Replacement Condenser Fan Motor Sizing

[DrChill]

Hi all,

I'd like to get your thoughts on replacing a worn out condenser fan motor with a slightly more powerful one.

The unit has a 6-pole, 1/8-hp motor, but those are expensive and hard to find. The motor is labeled 1,100 RPM.

I have a 1/4-hp condenser fan motor I would like to use. This motor is also a 6-pole but is labeled 1,075 RPM. Since a 6-pole motor has a synchronous speed of 1,200 RPM, it can't go over that. So, in the worst-case scenario, the fan would be turning 9% faster with the new motor. I would think anywhere between 1,075 and 1,200 RPM should be acceptable.

I'm not too worried about the possible increased current requirement. The motor is connected to the same contactor that switches the compressor, so it looks beefy enough to cope with an extra amp or two.

Does anyone see a reason why I shouldn't replace the 1/8-hp motor with the 1/4-hp one?

Thanks!

[Yuri B.]

If the fan remains the same, there is no reason at all worrying about the increased motor's power ; as to the decrease in rotational speed - it is negligeble 2,27%!

[chemi-cool]

The speed is irrelevant, the cfm is important.

[DrChill]

Thanks for your help. My old motor finally seized, and I had to install the new one. It seems to work fine. It only draws 1.2 of the 1.7 A is is specified to draw at full power.

[nike123]

Check difference between air in and out temperature, if it is 8-10K your fan moves enough air.

[taz24]

Hi all,

I'd like to get your thoughts on replacing a worn out condenser fan motor with a slightly more powerful one.

The unit has a 6-pole, 1/8-hp motor, but those are expensive and hard to find. The motor is labeled 1,100 RPM.

I have a 1/4-hp condenser fan motor I would like to use. This motor is also a 6-pole but is labeled 1,075 RPM. Since a 6-pole motor has a synchronous speed of 1,200 RPM, it can't go over that. So, in the worst-case scenario, the fan would be turning 9% faster with the new motor. I would think anywhere between 1,075 and 1,200 RPM should be acceptable.

I'm not too worried about the possible increased current requirement. The motor is connected to the same contactor that switches the compressor, so it looks beefy enough to cope with an extra amp or two.

Does anyone see a reason why I shouldn't replace the 1/8-hp motor with the 1/4-hp one?

Thanks!

I can't see you having any real problems.

Manufacturers and designers try to make out that the way the
condensing unit is put together is critical to the performance of the system.

In reality the condensing unit is capable of taking many of the compressors in the
companies range. It keeps costs down. So for most applications the cond is oversized
anyway and it is only critical on the largest of compressors in the range.


Just beware that if the fan is oversized it might be too effiecient in the winter
and drop the head preasure too low..

taz

.

[abbasi]

No problem we have as far as you increase motor power or rpm inturn increases CFM and forced co0nvection will be greater. We have done this many times and have decreased condenser area also by calculating proper heat rejection.
No wonder it worked fine

[ref717]

No problem, more advantages in replacing a higher motor input power, as long as you don't deviate too much with the existing design requirements especially the airflow CFM.
The 1,075 rpm is the motor nominal full load speed while the 1200 rpm is only the theoretical maximum speed the motor can operate. In actual operation, your motor will run at approx. 1,075 to 1150 rpm.

[hvacrmedic]

The 1/4 is twice the size of the 1/8. That makes it significantly oversized. That would be similar to replacing a 1/4 with a 1/2.

A common side effect of an oversized condenser fan motor is a excessive motor hum (noise).

[IMSTIEN]

Check the load amps on the 1/4 hp motor, if the amps are considerablely lower, than the unused hp will turn into heat energy and shut the motor from the over load protector.