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Capacitor interchange question

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jboehm

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I have a circulation pump for my hot tub that I suspect has a bad capacitor. It will not start on its own, but if I spin it by hand it goes. I can not find the exact replacement, but I can find one with what appears to be the same specs. Can I replace the rectanglular black one with the oval one. The original one is mounted in the tail cap, but I should be able to mount the oval one outside. How critical is to get one with the exact specs. Can I use one with more uf/mfd?
 

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rlitman

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I don't see a temperature rating on the oval one, but otherwise it looks like a fine substitute.
 
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exmaxima1

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wyliesdiesels

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exmaxima1 said:
Ummm, all components have some tolerance. 5% on a run cap is very good, and pretty much says it's not an electrolytic type.
Click to expand...

U misunderstood what i was saying.

The OP asked if the mfd on a replacement cap has to match the original.

And so i was replying to that...
 
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jboehm

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My recollection is for this setup it doesn't make a difference which wire goes to which tower. Is that correct?
 
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rlitman

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jboehm said:
My recollection is for this setup it doesn't make a difference which wire goes to which tower. Is that correct?
Click to expand...

That depends on the capacitor style, but in the case of AC metallized film capacitors (such as the oval one above), they are not polarized. The only consideration for these is that they need a little clearance room above the terminals when mounted. This is because the internal protection in the event of a fault is to have the terminal cap bulge a little (usually around 1/4"), which causes the terminals to disconnect from the film.

Electrolytic capacitors in DC service are rolled up in such a way that charging them with the correct polarity will tighten the winding, and charging them with the reverse polarity will try to make them unwind, which puts undue pressure on the case.
 
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Alchymist

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Electrolytic capacitors usually have quite wide tolerance range for capacitance value - often up to -10% to + 50% of marked value, tho most will vary on the + side.
 
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exmaxima1

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rlitman said:
Electrolytic capacitors in DC service are rolled up in such a way that charging them with the correct polarity will tighten the winding, and charging them with the reverse polarity will try to make them unwind, which puts undue pressure on the case.
Click to expand...

Putting reverse polarity on an electrolytic cap will also dissolve the anodized layer on the aluminum film and quickly short it out, boil the electrolyte, and "cause undue pressure" as well. I never knew about the winding tightening/loosening, and I appreciate that info. :beer:
 
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rlitman

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exmaxima1 said:
Putting reverse polarity on an electrolytic cap will also dissolve the anodized layer on the aluminum film and quickly short it out, boil the electrolyte, and "cause undue pressure" as well. I never knew about the winding tightening/loosening, and I appreciate that info. :beer:
Click to expand...

:) Yeah, they're coiled like a clock spring. With high voltage electrolytic capacitors you're supposed to slowly ramp up the charge voltage for their first use. You can actually hear them strain as they're charged the first time.
 
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exmaxima1

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rlitman said:
:) Yeah, they're coiled like a clock spring. With high voltage electrolytic capacitors you're supposed to slowly ramp up the charge voltage for their first use. You can actually hear them strain as they're charged the first time.
Click to expand...

I've ramped them up for years when I was building tube amplifiers. It was called "forming" and I thought I was re-anodizing the electrodes as the layer dissolves slightly over time with no potential on them. Interesting that there was a mechanical aspect to the procedure as well.
 
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wyliesdiesels said:
A start capacitor can have a 10% variance on mfd(micro farad).

A run capacitor cannot have any variance...
Click to expand...

Typical spec on run capacitor is +/- 6%, tighter than the start cap, but still allowable. I think where the confusion comes in is that the 6% is the allowable deviation FROM MARKED VALUE, not that you can substitute a cap that is 6% larger or smaller than the marked value.

Now if you could measure actual capacitance, you could search other caps of different values to find one within 6% of spec'ed value. Not many people have that capability.
 
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jboehm

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Well, cap is on and the motor starts and runs strong again. I guess I never noticed, but that motor sure runs hot.
 
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wyliesdiesels

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Alchymist said:
Typical spec on run capacitor is +/- 6%, tighter than the start cap, but still allowable. I think where the confusion comes in is that the 6% is the allowable deviation FROM MARKED VALUE, not that you can substitute a cap that is 6% larger or smaller than the marked value.

Now if you could measure actual capacitance, you could search other caps of different values to find one within 6% of spec'ed value. Not many people have that capability.
Click to expand...

Exactly :+1:

jboehm said:
Well, cap is on and the motor starts and runs strong again. I guess I never noticed, but that motor sure runs hot.
Click to expand...

Thats great u got it running.

Does it have a fan?

Do u have a clamp meter? If so measure the current that the motor is drawing. Is it close to the FLA listed on the nameplate???
 
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rlitman

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jboehm said:
Well, cap is on and the motor starts and runs strong again. I guess I never noticed, but that motor sure runs hot.
Click to expand...

Capacitor life can be inversely proportional to running temperature. Your cap may have failed because it was inside the case, and with the replacement being outside, it may last much longer.
 
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exmaxima1

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jboehm said:
Well, cap is on and the motor starts and runs strong again. I guess I never noticed, but that motor sure runs hot.
Click to expand...

Is the motor under load? It is normal for many capacitor-run motors to get hot (or even overheat) when run without the intended load on them.
 
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jboehm

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exmaxima1 said:
Is the motor under load? It is normal for many capacitor-run motors to get hot (or even overheat) when run without the intended load on them.
Click to expand...
Yes, it is under load all of the time. Even the jet pump motors are hot when they are running.
 
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