air con won't blow

[I_am_Yu]

I'm working on a roof unit in which the squirrel cage blower motor won't run. With power off, it spins freely by hand. With power on, the motor hums but does not run, and if I try spinning by hand there's a lot of resistance. Is this a bad motor?

[j-d]

I ordered this motor from Amazon and it was direct replacement. If you look at the Reviews, some say it comes with a new capacitor and some say not. Ours did NOT. Also did not come with wiring instructions but the colors were an exact match to the old ones.

[Bob_Landry]

wa8yxm wrote:

I am Yu wrote:
OK, it has a fan capacitor, a run cap, and a start cap. I only need to worry about the fan cap, right? (I can hear the compressor coming on.)

Fan cap is marked 7.5 microfarads +/- 10%. Measuring with a Fluke meter gives 6.51 microfarads. Does this mean it's bad... bad enough to cause the motor to lock up?

The fluke meter reading means nothing.. A capacitor can test out good on the fluke at say 9 volts peak (or less) and still be toast at 120 volts.

Maybe it's possible, but every one that my meter said was bad was bad and replacing it fixed the problem. I have never had a false reading on a cap check. I've checked many of them and have never had a bad one read good.

[wa8yxm]

I am Yu wrote:
OK, it has a fan capacitor, a run cap, and a start cap. I only need to worry about the fan cap, right? (I can hear the compressor coming on.)

Fan cap is marked 7.5 microfarads +/- 10%. Measuring with a Fluke meter gives 6.51 microfarads. Does this mean it's bad... bad enough to cause the motor to lock up?

The fluke meter reading means nothing.. A capacitor can test out good on the fluke at say 9 volts peak (or less) and still be toast at 120 volts.

[j-d]

I doubt the difference in uF between labeled value and meter reading will keep it from starting.
By "resistance" in turning the motor, is this with the power off? I just replaced a Coleman (RVP) motor with bearings that kept gumming up. Took it all apart, cleaned shaft and bearings, would run about half a day and stick again. I didn't have time to try to run down some new bearing bushings and just replaced the motor.
I found WD-40 loosened the thing up better than the good Zoom-Spout "turbine oil". On ours it was the bearing on the outdoor fan end of the motor that stuck. When it wouldn't turn freely, it also would not move in and out.
If it WILL spin freely, then please try the spin test Doug described.
I'm fortunate to have a Fluke DVOM that measures uF, it's a "73" I think. But I noticed a number of DVOM that will do this. Beginning about $30 at Harbor Fright.

[Bob_Landry]

Altern wrote:
You have a Fluke that measures capacitance? What model number is it?
Most multi-meters are not capable of this. If so, it must have cost a bunch $$$.

I have had two low end Fluke meters and both have had capacitor check capabilities. The Flukes are overpriced, IMO. Field Piece is coming out with some good test equipment.

[Altern]

You have a Fluke that measures capacitance? What model number is it?
Most multi-meters are not capable of this. If so, it must have cost a bunch $$$.

[dougrainer]

This is a simplified version from RVP. Doug
Set the system controller to call for cooling
operation. Temporarily restore power to the air
conditioner taking care to avoid moving parts. If
the motor will not start, while energized quickly
spin the motor by hand in the usual direction of
rotation. If the motor comes up to speed and
continues to run, remove power and check the run
capacitor. If a capacitor checker is unavailable,
substitute a known good capacitor of the same
rating. If the motor now comes up to speed
without assistance, the original capacitor was
defective. If the motor will still not start without
assistance, the motor start winding is open or
badly shunted and the motor must be replaced

[dougrainer]

Usually, with a bad fan capacitor, if you manually try to spin the blades, with the system powered UP, and the fan starts and runs, look to a bad capacitor. If the motor binds up when powered, but spins freely when no power, points to a bad fan motor. What year/brand/model AC unit? Doug

[I_am_Yu]

OK, it has a fan capacitor, a run cap, and a start cap. I only need to worry about the fan cap, right? (I can hear the compressor coming on.)

Fan cap is marked 7.5 microfarads +/- 10%. Measuring with a Fluke meter gives 6.51 microfarads. Does this mean it's bad... bad enough to cause the motor to lock up?

[Altern]

j-d is one smart cookie. My $$$ is on him for correct response...:)

[j-d]

More likely, the FAN side of a combo capacitor (other side being HERM for Hermetically Sealed COMPRESSOR) or a dedicated FAN capacitor. There may also be a Compressor Start Capacitor. Fan motors, even compressors, don't fail as often as capacitors.