you have. They do have an excellent technical department.

From the original questioner:
Aloha. So all went well today. The only motor shop here had a 375v 60uf cap. This replaced the 450v 60 uf. They said it would do just fine. I was hoping to get an exact replacement, not caring about brands. But was told if it only is for 220v then 375 will do. Made sense. See how it goes. I will note that it was $60. If I didn't need to use the machine I would have ordered via amazon. Like $8 and shipping. Bird in the hand…

I would like to thank everyone that leant a hand in me swapping out my first blown cap. I think the fear of god slowed me down a little, but no harm in being overcautious. Still skittish running the machine. It scared the sh*t out of me when it blew up. Good to keep you on your toes.

From contributor W:
I am not shocked at the price (local HVAC supply charges $80 for $10 caps if you don't have an account).

In any case, shorting the terminals of a capacitor with a screwdriver is a very good way to get hurt. A capacitor is capable of discharging all of its stored energy in an instant when shorted. Even a modest size capacitor can create an arc strong enough to send a glob of molten metal into your eye.

From the original questioner:
Good point. And I agree wholly. Shorting things out with screwdrivers isn't something I would like to do for all the obvious safety reasons. It is, however, very common among HVAC and electrical workers to do this, and was probably the most common response on other forums on how to discharge a cap.

I personally enjoyed the concept of the light build method lacking any more appropriate means to do so. I would be most concerned if I was discharging with a screwdriver and the damn thing blew up like it did when this one exploded. I still wince when I turn on the machine. Does anyone use "explosion proof" caps? Seems they would be a logical choice.

From contributor J:
I did recommend placing a meter across the terminals first. When the meter reads zero, it will be discharged. Then shunting the leads will not cause any discharge. Also, if it has melted, as the picture, it will not hold a charge anyway.

But, and it is a big but, your caution is legitimate, and thanks for voicing your concerns. Just because I do it, and it has never had any negative consequence, doesn't hold true for everyone. Electricity is funny that way.

From contributor W:
With subjects like this (at least in my opinion) it is a good idea to give the best practice advice, as you never know the skill level or understanding of those who will later read that advice.

For example: While it may be fairly safe to short a 20uF 120V cap, somebody who does not know the difference and reads the advice may end up shorting a cap charged to 600V with a capacity of 1200 or 1300 uF or larger. (Lighting ballast for example.)

I am a long time electronics hobbyist, tinker with the high voltage hobby (Tesla coils, spark gaps, etc.), and have many years of experience with single and three phase motors and drives ranging from fractional horsepower to those well over 1000HP.

As a teenager, I got whacked pretty hard (blown out of the back of the set) by a capacitor in a console TV flyback circuit. I was attempting to short it with a screwdriver. I have also blown up a few large oil filled caps in similar fashion and seen some downright scary arc flash accidents.

While a start cap on a 240VAC motor may not have the same ability to jump or create an arc flash large enough to blind or burn you, it is still best to avoid shorting it. In favor of a lengthy explanation of Ohm's law that will glaze over the eyes of those not interested and only looking for advice, it is often easier to give the safest advice possible that will allow them to get the job done.

Somebody mentioned reading the same advice in many other forums. That is what concerns me - advice often gets passed along by those who don't really understand the ramifications or context of the advice but take it at face value. That means (again at least in my opinion) that those truly in the know need to be careful how they convey advice regarding potentially dangerous subjects.

From contributor U:
Nothing like an exploding capacitor to get your attention.

Just a small addition to the thread: When replacing the cap or considering how to discharge it, note that this is a cap designed for AC applications to produce a phase shift (to cause the motor to start in the correct direction and rotate as previously mentioned), rather than a cap designed for DC applications. They are very different animals. Capacitors in DC applications can retain a charge after the power is removed (unless discharged by a bleeder resistor), and should be carefully discharged prior to handling as they can be lethal. A fully charged HV capacitor can retain a charge for days and provide very large amounts of energy when accidentally discharged.

On the other hand, while it's certainly possible, an AC capacitor in a motor start/run application doesn't typically retain a charge after the power is removed (or the capacitor explodes), so there's less risk. That said, experienced technicians typically place a screwdriver across the terminals of any capacitor as a final precaution before handling. I'd certainly agree that a more gentle discharge such as a light bulb or resistor is preferable (and less exciting), but the screwdriver is ultimately the final test (no risk of a blown light bulb not discharging the cap).

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