Start Capacitor #of Starts

[Kroll]

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

 

[Ulma Doctor]

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

 

[brino]

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

 

[John Hasler]

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.

 

[RJSakowski]

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.

 

[John Hasler]

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.

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).

 

[Kroll]

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?

 

[coolidge]

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.

 

[mksj]

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

 

[brav65]

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.