Replacing AC Condenser Motor?

[crabjoe]

I'm trying to figure out if I need a new motor or just a capacitor. How does one tell?

The symptom I have is this. The fan works, but you have to nudge it to get it started. No nudge, no start.

Is this a capacitor issue or a motor problem?

The motor is a AO Smith F48J66A48 in a York condenser.

Thanks!!

[buckofdurham]

If it was the capacitor it would not run at all even with a "nudge".
Sounds like the motor is almost dead.
Pour some oil in it.

[HayZee518]

turn on the power to the motor then shut it down and open the breaker. take a screwdriver and short out the capacitor. if you get a sharp spark, then the cap is ok. if you don't, the cap is shot.

[crabjoe]

The fan spins up normally. It's only an intermittent problem, so I can't just test the capacitor with a screwdriver.

Buckofdurham,

When you say to pour some oil in it. Where should I be pouring the oil?

Thanks all!

[HayZee518]

ok, intermittent. inside the motor is a spring loaded centrifugal switch connected in series with a winding and the capacitor. as the motor spins up a pair of "flyballs" move outward from the motor shaft and open a set of contacts. when this happens the motor run winding is actuated. if the flyballs do not move outward then both the start winding and run windings are energized. with this you have an increase in motor current and the possibility of burning out the start winding. I think buckof durham means add oil to the oil-lite bearings

[LazyPup]

As is true with nearly all AC motors, the A.O.Smith F48J66A48 is a PSC (permanent split capacitor) motor and it does not have an internal relay.

Although PSC motors do have two sets of windings, and one is commonly called the "run winding" while the other is commonly called the "start winding" in reality that is not correct.

When a motor has a true "start winding" the start winding is only energized momentarily during start in order to overcome inertia and establish the direction of rotation. Once rotation has begun and the motor spins up to its rated RPM either an internal governor relay or an external potential relay drops the start winding out of circuit.

In a PSC motor there are two identical sets of run windings, the inner winding which is called the "Run winding" and the outer winding, which is commonly called the "Start winding". When the power is first applied to the motor the inner winding is wired directly to the power source and is energized immediately. The power for the outer winding is then passed through a capacitor which causes a momentary time delay before energizing the outer winding. The time delay results in a slight differential between the theoretical center of the magnetic field in one winding in relation to the center of the magnetic field in the other winding. The differential between the magnetic fields establishes and maintains the direction of rotation thus in a PSC motor the "start winding" remains in circuit the full time the motor is running.

If a PSC motor will start when bumped as mentioned above that is a prime indicator that either the capacitor or the wiring for the start winding is defective. It should also be noted that if the motor is bumped and runs, it will overheat in a very short time.

The best method of testing a capacitor is to use a "capacitor tester" which has the ability to check the value of a capacitor under load, however as a rule capacitor testers are relatively expensive and seldom used in the real world outside of a shop.

A simple yet very reliable method of testing a capacitor is to use an analog VOM (volt,ohm.mili-amp meter).

You begin by disconnecting the main power source from the condensing unit then use a well insulated screwdriver to touch across the two poles of the capacitor to discharge any remaining charge it may be holding. (Do not take this step lightly, capacitors can store a tremendous charge. Absolutely never touch the contacts on a capacitor until you have discharged it, even if it has been out of circuit for months).

With the power off and the capacitor safely discharged you then remove the wires from the capacitor.

Now set your VOM to the highest resistance scale and hold one probe of the meter on one terminal of the capacitor while you touch the opposite terminal of the capacitor with the other probe. If the capacitor is good the needle on your meter will immediately swing up to about 1/3 to 1/2 scale, then begin falling back like a windshield wiper on your car. Reverse the probes and touch again, it should again repeat the upswing and drop off.

If the meter needle does not move the capacitor has a open circuit and needs to be replaced.

If the needle moves to full scale and stays there, the capacitor is shorted and also needs to be replaced.

When replacing capacitors on PSC motors we should make every effort to replace it with the exact value and when replacing a motor we must check the motor spec sheet to determine if the new motor uses the same value capacitor or if you will need a different one. (Personally, i make is a practice to always get a new capacitor matched to the new motor spec.) When it is necessary to replace a capacitor with a different value you must have a capacitance value equal to or greater than the original but you under no circumstances should you oversize more than 50% above the original rating for the motor.

The A.O.Smith motor, model number F48J66A48 listed above is a 1/4HP 208/230v 1Phase 850RPM PSC type motor and it is rated for a 5ufd capacitor.

[crabjoe]

LazyPup,

So you feel the problem is the capacitor. That's similar to what I've found, when doing a google search.

With that knowledge, I think I'm going to replace the capacitor to see what happens. They're only a few bucks and I don't like the ideal of testing one. Those things can be very shocking.

Thanks!

[Methodical]

LazyPup,

So you feel the problem is the capacitor. That's similar to what I've found, when doing a google search.

With that knowledge, I think I'm going to replace the capacitor to see what happens. They're only a few bucks and I don't like the ideal of testing one. Those things can be very shocking.

Thanks!

crabjoe what was the verdict. I am in the same situation.

Thanks

[crabjoe]

crabjoe what was the verdict. I am in the same situation.

Thanks

The capacitor seems to have done the trick, but I won't know for sure until the units been sitting off for a long while.

[Methodical]

I ended up replacing the motor and its been running fine so far.

[HayZee518]

I was on a service call to a guys house for an air handler motor in a heating unit. Now, no one would assume a motor to be bad, right? WRONG! Complaint was that the motor wouldn't run off the plenum limit. I found three wires melted together in the plenum ducting. So I figured I solved the problem when I replaced the wires. Fan still wouldn't run. There was a long three wires running from the splice box down inside the plenum to the motor. The motor was a "b***" to get at. I found a white wire melted to the plenum and shorting out, a blue wire an intermittent short, which would blow the breaker, and a black wire that didn't go anyplace. After replacing a bunch of wires I was able to get the fan to run at half speed off the plenum limit. The guy replaced the fan limit for about 86 bucks but the problem was still there. Come to find out, one motor winding, the one for slow speed had its field grounding out inside the motor housing. So no matter what speed the fan called for it would always go into slow speed. He ended up changing out the motor. Now he has high AND low speeds.