4 uF capacitor - whats realistically too low?

[bsaint]

Background story - outside unit is tripping my breaker and popping fuses intermittantly. I opened up the unit today. Blew out the condenser coil and checked my capacitors. The compressor unit's capacitor is good, but the fan's is rated for 4 and reading 3.18. +- 6% is 3.76. Should I replace the capacitor? Terminals are fairly corroded too.

 

[larry_g]

You can try but the motor may also be the problem. BTDT

lg
no neat sig line

 

[bsaint]

Well I'm going to clean up the terminals and put an amp clamp on the motor to see what its running amperage is.

 

[JimL]

Blew out the coil? Wash it with a hose and nozzle. Water not air. Replace the cap.

 

[bsaint]

I blew out the coil from the inside out using air because I didnt know how it would react to water pressure.

 

[nehog]

Unless otherwise marked, many capacitors are +100/-20% so you are probably OK.

 

[bsaint]

Yea its +- 6%. Also isn't the fan suppose to spin so the air goes from the outside out the top? I think the rotation is wrong on the condensor fan.

 

[JimL]

Air blows up, yes.

Don't use a pressure washer, normal household pressure will work.

95% of the units we work on are dirty and need a bath.

 

[bsaint]

So I learned it blows the fuses after about an hour of running. What does that tell you? Fresh out the gate I had 8 amps on the compressor and 0.5 on the fan. Both are perfect against a 18 amp RLA and .8 amp FLA respectively.

 

[JimL]

Fuses? if its on a breaker, could be a bad breaker. Kill the power, check all the high voltage connections going to the unit. Something could be loose. Contactor, disconnect, panel.

Give it a bath yet? Soak it down with your favorite cleaner first if you like.

What kind of unit is it?

 

[brewchief]

See if the disconnect is getting hot, we installed a 10 ton rooftop a few years back and the next summer it started blowing fuses after extended run times, the fuseholders in the disconnect were loose and getting hot causeing the fuse to blow. The best part was the customer wanted to save a few bucks and had there in house fixit guy wire it, he used an old used disconnect instead of a new one, I bet our service call was way more expensive then a new disconnect.

 

[pattenp]

At 3.18 uf your cap is off by more than 20%. You should replace the cap if it's down 10% or more.

 

[bsaint]

Its an older unit my landlord said is a 1985 unit. It has fuses and a breaker. Can I wire the fuse box out of series? I think it had both because it was installed before the modern electric service was put in

 

[racingtadpole]

Its taking an hour or so of running to get warm enough to start binding and loading the motor. You have a bearing letting you know its on its way south. DONT bypass the fuses, they are there for a reason.


Dont worry about the cap, its motor start only, value isnt that critical. Having said that, if you have to pull it apart to clean it and find another problem, then if it makes you feel better its only a couple of extra dollars.

 

[wb2vsj]

Landlord? Unless he's a good friend of yours, or you all have some sort of agreement, I'd be asking him to fix it. Keeps liability down.

1985? Is this a residential unit? (vs commercial) Damn! 27 years of run time is amazing (to me it is)

 

[bsaint]

I offered to take a look at it. Im a CNC controls service engineer. I don't deal much with this style electrical tho. I haven't had to use capacitors or mechanical relays like this in years. Everything I work on is 24vdc. Its a Trane residential. Like they say, nothing runs like a Trane.

So I logged the current for 2 hours running non stop. Starts off 12.276 amps and settles at 12.059 amps after about 15 minutes. Never tripped off with the fuses bypassed. The heating factor of the fuses can't be that much different than the circuit breaker? Im going to try new fuses and log it longer. I bet you its a bad disconnect.

The HVAC guy though made up some story to my landlord and he is buying all new system. Oh well.

 

[pattenp]

I think a general statement like that is not providing the OP with good info. Uf values are important, if off too far on a run capacitor can damage the motor. I think the cap at 4uf is a run cap and not a start cap. Start caps are usually listed at a range such as 189-227 MFD.

Its taking an hour or so of running to get warm enough to start binding and loading the motor. You have a bearing letting you know its on its way south. DONT bypass the fuses, they are there for a reason.


Dont worry about the cap, its motor start only, value isnt that critical. Having said that, if you have to pull it apart to clean it and find another problem, then if it makes you feel better its only a couple of extra dollars.

 

[kald]

At 3.18 uf your cap is off by more than 20%. You should replace the cap if it's down 10% or more.

I agree with this.

 

[racingtadpole]

I think a general statement like that is not providing the OP with good info. Uf values are important, if off too far on a run capacitor can damage the motor. I think the cap at 4uf is a run cap and not a start cap. Start caps are usually listed at a range such as 189-227 MFD.

OK so prompted by this rather blunt kick up the date, I went back and had a look at my book of knowledge from Trade School, and I am apparently wrong (cut me some slack, trade school was over 20yrs ago, and I dont work on AC motors anymore). Low value capacitance indicates a run cap, higher value cap for starting. Just re-learnt something I learnt 20+ years ago, guess that means Im not dead yet.

 

[acmikee]

check the voltage drop across your fusses. if it work with the fusses bypassed then your disconnect is bad.

 

[pattenp]

I should have added a , because it was meant as a polite kick.

OK so prompted by this rather blunt kick up the date, I went back and had a look at my book of knowledge from Trade School, and I am apparently wrong (cut me some slack, trade school was over 20yrs ago, and I dont work on AC motors anymore). Low value capacitance indicates a run cap, higher value cap for starting. Just re-learnt something I learnt 20+ years ago, guess that means Im not dead yet.

 

[bsaint]

So I was right. Turns out it was the disconnect. I replaced it with a freebie 16 amp one (half the original current) and its been running all week with 98 degree heat.

 

[pattenp]

It's good you figured it out with the heat we've been having. As preventive maintenance you should put in a new run cap if the one you posted is reading more than 10% low.

 

[EOC_Jason]

Yeah, heat kills capacitors...

That 27yr old unit is probably built orders of magnitude better than anything you can buy on the market today.

While you are there, check the fan motor, the old ones usually have two oil ports (little plastic caps). Wouldn't hurt to oil your fan motor.. Also you said the contacts were corroded, wouldn't hurt to clean them too if you haven't already. Obviously be sure to throw the power to the unit before doing any of that!

 

[jeff000]

Yeah, heat kills capacitors...

That 27yr old unit is probably built orders of magnitude better than anything you can buy on the market today.

Built better is arguable, but I would put money on a new one being far more efficient though.

My AC is the same tonnage (is the righ? same rated size) as my parents. mine draws 20% less then theirs, mines high end brand new and theirs is mid/low end and 19 years old. They are having it replaced next week.


If you are paying for power I would put that old disconnect back in and get the landlord to replace the whole thing.Or cut you a break on rent or something.

 

[EOC_Jason]

I didn't say anything about efficiency... Yes the newer models are more efficient, but when you do the math and figure you pay a little more each month during the hot month but your unit has lasted all that time, or you end up replacing your unit every 10 years...

Some manufacturers offer a 10 year part & labor warranty with a complete system. My recommendation is TO GET IT because the units today (at least down here in the south where we run them A LOT) will not last.

My father was an A/C tech all his life. The stories he tells... I helped him many a summer growing up. We still have a couple dozen A/C units at our business property that we maintain.