Capacitor question....................(Starch)

Matt Undy

I don't really have a good answer. You could find a good motor shop and ask them. Frequently the capacitor value is on the ratings plate somewhere. You could also go by the horsepower rating and the NEMA code that designates its starting current(which I don't remember the specifics of, there are 2 codes on the ratings plate, one that relates to the tempurature rating of the motor and the other that relates to the locker rotor current...or some motors just state the locked moter current.) Check the motors chapter of "Practical Electrical Wiring" or "Modern Refrigeration".

If it were my motor, I would find a similar motor in a catalog, see what capacitor it required, and go wihth a 440 volt AC capacitor of the next capacitance range up.

You also need to determine if it is just a start capacitor or if it is a run capacitor. If there was only one capacitor and it has a centrifugal switch on end of the motor shaft(you usally can see a spring, flyweight, and spool assembly inside the housing on the terminal end if it is an open type motor), it is probably capacitor start, induction run. You will need to select a capacitor that can take continuous duty if it is a capacitor run motor.

You may be able to use a hard start if there are ones designed for use with centrifugal switch type capacitor start motors(most are designed for use with hermetic compressors which use a current realy instead of a centrifugal switch because the motor shaft is sealed inside the compressor casing). Try looking at a Supco catalog....

If the motor is capacitor start, the likeleyhood of causing problems if you're wrong is less than if its capacitor run.

Basically, make sure the motor gets up to speed, look at its current once its runing, make sure its at or below the nameplate rating, and make sure that neither the motor nor the capacitor get too hot.

There may be somewhat less protection from a locked rotor or faliure to start completely because of overload condition if the cap is oversized, it may have more of a tendency to burn out the start winding instead of blowing the internal fuse in the capacitor.

Also watch it for the first few weeks for signs of overheating or failure to start.

Joe Brix

Please help my incapacitated brain.........

....by answering my question about the capacity of capacitors!

If you have a dual capacitor for a condensing unit, say 35/5 microfarad, and it's rated for 370 volts, is there any problem with installing a replacement capacitor rated for 440 volts????


Starch

Mitch_4

so long as

the microfaarad rating is the same, you can use a higher voltage rating, but not a lower one.

Mitch

Johnboy

microfared

You can sub the voltage higher but not lower. as far as microfareds, you can use one that is + or - 10% of the rating.

Joe Brix

Thanks, Johnboy and MitchB.

I thought that was the case, but could not find any "proof."

Paul Fredricks_3

+ or - 10% is true, but I never understood why they said that. Try and find a 31.5 or 38.5 UF cap. Must have more to do with electronics.

Joe Brix

I always was told...

....that if your actual microfarad reading was within 10% of the rating, the cap should be ok. Beyond that tolerance, then you should replace it.

jim lockard

higher volts

Some guys only carry the 440 volt caps saves space and inventory. J.Lockard

Joe Brix

And that, Jim.....

....was the exact motivation for my question! I'm trying to reduce shop and van inventory!

Starch

jim lockard

Caps

Glad to have helped. J.Lockard

Bob Bona_4

A must have on the truck

Amrad makes a universal cap-the Turbo200- that fits just about everything depending on which terminals you jumper. This things is the answer to my prayers. Indispesible. I've already used two of them this month, got the A/C on and got paid in one trip.

Cosmo_3

turbo 200

I love this cap, saved my butt many times!

Cosmo

Paul Fredricks_3

Oh

That makes more sense.

Jeff Lawrence_25

I've put three

Yep, 3 of them on in a week.

My 'favorite' supplier is out of stock on them. Lotsa local thunderstorms lately....

Tom Hopkins

The tollerance of the capacitance of most electrolytic capacitors is +-20% when they are new from the factory. This varies with manufacturing variences and things like ambient tempurature. Many capacitors manufactured for motor start/run applications are marked with a capacitance range instead of a specific value and tollerance.

It is probably not the cause of your problem unless its capacitance is far lower than its rating.

The working voltage simply refers to the voltage the dialectric is designed to withstand, so a higher voltage rating isn't a problem.

A capacitor in this application is just passing a current and shifting its phase 90 degrees. The shift is always 90 degrees, regardless of capacitance. This produces a second magnetic field in a second set of windings to encourage the motor to rotate in a specific direction. The capacitance in this instance relates to the amount of current that the capacitor is capable of carrying. As a result you can repalce a capacitor with a capcitor with a higher capacitance. Of course over multiple iteratiosn either increasing the voltage or capacitance could result in somethig similar to oversized heating plant syndrom.

Also be sure to pay attention to if the capacitor is in a start or run application and what the rating of the replacemnt is. A capacitor that is designed for a start applicaiton is only designed for intermittant duty while the motor is starting and is switched out by a centrifugal switch or current relay once the motor gets up to speed. This type of capcitor will overheat and fail if used as a run capacitor where it stays in the circuit continuously, but a run capacitor could be substituted for a start cpacitor.

Also remeber that some older capacitors were filled with PCB.

Joe Brix

Thanks, Matt.

That was some very useful information!

Starch

Unknown

Hey Matt,

seems like you really know about capacitors, so I have a question. If you have a motor say from a jet-pump, and it`s cap is completely missing, is there a way to find-out which size it requires?

Dave

Darren_11

Multi tap

Hey starch have you tried any of the multi tap capacitors they do about 20 different values and they'll get you out of a jam on a service call. Darren

rucomfy

Are you sure about that tolerance........??

Many of the capacitors that I see on both old and new equipment usually are stamped or labeled with the rating and a tolerance of plus/minus 6% (+-6%). That is the criteria I use to test a capacitor. And any capacitor with a bleed resistor must have it temporarily removed for an accurate assessment. Twenty percent seems high to me.

If I have a 35mfd then 28mfd is OK (35 x .80 = 28). It would seem to be asking for trouble during the peak cooling seasons and may prompt a callback in July.

Me thinks I stick with 6% as stamped or labeled. Check it out.

Also have used Turbo cap. Be sure to use correct jumper and mark cap so next guy knows what you took out.

Watch for the common leg on those three wire condensing fan motor hookups.

Bruce Stevens

I was always taught

plus or minus 10 percent, I also was taught the percent stamped on the cap was what was allowed as factory allowance when manufactured. If you had a cap that was at 6 percent new it was still operational until it reached the 10 percent mark.