Testing CFM of Refrig Condenser fan. Is my motor slow?

etbrown4June 7, 2012

Other than putting you hand in front of the fan, is there a reliable way to test the cfm output of a refrig condenser fan, or know what it should be?

I've seen a digital cfm tester at Lowes.

After cleaning the condenser coil, I still think I am not getting enough air through that coil.

It's on a SubSero 700, but the question really applies across the board to all refrigeration.

I know all ac motors are supposed to run at their synchronous speed, but I've heard of fan motors needing replacement from running slow.

Have others encountered a slow condenser or evaporator fan motor?

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AC induction motors (most are) do not run at synchronous speed. They always run slower than that.

    Bookmark   June 7, 2012 at 2:20PM
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And the SubZero fan may be controlled by electronics to run at a deliberately slow speed under certain conditions. SubZero customer service should be contacted about this.

If you could measure the cfm flow rate achieved by the fan (which is not so simple to do), what would you do with the information?


    Bookmark   June 7, 2012 at 7:28PM
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It would be great to compare the actual cfm off of the existing fan and motor with the mfg's spec!

Otherwise it's all guess work, and I'd like to avoid that. For that reason, it would be great to know what some pros have done when they have suspected an under performing fan and motor.

    Bookmark   June 8, 2012 at 10:12AM
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I'd be happy to at least verify the actual rpm of the motor, and from there we could at least deduce that if the rpm is right then the cfm's must be! (assuming no damage to the blade)

I know the rpm is stamped on the nameplate. (I'm also aware that the motor might be variable speed.)

Is a meter available which will measure the rpm's of an electric motor? Anybody used one?

    Bookmark   June 8, 2012 at 10:19AM
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A strobe light with calibrated frequency would allow measurement of the blade rate (per second). Dividing by the number of blades and '60' would yield the motor speed (rpm). Or, put a dot on a single blade and sync to it to get motor speed.

I wasn't aware SubZero specified their condenser fan cfm.

Measuring air velocity would allow computation of cfm, but only if flow were uniform over some relevant area. Non-uniform flow combined with blade tip spillage would make the measurement difficult unless the cfm were specified and measured on the other side of the condenser from the fan and integrated over the area of the condenser.


    Bookmark   June 8, 2012 at 12:22PM
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Problem hopefully solved, for now.

Ebay is loaded with digital tach's for $13 including shipping.
Once it comes in, I'll share my experience with it and a comparison of the measured rpm with the nameplate.

Not sure exactly how it works but I think there is a light or laser which reflects on reflective tape applied to one of the fan blades.

No strobe required and hopefully no math.

    Bookmark   June 9, 2012 at 11:30PM
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Digital Laser RPM meter worked like a charm!

Subzero says it's a 1360 rpm motor and I got that plus or minus a few!

At least the condenser fan is ruled out, and I have a great little rpm meter.

Will have to look further to see why I get a service light most of the time. The service light on this unit is supposed to signify that the compressor is running for longer periods that it should. (The condenser coil could not be cleaner.)

Possibilities are: a thermistor, the sealed system and charge, and possibly a few others, I suppose.

    Bookmark   June 17, 2012 at 6:10PM
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