Delta Rockwell band saw motor will not start

[Janderso]

I am so incompetent when it comes to electric motors but I think this is a no-brainer.
I bought this beautiful Delta-Rockwell 14" metal/wood band saw off Facebook Marketplace yesterday.
When I went to look at it I knew I had found something special. It's very clean and has seen little use.
The seller told me it runs but you need to give it some help. Sure enough, spin the blade pulleys and off she goes.
This morning I found this 216-259 mfd start capacitor burnt out, smelly and I have no change when I touch my Fluke T5-1000 to the leads. I get 0L.
I found one from Graingers I just hope UPS will get it to me.

Question,
Why do they continue to use these capacitors? They don't last. Is there a brand that will last the life of the motor?

 

[kb58]

Short answer: no.
Longer answer: Yes, but it would cost a lot more and be bigger.

BTW, any good capacitor will always read "OL" on an ohmmeter because of how capacitors are constructed. There will be an initial inrush of current, but after that, zero conduction. An ohmmeter is only good for checking for shorts.

 

[silence dogood]

A capacitor is basically made of two metal plates separated by an insulator. Won't go to much into detail, but a cap can store electricity and then release it when necessary. The problem is the insulator which is also called a dielectric. To get a capacitor that can hold a charge big enough to start an electric motor would be the size of a room. So a special dielectric is made that works so well the capacitor can be made much smaller. Unfortunately, this dielectric is a form of paste that can over time dry out or degrade. That is why I always recommend to get the newest caps on the market, they even have a shelf life.. Maybe with more development on these new aeogel capacitors, the problem will be solved.

 

[markba633csi]

Jeff: Not sure if you did this but you should reverse the leads of your meter a couple times when using the resistance range of the meter to test.
You should see the meter swing towards zero ohms then climb towards infinity each time you swap leads as the cap charges/discharges. This test won't detect changes in cap value, only open or shorted caps. A quicky test. Old fashioned needle style analog meters are great for this.
The way these caps are constructed they only last a few years for what they cost. The older US made units would sometimes go for decades.
Newer import caps seem to fail more frequently. From the looks of it yours had leaked/failed after many years of use.
-Mark

 

[SLK001]

Question,
Why do they continue to use these capacitors? They don't last. Is there a brand that will last the life of the motor?

It's a start capacitor - it changes the field angle enough to get the motor running, then it is switched out of the circuit. You can tell that it is a start cap, because you could "push start" the motor by hand. Having a start capacitor makes the motor smaller, because you don't have to have a separate start winding - the main winding is doing double duty. A good analogy is a car transmission. To get a car rolling, you use a low gear (ie, have a start capacitor). To run a car efficiently, you use a high gear. A motor is wound for "high gear" and uses a start capacitor (low gear) to get the motor running fast enough to shift into high gear.

 

[Janderso]

Jeff: Not sure if you did this but you should reverse the leads of your meter a couple times when using the resistance range of the meter to test.
You should see the meter swing towards zero ohms then climb towards infinity each time you swap leads as the cap charges/discharges. This test won't detect changes in cap value, only open or shorted caps. A quicky test. Old fashioned needle style analog meters are great for this.
The way these caps are constructed they only last a few years for what they cost. The older US made units would sometimes go for decades.
Newer import caps seem to fail more frequently. From the looks of it yours had leaked/failed after many years of use.
-Mark

Mark,
I didn't know my Fluke wouldn't test a capacitor. I see they make specific products to test these things.
How about a visual.
This thing is toast. It stinks to high heaven too.

 

[kb58]

Right, so no need for testing.

 

[Janderso]

Right, so no need for testing.

I still wanted to verify but it seems I don't have the correct device.
I learned that much.

 

[kb58]

Even if it had tested fine, no way one with that sort of damage should be used. Too much risk of shorting or starting a fire.

 

[darkzero]

A bit of repeating but no, capacitors won't last "forever", they eventually dry out). Of course some are made better than others & may last longer.

Mark already said it but if you use the OHM function, the resistance will climb then eventually show open (OL). Reverse the leads & it will repeat. But this doesn't actually tell you the cap's reading.

Not sure what model Fluke you have but some MMs have a capacitance function. On my Fluke this is the symbol

. Sometimes it may be in blue meaning it's an alternate function & you have to press the blue button to switch to that function.

I've had good luck with Grainger's Dayton brand caps. Start cap on my mill blew only a yr old. Wanted to fix it ASAP cause I was in the middle of something so I picked one up from Grangier next morning. Well I replaced the Run cap too just for the heck of it. They were made in China but it's been 7 yrs now trouble free.

EDIT: Oh & in the future, if you are going to measure a pretty good/large size cap, be sure to discharge it first.