# Increasing starting capacitor size



## maninthesea (Nov 11, 2008)

First an explenation of the situation.

An off island customer is haveing a problem with their compressor motor. Its a 7.5HP single phase 3450RPM motor 213T frame. Sorry I dont recall the nameplate voltage but I would assume its running on about 212-213VAC(Should be 220) Thats what most of the islands seem to have around here.
They were blowing fuses andl had their electrician have a go at it. On his recomedation they asked me if I could find replacement springs for the centrifugal switch and a replacement capacitor. I found springs I think shold work and found the 1020-1200MFD capacitor 125VAC. Now their electician has asked for a "larger" capacitor. Which I can mostl likely get & will send to them. 
I am just wondering how adding capacitance will affect things. I am trying to remember what the navy taught me 20 years ago and all I recall is ELI the ICE man & countermotive force. 
Can anyone explain in slightly above laymans terms how increasing capacitance affects the motor?
Thanks in advance?
Jim 
PS they are considering a replacement motor. Anyone recomend any supply houses that are better priced than grainger?


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## GettingBy (Aug 19, 2010)

maninthesea said:


> I am just wondering how adding capacitance will affect things.


You can infer a rule about this by looking up motor sizes and capacitor sizes for them in the 6 to 8 hp range. The uF vs. hp may have been determined empirically.


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## maninthesea (Nov 11, 2008)

GettingBy said:


> You can infer a rule about this by looking up motor sizes and capacitor sizes for them in the 6 to 8 hp range. The uF vs. hp may have been determined empirically.


Sorry I am not following you. Are letting me know how to determ ine correct capacitor? I know for sure what capacitor came out and am just wanting to educate myself on what would be the results of putting too big of a starting capacitor back in? 

I am guessing one size up will not be catastrophic but would it even be benificial? What are the effects of a grossly oversized starting capacitor. I did a lot of google searches and most of what I found was how to estimate the correct starting capacitor for a motor when the original value is unknown. The general concensus was using a chart(which I cant find and the links were defunct) or a rulle of thumb 100MFD per HP which would put the existing start capacitor oversized.


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## DuMass (Feb 6, 2008)

Lots of guy’s have done that before, but it's generally not a good idea. 

http://gearmotorblog.wordpress.com/2010/01/26/hotrodding-capacitor-motors/


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## GettingBy (Aug 19, 2010)

maninthesea said:


> Sorry I am not following you. Are letting me know how to determ ine correct capacitor? I know for sure what capacitor came out and am just wanting to educate myself on what would be the results of putting too big of a starting capacitor back in?


The link above answers that. 
The best you can do is know the tradeoffs and decide if you can live with the downsides of using larger capacitors. But don't use a larger cap. as a bandaid to work around some as-yet-undiscovered root cause for the fuses blowing.

Making a scatterplot of motor hp vs. uF will give you some idea of the normal range of values. The trendline probably will not be a straight line as is implied by 100MFD per HP.


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## maninthesea (Nov 11, 2008)

Thanks Dumas Thats exactly what I was looking for. I want to advise the customer what the possible detrimental effects of oversized capacitor is.

@ gettingby 
I belive the cause for blowing fuses was blown capacitor so motor would sometimes lock & hum then blow the fuse. From what I understand the centrifugal switch was not cutting the starting capacitor out when the motor came up to speed (rusted springs and mechinism) So...... New springs and cleaned the mechinism should keep from leaving the start capacitor online and burning it up & then blowing fuses on subsiquent starts. 
All this is what I understand from the information I have recieved which was from philipino electrician speaking english as second language to Japanese shop owner speaking english as second language sending me emails and short phone conversations. This machine is 60 miles away(by boat) or $150 by plane so if I can sort the customer out without haveing to make a trip I usually do. 
Since my initial post I have found out the capacitor that we are replacing is not the original capacitor so it may be that it is undersized, correct or oversized.
Now my mission is to find the correct capacitor using the nameplate data from the motor.

Cheers Jim


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## GettingBy (Aug 19, 2010)

maninthesea said:


> All this is what I understand from the information I have recieved which was from philipino electrician speaking english as second language to Japanese shop owner speaking english as second language sending me emails and short phone conversations. This machine is 60 miles away(by boat) or $150 by plane so if I can sort the customer out without haveing to make a trip I usually do.


And I thought I had problems! :laughing:


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## Inner10 (Mar 12, 2009)

That is an interesting read on capacitors, when I installed laundry equipment I recall the single phase machines always had a big line of capacitors and jumpers you could alter if you needed more starting power. If you hooked them up to 208V you had the entire line connected and said a prayer every time they started.


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## GettingBy (Aug 19, 2010)

Inner10 said:


> a big line of capacitors and jumpers you could alter if you needed more starting power. If you hooked them up to 208V you had the entire line connected and said a prayer every time they started.


So an acceptable range of capacitance values could be vary over 1:2 or a 1:3 range? The tolerance alone on a single cap may be -10%, +50%.


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