CAPACITOR SIZES FOR CAP-START and/or RUN MOTORS

Stop Yelling (turn off All Caps).

The correct size for a capacitor for a given motor will be listed on the information tag attached to the motor. There's no set size of capacitor for given horse power, it varies by manufacturer and design.
Generally using the correct capacitance for a given motor is best, depending on the source and your personal experience there is zero tolerance for run capacitors and + or -10% for start capacitors. I have also heard + or -10% for run and 25% for start. Personally if I cannot come up with the correct size run capacitor I don't have a problem going 5 or 10% over the correct rating. Start capacitors I try to never undersize and in some situations it can be advantageous to oversize the start capacitor.

The drawbacks to incorrect capacitor size. On a permanent split phase capacitor motor if the capacitor is too small the motor will never come up to speed, to large capacitor and you can burn out the windings in the motor. If the start capacitor is to small the motor will never start. Since the start capacitor is only in the circuit for a short period of time over sizing the capacitor is less critical, however dramatically over sizing a start capacitor can burn out the start windings. Sometimes start capacitors are intentionally oversize (many times by the addition of a hard start kit) to make up for conditions such as low-voltage or excessive starting load. The downside to over sizing the start capacitor is that it can reduced equipment life.

I found this link you might be interested in.

http://dnr.louisiana.gov/sec/execdiv/TECHASMT/ecep/hvac/d/d.htm

Thanks altench:

I think I would try and find a capacitor closer to the original value, just make sure that the voltage rating also matches or exceeds the value of the original, and that you use an AC rated capacitor. Capacitors are a lot cheaper than garage door openers. Good luck

IF I REMEMBER MY NAVY TRAINING DAYS CORRECTLY, CAPACITORS ARE USED PRIMARLY TO CORRECT THE POWER FACTOR SHIFT CAUSED BY AN INDUCTIVE LOAD (ELECTRIC MOTOR). WITHOUT THIS DEVICE THE VOLTAGE WOULD LEAD THE CURENT AND THUS BE OUT OF "PHASE" THEREBY MAKING THE MOTOR VERY INNEFFICENT, RESULTING IN OVERHEATING,ETC.

THUS A CAP START MOTOR HAS A CAP CALCULATED TO CORRECT THE POWER FACTOR OF BOTH WINDINGS UNTIL IT IS REMOVED-GENERALLY BY A CENTRIFUGAL SWITCH-THE RUNNING WINDING , IN THIS CASE, HAS NO PF CORRECTION AND GENERALLY WILL HAVE A RELATIVELY LIGHT RUNNING LOAD.

A CAP START/RUN MOTOR HAS CAPACITORS TO CORRECT THE PF FOR BOTH THE START AND RUN FUNCTIONS OF THE MOTOR AND GENERALLY HAVE A HEAVY RUNNING LOAD.

THE MFG HAS COMPUTED THE INDUCTANCE OF THEIR WINDINGS WITHIN THE MOTOR AND PROVIDED APPROPIATE CAPACITANCE TO CORRECT THE POWER FACTOR SHIFT CAUSED BE THE INDUCTIVE LOAD. THE RESULT OF USING THE WRONG CAP SIZE IS OBVIOUS-THE MOTOR WILL BE RUNNING "OUT OF PHASE" (VOLTAGE AND CURRENT) WHICH CAN RESULT IN DAMAGE EVENTUALLY.

LOOKING UP ALONG THE POWER COMPANIES SERVICE LINES, YOU'LL NOTICE AN OCCASIONAL CAPICATOR BANK MOUNTED ON THE POLES--THESE ARE REQUIRED TO CORRECT THE POWER FACTOR (LEADING OR LAGGING OF THE VOLTAGE/CURRENT) CAUSED BY THE INDUCTIVE LOADS (MOTORS PRIMARLY)--THEIR OTHER LOADS ARE RESISTIVE ( LIGHTING, HEATING ELEMENTS,ETC) WHICH DO NOT REQUIRE PF CORRECTION.

TELEPHONE COMPANIES USE "INDUCTORS" TO CORRECT THE CAPITANCE INHERANT IN "CABLE PAIRS" IN LONG RUNS TO THE CUSTOMER--WITHOUT THIS INDUCTOR THE CUSTOMER WOULD EXPERIENCE AN "ECHO" ON THEIR LINE WHILE TALKING , BECAUSE OF THE "OUT OF PHASE" CONDITION CAUSED BY CABLE CAPACITANCE.

YOU SHOULD GET THE INFORMATION NECESSARY OR BE CLOSE TO THE ORIGINAL VALUES TO PROTECT YOUR INVESTMENT.

DIDN'T MEAN TO GET OFF ON A TANGENT

DONZI

I really don't think anyone answered the original question. I have the same problem. Re-furbishing an older model air compressor and the capacitor needs replacing. SO HOW do you size for the right uf capacitor for the motor?? Don't tell me the tag on the motor- if I could read it, I wouldn't be asking this question, I would just get the number and order a replacement. The old capacitor has no markings as does the motor. So is there a chart to match capacitor size to a motor-that's the question- where do you get information if you have no numbers to work with?????

Sorry, but post #4 is incorrect in that you are confusing Power Factor Correction Capacitors, typically used with 3 phase industrial loads, and motor starting capacitors used in single phase motors. Apples and oranges.

Brief version: A single phase AC induction motor will not spin inherently, there are no relative rotating magnetic fields, there are just two fields that are expanding and contracting. The rotor may eventually spin, but the direction will be random and the acceleration will be so slow that you likely will not get it to full speed before the windings burn out because of excess current. So to START an AC induction motor spinning in a CONSISTENT direction, they use several tactics, several of which involve capacitors. I'm not going to go into detail on that here, that's what search engines are for. But the upshot is, the capacitors are generally used to create a delay in the formation in one of the magnetic fields by virtue of the Capacitive Time Constant, which states that a capacitor can only charge at a specific rate. So in the motor, by placing a capacitor in one set of windings, the delay it causes in the resulting magnetic field created by that winding makes for a relative shift in timing of that field getting to full strength. The winding that gets power first induces the rotor to create it's own field, the stator field that gets to full strength a few cycles later makes it repel the rotor field and start spinning. Once spinning, the process perpetuates itself, that's why the starting capacitors are taken out very quickly.

As to sizing, as was mentioned it is all over the map, because the motor designer has to take into account the rotor mass, friction, windage, torque requirements, magnetic permeability of the laminated steel cores of the stator and rotor, wire resistance, etc. etc. etc. In most cases when a NEW motor design is created, this is actually determined by testing, essentially trial and error. Remember, for them, they only need to do that once, then repeat a million times.

There are sometimes two sets of caps in a single phase motor, a Start cap and in some designs, a Run cap. Start caps can be +-10% of whatever was selected, they are not in the circuit very long. In fact, some motors, like refrigeration motors, sometimes have options tor "hard starting caps" that exceed that +10% value to take care of the need for higher torque because of having to start against a closed head. The trade off is added motor heat, but in refrigeration compressors, moving heat around is what they do! But doing that on everyday motors can cause premature failure, so stick to the +- 10%.

Running caps MUST be the correct size, otherwise you can kill the motor windings by over or under exciting them continuously. Over exciting causes increased heat with no increase in work, which eventually cooks the insulation; under exciting causes a loss of torque at the square of the decrease in relative voltage, so the motor under performs and overloads.

If you have a motor with no nameplate data and bad capacitors with no makings, you have a boat anchor. Sorry.

This would make sense, and the original run cap was marked 15uF,, but all I had was a 40uF run cap to start with.

The motor is a Harbor Freight "5 HP" single phase, 3450 RPM, 208-240 volt, 15 amp FLA, compressor duty. I know, I know.... a wanna be 5 hp chinese motor... it was free to me.

When I am using the 40uF, I measured approx 11 amps max (full tank.. 110psi).

I tried a 15uF that was on another motor and the current went up to 14 amps, and 5 uF it was 16 amps.

Since the current was the lowest with the 40uF cap (~11 amps), couldn't I use the 40 without damaging the motor or is the larger cap hurting something else, like PF or VA?

The original 15uF was open.

Well formed reply...

Particularly like the last sentence... re boat anchor. I burst out laughing at it's accuracy and humor...

Well done.

Simple. Contact the motor manufacturer directly. Please turn off Caps Lock first, especially if using the telephone.

Formula to determine

capacitor size for single phase induction motor,

HERE:

P = Power in kW ,

n = efficiency of motor ,

V = supply voltage ,

C = capacitor in micro ferad

Don't know if this works, but you could try it and see...

That doesn't look right to me. I assume the capacitor size varies as electrical input, so the efficiency n would be in the denominator.

Also trying typical figures, P = 1kW, V = 230V, comes to C 0.0004μF. I'd expect something like 5μF for that size of motor.

μf = (1000 x 70 x 1000) divided by (52,900 x hz 60)

(70,000,000) divided by (3,174,000) = 22μf for a 1 1/3 hp motor

Yeah still seems a little sketchy...

https://www.electricneutron.com/electric-motor/single-phase-capacitor-sizing/

https://www.hvacproblog.com/correctly_sizing_a_capacitor#:~:text=The%20run%20capacitor%20should%20have,from%2C%20for%20Start%20Capacitors%20ONLY!