Possible that someone replaced the motor with an 850 rpm instead of 1145 rpm?
That one has caught me before.
Same with run caps. In fact today I called carrier to confirm capacitor size because I had a slow condenser fan on a rooftop unit.
Someone had installed a 7.5 mfd when the motor needed a 10.

 

The motor I installed was the same as what was removed which was 1175 rpm. Funny thing is the condenser next to the one I’m have problems with also had bad bearings in the condenser fan and I also replaced that motor too. Exactly the same motor and capacitor. New capacitors on both per motor requirements

 

Might start checking connections all the way back to breaker panel and see if ya can find anything not quite right.

 

That’s the kicker all three phases are good at L1,2 and 3 at the contacts. Even dropped the lines off the load side of the contacts and put a call to close the contact and voltage was fine

 

Is one leg actually grounded and motor is only getting 120 vac. Seen this a couple times on split system condenser fans. Fam running at half speed. Ohm the wires.

 

Plugged cond coil?
What was hi side press w/ new motor?

 

You said tandem compressors ... so you most likely have 2 fans ?

Something might be going on with the other fan

 

Yes it is tandem but only the one fan.

 

It does not have a speed controller somewhere in the circuit does it? The behavior sounds like what happens when the pressure transducer goes bad on a Johnson Controls p266 head pressure controller. It goes into "limp home mode" and runs the fan at half speed.

 

Is there a fan cycling switch or a motor master for the cond fan. If so bypass it temporarily for a test. Also try hooking the fan up by itself to the line side of the contactor, if same results try moving the wires to diffent phases. Something is wrong , just haven't found it yet...is the motor pulling max amps at 1/2 speed?

 

edit

 

Have you checked voltage AT the motor?

 

No fan cycling controller it’s running directly from the contacts. Was planning to swap legs of power today if I get back to that job. Amps are running max spot on to name plate on motor.

 

Run wires from L1 L2 of contactor direct to cap and motor. What happens? What does motor tach at?
Maybe label has wrong rpm stamped.
What rpm CFMotor , does unit call for?
Does it speed up when the condenser access door is removed?

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Please post the model and serial number of the unit. This should always be included in the first post of any request for assistance.

Thanks.

 

Problem solved. Had a transformer and ambient switch installed aftermarket that was not included in the electrical schematic and I was assuming all legs were going directly to the motor per the print. Long story short ambient switch failed causing fan to run half speed through the transformer all the time regardless of the outside temperature.

 

The old choke coil trick . York used them a-lot on the LCW (?) chillers for low ambient fan control . The transformer was used to drop the voltage to the motor to slow it down.


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