AC single-phase fan motor works (almost well) without start capacitor, but not with the correct capacitor connected?

Question

I have a fan motor from a mobile a/c unit here which stopped working. It is for the fan that blows the warm/hot air trough the a/c condenser (exhaust air).

240V AC motor with a 4uF (+/-10%) 450V starting capacitor that has two speeds (high and low). 5 connections/wires:

blue, red (low speed), yellow (high speed), brown and white (which connect to the capacitor terminals)

Not running on either speed. No humming, motor does not run even if given a manual kick start turning. 240V AC present and good.

I tested the capacitor with a capacitance meter and it read at less than 2.8uF which is well below specification.

Replaced it with a new capacitor 4uF 500V testing at correct capacitance.

Motor still not running (minimal humming sound), even with manual kick start.

I removed the capacitor from the wiring and bridged the two capacitor terminals.

This way, eliminating the capacitor from the wiring completely, the motor runs now in both speeds perfectly!

Well, kind of: in low speed connection it requires a slight tip to get it running (as is to be expected without a start capacitor). Without kick start there is just the typical and well noticable humming sound of the motor.

In high speed the motor just about manages to start on its own, gradually starting up and then comes to full speed.

Any ideas what the explanation for this is?

My understanding is that in these types of single-phase AC motors the capacitor is wired in parallel to the auxiliary winding.

Would it be possible to bridge the two capacitor terminals in parallel to the capacitor? Can that work or are there other problems with that?

The motor has a thermal fuse in the main winding, I already check that and it is not blown. The blown thermal fuse is a known problem on these types of motors, I had that before. But even in that case the motor would not run with the capacitor terminals bridged.

EDIT:

I checked the wiring again and it is

• red - blue OL
• yellow - blue OL
• red yellow 67 ohms
• white - yellow 30 ohms
• white - red 98 ohms
• white - blue OL
• brown - blue 0.5-1 ohm
• brown - red OL
• brown - yellow OL
• brown - white OL

-> Motor doesn't run, main windings open

If brown and white are bridged (capacitor connections) it is

• red - blue 95 ohms
• yellow - blue 30 ohms

With this bridge the motor runs in both speeds correctly, but does not start without a little manual push in low speed (capacitor not connected in that szenario)

Answer 1

The capacitor is connected in series with the aux winding, not parallel. It seems most likely that the aux winding circuit is open. That could mean an open winding or an open centrifugal switch or other mechanism for disconnecting the aux winding once the motor is at full speed. The means of disconnecting the aux winding could be external to the motor. Determining that would require a circuit diagram for the a/c unit or tracing the wires coming out of the motor.

If the motor is really the capacitor-run type rather than the capacitor-start type, the problem would most likely be an open aux winding. I suspect it is a capacitor-run motor because the capacitor value seems low for a start capacitor and capacitor-start motors are not often used (likely never) for fans.

In either case, there might be some reason that the aux winding is connected through the switch, but that seems extremely unlikely.

Answer 2

The cable colour code, for connecting the motor, is as follows:

Red - connection to line, through selector switch, for low speed

Yellow - connection to line, through selector switch, for high speed

Blue - connection to neutral

Brown - connection to capacitor

White - connection to capacitor

The final schematic, based on your resistance measurements, shows that a break in the winding, marked 'X' in red, is the root cause of the motor refusing to start.

However, with the capacitor bypassed (brown and white cables interconnected) and the shaft manually rotated, the motor will start and run at both the speeds.