HVAC-Talk: Heating, Air & Refrigeration Discussion banner
Cancel
Create thread New Forums

Rundhaug- need capacitor size

Jump to Latest
13K views 75 replies 18 participants last post by  hedrash  
K
#1 ·
Ancient Westinghouse

M: SP018CAW
Style: 3958A46G01
Serial:MF63004

Big capacitor from pub era.
Thanks.
 
D
#7 ·
Unfortunately, if the motor isn't running then you have one equation with two or possibly three unknowns. The unknowns being Voltage at the capacitor terminals, and the Mfd of the cap.. The third unknown is the FLA of the motor if the nameplates can't be read.
 
Save
Like Like
K
#27 ·
the model and serial#'s are stamped in metal, but nothing readable on the capacitor or compressor... Its a recip, and I cannot tell you which brand... Too many scrolls out there lol

Cimerian said:
With no info make an educated guess. 35uf probably a good start. Then if compressor starts take amp on common and run together then on herm alone. If they agree within .5 amps your good. If not try a different size.
Click to expand...
this is interesting, i always assumed the amps on S + R would equal the amps on C... i guess common and run together would be like taking common minus run.... very interesting if this actually works.

Poodle Head Mikey said:
No, wait; don't give up so soon.

Let's use what we Do know:

018 in the M# almost certainly indicates an 18,000 BTU or 1 1/2 ton unit.

At comfort cooling conditions it takes about one horsepower per ton of cooling.

My general ROT is 12 amps per HP at 120V

So we can probably go with the idea that this system runs on 240V - so: 6 amps per HP. Or: a 9.0 FLA for the compressor.

PHM
-------
Click to expand...
Thanks for chiming in Mikey...
the RLA on this compressor was 11.6

I started with a 25/5... The compressor and fan used separate capacitors. So i just used Herm and common... The compressor started, and ran at 10.2 amps...

then i jumpered across Herm and Fan on the capacitor, making it a 30 mfd capacitor, and got 9.8 amps. Since recips generally take smaller capacitors than scrolls, and many two ton units that i service have 35mfd capacitors, i decided to call it good.
voltage from c to herm was 361
capacitance under load, using 'rash's formula, was 38.8Mfd...
Thanks again for the help guys....
:cheers:
 
S
#11 ·
That's incorrect. That only gives you the operating capacitance of the capacitor when the motor is running. The amperage used is the start winding and the voltage used is the voltage between the start and run windings.

Sent from my SCH-I545 using Tapatalk
 
C
#9 ·
With no info make an educated guess. 35uf probably a good start. Then if compressor starts take amp on common and run together then on herm alone. If they agree within .5 amps your good. If not try a different size.

Sent from my SM-G900P using Tapatalk
 
I
#30 ·
I read that somewhere recently too. It was as a short cut to cap checking. If your readings were within 0.1 amp, held as accurately as you can, then the cap is good. If off by more than 0.2 or 0.3 range then you'd better shut er down and check the cap specifically. I tried that the next day and found 2 weak caps with that method. Kind of nice to do while the unit is running.
 
D
#10 ·
Never tried this, but I might play with my own system.

If we are talking about the condensor fan motor, what about trying a 5 mfd. If it starts the motor, then read volts, and amps. on the capacitor.

If the voltage is over the capacitor's rating then try a 7.5 mfd. Again, if the motor starts then read volts and amps..

Find the capacitor that has voltage within its rating and amperage...

Oh, never mind... we still wouldn't have any idea if amperage was in line or not unless maybe the motor over heating and seized. Then we would know we got it wrong.
 
Save
Like Like
#14 ·
No, wait; don't give up so soon.

Let's use what we Do know:

018 in the M# almost certainly indicates an 18,000 BTU or 1 1/2 ton unit.

At comfort cooling conditions it takes about one horsepower per ton of cooling.

My general ROT is 12 amps per HP at 120V

So we can probably go with the idea that this system runs on 240V - so: 6 amps per HP. Or: a 9.0 FLA for the compressor.

PHM
-------
 
Save
Like Like
H
#18 ·
Check link I posted. Ridiculous amount of math to it but had a few rule of thumb charts. Posted wrong picture while I was trying to explain to a guy that if you have 115v from T1 to ground & 115v from T2 to ground does not mean you have 208/230 from T1 to T2. Some of these "techs" scare me to death. Had 1 leg of power back feeding through compressor

Sent from my SM-G920V using Tapatalk
 
#23 ·
If this was my problem to solve I'd try starting the compressor with a smallish run cap - just connect it up with jumper wires.

Start with some clearly too small capacitor, maybe 15uf. Let the run amps settle and then write down:

15 = _______

Then try a 20, a 25, a 30, a 35 and etc.

Write all them down the same way. I would expect the running amps to decline as the capacitance increased. When it started to increase I would go back to the one-previous size, make sure the cap terminal voltage was within limits, and call it a day.
 
Save
Like Like
#29 ·
After you do email them to yourself - then what happens? How do you get a picture you have emailed yourself into a post on this forum?

PHM
-------
 
Save
Like Like
#50 ·
How could you tell that I wasn't just showing off my new turban? <g>

But actually, with the provided directions, I just grabbed some random pics off my computer.

PHM
------
 
Save
Like Like
H
#43 ·
Yeah it's definitely a good way to go. I'm trying to decipher the other formula, which is pretty extensive on how to calculate a run cap for an unknown. It's a lot of math, but the one link is relatively hard to read. Going to put in some variables at work & see if I can simplify it a bit.

Sent from my SM-G920V using Tapatalk
 
I
#44 ·
I would think it's very important that the amp draw is taken in the crosshairs of the clamp. Also that there are no wires close to the clamp that may affect the reading because you are dealing with a few variables here, actual voltage read at capacitor and a very accurate amprobe reading. A little variance and either one will change your actual capacitance as derived by the formula.
 
K
#58 ·
Last fall I bought a different house. The sellers last name was, "Rundhaug". Many emails contained that name.....

So naturally, when I tapped "Rundawg" in the sun, by the truck, my phone "corrected" it to Rundhaug..... :) lol gotta love technology...
 
You have insufficient privileges to reply here.
HVAC-Talk: Heating, Air & Refrigeration Discussion
posts
5M
members
275K
Since
2000
The #1 HVACR Community
Full Forum Listing
Explore Our Forums
AOP Residential HVAC/Refrigeration(Beenthere Zone) General Discussion Tech to Tech Chat - Residential Controls Refrigeration & Ice Making