A/C Fan Motor

[Rontx]

I replaced the fan motor (3-wire) and capacitor in my condenser unit. I was uncertain if I needed to keep one side of the cap. connected to Line as the original cap. was as the new motor (4-wire) did not show this. I tried it without this connection and all seemed well for a couple of days then the fan stopped and was very hot.
I reconnected the Line connection to the cap and the fan ran again but seemed to run slower. After several minutes it stopped again and was very hot. I checked the cap with an analog meter and it reads open. I am wondering if running the motor without the line connection to the cap may have ruined the cap and possibly also the motor? I can try a new cap but do not know how to test the motor. I have made some resistance readings but do not know what I should be reading between the leads. It has brown and brown/white to the cap and a black and a yellow to the line. Does it matter which line connection I put the wires on as long as I am on L1 and L2? Also, could I have damaged the compressor? I do not know how to test them. I was told that there may be a reset switch on it but have not noticed one.
Thanks for any assistance.
Rontx

[LazyPup]

There is an industry standard color code but there is no official requirement for the manufacturer to use it, so it is very important to check the motor data plate or the data sheet that comes with the new motor before making any connections.

Normally for a single speed, non-reversing motor the Black wire is Line 1, the yellow or white wire is Line 2 and the Brown wire is the capacitor wire.

Typically there is a wire connected from L1 on the contactor to the capacitor and the Brown wire from the motor is attached to the opposite capacitor terminal.

Some capacitors are also polarized, which means you must connect the wires to the appropriate terminal on the capacitor. When we have a dual capacitor the center terminal is the Common terminal, which is connected to Line 1 on the contactor. The terminal marked 'H" or "Herm" is the capacitor connection for the Hermetic compressor and the terminal marked "F" or "Fan" is the correct terminal for the Brown wire from the fan motor. If you have a single capacitor that it polarized you will see a small "RED DOT" or a + sign painted or embossed into the capacitor housing near one terminal. That index indicates the Common side of the capacitor. Here again, they typically run a wire from Line 1 on the contactor to the identified terminal on the capacitor. The Brown wire from the fan motor is then attached to the non-identified terminal.

Some motors such as yours will have both a Brown wire and a Brown/white stripe wire. When the motor has both a Brown and Brown/white stripe wire, the wire from the contactor L1 terminal to the capacitor is not connected. The Brwon/White stripe wire is connected to the capacitor identified terminal and the Brown wire is connected to the opposite terminal on the Capacitor. (The brwon/white stripe wire is internally connected to line 1 in the motor therefore the wire from the contactor is not necessary.)

In order for a capacitor run motor to initially establish rotation the start winding in the motor is rotated slightly from the run winding position. The capacitor then causes a slight momentary phase shift of the incoming current so that the current in the start winding is at full potential when the run winding in minimal and vice versa. (In a 60hz motor this phase shift takes place 60 times per second). The actual amount of phase shift required is determined by the actual position of the windings and not all motors are the same therefore it is critical that the capacitor be the correct value for the motor at hand. For this reason, it is a very good practice to always change the capacitor when changing a motor to insure you have the correct capacitor for the new motor, in fact, in most HVAC supply houses if you ask the counter guys to pull a motor they will automatically set a new capacitor on the counter with it. Now, whether you choose to purchase the new capacitor is up to you, but capacitors are cheap and most trained techs will take it without question.

Now while a defective capacitor is a leading cause of motors overheating, it is by far not the only cause.

A motor produces energy by creating electro-magnetic fields in the motor windings. Primarily the amount of energy the motor draws is then determined by the strength of the magnetic fields, which is determined by how the windings are made.

There is a firm law in physics which states that energy cannot be created nor destroyed but only changed in form. In an electric motor the electrical energy if first transformed to magnetic energy which is then transformed to mechanical energy by the rotation of the motor. This is a finite balance as 746watts of electrical energy is equal to one horsepower of mechanical energy. When a motor is properly sized the incoming electrical energy is transformed to mechanical energy and consumed by the load, however, if a motor is oversized it will draw a current equal to its rated horsepower and will produce the magnetic fields equal to the rated horsepower but the amount of energy extracted from the motor in the form of mechanical energy is less than the applied energy so the remaining balance of energy is dissipated in the form of heat energy, and voila', an over sized motor burns out quickly leaving the installer scratching his/her head and wondering why.

As you can see, it is important that the load must be equal to the output of the motor, therefore when changing a fan motor it is also vitally important that the fan blade be positioned on the shaft correctly. The fan blade must be positioned on the shaft so that the blades are aligned in the center of the fan shroud in the condenser housing, otherwise the fan cannot pull the air correctly, resulting in a reduced load on the fan motor, ergo the motor overheating and an increase in the compressor head pressure, which puts a severe strain on the compressor. When selecting a replacement motor it is very important that we get the same horsepower, rotation and RPM as what the load is rated for, otherwise you can expect the motor to fail in a short time.

Whenever we replace a motor or capacitor it is a good practice to check the run amperage of the motor to make sure it is operating within its designed rating. Normally a high amperage reading would indicate the load is too great for the motor, but a lower than normal reading would indicate the load is not great enough. This may mean the fan is miss-positioned or it would indicate the motor is too big.
(See Attached illustration).

[dnpeters]

LazyPup - I enjoy reading your electrical explanations, and have gained a greater understanding of electrical theory as a result. I am a little confused, however, by your explanation that a 3/4 hp (my figures) motor would create substantially more heat than a 1/2 hp motor driving the same load (fan). If heat is a byproduct of work, wouldn't both motors be doing the same amount of work and thus produce about the same amount of heat? Again, thanks for the continuing education. Good luck. - Dave

[Rontx]

A/C Fan Motor

Thanks for the good information.
When I purchased the new motor I took the old one with me. Before I installed it I verified the spec's were the same for the new motor. The counterman also pulled a new capacitor as you mentioned. I verified it also with the old one and the data for the new motor. The capacitor they sold me was nonpolarized and I was told that it didn't matter as they make them now so they can be installed either way. Someone had mentioned the fan blade position so before I took it apart I marked the blade position relative to the shroud. I reinstalled it in the same position. This unit was in place when I bought the house in 1975 and I believe it is the original parts.
The wiring diagram on the motor simply shows yellow and black to line without regard to L1 or L2 and brown and brown/white to cap without regard to an indicated terminal.
I did happen to get black to L1 and yellow to L2. As the cap is non-polarized I simply hooked the brown and brown white without regard to terminal position.
Unless the fan is actually somewhat out of position I am confused as to what caused the failure. Since the fan motor appeared to be running slow and quickly overheated I am wondering what my next step should be.
I do have an ampmeter and could try to run a test on the motor before it overheats.Thanks for your help and will welcome any other thoughts.
Rontx

[LazyPup]

If we were examining an internal combustion engine we could say that heat is a product of work, and if the motor is oversized it will basically play with the load, therefore it would run cooler than a motor running at full rated load, however such is not the case in an electric motor.

In an electric motor energy is consumed by making electro-magnetic fields in the motor windings. Regardless of what size of load we have the size of the windings remain the same, thus the amount of magnetic flux produced remains nearly constant. If the magnetic flux is induced into an armature it is then dissipated in the form of mechanical motion by the rotation of the motor however the mechanical energy is still in the motor unless we attach a load to which the motor can transfer the energy. When the amount of electrical energy being drawn into the motor is in equilibrium with the amount of mechanical energy being drawn out of the motor the only heat generated is the heat of bearing friction and a slight loss to hysterisys currents or electrical eddy currents in the motor windings.

Now let us consider what happens if we oversize a motor.

746watts of electrical energy equal 1 Horsepower of mechanical energy.

Let us use your example of a 3/4HP motor in the place of a 1/2HP motor.

If 746watts = 1HP we can then say 3/4HP = 746 x .75 = 560watts and 1/2HP = 746watts x .50 = 373watts.

The 3/4HP motor windings would now draw 560watts of energy which they convert to magnectic flux. The load is only equal to 373watts of energy so we have a remaining balance of 560w - 373w = 187watts of energy

Understanding that energy can neither be created nor destroyed it stands that the excess energy must go somewhere. In this case the 187watts of excess energy entering the motor cannot be consumed by the load so it has no alternative but to dissipate in the form of heat within the motor windings. To give you a rough estimate of how much heat that is, consider that a MR.Coffee coffee maker has a 175watt hotplate to make it function so in a sense we could say that by replacing a 1/2HP motor with a 3/4HP motor we are expecting the motor to operate with a 187 heating element buried in the windings.

[dnpeters]

Very understandable explanation, even for this amateur. Again, thanks.

[suprheat]

Since you originally hooked it up wrong, you may have damaged the motor and/or the cap. You said the cap read open. If it is then the motor will run slower, if it runs at all, and will overheat. You probably need a new cap. This is a PSC motor and doesn't have start windings, also we're talking A/C not D/C so polarity of a cap has no bearing.