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Odd question: will an old Sprague oil cap work in furnace?
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Author:  TubeDriver [ January 15th, 2016, 1:04 am ]
Post subject:  Odd question: will an old Sprague oil cap work in furnace?
So my furnace blower stopped working. I found a blown relay and circuit board trace in the blower control board and replaced the board and everything works fine. However, I tested the blower run cap and it is off. It is a 7.5uf 340V cap and it tests at about 5uf. I ordered a replacement but it may be a week before I see it. Just by chance, I have some old Sprague 8uf 600v oil caps. One measures at 7.8uf which is pretty close. I was wondering about using this cap until the replacement arrives. Would an old oil cap work in this situation?

Would this make the motor sound better? :lol:
Author:  Stuart Polansky [ January 15th, 2016, 5:23 am ]
Post subject:  Re: Odd question: will an old Sprague oil cap work in furna
You didn't mention AC or DC rating of the cap. .... otherwise okay; tolerances are very loose. Frequently+/- 20%.

If the furnace is on a 240 volt circuit, then 600DC is not high enough. 2.828 x AC minimum if rating is not on the cap.
Author:  SamB [ January 15th, 2016, 6:50 am ]
Post subject:  Re: Odd question: will an old Sprague oil cap work in furna
Usually, runcap are 370volt, they are very cheap andyyou can find them at R.E.Michel.
Author:  TubeDriver [ January 15th, 2016, 9:23 am ]
Post subject:  Re: Odd question: will an old Sprague oil cap work in furna
Hey Stu,
It is 120v circuit so the Spraque should be ok (it is 600v DC).

I figured if the cap is off enough, the motor is running hotter and could be pulling more current which might explain the blown relay? On the other hand, the blower boards fail pretty regularly so there does not have to be a secondary cause. I figured replacing the cap with something closer to spec would make sense.

Stuart Polansky wrote:
You didn't mention AC or DC rating of the cap. .... otherwise okay; tolerances are very loose. Frequently+/- 20%.

If the furnace is on a 240 volt circuit, then 600DC is not high enough. 2.828 x AC minimum if rating is not on the cap.
Author:  TubeDriver [ January 15th, 2016, 9:25 am ]
Post subject:  Re: Odd question: will an old Sprague oil cap work in furna
You are right, it is a 7.5uf 370v cap. I ordered a new 7.5uf 440v cap but it will be a couple days till it shows up.


SamB wrote:
Usually, runcap are 370volt, they are very cheap andyyou can find them at R.E.Michel.
Author:  Stuart Polansky [ January 15th, 2016, 9:47 am ]
Post subject:  Re: Odd question: will an old Sprague oil cap work in furna
Mr. Driver,

I could definitely be wrong, but I think the run cap is power factor correction.

http://ecmweb.com/content/power-factor- ... ion-motors

If that's the case, the motor should only run hotter if the current draw on the air handler circuit is excessive and causes voltage drop.

I'm not sure about your individual motor, but in some light fixtures, the power factor is so low that the load of a 150 watt, 120 volt fixture exceeds 5 amps (!). Adding the cap (usually around 10uF) reduces current to about 1.5-1.6 amps. True power vs. apparent power. On a long run, that can be a lot of voltage drop.

Otherwise, I'm not sure that the motor operates any differently...

Stuart

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Author:  J-ROB [ January 15th, 2016, 9:56 am ]
Post subject:  Re: Odd question: will an old Sprague oil cap work in furna
That Sprague oil cap should sound great in your furnace.

Bypass with some .01 MKPs or silver micas for a bit more definition in the highs.
Author:  Stuart Polansky [ January 15th, 2016, 10:03 am ]
Post subject:  Re: Odd question: will an old Sprague oil cap work in furna
J-ROB wrote:
That Sprague oil cap should sound great in your furnace.

Bypass with some .01 MKPs or silver micas for a bit more definition in the highs.


All caps sound the same. It's all in your head. Superstitions and audio-phools.


Hey, wait, maybe J-Rob is on to something here. Replace your furnace blower motor with one of these!

http://www.eminent-tech.com/main.html

TRW-17
Author:  tomp [ January 15th, 2016, 10:08 am ]
Post subject:  Re: Odd question: will an old Sprague oil cap work in furna
Stuart Polansky wrote:
You didn't mention AC or DC rating of the cap. .... otherwise okay; tolerances are very loose. Frequently+/- 20%.

If the furnace is on a 240 volt circuit, then 600DC is not high enough. 2.828 x AC minimum if rating is not on the cap.


This is not a rectified DC circuit so the peak voltage on the cap in either direction would be 1.4.4 times the RMS AC rating. 600 would be high enough. The problem with using a DC cap in an AC circuit is potential heating because the capacitor may not have a high enough ripple current rating.
Author:  SoundMods [ January 15th, 2016, 10:22 am ]
Post subject:  Re: Odd question: will an old Sprague oil cap work in furna
TOMP nailed it. Motor start caps are designed for the purpose and any substitutes need to be able to tolerate line voltage and current.

To quote: "Start capacitors briefly increase motor starting torque and allow a motor to be cycled on and off rapidly. A start capacitor stays in the circuit long enough to rapidly bring the motor up to a predetermined speed, which is usually about 75% of the full speed, and is then taken out of the circuit, often by a centrifugal switch that releases at that speed. Afterward the motor works more efficiently with a run capacitor."
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