electric motor start capacitor

I have an electric air compressor that occasionally won't start, it just sits there and hums. If I tap the round hump on the motor ( start capacitor) it takes right off and might be good for another hundred starts. I think I need a new start capacitor but I can't find a replacement when I search the net. Can anyone cross reference these numbers? HI TORQ CSP 704S 704-845 MFD Thanks in advance
 
Here's picture
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The key numbers are "704-845 MFD". "MFD" stands for "microfarad", sometimes abbreviated as "UFD". So your capacitor is probably rated at 704 microfarads, with a tolerance of +20/-0 percent. If you look at <a href="https://www.grainger.com/category/capacitors/motor-supplies/motors/ecatalog/N-9yp?redirect=capacitor&searchRedirect=capacitor&searchBar=true#nav=%2Fcategory%2Fcapacitors%2Fmotor-supplies%2Fmotors%2Fecatalog%2FN-9ypZ1yzaxit%3FsearchRedirect%3Dcapacitors&cm_sp=CS_Banner-_-Capacitors_L3_CS-_-Motor_Start_Capacitor">this list at Grainger</a>, you'll see there's no "704-845 MFD" capacitor. But there are several rated "708-850", which is close enough. You'll need to check the dimensions of your cap to see which replacement will fit.

I'm a bit dubious the problem is the capacitor itself. It seems more likely there's a bad connection at one of the capacitor terminals, or maybe the centrifugal switch on the motor (if it has one) is going bad.
 
I would bet it's the centrifugal start switch mechanism sticking over a 'lytic that responds to tapping, but and electric motor shop should have a capacitor if you're convinced that's the problem.

They are under $20 from Grainger.

Go to grainger and enter 110-125vac voltage round motor start capacitor, 708-850.

Heck, there's a whole slew of 'em on fleabay, starting @ $4.99. Use the same search string.
 

Along the same line. I have a 3/4 hp motor. It will also not start makes a loud hum, can't turn it . The start switch is not burned and has a connection. ,
Tried a new start capacitor. Do not see any windings that look burned.
any ideas?

Thank you,
Ted
 
If tapping makes it start, I also suspect its more likely a bad loose or burned connection instead of the capacitor itself or else the centrifugal start switch is burned and carboned or not closing good. Remove the round cover and check the capacitor terminals and connections for indications of burning. Often removing and cleaning and tightening those connections cures the problem (unless its the switch)

John T
 
Very simple take the old one with you to the store you plan to buy it form and they can or should be able to test it and also be able to get you what you need. But if you have a motor like I had on my compressor it is not the cap. it is the motor it self. The motor I had on mine was sold by H.F. 20 plus years ago and rated at 6HP and did real good up till a couple years ago.
 
Hello mptaszynski,

Sounds like the starter contacts need a good cleaning. Could be a lose connection as well at the same location. It is not the start capacitor! Hitting it does not make it come to life, unless you have a bad connection there.

Guido.
 

Those numbers are minimums. Any micro farad rating that is higher is better and any voltage rating that is higher is better. All it has to do is fit under the cover.
 
Guido
I agree, I've found more end switches that need a very good cleaning and very few bad start caps.
geo
 

Motor turns free until you power it. Was working fine, just would not start one day. Makes a fairly loud hum when it wont start.
 
Hello Geo-TH,In,

Having more info, like the age of the unit, would have made for better guessing, later......


Guido.
 
Ted in NE:

Sounds like you may have a short in one of the motor coils. Completely disconnect the motor from the power source; remove the motor and carefully disassemble it. First, check to make sure that BOTH Brushes are contacting the Commutator, as only one brush contacting can cause those symptoms. Next, check for any arcing shorts (bridging) between any of the pads on the Commutator. If NO shorts between the Commutator pads; then visually look for burned or over-heated spots on any of the coils. Then check out the coils with an Ohm-Meter. If you find a shorted/burned-out coil it's time to buy a new (or GOOD, used) motor. - A friend of mine who worked for many, many years rewinding coils for electric motors once told me that "unless it's a very large, or high torque motor; that it's cheaper to buy a new motor than to have one rewound". Very labor intensive.

Doc
a243679.jpg
 
Dr. Walt, Ted said his motor has a starter capacitor. That is a picture of a universal motor. Universal motors do not have starter capacitors. Capacitor-start induction motors do not have brushes.
 

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