ac capacitor VS run capacitor 2

[jarvis]

I need a 4mf capacitor for a small motor. I have a 4mf capacitor that came from an old power supply. It looks just like a motor run capacitor and is rated for more than what I need. Is there any difference between an ac capacitor and a motor run capacitor? Can I use this? Thanks for any help you can give.

Ben Englund

 

[Skogsgurra]

Hi,

"from an old power supply"?

Hmm.. Just make sure that it can take the motor voltage and ABOVE ALL that it is an AC capacitor and not an electrolytic (has + and - markings on the terminals and will explode).

If it is an AC capacitor and the voltage is OK, then there should not be any problems. Compensation capacitors for flourescent lighting do also work well for single phase 200 - 250 V motors.

 

[jarvis]

It is definately an ac capacitor. It was part of the power supply of VERY OLD IBM computer.

Unfortunately I tried it and the motor still does not run. It only buzzes and turns very slowly. If I turn it by hand it turns hard and does not continue turning. Though it seems if I could turn it faster it would continue. Without power to it it turns freely. Could my old capacitor be weak? I tested the capacitor with a meter and it is not shorted or open. Any Ideas? If the start coil was burnt out it should not turn at all by itself should it? How do I check it with only the two capacitor leads and 110 plug cord coming out of the housing? Thanks for your help.

Ben Englund

 

[Skogsgurra]

I guess that you might need a larger capacitor. How big is the motor? Are you, BTW, a distant relative?

Gunnar Englund
Sweden

 

[jarvis]

The original stock capacitor was 4mf(4micro farad). My replacement is also 4mf. Is it possible for a capacitor to lose capacitance without showing open? Or could the internal leakage be getting too great?

skogsgurra:
My great grandfather Ernie Englund came from Sweden and spoke in swede when he was mad. I believe his parrents brought him over as a young boy. So I guess we could very well be related:>)
It used to be that I did not see any Englunds here in Minnesota, but I am seeing more and more.

 

[ScottyUK]

[blue]I tested the capacitor with a meter and it is not shorted or open.[/blue]

What sort of meter did you use, and what were the results? A capacitor should be open when tested with a normal multimeter.



-----------------------------------

Start each new day with a smile.

Get it over with.

 

[jarvis]

When using an analog multi-meter on its highest resistance setting with test leads across a good capacitor that is discharged should make the needle deflect almost all the way to zero resistance and move toward infinate resistance starting out fast and slowing down as resistance increases. The higher the capacitance, the slower the needle will move. It varies with different meters and capacitor types.

That said..I should have said that the capacitor tested ok although I am not able to test the leakage current.

 

[electricpete]

It might be dangerous to try to turn an energized motor by hand if it does start.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[ScottyUK]

See if you can find another capacitor from a scrap fluorescent light fitting as skogsgurra suggested. They normally contain a capacitor in the 4 - 6uF range. These caps are of a similar construction and duty to the motor capacitor, so they are loosely interchangeable.

Regarding capacitor testing, your observations are absolutely correct. Sometimes I find that by asking silly questions, something obvious is picked up.

Another silly question: can you describe how the capacitor and motor windings are connected? What measurements have you made on the motor, and what were the results?



-----------------------------------

Start each new day with a smile.

Get it over with.

 

[jarvis]

See "motor buzz"

Thanks

Ben Englund

 

[ScottyUK]

Hi Ben,

Naughty practice, double posting! Someone will red flag that if they are srict.

Third daft question: is the thermal protection definitely ok? Some of these little motors have non-resettable thermal fuses which can be tricky to replace.



-----------------------------------

Start each new day with a smile.

Get it over with.

 

[jarvis]

I was hoping people would respond to the other post with the more appropriate name. I posted this one first before I tried the other capacitor.....

I have no idea if the thermal protection is okay or not. I seem to get a low resistance reading from one coil and a high one(like 500ohms can't remember now) from the other. maybe the thermal protector is shot and causing a higher reading on that coil? Would it be connected to both coils or just one?

NOTE: I'm not sure how the coils are hooked up, I made ohm checks and guessed from there.

Thanks again
Ben Englund

 

[busbar]

On a PSC motor, the thermal protector is in the 'common' lead, versus the 'run'- and 'aux'-winding lead. It almost always fails catastrophically… not much ”in between.”

Check line voltage at the motor—then line current—then possible mechanical binding [motor off.] Note a ‘cooked’ aroma is the kiss of death.

 

[jarvis]

I just plugged the cord into the wall. It is in the same wall socket as a computer which works fine even with the motor plugged in too, so the line voltage should be fine. There seems to be no smell. With the power off I spin it by hand and it continued for another rotation or two, so I don't think it is binding. When plugged in, it turns only about 60 rpm and it is not steady. It kind of jerks part of the revolution and makes a loud buzzing noise. When I try to force it to go faster by hand, it buzzes louder and resists being turned faster. Any insight is greatly appreciated. thanks

Ben Englund

 

[jbartos]

Suggestion: Visit

http://www.lmphotonics.com/single_phase_m.htm

for various single phase motors and their associated capacitor(s)

http://www.engin.umich.edu/labs/csdl/ME350/motors/ac/induction/1-phase/

for: a bit of single phase motor theory

http://www.gewater.com/pdf/TechNotes/TN 130 Evaluating Condition of Single Phase Submersible Mo.pdf

for: motor capacitor test

http://toad.net/~jsmeenen/capacitor.html

for: motor run capacitor test chart
etc. for more info

 

[jarvis]

Thanks Jbartos for those great links. I really like the capacitor test circuit, its a good idea.

Ben Englund

 

[SteveKW]

When I try to force it to go faster by hand, it buzzes louder and resists being turned faster

It only buzzes and turns very slowly. If I turn it by hand it turns hard and does not continue turning.


Both of those lines are classic signs of BAD BEARINGS!
Energizing the stator just makes the rotor drag because of bad bearings, worn shaft, worn sleeve, or bearing fit! Take your pick.

 

[jarvis]

Could it still turn freely when not powered if the bearings were shot? I am positive the capacitor was bad, could bad bearings cause the cap to fail? Or maybe a coincidence? Whould I notice the slop in the shaft by wobbling it by hand?(have not checked that) If I oil it will that help to determine if it is the bearings? Thanks

Ben Englund

 

[SteveKW]

Yes, the motor can turn just like new with bad bearings!
Especially if it is a sleeve bearing and fresh oil has been added!

Bad bearings possibly could cause the capacitor to fail but good run caps can stay across the line all day long with no problems!

Slop in the shaft can be very hard to spot (I work on motors all day long and it can take a good minute to "feel" the slop in a bad bearing/fit in some motors).

The big clue for you is when you apply power the motor has "hard" spots that makes a metal to metal buzz.
When the bearings are good the only resistance you would have is a MAGNETIC type resistance to its turning. (This would feel similar to turning the shaft on a small hobby type permanent magnet DC motor)

 

[busbar]

General comments referring to product style at:

http://www.grainger.com/Grainger/wwg/viewCatalogPDF.shtml?CatPage=128

Permanent Split Capacitor — Band-/Stud-Mount — 5.63-inch OD — "Fan Duty" HVAC motor
NEMA Frame: 48YZ {old GE 39 fr}

In many cases...

Sleeve bearings are susceptible to wear and resultant shaft scoring—not usually economically repairable.

Replace motor if relatively small shaft sideplay is found. {Endplay is acceptable.}

Only reliable check running current compared to nameplate {with voltage/temperature/frequency in tolerance.}