# Start Capacitor #of Starts



## Kroll (Jun 12, 2015)

Guys afew weeks ago my motor just quit working while I had been using it for couple of hrs.But there were no capacitors (so I thought) mounted on the outside,so I posted a question about what to do and I then discover two start capacitors mounted on the inside of the motor.So I went shopping for two more caps,so today I mounted the new caps and now the lathe is back to normal.But I was wondering what took out the caps till I read the specs sheet on several brands which stated that they are rated for 20 starts per hr. Well I be,I did not know that which explains what happen cause I was making cuts then stop, check the OD, start back up again.Does this sound right to you other Sears/Atlas owners,or maybe it was something else?????----Carroll


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## Ulma Doctor (Jun 12, 2015)

start capacitors are much more frail than run capacitors, generally speaking.
a run capacitor is made to be used in the motor run circuit continuously as the motor runs.
the start cap is  momentarily actuated  and stays in the motor circuit only for a couple seconds.
start capacitors will have higher ratings (bigger bang)  than a run capacitor.

electrical components can fail at any time without warning


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## brino (Jun 12, 2015)

Kroll said:


> But I was wondering what took out the caps till I read the specs sheet on several brands which stated that they are rated for 20 starts per hr.



Hey Kroll,
I have never seen that spec.ed before. 
Thanks for posting it, I learned something today.
I also do a lot of cut and measure..........maybe I need to change my ways......
-brino


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## John Hasler (Jun 12, 2015)

Kroll said:


> Guys afew weeks ago my motor just quit working while I had been using it for couple of hrs.But there were no capacitors (so I thought) mounted on the outside,so I posted a question about what to do and I then discover two start capacitors mounted on the inside of the motor.So I went shopping for two more caps,so today I mounted the new caps and now the lathe is back to normal.But I was wondering what took out the caps till I read the specs sheet on several brands which stated that they are rated for 20 starts per hr. Well I be,I did not know that which explains what happen cause I was making cuts then stop, check the OD, start back up again.Does this sound right to you other Sears/Atlas owners,or maybe it was something else?????----Carroll


Starting requires much more capacitance than running.  Fortunately the start cap is in the circuilt only for a few seconds.  This allows motor designers to make an engineering tradeoff by using cheaper and smaller nonpolar electrolytic capacitors for starting.  These are much less efficient than the oil caps used for running.  They heat up while in the circuit but are switched out before they get too hot, and cool off before the next start if it doesn't come too soon.


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## RJSakowski (Jun 12, 2015)

It is the first time that I have heard of such a spec.  I guess that it makes sense in that the start capacitor in an ac induction motor circuit is constantly charging and discharging as long as the start circuit is complete.  This process will generate heat in the capacitor. which has to be dissipated.  A higher duty cycle for the start winding, as is created by frequent starts, could put more heat into the capacitor than can be dissipate resulting in overheating and a shortened life.

As a physics student, I remember a problem where a capacitor was charged to a voltage V and then partially discharged into an identical capacitor.  The resultant voltage on the two capacitors is V/2 and the energy stored in the two capacitors is one half the initial energy which means half the energy was dissipated somehow.  The prof said radiation from the spark that jumps when connect the two but offered no proof.  What you describe makes more sense.

It is useful information for all of us doing lathe work with capacitor start motors.  I know that I had done exactly what you have described.

Thanks for sharing the info.


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## John Hasler (Jun 12, 2015)

RJSakowski said:


> It is the first time that I have heard of such a spec.  I guess that it makes sense in that the start capacitor in an ac induction motor circuit is constantly charging and discharging as long as the start circuit is complete.  This process will generate heat in the capacitor. which has to be dissipated.  A higher duty cycle for the start winding, as is created by frequent starts, could put more heat into the capacitor than can be dissipate resulting in overheating and a shortened life.
> 
> As a physics student, I remember a problem where a capacitor was charged to a voltage V and then partially discharged into an identical capacitor.  The resultant voltage on the two capacitors is V/2 and the energy stored in the two capacitors is one half the initial energy which means half the energy was dissipated somehow.  The prof said radiation from the spark that jumps when connect the two but offered no proof.  What you describe makes more sense.
> 
> ...


Nonpolar electrolytics such as are used as start capacitors have much more internal resistance than the oil filled types used as run capacitors. 

In the experiment you describe the capacitors and the stray inductance (their is always some) of the circuit form a resonant circuit which rings at its resonant frequency.  The ringing decays exponentially as the energy is dissipated in the resistance of the circuit (which includes the internal resistance of the caps).


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## Kroll (Jun 12, 2015)

DAYTON 2MDT2 Motor Start Capacitor, 243-292 MFD, Round
DAYTON
Item Specifics
Item
Motor Start Capacitor

Starts per Hour
20

Max. Power Factor (%)
10

Shape
Round

Hz
60/50

MFD Rating
243-292

Voltage
110-125VAC

Dia. (In.)
1-7/16

Overall Height (In.)
3-3/8

Temp. Range (C)
-40 to 65

Construction
Sealed Molded Phenolic Resin Case

Item
Motor Start Capacitor

Terminals
2-blade Quick-connect Insulated

Standards
cULs Recognized (E224674)

Case Height (In.)
3-3/8

Temp. Range
-40 Degrees to 65 Degrees C

Overall Height
3-3/8"
This is the info on the cap,I never heard of this before so I was just wondering if what I am reading is true?


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## coolidge (Jun 12, 2015)

Lack of use is generally the cause of death for start caps. If they sit there flat with no use for months or longer periods of time. Electrolytic caps also just dry out and fail over time. New caps should last a good long time though, years.


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## mksj (Jun 12, 2015)

coolidge said:


> Electrolytic caps also just dry out and fail over time.


Yep.

Lack of use and high temperature are factors, mostly for electrolytic type capacitors. But too frequent start/stop cycles will overheat the start capacitor (which is usually an electrolytic because of the uF and voltage rating), and capacitors running at or over their ratings will significantly shorten their life expectancy. One would surmise that since the start capacitor is in series with the coil when starting, that the starting load (Amperage x Time) would play a part in the energy dissipated in the capacitor. A saw would have a low starting load vs. a compressor which would have a much higher load.

That being said, a good quality capacitor will last a lot longer, and the replacement cost is negligible relative to the usable life cycle of  30,000-60,000 hours. Higher voltage capacitors have heavier films/insulators, and can hold up longer, but size is an issue.

Motor Capacitor FAQ
http://www.temcoindustrialpower.com/product_selection.html?p=motor_capacitor_faq


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## brav65 (Jun 12, 2015)

Kroll said:


> Guys afew weeks ago my motor just quit working while I had been using it for couple of hrs.But there were no capacitors (so I thought) mounted on the outside,so I posted a question about what to do and I then discover two start capacitors mounted on the inside of the motor.So I went shopping for two more caps,so today I mounted the new caps and now the lathe is back to normal.But I was wondering what took out the caps till I read the specs sheet on several brands which stated that they are rated for 20 starts per hr. Well I be,I did not know that which explains what happen cause I was making cuts then stop, check the OD, start back up again.Does this sound right to you other Sears/Atlas owners,or maybe it was something else?????----Carroll



I think it is like how many licks does it take to get to the center of a Tootsie pop.  Sometimes it's 1, 2, 3 crunch...other times they go for ever.


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## Silverbullet (Jun 12, 2015)

Hey brave65' have a question ,how long is an  [ ever] , ha just funnin , but ya never know right an ever could be a lifetime.


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## cdhknives (Jun 12, 2015)

Motors can also be rated in starts per hour.  The big boys I work with (4160 nominal Volts) generally get 2 start/restart tries before a mandated 20+ minute cooling period.  For your lathes 5-10 starts over a 10 minute time is probably okay if you unload your spindle every time and let it run 10 minutes or so after.  Running pumps cooling air through or over it.  This is why you always start your lathe with the spindle disengaged...the extra time to spin up the extra mass is all going to greater heat in the starting circuit and motor windings.  A 1.15 service factor motor helps, as does a larger Hp motor.  Most of this is beyond the scope of out little 1/3-1 Hp motors on our Atlas lathes, but some of the big boys y'all may also work with will want to consider this.

Heat is the #1 enemy of electrical parts.  Bar none.  Electrolytic caps dry out faster with use as they are warmer.  The reason for the myth of the 'dry out in storage' is time...some time during storage it dried out that last bit to fail, in use it would have died even sooner, but it wasn't being used so no one noticed.  In other words, it may have died in 1 year from last use if still in weekly service, it did die in year 3, but wasn't turned on until year 5 so 'that electrolytic cap dried up from non-use'!


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## silence dogood (Jun 12, 2015)

These capacitors do have a life span and that includes the shelf life.   I realize that the specs say the cap can take -40 to 65 degrees C.  Still,  I would not let them freeze or sit in the hot Texas sun.. There is a paste inside of them.  And while you are replacing the start cap,  check the contacts on the centrifugal switch.  If the contacts are dirty with carbon,  the motor may not get up to speed.   If the contacts don't open,  you will be blowing caps.   One final note,  a guy gave me a old Rockwell 9" table saw.  Aside from some cleaning and adjusting.  I replace the start cap,  it was in a box separate from the motor.  It also had a start relay instead of a centrifugal switch.   It was a little rectangular black box that said Klixon 4CR on it.  Unfortunately,  these things aren't  available anymore.  An electrician told me to wire in a 20 amp momentary on switch in series with start cap to the start winding( red wire in this case).  So now when I turn on the saw, I press both switches at the same time and hold approx. a sec. for the motor to build up speed.   I must say even though this saw was a cheapy in it's day,  it's still a lot better than this $200 or less crap that one buys in the store now a days.   Mark


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## John Hasler (Jun 12, 2015)

cdhknives said:


> Motors can also be rated in starts per hour.  The big boys I work with (4160 nominal Volts) generally get 2 start/restart tries before a mandated 20+ minute cooling period.  For your lathes 5-10 starts over a 10 minute time is probably okay if you unload your spindle every time and let it run 10 minutes or so after.  Running pumps cooling air through or over it.  This is why you always start your lathe with the spindle disengaged...the extra time to spin up the extra mass is all going to greater heat in the starting circuit and motor windings.  A 1.15 service factor motor helps, as does a larger Hp motor.  Most of this is beyond the scope of out little 1/3-1 Hp motors on our Atlas lathes, but some of the big boys y'all may also work with will want to consider this.
> 
> Heat is the #1 enemy of electrical parts.  Bar none.  Electrolytic caps dry out faster with use as they are warmer.  The reason for the myth of the 'dry out in storage' is time...some time during storage it dried out that last bit to fail, in use it would have died even sooner, but it wasn't being used so no one noticed.  In other words, it may have died in 1 year from last use if still in weekly service, it did die in year 3, but wasn't turned on until year 5 so 'that electrolytic cap dried up from non-use'!


*Old* electrolytic capacitors, as in pre-WWII, did dry out in storage (they dried out even faster in use).  However, I have electrolytics from the 1970s in my junk collection upstairs that are as good as new.  The start cap in the static converter on my mill came out of a thirty year old air conditioner that had sat in my machine shed for at least a decade.


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## cdhknives (Jun 13, 2015)

I have to disagree.  There is no such thing as a perfectly sealed electrolytic capacitor.  Well made modern electrolytic caps may have a lifespan of 50+ years, but the slightest manufacturing defect in the case or seals on the leads going into the can and that lifespan is greatly shortened.  Most electronics won't even survive long enough to know the difference.  Motor caps, with the heat and torture they endure, can and do fail from simply drying out.  BTW, there are 'dry' electrolytic capacitors, we're only talking about so called 'wet' electrolytic capacitors as the dry type are significantly more expensive.  Bottom line is no electrolytic cap case is absolutely perfectly sealed against all permeability, and well made units are probably going to be killed by misuse (power surge, for example) long before they dry out.

Also, most people see a cracked capacitor case and assume the crack caused it to fail.  The case rupture is usually from internal overpressure as it overheats.  Metal can caps have a blowout tab, usually on top.  Plastic cases just crack.  When I taught a second year college electronics lab we had a tradition of signing the ding when you blew a cap and it hit the wall...usually from polarity reversal but a couple of times the only explanation I could find was infant mortality.


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## wa5cab (Jun 15, 2015)

Knives,

Although you are correct that there are (or were) both "wet" and "dry" electrolytics, I don't think that any "wet" ones have been made since the 1930's.  At least I I routinely work on vintage US military radios made mid 1930's and later and don't think that I have ever actually seen a "wet" one.

However, as applied to "dry" electrolytics, "dry out" is misleading.  The dielectric layer in dry electrolytics is created during manufacture.  Unfortunately, the process is reversible by time and temperature.  The degradation process has for decades been commonly referred to as "drying out".  But it is actually electrochemical in nature.  And is itself sometimes reversible.  At least with single polarity capacitors.  The repair process is commonly referred to as "reforming", and involves applying a current limited voltage across the capacitor and monitoring either the voltage or the current during reforming.  But I don't think that you can reform a non-polarized capacitor.  So the obvious repair procedure for AC motor start or run capacitors is replacement.


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## Alan Douglas (Jun 15, 2015)

There is, or was, an online video from a large capacitor manufacturer in the US, showing the etching and forming process. It involves large rolls of aluminum foil and heated tanks of electrolyte. The formed foil is then wound into capacitors.

I presume the improved lifetime of modern electrolytics is due to increased purity of the aluminum, rather than "drying out."  Certainly modern electrolytics show far less leakage current than older ones.  Electrical leakage is from defects in the dielectric layer.

There may have been some liquid-filled electrolytics made post-war but if so, they weren't common.  Military gear never used them at all, I presume because they wouldn't work at Arctic temperatures and in all positions.  Their advantage was mainly in home radios where they would absorb the voltage surge at turn-on.


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## coolidge (Jun 15, 2015)

Then there is the issue of counterfeit capacitors...outwardly the cap looks like a higher priced capacitor and you peel off the outer cover and find a cheapo China capacitor inside. (face palm)


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## RJSakowski (Jun 15, 2015)

Alan Douglas said:


> There is, or was, an online video from a large capacitor manufacturer in the US, showing the etching and forming process. It involves large rolls of aluminum foil and heated tanks of electrolyte. The formed foil is then wound into capacitors.
> 
> I presume the improved lifetime of modern electrolytics is due to increased purity of the aluminum, rather than "drying out."  Certainly modern electrolytics show far less leakage current than older ones.  Electrical leakage is from defects in the dielectric layer.
> 
> There may have been some liquid-filled electrolytics made post-war but if so, they weren't common.  Military gear never used them at all, I presume because they wouldn't work at Arctic temperatures and in all positions.  Their advantage was mainly in home radios where they would absorb the voltage surge at turn-on.


In doing some reading on the subject, I discovered that apparently the Japanese company, Rubicon, reintroduced wet electrolytics in the 1990's in an attempt to reduce ESR.  Because they didn't get the formula right, it led to a rash of exploding capacitors in the early 2000's.  My electronics experience goes back several decades and I, like you, thought that wet electrolytics went out before WW II.


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## John Hasler (Jun 15, 2015)

RJSakowski said:


> In doing some reading on the subject, I discovered that apparently the Japanese company, Rubicon, reintroduced wet electrolytics in the 1990's in an attempt to reduce ESR.  Because they didn't get the formula right, it led to a rash of exploding capacitors in the early 2000's.  My electronics experience goes back several decades and I, like you, thought that wet electrolytics went out before WW II.


The Rubicon caps worked fine.  The ones that popped were made by Chinese companies that tried to copy them but bungled the formula.


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## RJSakowski (Jun 15, 2015)

John Hasler said:


> The Rubicon caps worked fine.  The ones that popped were made by Chinese companies that tried to copy them but bungled the formula.


The article I saw mentioned the stolen recipe but didn't say who it was stolen from or who did the stealing.  Thanks for clarifying.


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## wa5cab (Jun 15, 2015)

Hmm.  I had somewhere heard the story about the Chinese piracy but didn't know or it didn't register that the exploding caps were wet electrolytics.


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## ogberi (Jun 16, 2015)

I work in IT and we had a literal plague of the knockoff caps blowing out in equipment.  Motherboards in workstations, two HP (yech) servers, tons of video cards and power supplies, power supply boards in LCD monitors, etc.  IF they died quietly, they just puffed up and leaked electrolyte.  Sometimes they spat and fizzled, a few times they detonated.  Not much stinks worse than that.    
Although we did have two servers die completely, there were redundancies in place so the services they provided had minimal downtime.   Mostly it was workstations and monitors that died.  Though we did drop a core switch because of bad caps.  That wasn't a pleasant automated call at 5am...


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## wa5cab (Jun 16, 2015)

Yeah.  The mother board in my wife's computer apparently died because of this.  At least several of the same size PC mounted capacitors swelled up.


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