Capacitor of a 230V Single phase motor—Can increase ?

Thread Starter

ranatungawk

Joined Oct 30, 2008
172
Sometimes my water pump doesn’t start at once. I Have to support it by rotating the motor fan a little.

What I inspected and did:

  1. Replaced the starting Capacitor with a similar new one (15MFD 400V)
  2. Wire joints clean and re tight-No connectivity issues
  3. Internal bearings are in good condition (no extra resistance)
  4. Voltage to motor is OK (230V)
  5. No voltage drops while rotating

However, issue still remains same. If I increase the capacitance a little (with a 20MFD ) will it help ?
 

MisterBill2

Joined Jan 23, 2018
9,474
I have had to add capacitance to two furnace blower motors for a similar problem. So yes, adding another capacitor may be the fix. In fact, if you still have the one that you replaced you could try both in parallel and see if that works. Some of those capacitors from some manufacturers may not be as rated.
 

MaxHeadRoom

Joined Jul 18, 2013
23,782
Any induction motor run (or start) capacitor should be AC induction motor run/start rated, do not use a capacitor that is meant for any general usage.
There should be no need to replace one with a value different from the OEM part.
Advice: Do not use capacitors that are of Chinese origin.
If this occurred after the pump has been running for any length of time, it points to maybe something else being the cause.
Max.
 

MisterBill2

Joined Jan 23, 2018
9,474
The presumption that a capacitor value is exactly as marked on the outside is a really huge leap of faith. Especially for capacitors that are used, or from some sources.
And I once provided a fix that lasted aboy 15 years by adding a number of 0.47 mfd/400 volt capacitors across an insufficient motor run capacitor. The furnace eventually was replaced with a more efficient model, although the blower still worked.
 

Thread Starter

ranatungawk

Joined Oct 30, 2008
172
The presumption that a capacitor value is exactly as marked on the outside is a really huge leap of faith. Especially for capacitors that are used, or from some sources.
And I once provided a fix that lasted aboy 15 years by adding a number of 0.47 mfd/400 volt capacitors across an insufficient motor run capacitor. The furnace eventually was replaced with a more efficient model, although the blower still worked.
Thanks for the comment .

does adding 20MFD cap insted of the original 15MFD make a bad effect ?
 

Dodgydave

Joined Jun 22, 2012
10,062
Just that it will take more current than needed ,and may make the motor windings warmer.. that's 40 Ohms less on a 60Hz supply, 52 Ohms on a 50Hz.
 
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MaxHeadRoom

Joined Jul 18, 2013
23,782
Any increase OR decrease over the optimum value can cause a degenerative effect.
You said 'replaced with a similar one', was this an AC motor start type of reputable manufacture?
Otherwise it is just guess work, and changing the value does not always guarantee it is correct.
What is the nature (job) of the pump?
Max.
 

Thread Starter

ranatungawk

Joined Oct 30, 2008
172
Any increase OR decrease over the optimum value can cause a degenerative effect.
You said 'replaced with a similar one', was this an AC motor start type of reputable manufacture?
Otherwise it is just guess work, and changing the value does not always guarantee it is correct.
What is the nature (job) of the pump?
Max.
This is a water Pump: 0.75Hp, 230V/50Hz, Single phase motor (27 years old)
I replaced with an exact slimier one which bought from the original pump manufacture. ( i tested it with a capacitor meter and found good)
 

MaxHeadRoom

Joined Jul 18, 2013
23,782
A capacitor tested on a meter is not the same conditions it experiences in a AC motor start application.
Is the original and replacement of Chinese origin?, where is the pump made?
AC induction Motors of Chinese origin are notorious for early failure of the start or run capacitors.
If it has ran OK with this value of cap for 27 years, it hardly seems likely the cap is the reason.
Max.
 

MaxHeadRoom

Joined Jul 18, 2013
23,782
probably no bad effect,
Bad advice IMO.
If it has ran for 27yrs it is most likely that there is some other issue, mechanical etc, that suggests at least a strip down and check for wear first before adding parts that were non original.
Is the pump a submersible variety?
Is it oil filled? if so, check for correct tranformer oil level.
Max.
 
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MaxHeadRoom

Joined Jul 18, 2013
23,782
Motor capacitors age, both in use and in the box.
Generally applies to the electrolytic start versions, the one's I buy are CDE (Dubilier) and are oil filled paper run capacitors and have a very long shelf life that i would never be concerned on that issue.
I was referring to the suggested change of value to be fitted.
Max..
 
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MisterBill2

Joined Jan 23, 2018
9,474
I have had to replace motor capacitors that I presume were the kind that Max claims never fail. But they do sometimes appear to lose capacitance, although it may be that they just suffer from increased series resistance.

In any case, replacing the cap caused the motor to once again start quickly and run correctly. Note that I did not dis-assemble the old capacitor to find out the failure mechanism.
 
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