higher value start capacitor

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I have replaced a 12 uf +/-10% start capacitor in a pump motor with a 15 uf
+/- 5% cap. The supplier did not have a 12 uf in stock
Should I order a  12uf or leave the 15uf  in place ? I have read elsewhere
that it will just slightly increase the strarting torque and should not be a
problem. The motor is working ok now. Would there be any adverse effects?
....Opinions ?


Re: higher value start capacitor



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Shouldn't be a problem.



Re: higher value start capacitor


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If it's a start cap with centrifugal switch it should be ok but if it's
a run cap it can overspeed and heat the motor. Pumps usually don't need
the extra torque of a start winding so it may be in the circuit all the
time like some of the pumps I've seen.

Re: higher value start capacitor



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15 uf
elsewhere
be

Overspeed??

All the best
Ian Macmillan



Re: higher value start capacitor


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try playing around with the cap values on a cap run motor
the speed will vary and the motor runs much hotter at higher
speeds.

Re: higher value start capacitor



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It is a davey xp900h and looking up the specs says it is  P.S.C. start.
ie permanent split capacitor which according to Wiki , is in the circuit all
the time .
and is a start and run capacitor.no centrifugal switch
other specs... 2 pole, 0.8kw, 2850 rpm
I haven't noticed it running any hotter. It may be a bit faster. The cap I
pulled out of it was measuring only 6.7uf tho marked 12uf.
Pump is not new and gets a lot of work stopping and starting as a household
water supply.
Pump was failing to start sometimes even tho the pressure switch was on. now
ok with new cap.
I might try and pick up a 12 uf as it is no trouble to change and only $12.
regards


Re: higher value start capacitor



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In that case you might want to pop in the right value before summer, just
so the motor is run to spec., especially if the pump runs for long periods.  

The 15uF is letting through more current to the start winding, gonna
come out as heat, perhaps a little more work done.  Cost you a little
more in power.  I doubt it will kill the motor unless there's already
a heat problem.

Grant.

Re: higher value start capacitor


bristan Inscribed thus:

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I wouldn't waste my money !
Look at it this way. Assume that the original cap was at the top of its
spec, ie 12uf + 10% = 13.5uf and assume that the 15uf is at the bottom
of its spec, ie 15uf - 10% = 13.5uf.
It ain't worth messing.

--
Best Regards:
                     Baron.

Re: higher value start capacitor



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It's just as valid to consider the pessimal worst case too, if the
pump's start winding cooks this summer, OP is out of pocket a fair
bit more than a $12 cap.

Grant.

Re: higher value start capacitor


Grant Inscribed thus:

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I agree, I would bet that its low rather than high, but only by
measuring the caps actual value would confirm that.

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--
Best Regards:
                     Baron.

Re: higher value start capacitor



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measured it at 15.13uf
there has been a very noticeable increase in water pressure since replacing
the old cap which had deteriorated to only 6.7uf.
Pump seems to run more quietly also. Slighly warm to touch after running for
a while in winter temps.



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Re: higher value start capacitor


bristan Inscribed thus:

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Mmm. I'm slightly surprised its high.

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That is the behavior I would expect with the new cap.  There is more
current flowing through the windings and therefore more torque to drive
the impellor, so the motor isn't slowing down under load like it did
with the duff (old) cap.  There may be an over temperature cutout in
the motor windings to protect it from stall conditions.

--
Best Regards:
                     Baron.

Re: higher value start capacitor



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For a brand new 5% cap?  I'm not ;)  Is there an industry norm of
supplying caps at bottom of error range?
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One would expect so.  But constant higher temp operation may shorten
life...  Then again, it might keep the pump warmer and drier for less
corrosion?

Grant.

Re: higher value start capacitor


bristan Inscribed thus:

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It will be just fine.  Leave it be...

--
Best Regards:
                     Baron.

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