BFQ110 motor runs but does not start

[Cubytus]

Hi there,

I was given a non-running BFQ110 exhaust fan motor to diagnose. Supposedly, after a dust clean-up, the owner put it back and it didn't run.

I went first by looking for a possible datasheet on this model, wich is really a Hung Hsing 2A2561102, to no avail. It is a single-speed, 120VAC fan running at 1625RPM (nominal) and uses up 0.7A and has a running capacitor 3.5µF/250VAC (non-split).

I did a few basic tests:

Current measurement

First connected to a power-monitoring smart plug (just because I already had it, not because it's the most efficient tool for the case) to see if any current was flowing. I also checked the shaft could freely rotate and was not tightened like an old, dried-up fan motor.
Result: no current flow whatsoever, shaft feels "heavy" but can be rotated by hand. I don't have any point of comparison to know if it's normal or tight.

Coil resistance from terminals

Tested coil resistance across 120VAC terminals.
Result: infinite resistance (>20MΩ)

Coil resistance from inside

Opened the motor shell to look for anything out of the ordinary. I carefully removed the tie-wraps as well as insulating covers to test the coil directly. Multimeter gives around 6Ω, which isn't the true resistance as we're dealing with a motor, but at least tells us the coil isn't broken. Without a datasheet, I can't know the nominal values, though.
Result: thermal overload protector burned, a 1$ part-with-2-hours-install-time. Classic.

But burning a fuse (thermal or otherwise) is only a symptom, not a cause.

Powering the coil directly

So I shorted the open fuse, assembled back the motor, powered it…
Result: shaft doesn't rotate, yet power used is 50W.

When rotating the shaft by hand, it immediately started running and power used dropped to 45W. I can't tell if it's spinning at the nominal speed, though, as I don't have a strobe light. That would have told me whether shaft resistance was normal or not.

After a few minutes of running freely, though, the coil got very hot. Again, I had no clue if it was within normal parameters, but I was under the suspicion that a motor running without a load shouldn't get that hot.

Questions:

What prevent a motor from starting, but not running?
On a dead DC motor I was given once, I remember the shaft oscillated but couldn't spin, which I somehow traced to a dead running capacitor (all sealed, so I had to trash it). But this is an AC motor, not DC.

The only part I haven't tested yet is the capacitor. However, as it is a high-voltage capacitor that may hold a charge for a while and there's no apparent bleed resistor across its terminals, I held back. I have a wide range of low-power resistors, but currently don't know how to calculate a bleed resistor value.

Any insight appreciated.