Single phase 3-speed AC motor won't start spinning or spins in wrong direction

Question

I have a Danby DPAC8399 air conditioner, and its fan motor started acting strangely. Sometimes, it it will only do a hum and won't start, and other times it will do that hum for a few seconds and then start in the wrong direction. I can help the motor by giving a push to the fan so it starts in the proper direction. From these symptoms, I though the start/run capacitor was bad. So I measured it with a multimeter and it displayed 7.72 μF, which is in range for an 8 μF ±5% capacitor. Still, I replaced it just in case, but the motor is still having the same behavior.

Now I am wondering what to check next. Is there a way to know whether I have to replace the PC board or the motor itself?

Here is a picture of the motor and its plate:

In case it can help diagnosing the problem, I measured the resistance between the different wires (Updated with values measured while motor disconnected prom PC board):

Color	Yellow	Blue	Black	Brown (Neutral)
Red	11.2 Ω	15.1 Ω	∞	21 Ω
Yellow		5.1 Ω	∞	31.5 Ω
Blue			∞	46.5 Ω
Black				∞

The fan has 3 speed (Low, Med, High) and I measured the voltage between the ground and these wires for all the speeds:

Speed	Yellow	Yellow	Blue	Black	Brown (N)
High	123.5V	124.1V	124.5V	20.15V	0.5V
Med	105.8V	123.5V	127.0V	12.96V	0.425V
Low	94.2V	113.5V	123.5V	12.55V	0.4V

The air conditioner has 4 modes (Cool, Heat, Dry and Fan), but I did all my test in fan mode only.

Picture of the PC board:

Picture of the PC Board on which you can see the capacitor at the bottom:

On the right of this picture, you see the 5 colored wires going to the motor. Outside these wires there is a ground wire, and an orange and a white wire going to the capacitor:

The original capacitor and its specifications:

UPDATE:

I am also wondering why the black wire has infinite resistance with the other motor wires and has only 20 volts or less. And why the brown wire has only 0.5 volts or less? Does this look normal for such a motor?

UPDATE 2:

Well, it was quite a bit of work taking the motor out of the AC. It is definitely not meant to be serviceable unfortunately. And no luck finding a centrifugal switch. See the pictures of the open motor:

Also, I am wondering what the symbol means on the brown wire on the diagram:

UPDATE 3:

I ordered a new PC Board and swapped the PC Board, but that did not fix the issue. The motor still sometimes starts in the wrong direction.

UPDATE 4:

I managed to find a new motor and swapping the motor fixed the issue.

Answer 1

An AC motor which uses a start capacitor will exhibit this behavior if the capacitor is not connected; it is possible that this motor has a centrifugal switch, which should start the motor by connecting the capacitor until spinup is achieved; dirty contacts on such a switch will make it hard to start, and sometimes just hum. The centrifugal switch will be affixed to the rotor, inside the motor bell housing.

It is also possible that some other switching mechanism (maybe one of the relays?) is intended to be a starting switch, so it might be useful to trace the capacitor's wiring and see if it has a relay connected, which does not have electrical continuity from its COM to NC terminals (this can be checked with power disconnected using an ohmmeter).

Answer 2

A bad crimp on the connector of the orange or white wire could account for the failure to start because the capacitor would be disconnected. See if you can validate continuity between each connector and the conductor in its wire. Note that the orange wire is under stress and continuity may be intermittent depending on stress and position.

Answer 3

A dead capacitor or a dead connection between the capacitor and the motor.

The capacitor-phase-shifted winding determines the motor direction. Without it, the motor can run either direction, provided you "show" it in which direction to start. If the motor is not "shown" where to go, it hums until either overheats or starts in random direction.

In some motors, the "starting" capacitor is disconnected by a centrifugal switch or a timer. Both the switch and the "start" winding may be defective as well, but the capacitor is the usual element to swear at and replace.

The underlying reason:

The easiest to understand motor is three-phase. The three windings powered by these 3 phases create a rotating magnetic field and the rotor follows it. In the single phase motor, the field doesn't really rotate, it just alternates between the two directions. It looks like rotating and the rotor can follow it pretty well, provided it already runs. And for starting in a particular direction, there is a separate winding wound at angle to the main one and powered by a phase-shifted voltage/current.

(There is a good example for a motor that starts in random direction by design in the microwave ovens. The table turns in whatever direction it feels like.)